Daily Archives: May 3, 2021

INTRODUCTION

Excitation of waves by a moving object is ubiquitous in many areas of physics, such as electrodynamics (1), acoustics (2), and hydrodynamics (3)examples are the Cherenkov effect, sound waves, and ship wakes. These processes are thought to be successfully explained by classical physics, wherein wave interference is often critical for describing the phenomena. For example, in electrodynamics (1), radiation emission patterns are predicted by Maxwells equations.

Such is the case for Cherenkov radiation (CR): the emission of light by free charged particles moving faster than the phase velocity of light in a medium (4). Since its discovery in 1934, a long-standing hallmark of CR is its manifestation as a shock wave of light (1, 59), resulting from the coherent temporal interference of radiation at a wide spectral range. Despite the wide applicability of this effect, no experiment has ever directly observed the shock wave dynamics emitted by a single particle. As one of the implications of this work, we shall see that the underlying quantum nature of CR fundamentally limits the shock wave duration in many existing experimental settings, which can be understood in terms of the entanglement of the light with the emitting particle.

Looking at the bigger picture in electromagnetism, light emission by free charged particles constitutes a family of effects (10, 11) including, for example, transition radiation (12), Smith-Purcell radiation (13), undulator radiation (14), and CR. Often called coherent cathodoluminescence (10) (CL), these phenomena are used in many areas of physics and engineering, from electron microscopes (10, 15), particle detectors (16, 17), free-electron lasers (18), engineerable light sources (19, 20), and medical imaging (21). As the spectral range of emitted light can be straightforwardly tuned by varying the particle energy, coherent CL is a promising platform for light generation in otherwise inaccessible regimes (18, 22), such as at terahertz, ultraviolet, and x-ray frequencies.

The broad tunability of coherent CL, alongside recent advances in shaping (23, 24), coherent control (2527), and entanglement (28, 29) of free electrons, makes it a probe of fundamental light-matter interaction (15, 30) and a prominent candidate for quantum measurement (15). These advancements brought about fundamental questions regarding the role of the particle wave function (3034) in coherent CL. However, in all relevant experimental settings, coherent CL is still considered as classical (3539) or semiclassical (31, 34, 40, 41). The general expectation from a quantum theory is that when the emitting particle is not directly measured (42), the quantum features of its wave function (4345) cannot leave a detectable mark on the emitted light. A milestone of fundamental importance would be, therefore, to identify observables of coherent CL radiation that are both detectable in practical settings and directly depend on the quantum state of the emitting particles. This observation has implications also for general wave phenomena, such as any mechanical waves excited by free moving objects. Can fundamental quantum aspects of a particle affect the patterns of waves in seemingly classical regimes?

Here, we introduce the quantum optical paradigm to describe coherent CL and identify the specific measurements that depend on the quantum wave nature of the emitter. By formulating a general quantum theory of spontaneous light emission by charged particles, we show that already in what are generally assumed to be classical regimes, coherent CL can be dominated by quantum features such as wave function uncertainty, quantum correlations, and decoherence. These effects can be exposed in quantum optical measurements, such as first-order correlation measurements, even in seemingly classical features such as the emitted pulse duration. Although the concept of coherence transfer was thoroughly studied in quantum optics for nonrelativistic bound-electron systems [for example, in effects such as quantum beats (46)], it was never applied to light emission from relativistic free charged particles, still commonly described in classical or semiclassical terms. Hence, new insight is gained by analyzing the optical coherence of such system through the prism of quantum optics.

As an unexpected implication for the Cherenkov effect, we find that quantum decoherence imposes a fundamental lower bound for the Cherenkov shock wave duration, predicting an uncertainty principle that connects it to the particle momentum uncertainty. Quantum coherence is the ability of a quantum system to demonstrate interference. The coherence between different parts of a wave function (in momentum or real space) allows for the famous double-slit interference and the formation of short quantum wave packets propagating in space. Quantum decoherence, as its name suggests, is the loss of quantum coherence, hindering the visibility of interference. Most commonly, this process happens when an open quantum system interacts with its surrounding environment (47). The underpinning mechanism for decoherence is the entanglement of the observed subsystem (for example, an emitted photon) with another, unobserved subsystem (for example, a charged particle). In our context, we identify many practical scenarios in which CR is not a shock wave, owing to the underlying quantum decoherence of the emitted light.

Our quantum theory of coherent CL has new applications, such as detecting the shape, size, and coherence of the emitters wave function by measuring the spectral autocorrelations of the light it emitsthereby gaining information on the wave function uncertainty. Our findings can resolve a question, which, with the advent of ultrafast electron microscopes, has been frequently asked: What part of the measured energy spread of an electron beam is due to coherent energy uncertainty, and what part is due to incoherent uncertainty? Moreover, our work sheds light on fundamentally new capabilities to measure quantum properties of charged particles that can serve as an alternative to matter wave holography, which is especially important for many high-energy particles observed in Cherenkov detectors, where holographic techniques do not exist. The results presented in this work pave the way toward novel tunable light sources and measurements sensitive to the wave function of free charged particles.

In classical physics, waves interfere coherently when they are generated from different point particles constituting an emitter (48), so long as the different emission points are perfectly correlated with each other (Fig. 1A). In particular, the emission from each individual particle is considered to always be coherent with itself. In quantum mechanics, an emitter is described by a spatially varying wave function. Following the emission of wave quanta, the particles and waves are in an entangled state, known to cause quantum decoherence (Fig. 1B) (47) if one of the constituents of the bipartite system is not measured. As spontaneous emission of light by free charged particles is usually described classically (3539), it is generally assumed that the abovementioned effect is negligible, on the grounds that the correspondence principle (49) is always valid. This assumption is backed by the small quantum recoil (10) exerted by the photon, amounting to only minor corrections (4345). It is the purpose of the following analysis to show that under certain common conditions, quantum mechanics fundamentally modifies light emission, even in regimes that are traditionally seen as classical.

(A) Classical wave dynamics. A point particle with velocity v passes through an optical medium and emits waves that may interfere coherently. The classical emitter current density J(r, t) = ev(r vt) emits a temporally coherent shock wave. (B) Quantum description. A quantum particle is described by a delocalized wave function (r, t). A current operator J(r,t) is then associated with the particle. Even when the initial particle is only described by a single momentum ki, it may spontaneously emit many wave quanta (momenta q, q, ). The waves are then entangled with the particle because of momentum conservation (leaving the final particle having momenta kf, kf, respectively). When only the emitted waves are observed, this entanglement can lead to quantum decoherence and lack of interference visibility, resulting in the emission of incoherent radiation.

Recent works considered in depth the effect of the wave function size and shape on the spontaneously emitted radiation by free charged particles. Investigations based on semiclassical analysis (31, 34, 40, 41) imply size- and shape-dependent effects on the emitted power spectrum, while a quantum analysis (32, 33) suggested no such effects. Importantly, experiments have demonstrated wave function dependence upon postselection of a final electron state (30), while no such effects were observed when only the light was measured (no postselection) (33). The findings detailed below determine between the contradicting results, showing explicitly that without postselection, the wave function does not affect the power spectrum of spontaneous emission, while suggesting a new observablespectral coherencewhich explicitly depends on the wave function, and how the size and shape of the latter could be extracted from it. In this context, our findings can help promote the fast-growing field of free-electron quantum optics (50, 51) and emphasize the effect of the electron wave function in ultrafast electron beam spectroscopy experiments (15).

Without loss of generality, consider the emitting charged particles to be free electrons. We also consider the emitted electromagnetic field to be in a general optical environment. The initial state is described by a density matrix i, where the electrons have a reduced density matrix e, and the radiation field is found in the vacuum state 0, such that the initial state is separable i = e 00. The interactions between the electrons and the electromagnetic field are governed by the Dirac Hamiltonian: Hint = ec A, where e is the electron charge, c the speed of light, i=0i are the Dirac matrices, and A is the electromagnetic vector potential operator. Considering a weak coupling between the electrons and photons, the final quantum state of the system, f, is found by first-order time-dependent perturbation theory (see section S1).

In general, after the interaction, the electrons are entangled to many photonic modes because emission is allowed for different directions and at many different frequencies. For example, starting from an arbitrary initial wave function of a single electron and zero photons, i = kikiki0, and if momentum is conservedas in CRthe photon can be emitted with different momenta q = ki kf, giving an entangled final statef=kikikfMkikf;qeiEft/eiqtkfq=kikf(1)where Mki kf;q is the transition amplitude. Information regarding the electron initial state ki can be extracted by measuring the photon momentum q = ki kf in coincidence with (or postselection of) an electron momentum kf. However, this is not the experimental situation of CL, where only the light is measured, and the electron degrees of freedom are traced out. In this case, both experimental and theoretical evidence suggest that the initial electron wave function has no influence on observables of the emitted radiation (32, 33, 44), such as the power spectrum. Below, we will examine this situation carefully and show how the emitted light autocorrelations can be strongly influenced by the single electron wave functionalthough the power spectrum is not, suggesting a ubiquitous, hidden quantumness to the radiation by free electrons.

To describe the photonic final state in the experimental scenario of coherent CL, we calculate the reduced density matrix of the electromagnetic field, ph = Tre{f}, with Tre denoting the partial trace over the electronic state. The electric field autocorrelation is determined by the final photonic state, ph, via the quantum mechanical expectation value E()(r, t)E(+)(r, t) = Tr{E()(r, t)E(+)(r, t)ph}, where E(+)(r, t) and E()(r, t) = (E(+)(r, t)) are, respectively, the positive and negative frequency parts of the electric field operator. Instead of the simplified momentum-space picture of Eq. 1, which strictly holds only for CR (see section S8), we use a more general formalism. On the basis of quantum electrodynamical perturbation theory, the formalism holds for all coherent CL processes and for an arbitrary number of electrons (see section S1 for derivation), yieldingE(r,)E(r,)=02d3Rd3RG(r,R,)G(r,R,)j(R,)j(R,)e(2)where G(r, r, ) is the Dyadic Greens function of Maxwells equations for the dielectric medium (52), and where E(+)(r,t)=0deitE(r,). The quantity j(r, )j(r, )e = Tr{ejj} is the expectation value, with respect to the emitter initial state, of the correlations in the current density operator j(r, t) = ec, where (r, t) is the emitter spinor field operator described in second quantization (see sections S1 and S2). From here onward, we assume that the particles propagate as wave packets with a well-defined carrier velocity v0 (the paraxial approximation, where the particle dispersion is linearized about its mean momentum/energy).

Now, let us constrain the discussion to the seemingly classical regime, where photon recoils q are much smaller than electron momenta pe. This constraint is applicable to a vast number of effects, including all cases in which the emitter is relativistic, all current free-electron nanophotonic light sources, and all free-electron sources in the microwave and radio frequency ranges. In general, this derivation applies to both the single- and many-particle emitter states, described via second quantization of the emitter. The current correlations in Eq. 2 can then be written as (see section S2 for derivation)j(x)j(x)=e2v0v0[Ge(2)(x,x)+(xx)Ge(1)(x,x)](3)where x = r v0t and x = r v0t. In Eq. 3, we define the first- and second-order correlation functions of the emitter Ge(1)(x,x)=Tr{e(x)(x)} and Ge(2)(x,x)=Tr{e(x)(x)(x)(x)}, respectively, where (x) are operators corresponding to the particle spin components = , . Equation 3 is valid for both fermionic and bosonic statistics, under the approximations detailed above.

The current correlations comprise two terms: a pair correlation term proportional to Ge(2)(x,x), giving rise to spatially and spectrally coherent spontaneous radiation (henceforth called coherent radiation) when substituted into Eq. 2, and a term proportional to the probability density Ge(1)(x,x), contributing a spatially and spectrally incoherent spontaneous radiation (33) (which we refer to as incoherent radiation). In this work, we focus on the case of a single particle, wherein Ge(2)(x,x)=0, and discuss the nature of quantum decoherence of the light it emits. A derivation of the effects of many-body quantum correlations [Ge(2)(x,x)0] on the radiation will be reported in a separate work (53).

CR is characterized by a directional, polarized, cone-shaped radiation pattern with opening semi-angle c satisfying cos c = 1/n(), where = v/c is the speed of the particles normalized by the speed of light and n() is the refractive index of the medium. We assume that the emission is detected with a far-field detector located at a specific azimuthal angle on the cones rim, providing broadband detection of all frequency components [note that in certain practical situations, the entire emission ring (over all azimuthal angles) could be collected using special optics (54)thereby increasing the signal level]. Using the far-field expression for the dyadic Green tensor of a uniform dielectric medium (52), G(r,r,)=eiqr4r(Irr)eiqr, and assuming weak material dispersion, we find from Eqs. 2 and 3 that the radiation field projected on the detector is described by the following frequency-domain quantum autocorrelation (see section S3 for derivation)E()(r,)E(+)(r,)=U02ei(qq)r2n0cr2d3xei(qq)xGe(1)(x,x)(4)where we denote q=rcq, with rc being the observation direction on the Cherenkov cone, q = n()/c, and with U0 = sin2 c. Equation 4 implies that for a single emitting particle, the first-order autocorrelation of CR is intimately relatedthrough a Fourier transformto the probability density of the particle wave function. The same conclusionyet with more complex expressionsapplies to all coherent CL processes, such as Smith-Purcell and transition radiation. Smith-Purcell radiation (13) occurs when an electron passes near a periodic grating. The grating introduces a boundary condition defining periodic photonic Bloch modes u(r) ( standing for all relevant indices such as Bloch vector, band number, and polarization). The periodicity of the photonic near field allows simultaneous energy and momentum conservation in the emission process, which results in the emission to the far field. A possible way to obtain the dyadic Green function of Eq. 2 is via mode expansion (52) G(r,r,)=c2u(r)u*(r)/(22), and a result similar to Eq. 4 could be derived.

The emission of classical shocks from a point charge (9), for which j(r, t) = ev(r vt), is optically coherent over an arbitrarily wide spectral range only limited by the optical response of the medium. As such, the measured duration of the shock wave intensity envelope E(t)2 is only limited by the material dispersion and/or the detection bandwidth, theoretically enabling shock waves on the scale of femtoseconds and below (8, 9). The quantum description, however, incorporates the finite-sized single-particle wave function through Eq. 4. The incoherent emission from different points on the wave function (resulting from the delta-function term in Eq. 3) is a manifestation of quantum decoherence of the emitted light, expected to inhibit interference visibility and stretch the shock duration. Here, the photon acts as the observed subsystem, and the electron it was emitted from acts as the unobserved subsystem (47). As these two subsystems are entangled in momentum and the electron degrees of freedom are now traced out, the different photon momenta tend to a classical mixture instead of a pure quantum superposition. As a result, the quantum coherence of that photon is hindered, and the interference visibility can be greatly reduced.

For CR, this observation manifests itself in a rather straightforward manner. Considering weakly dispersive media and wide detection bandwidths, the shock wave power envelope P(t) = 2r20ncE()(t)E(+)(t) travelling at a group velocity vg is given by the equal-time temporal Fourier transform of Eq. 4. The probability cloud Ge(1)(x,x) is projected along the direction of observation rc on the Cherenkov cone. From this relation, we find that if the emitting particle wave function has a momentum uncertainty pe in the direction of CR, then the position uncertainty of the shock wave, xshw, satisfies a generalized uncertainty principle (see section S3 for derivation)xshwpe2(5)where the inequality becomes a strict equality for a minimum-uncertainty particle (satisfying xepe = /2) and for broadband detection. The intuition behind Eq. 5 is the following: If the particle has a position uncertainty along the emission direction, the shock wave emitted from it will demonstrate this same position uncertainty. For a classical particle, this uncertainty approaches zero, giving a classical shock. However, for a quantum particle, the Heisenberg uncertainty principle (55) defines a lower bound to the position uncertainty, thereby affecting the presumably classical light.

The seemingly elementary result in Eq. 5 represents a rather deep conclusion: It demonstrates how the well-known classical wave interference can only be generated by a quantum particle that has a certain momentum uncertainty. This result also provides a fundamental quantum lower bound on the interference (the shock wave duration) that cannot be captured within a classical theory considering point particles or with a semiclassical theory treating the wave function as a coherent spread-out charge density (33, 41) (see discussion below). As a concrete example, Fig. 2 shows how the momentum coherence pe pe (or coherent momentum uncertainty) determines the tight lower bound on the shock duration. For example, particles in a mixed quantum state in momentum space, with low coherent uncertainty pe pe, emit temporally incoherent light and, consequently, a longer shock wave. These kinds of considerations also show why low-frequency radiation (radio frequency, microwave, etc.) will generally be classical.

(A) A quantum particle with a coherent momentum uncertainty pe that equals its total momentum uncertainty pe displays a broad quantum coherence between its initial momenta pi (yellow glow). When the particle transitions to any final momentum pf, the emitted wave inherits this initial coherence because of the which path interference between the initial particle states. Hence, different wave vector components of the wave are coherent (red glow). (B) A quantum particle in a mixture of momenta (total uncertainty pe) with low coherent uncertainty pe pe emits temporally incoherent waves. The limited interference inhibits the pulse formation, and its length exceeds the classical prediction. (C and D) The temporal field autocorrelations, 2r20ncE()(t)E(+)(t) (in W), for 1-MeV electrons in silica in the visible range. The electrons are modeled as spherical Gaussian wave packets with coherent energy uncertainty (A) e = 3.72 eV (wave packet radius ~50 nm) and (B) e = 0.19 eV (wave packet radius ~1 m). The diagonal (t = t) indicates the temporal power envelope, P(t), being transform-limited in (A) and incoherent in (B). Insets show a scaled comparison between P(t) and the degree of first-order coherence of the light, g(1)(). For both (A) and (B), the classically expected shock wave full width at half maximum is 1.4 fs.

Experimentally, the decoherence effect best manifests itself in the temporal (or spectral) autocorrelations E()(t)E(+)(t) [or E()()E(+)()], where the off-diagonal (t t or ) terms relate to the coherence. Temporally coherent CR results in a transform-limited shock wave, as the classical theory suggests. However, the quantum corrections may alter the temporal behavior: Coherent (incoherent) shock waves exhibit g(1)() wider (narrower) than the pulse envelope. Figure 2 (C and D) demonstrates this behavior by simulating CR emission from 1-MeV electrons in silica for varying uncertainties. We note that this energy was chosen because it is closer to values often considered in high-energy physics to quantify Cherenkov detectors (with a relativistic particle velocity = 0.94 close to 1), while being of a similar order of magnitude to what one finds in a transmission electron microscope (TEM). Lower electron energies (such as those useable in TEMs) could readily be considered, keeping in mind the experimental limitations detailed below in the Experimental considerations section.

In this context, it is noteworthy to mention that classical and semiclassical theories predict that the emitted radiation is always perfectly coherent, both temporally and spectrally. The reason for this lies in the treatment of the electron probability density Ge(1)(x,x) as a classical charge density that emits light coherently from different points. While this approximation holds in the limit of a point particle, it fails when the electron wave function is delocalized such that it exceeds the photon wavelength. Subsequently, it can be shown that these theories do not satisfy the quantum uncertainty principle (Eq. 5) in practical experimental situations of CR (e.g., in standard electron microscopes)as they always predict a larger optical coherence in the semiclassical picture (because quantum decoherence is ignored). One is able to amend the semiclassical picture by adopting an ad hoc probabilistic approach (33) demanding that the electron emits light incoherently from different points, although a fully quantum treatment is necessary to unveil other important aspects such as quantum correlations (53, 56). An elaborate comparison between these theories can be found in section S7 and other works (32, 33, 41).

Quantum optical measurement of the spectral autocorrelations may unveil information about the emitter wave function itself and provide an unprecedented analytical tool for particle identification. Equation 4 provides a direct relation between the frequency-domain autocorrelation E()()E(+)() and the spatial Fourier transform (or structure factor) of the emitter probability density Ge(1)(x,x). This structure factor is equivalent to a momentum coherence function of the particle e(q q) = d3k e(k + q, k + q), namelyE()()E(+)()d3xei(qq)xGe(1)(x,x)=e(qq)(6).

Equation 6 implies that a spontaneously emitted photon is only as spectrally coherent as the emitting particle it originated from (see Fig. 2, A and B). Spontaneous CR can, therefore, be used to map the structure of the emitter wave function and its momentum coherence by analyzing the correlations of emitted photons. Note that only the spectral coherence of the photons plays a role here, namely, the width of the off-diagonal part of the autocorrelations (see Fig. 3C). The diagonal part (optical power spectrum), E()()E(+)(), is wave function independent.

(A) A charged particle wave packet (r, t) of finite size and carrier velocity v0 impinges on a Cherenkov detector with material dispersion n(). The particle spontaneously emits quantum shock waves of light into a cone with opening half-angle c() = acos[1/n()]. Collection optics is situated along the cone in the direction rc in the far field. (B) Detection scheme for measuring the spectral field autocorrelations E()E() using an interference between spectrally/temporally sheared fields (57). (C) The reconstructed photon density matrix determines the spatial probability distribution (r)2. (D and E) Simulation of particle wave function size reconstruction from the photon density matrix. A single 1-MeV electron ( = 0.94) in a silica Cherenkov detector [dispersion taken from (77)] emits CR that is collected within the visible range ( = 400 to 700 nm, centered at 0 = 550 nm). The electron wave function envelope is Gaussian and spherically symmetric, with position uncertainty of (d) xe = 254 nm and (E) xe = 1016 nm (bottom insets). In both (D) and (E), the measured photon density matrix, ph(, ), is plotted. The wave functionindependent diagonal ph(, ) that denotes the photodetection probability is the same for both cases. However, the off-diagonal spectral coherence ph( ) is strongly dependent on the wave function. Measuring its width coh (top insets) and using the approximate Eq. 7 provide the estimates (D) xe=290nm and (E) xe=1006nm.

The wave function size and shape can be estimated, for example, by assuming a spatial variance matrix for the emitter probability cloud given as ij2=Tr{rirje}. The photons are collected at an observation direction rc on the Cherenkov cone, and the width of their spectral coherence is measured (see Fig. 3, A to C). It then gives an estimate for the wave function dimensions along the observation direction (see section S4 for the derivation)rcT2rc=vg22(7)where vg denotes the shock wave group velocity. If the electron wave function is not spherical, we can further reconstruct the three-dimensional by measuring the spectral coherence along different Cherenkov cones rc [which can be done by measurements of multiple particles with the same wave function moving through media of different refractive indices n, as done in threshold detection (16)]. At least two such measurements are necessary to find both the longitudinal and transverse sizes of the wave function.

The quantum optical measurements necessary for the reconstruction of the photon density matrix in the frequency domain have been demonstrated experimentally for single photons (5759). Combining these quantum optical reconstruction techniques with Cherenkov detectors may allow for completely new and exciting capabilities. Currently available techniques for particle identification in Cherenkov detectors are limited to measuring velocity or mass (16, 17). Our proposed scheme further enables the measurement of the wave function dimensions and coherences of naturally occurring particles such as in cosmic radiation and beta decay (16), as well as the characterization of charged particle beams (for example in microscopy). Figure 3 (D and E) shows an example for such measurement scheme for the case of 1-MeV electrons. This method can provide an alternative to matter wave holography [used in electron microscopes (60) to measure the transverse wave function], which is currently unavailable for high-energy charged particles, such as muons, protons, kaons, and pions. In contrast, the measurement we propose is relevant for these particles and can be used as part of Cherenkov detectors, which also have the advantage of being a nondestructive measurement.

Beyond the capability of reconstructing the wave function size, our technique can be used to detect the signature of non-Gaussian wave packets (in energy-time space), such as coherent electron energy combs produced in photon-induced near-field electron microscopy (PINEM) (61), ultrafast TEM (27), and other methods (see Fig. 4) (6264). In PINEM, a free electron traverses a near-field optical structure and interacts with a coincident laser pulse. As a result, the electron wave function is modulated and given by a coherent superposition of energy levels. Following free-space propagation, the electron wave function takes the form of a pulse train (27). When this electron emits CR, notice how the interference fringes due to its shaped wave function appear only in the photon spectral autocorrelations (off-diagonal) and not in the radiation spectrum (diagonal).

(A) A free electron wave function is shaped by the interaction with a strong laser field of frequency (here, = 2 200 THz), as done in photon-induced near-field electron microscopy (61). The result is a coherent electron energy ladder, manifested as a temporal pulse train. (B) Cherenkov photon autocorrelations reveal the electron wave function spectral interference pattern, matching the laser frequency. The measurement scheme is the same as in Fig. 3 (A to C).

Here, we briefly discuss some important considerations for realizing our predictions in an experiment. For the analysis discussed in the previous section, the electrons coherent interaction length Lint must be in the range /n Lint (n/n)(/), where n = n ng is the difference between the refractive and group indices of the material, and denotes the wavelength band collected by the detection system (see section S5). The lower limit ensures that the Cherenkov angle is sharply defined, while the upper limit ensures that the material dispersion has a weak effect on the correlation between different frequencies. For standard materials and optical wavelengths, Lint is in the order of a few micrometers.

For CR in bulk media, other scattering processes with mean free paths smaller than Lint can readily broaden the particle spectrum e(k, k). In section S6, we show that for a general uniform medium of optical response function Im G(q, ) (which encompasses all types of inelastic processes, such as scattering by phonons and plasmons, and excitations of electron-hole pairs), the momentum coherence function e(q q) of Eq. 6 remains unchanged. As the latter quantity is the one responsible for the Cherenkov autocorrelation through Eq. 6, we expect the signature of the wave function to persist. In electron microscopy, one can avoid these scattering processes by using an aloof beam geometry having electrons that propagate in vacuum near an optical structure, such as in Smith-Purcell experiments or in emission of Cherenkov photons near dielectric boundaries (65).

Here, we investigated light emission by free charged particles from a quantum-optical viewpoint, by using a fully quantum formalism of light-matter interaction. Our conclusions take into account the experimental situation that the emitting particle itself is not measured. In this situation, recent studies show that the particle wave function has no influence on the emitted spectrum. We complement this realization by showing that quantum optical observables such as the emitted pulse duration and optical autocorrelations are all strongly influenced by the particle wave function. Moreover, all the quantum features of the particle such as coherence, uncertainty, and correlations embedded in the emitter wave function play an important role in determining the properties of the emitted light.

As an example, we considered the Cherenkov effect, and its characteristic optical shock wave, envisioned classically for almost a century as a coherent, transform-limited pulse of light. Instead, we found that it is fundamentally limited by the particle quantum uncertainty, satisfying a generalized uncertainty principle. The smaller the coherent momentum uncertainty of the particle is, the longer (and less coherent) the shock wave becomes. We further showed how this uncertainty relation can be harnessed to unveil information about the particle wave function, allowing unprecedented capabilities for particle detection. For example, Cherenkov detectors together with a quantum-optical measurement of the emitted light can be used to reconstruct the particle wave function size, shape, coherence, and quantum correlations.

Our findings can be used to resolve an important fundamental question of practical importance: What part of the energy uncertainty of a free electron is coherent, and what part is incoherent? This property can be measured from the radiation autocorrelations and spectrum. With the advent of laser-driven electron sources, for example, in ultrafast electron microscopes (2527, 63, 66, 67), the particle coherent energy uncertainty is believed to be dictated by the laser linewidth (26, 68), e.g., spanning tens of millielectron volts for excitations with femtosecond lasers. With the ability to coherently control the spatial electron wave function (23, 24), the transverse momentum uncertainty can be further lowered. Such conditions allow for the predictions of our work to be tested experimentally under controllable settings.

Considering the outlook for using free electrons as quantum probes (15, 51, 69), our work paves the way toward quantum measurement of free electrons and other charged particles based on spontaneous emission. One interesting direction for extending the research is to consider light emission from low-energy (tens to hundreds of electron volts) coherent electrons (70, 71), for which the zero-recoil approximation is no longer valid. In addition to recoil-induced quantum corrections in the emitted light (45), we expect the coherence of such light to be limited by the high spatial coherence of the electrons. Furthermore, our results may readily be generalized to other physical mechanisms of wave emission, for example, analogs of the Cherenkov effect (72, 73), as in Bose-Einstein condensates. Similar effects can be explored with any photonic quasiparticle (74), and even with sound waves, and phonon waves in solids (75), which all have the same underlying quantum nature and must have exact analogous phenomena.

Another intriguing question is the effect of many-body correlations (as manifested by the second term in Eq. 2) on such radiation phenomena, giving rise to yet unexplored quantum super- and subradiance regimes of coherent CL. These arise from coherent interference of multiparticle wave functions, which will be discussed in forthcoming work (53).

Note added in proof: This work was first presented in the Conference on Lasers and Electro-Optics in May 2020 as a conference presentation (76). A related paper (doi: 10.1126/sciadv.abf6380) appears in Science Advances.

Acknowledgments: Funding: This work was supported by the ERC starting grant NanoEP 851780 and the Israel Science Foundation grants 3334/19, 831/19, and 1415/17. A.K. acknowledges support by the Adams Fellowship of the Israeli Academy of Sciences and Humanities. N.R. was supported by the Department of Energy Fellowship DE-FG02-97ER25308 and by a Deans Fellowship by the MIT School of Science. Author contributions: A.K., N.R., A.A., and I.K. conceived the idea and contributed to writing the paper. A.K. performed the theoretical derivations. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Read the original post:

The coherence of light is fundamentally tied to the quantum coherence of the emitting particle - Science Advances

I read the online version of Oliver Burkemans long read that raises the question of whether free will is an illusion, and shortly afterwards read the same article again in print (The clockwork universe, Journal, 27 April). I was surprised when I realised that the brief reference to quantum physics online was missing in the printed version. Was it simply a matter of space, or was it left out because it made the whole argument too complicated? Either way, its omission was unfortunate.

Most physicists would not regard the events in our universe as deterministic, or clockwork if you insist; they are in fact considered probabilistic and would leave Laplaces demon scratching its head when attempting to make any long-term predictions. Quantum theory is fundamental to our understanding of reality, and those tiny fluctuations that the article mentions are an essential part of our reality. They allow the stars to shine, for instance. The argument for determinism implies a first mover, the unmoved mover, as Thomas Aquinas put it. Perhaps the quantum universe injects a multiplicity of unmoved movers, all that is needed to disrupt a predictable, in theory at least, deterministic universe and restore the possibility of free will.Andrew BromilowWaterloo, Merseyside

Oliver Burkemans common-sense argument against free-will scepticism its just at odds with too much else that seems obviously true about life is persuasive. Every hard determinist Ive ever met seems to agree in practice. Ive yet to meet one who didnt look both ways before crossing the street.Rev Carl HardingBarrow-in-Furness, Cumbria

The latest resurgence of scepticism [about free will] has been driven by advances in neuroscience, Oliver Burkeman says. What this shows, however, is that most people still dont really get the problem. You dont get it until you see that nothing in science could ever make any difference, for reasons that Burkeman gives. That said, its worth listening to Albert Einstein echoing the 17th-century philosopher Baruch Spinoza in his contribution to The Golden Book of Tagore, published in 1931: If the moon, in the act of completing its eternal way around the earth, were gifted with self-consciousness, it would feel thoroughly convinced that it was travelling its way of its own accord on the strength of a resolution taken once and for all. So would a Being, endowed with higher insight and more perfect intelligence, watching man and his doings, smile about mans illusion that he was acting according to his own free will.Galen StrawsonLondon

Those who suggest free will is an illusion are ignoring the gap in our understanding of consciousness and its relationship to the quantum realm in which randomness (ie indeterminism) has been proven to be real by the National Institute for Science and Technologys work on a randomness beacon. Causal determinism in the human brain is on a shoogly peg, and it is likely that Laplaces demon could no more make accurate predictions about the universe than about a football match, or any situation where the outcome is influenced by human consciousness. Given our lack of understanding about consciousness, any argument against free will is at best incomplete and at worst requires a quasi-religious leap of faith around something we do not understand enough to make such bold claims.Joe HindEdinburgh

The theory that our choices are determined by forces that go back to prehistory is pointless and dangerous. Pointless because any decision taken can be explained by the theory of predetermination or by the common view that people are able to make choices based on their beliefs, ethical standards and experience. Neither explanation can be proved or disproved. It should therefore be rejected as of little use for individuals or society. It is dangerous not only because it rids individuals of the responsibility for their actions but because it can also lead to the same conclusion when nations act irresponsibly towards each other.

This is not just a question for the academic world. There are political realists who contend that a rising power like China will inevitably clash with the established world power that is the US. This is where the clockwork world can lead us.

Our environment and genes are the main factors that shape who we are, but we know that there are many crossroads in life where the road less travelled was the one that we should have taken. Free will needs to be defended against the determinists.Derek HeptinstallWestgate-on-Sea, Kent

I bought the Guardian on Tuesday after I saw the front-page trail: Is free will just an illusion? I dont buy the Guardian often, simply because I cant afford it, but I couldnt ignore this. Surely according to quantum physics all possibilities exist at every moment in time, so the possibility of the existence of free will exists simultaneously with the possibility of not free will?Margaret ForbesKilmacolm, Renfrewshire

The problem with the assertion that free will is an illusion is that it carries inescapable consequences for the ability of our intelligence to arrive at an adequation of the truth. Notwithstanding that education, cultural background and character affect our cognitive reasoning, to have any hope of knowing anything truly, we must be capable of objectivity and therefore, by definition, free in our thought to some degree. If we are always deterministically fated to be led down a wired neural pathway towards a conclusion that we cannot escape, we have no real basis for assuming that it bears any relation to the actual nature of things.

If free will is an illusion, then so is true knowledge. And this would then also apply to our capacity to know the true nature of free will. Is Oliver Burkeman not fatally determined to arrive at the conclusion he has, regardless of whether it is actually true or not? In which case, Im not sure we can really say his conclusion is true.Khalid NaqibChilton, Buckinghamshire

I read Oliver Burkemans article with increasing frustration. To propose a mechanistic universe in which every effect of a cause is precisely determined ignores modern scientific thinking. Chaos theory tells us of the butterfly effect, whereby even a very small variation in initial conditions can lead to an extremely large difference in later results. And quantum theory tells us that, at the subatomic level, we can only say that a given range of effects has a probability (which we might be able to calculate) of resulting from that cause. Schrdingers cat has a 50% chance of survival. We do not know, even given the most precise knowledge conceivable of todays conditions, what will happen tomorrow.

Burkeman is contrasting two extreme theories, of unfettered free will and Laplaces demon, when the truth lies somewhere in between. Surely the truth is that we have free will, but the options available to us and our likely preferences are influenced to varying degrees by things beyond our control?Robert DimmickCaversham, Reading

The free will debate is surely the most fruitless in all philosophy. Imagine a world with free will. Now imagine a world without. Would there be the slightest difference? And the argument that if there is no free will then there is no responsibility and so punishment is unjustified is spurious. In such a world we would also lack the free will to choose whether to punish. I feel I have no choice but to believe in free will.Jim WatsonStroud, Gloucestershire

The long read reminded me of the test to distinguish between a scientist and an engineer. With the candidate at one end of a long room and a highly desirable prize at the other, the candidate is allowed to travel half of the intervening space on each command to move. The scientist, of course, instantly recognises that it is impossible to reach the goal in a finite time and remains standing, whereas the engineer moves smartly forward to get close enough for all practical purposes as soon as possible. The principle that every event has a cause is sound, but does that translate to every event being inevitable from the origin of time? Sure, we can do a thought experiment tracing every event back through its sequence of preceding causes to whatever origin we choose. If, though, we then allow the system to rerun, the sheer enormity of possible outcomes ensures that the slightest perturbation at any point will result in a radically different outcome. For all practical purposes, Im an engineer on this one.David WoolleyChipping Norton, Oxfordshire

In my 95 years, I have frequently pondered on the subject question about which much can be said, as Oliver Burkeman demonstrates. We find it easy to define determinism, but what do we mean by free will?

Say, for example, that I am a beginner at chess and face a checkmate from just two options. My tutor points out a third, which recovers the situation. Was my free choice the same as theirs?

My hopeless situation induces me to resign from the game. My opponent insists I continue to the end. I comply and unexpectedly achieve a draw. Would I have offered to resign had I known the eventual outcome?

Few choices in life are between an apple and a banana. Almost always, we dont know all the options, or their outcomes. The real choice is between the outcomes, which requires us to know the future. We have to guess that. My conclusion is that determinism is so complex that for all practical purposes it may be discarded, but that for the same reason we must discard the notion of free will.HS GrnewaldPinner, London

As a physicist and engineer, I have my own views on free will and religion. Quantum mechanics was not as issue for the earlier philosophers who considered a Newtonian world of perfect prediction given perfect knowledge. That is no longer considered true. Apart from the obvious quantum truth that nothing can be measured perfectly, any interaction between particles can have only probabilistic outcomes.

For example, in the article two examples are given in which a person acts strangely as a result of cancer. Cancer is generally the result of a chemical or radiation incident, which is totally random and unpredictable. The human body has many mechanisms that may or may not repair the original cancerous cell. This could not be predicted a year in advance, certainly not at birth, and absolutely not as the outcome of generations of procreation, each of which involves one sperm out of millions, chosen at random.

Coming from a different angle, if someone commits a homicidal outrage, which may or may not be the result of free will, surely I and my community should have equal freedom to react with a punishment, which may or may not be the result of a free-will decision. If a person is driven to antisocial action, their actions, whether a result of free will or not, are likely to take into account the likelihood of punishment. If society regards actions as punishable, those actions will be somewhat less likely than if not.

I regard punishment as less important than deterrence and prevention; in many cases, there is no real gain but a high cost in imprisonment. In other cases, the important function of imprisonment is isolation from society, thereby avoiding further harm. But I am a scientist and engineer, not a moral philosopher and legislator or judge.Rod DalitzEdinburgh

Have an opinion on anything youve read in the Guardian today? Please email us your letter and it will be considered for publication.

See the original post:

The battle for free will in the face of determinism - The Guardian

Todays most advanced clocks keep time with an incredibly precise rhythm. But a new experiment suggests that clocks precision comes at a price: entropy.

Entropy, or disorder, is created each time a clock ticks. Now, scientists have measured the entropy generated by a clock that can be run at varying levels of accuracy. The more accurate the clocks ticks, the more entropy it emitted, physicists report in a paper accepted to Physical Review X.

If you want a better clock, you have to pay for it, says physicist Natalia Ares of the University of Oxford.

Time and entropy are closely intertwined concepts. Entropy is known as the arrow of time, because entropy tends to grow as time passes the universe seems to consistently move from lower entropy to higher entropy (SN: 7/10/15). This march toward increasing entropy explains why some processes can proceed forward in time but not in reverse: Its easy to mix cream into coffee but exceedingly difficult to separate it again. Machines also increase disorder as they operate, for example by giving off heat that boosts the entropy of their surroundings. That means even a standard, battery-powered clock produces entropy as it ticks.

Headlines and summaries of the latest Science News articles, delivered to your inbox

Physicists had previously calculated that, for tiny quantum clocks, theres a direct relationship between the maximum possible accuracy of their ticks and the amount of entropy emitted. But larger clocks are too complex for such calculations. So it wasnt clear if such a rule held for other types of clocks, too.

To test how much entropy was released in the ticking of a simplified clock, Ares and colleagues made a clock from a thin membrane, tens of nanometers thick and 1.5 millimeters long, suspended across two posts. An electrical signal sent into the clock jostled the membrane, causing it to flex up and down. This bending motion repeated at regular intervals, like the steady ticks of a clock, and an antenna registered that motion. The more powerful the electrical signal was, the more accurately the clock ticked. And as the clocks accuracy increased, the entropy a result of heat produced in the antennas circuit increased in lockstep.

That result suggests that the theoretical relationship for quantum clocks also applies to other types of clocks. Its nice to have that, says physicist Juan Parrondo of the Complutense University of Madrid, who was not involved with the study. What Im not so sure of is how universal is this type of relationship that they find. The researchers studied only one variety of clock. Its not yet clear whether the relationship between accuracy and entropy applies to clocks more generally, Parrondo says.

But some scientists suspect the relationship may be universal, revealing a fundamental aspect of how clocks function. The new study would push us even more in this direction, says quantum physicist Ralph Silva of ETH Zurich, who was not involved with the research. Its a data point in favor that its probably the case for all clocks. But thats not been proven.

In order for a clock to operate reliably, it must undergo a process that has a preferred direction in time. If the clock didnt create entropy, it would be just as likely to run forward as backward. And the more entropy the clock creates, the less likely it is that the clockwork will suffer from fluctuations temporary backward steps that would degrade its accuracy.

So if the accuracy of all clocks does come at a cost of increased entropy, that trade-off may reflect a close link between the passage of time and its measurement.

The rest is here:

A clocks accuracy may be tied to the entropy it creates - Science News Magazine

Tulane scientists are part of team of Louisiana researchers looking at how smart quantum technology can improve communications in the military.

A team of Louisiana researchers, including a group from the Tulane University School of Science and Engineering, has developed a smart quantum technology that could have real-world applications to quantum networks and future quantum communications systems used in the military.

Ryan Glasser, an associate professor of physics at Tulane, and his team in the Department of Physics, collaborated on the study with researchers from Louisiana State University. The study was featured on the cover of the March 2021 issue of Advanced Quantum Technologies.

Recent developments in optical technologies have resulted in extremely high information transfer rates using the spatial properties of light i.e. images (and more complex structured beams), Glasser said. However, a difficulty in such communications using light through free-space is that turbulence can severely distort the beams, resulting in errors in the communication.

The teams result is an exciting step forward in developing this understanding, and it has the potential to ultimately enhance the Armys sensing and communication capabilities on the battlefield.

Sara Gamble, program manager at the U.S. Army Research Office

To correct the errors, researchers developed an artificial intelligence scheme to help overcome the negative effects of turbulence on light that propagates through the atmosphere. The system corrects for spatial distortions of laser light.

We showed the systems efficacy first in the classical regime by using simulations, Glasser said.

The work was funded through the U.S. Office of Naval Research to develop artificial intelligence techniques to help create robust communications networks, under program officer Santanu Das. Tulane then collaborated with the LSU team, which implemented an experiment to show that the AI (artificial intelligence) approach can be adapted to work using quanta of light, or single photons.

The experiment our LSU collaborators performed shows that we can overcome the destructive effects of turbulence on single photons, which will aid in the real-world implementation of free-space quantum communication links, Glasser said. Such technologies are crucial to future quantum technologies, including quantum networks and quantum imaging. Were excited to be doing research that combines the flourishing fields of quantum technologies and artificial intelligence.

Sara Gamble, program manager at the U.S. Army Research Office, said the research is still in the early stages of understanding the potential for machine learning techniques to play a role in quantum information science. But, she said, the teams result is an exciting step forward in developing this understanding, and it has the potential to ultimately enhance the Armys sensing and communication capabilities on the battlefield.

Other researchers from Tulane include Sanjaya Lohani, a postdoctoral researcher and Erin M. Knutson, now a postdoctoral fellow at Santa Clara University. The LSU team includes PhD candidate Narayan Bhusal, postdoctoral researcher Chenglong You, graduate student Mingyuan Hong, undergraduate student Joshua Fabre and Omar S. Magana-Loaiza, an assistant professor of physics. Pengcheng Zhao of Qingdao University of Science and Technology also participated in the study.

The Louisiana researchers are part of the Louisiana Quantum Initiative, a statewide endeavor to advance the research and technology of quantum systems in the context of the second quantum revolution and develop the strategy and technological infrastructure of quantum-driven networks and devices. The initiative is an ecosystem of research that relies on emergent and dynamic associations and efforts among institutions as well as individual members.

Go here to see the original:

Tulane part of Navy/Army-funded research on improving communication - News from Tulane

Dearest readers, this week, the one and only Campus Times brings you the most exclusive insider scoop: a look behind the scenes at the only thing holding up students rapidly decomposing will to open that godforsaken Zoom application.

The Wellness Wednesday emails that students receive on a weekly basis consist of updates to current campus goings-on, plans for the Universitys future, and suggestions for students concerning their mental health.

Recently, one of our dedicated researchers came across a stash of emails, buried deep within the Universitys drafts. Clearly, someone had been trying to dispose of the evidence. These emails, after careful examination, found to be a collection of scrapped Wellness Wednesday concepts.

The following titles were among the most shocking discoveries:

Stressed? Drop the goddamn class!

Tired? Why not try a new approach to beating insomnia? Drop by a lab and swipe some chloroform!

Why meditate? Drugs are easier!

Counselors available whenever you need (who will likely suggest revolutionary methods of coping such as: trying yoga, and talking about how your childhood is the reason you dont understand, and the class Honors Quantum Physics 719, which runs MWF 23:59 05:00)!

Karen, Im not going to tell the students to try essential oils; no, I will not bring them into your pyramid scheme

Keeping a journal: the experts say to do it, so do it

Beat that burnout by buying! Student discount on coffins!

Reject modernity; return to monkey

And finally, the one that stunned every editor in our office with its baffling brilliance:

FEELING BAD? DONT

These astonishing findings are just the tip of the iceberg when it comes to the insight that Wellness Wednesday brings. I myself as a student can say with full certainty that I have definitely, most likely, 100% probably noticed at least one of these emails in my inbox this semester.

The mental health crisis among students is reaching a boiling point. However, when asked about this, admin denied this was the case by saying its still cold outside, therefore a boiling point could not have been reached.

Truly we are unmatched in this battle of intellect. The almighty figureheads know what is best for the students. With the pandemonium of the pandemic continuing into this semester, executive decisions were made to cater to the needs of the creatures known as college students. An entire two days were given for rest and recuperation. Not together, silly. Then they might have time to do something, or rest, and we cant have that, god.

The University sleeps soundly knowing they have exhausted every available resource to maintain the well-being of its students. That is, of course, the most important part of a human being. Wait, these are college students, not people. Thats why they dont need sleep!

With the stellar outcomes of these never-before-heard-of techniques like telling students to Keep Going!, its a wonder every institution of higher education hasnt adopted this approach. After all, what students need above all else is another voice telling them what to do.

Read the original:

Wellness Wednesday advice: If going to be sad? Don't! - Campus Times

Sheena Brown, Correspondent Published 2:47 p.m. ET May 2, 2021

This school year was unconventional, to say the least. Though in-person learning was an option at our school, our family chose the virtual route. What we hoped would be a couple months turned into nine.

Nine long months of navigating virtual platforms, finding a workflow that worked for us all, and suffering weekly through sixth-grade math. I, for one, have learned A LOT! Time will tell if my kids feel the same. One thing I am sure we all agree on, it has not been ideal.

As the weather warms and we stare down a manageable number of modules left to complete, I feel an excitement for Summer not known since my own days in school. Yet, amidst the giddiness for nights catching fireflies and long afternoons floating in sparkling pools, I know that summer break will not be a break for me. As a parent, especially a parent this year, I am worried about my children backsliding. The fear is more solid this summer since I know their teacher so personally and if it wasnt for Google and YouTube, she would have hung it up nine months ago!

Thankfully for me, and for you if you find yourself in a similar predicament, there are ways to combat the dreaded summer slide. Fun ways. Ways that may not even feel like learning if you present them just right.

1.Have conversations. Maybe your house is different, but no one around here is kicked back pondering the Big Bang or casually mulling over quantum physics. However, there is real value in simply talking to each other. The content, it turns out, isnt as important as the act itself. So even just talking about the weather counts.

2.Do real world math. How much stain to seal a deck? How many cups of flour to bake a dozen cookies? How far is it to the store and how long will it take to get there? Anything that gets the old gears grinding is good!

3.Engage in short bursts of skill review every day. A math problem, a crossword puzzle, even brainteasers. Little chunks of learning, especially when it's a family affair, can really add up.

4.Write. Write in a journal, help write a grocery list, write a story, or write letters in the mud with a stick. Putting utensil to whatever is a great way to be creative, practice skills and possibly find a new passion all at once.

5.READ. No matter how much research I did, the most repeated and revered piece of advice for keeping minds sharp is to read! This may seem too simple or possibly made up because I work for the library, but its true and its importance cannot be overstated! Encourage kids to read alone, read together, read aloud, just read!

If youre not sure what to read, check out a prepackaged Mystery Bag handpicked by our amazing Youth Services staff. Or visit crcpl.org to use our Book Match Service where well match you with the story of your dreams.

Hopefully these suggestions, along with the super cool Bookworm events happening at the Library (also listed on our website) will keep my kids, and yours, learning all summer long.

Sheena Brown is a horrible virtual schoolteacher and part-time clerk at the Main Library.

Read or Share this story: https://www.chillicothegazette.com/story/life/2021/05/02/beyond-books-creative-ways-combat-summer-slide/7401322002/

Go here to read the rest:

Beyond Books: Creative ways to combat the summer slide - Chillicothe Gazette

QED stands for quantum electrodynamics, which describes how light and matter interact and links the theories of quantum mechanics and special relativity; not exactly an easy field to understand - or to explain - unless you are Richard Feynman. The book QED is a collection of transcripts of a series of lectures Richard gave to the general public; for people without any technical or academic background to quantum or theoretical physics about his field and work.

The extraordinary thing about these lecturers is how easy to follow and understand they are, in spite of the complexity of the subject matter covered. Richard had a gift for explaining intricate ideas in human terms, without dumbing them down.

All too often, marketing messages are obscured by flowery language, smug jargon, and clever ideas that make it difficult for ones audience to understand what it is that you are actually trying to say and what it is that you would like your audience to do after receiving your message.

All too often, this confusion is a result of the marketer or advertiser not being sure of what it is they want to say or why they are saying it.

If Richard Feynman could get non-physicists to understand what quarks are and what we do and dont know about how electrons and protons behave (even when they are behaving badly), then there is no excuse for corporate communication that fails to communicate a clear message and call to action.

If you are unable to communicate clearly and convincingly, it is probably because you dont know what it is you are actually trying to say.

View post:

#PulpNonFiction: Advertisers, be clear about what you want to say and why! - Bizcommunity.com

In 2001 at the Brookhaven National Laboratory in Upton, New York, a facility used for research in nuclear and high-energy physics, scientists experimenting with a subatomic particle called a muon encountered something unexpected.

To explain the fundamental physical forces at work in the universe and to predict the results of high-energy particle experiments like those conducted at Brookhaven, Fermilab in Illinois, and at CERNs Large Hadron Collider in Geneva, Switzerland, physicists rely on the decades-old theory called the Standard Model, which should explain the precise behavior of muons when they are fired through an intense magnetic field created in a superconducting magnetic storage ring. When the muon in the Brookhaven experiment reacted in a way that differed from their predictions, researchers realized they were on the brink of a discovery that could change sciences understanding of how the universe works.

Earlier this month, after a decades-long effort that involved building more powerful sensors and improving researchers capacity to process 120 terabytes of data (the equivalent of 16 million digital photographs every week), a team of scientists at Fermilab announced the first results of an experiment called Muon g-2 that suggests the Brookhaven find was no fluke and that science is on the brink of an unprecedented discovery.

UVA physics professor Dinko Poani has been involved in the Muon g-2 experiment for the better part of two decades, and UVA Today spoke with him to learn more about what it means.

Q. What are the findings of the Brookhaven and Fermilab Muon g-2 experiments, and why are they important?

A. So, in the Brookhaven experiment, they did several measurements with positiveand negative muons an unstable, more massive cousin of the electron under different circumstances, and whenthey averaged their measurements,they quantified a magnetic anomaly that is characteristic of the muon more precisely than ever before. According torelativistic quantum mechanics, the strength of the muons magnetic moment (a property it shares with a compass needle or a bar magnet) should be two in appropriate dimensionless units, the same as for an electron. The Standard Model states, however, that its not two, its a little bit bigger, and that difference is the magnetic anomaly. The anomaly reflects the coupling of the muon to pretty much all other particles that exist in nature. How is this possible?

The answer is that space itself is not empty; what we think of as a vacuum contains the possibility of the creation of elementary particles, given enough energy. In fact, these potential particles are impatient and are virtually excited, sparking in space for unimaginably short moments in time. And as fleeting as it is, this sparking is sensed by a muon, and it subtly affects the muons properties. Thus, the muon magnetic anomaly provides a sensitive probe of the subatomic contents of the vacuum.

To the enormous frustration of all the practicing physicists of my generation and younger, the Standard Model has been maddeningly impervious to challenges. We know there are things that must exist outside of it because it cannot describe everything that we know about the universe and its evolution. For example, it does not explain the prevalence of matter over antimatter in the universe, and it doesnt say anything about dark matter or many other things, so we know its incomplete. And weve tried very hard to understand what these things might be, but we havent found anything concrete yet.

So, with this experiment, were challenging the Standard Model with increasing levels of precision. If the Standard Model is correct, we should observe an effect that is completely consistent with the model because it includes all the possible particles that are thought to be present in nature, but if we see a different value for this magnetic anomaly, it signifies that theres actually something else. And thats what were looking for: this something else.

This experiment tells us that were on the verge of a discovery.

Q. What part have you been able to play in the experiment?

A. I became a member of this collaboration when we had just started planning for the follow-up to the Brookhaven experiment around 2005, just a couple of years after the Brookhaven experiment finished, and we were looking at the possibility of doing a more precise measurements at Brookhaven. Eventually that idea was abandoned, as it turned out that we could do a much better job at Fermilab, which had better beams, more intense muons and better conditions for experiment.

So, we proposed that around 2010, and it was approved and funded by U.S. and international funding agencies. An important part was funded by a National Science Foundation Major Research Instrumentation grant that was awarded to a consortium of four universities, and UVA was one of them. We were developing a portion of the instrumentation for the detection of positrons that emerge in decays of positive muons. We finished that work, and it was successful, so my group switched focus to the precise measurements of the magnetic field in the storage ring at Fermilab, a critical part of quantifying the muon magnetic anomaly. My UVA faculty colleague Stefan Baessler has also been working on this problem, and several UVA students and postdocs have been active on the project over the years.

Q. Fermilab has announced that these are just the first results of the experiment. What still needs to happen before well know what this discovery means?

A. It depends on how the results of our analysis of the yet-unanalyzed run segments turn out. The analysis of the first run took about three years. The run was completed in 2018, but I think now that we weve ironed out some of the issues in the analysis, it might go a bit faster. So, in about two years it would not be unreasonable to have the next result, which would be quite a bit more precise because it combines runs two and three. Then there will be another run, and we will probably finish taking data in another two years or so. The precise end of measurements is still somewhat uncertain, but I would say that about five years from now, maybe sooner, we should have a very clear picture.

Q. What kind of impact could these experiments have on our everyday lives?

A. One way is in pushing specific technologies to the extreme in solving different aspects of measurement to get the level of precision we need. The impact would likely come in fields like physics, industry and medicine. There will be technical spinoffs, or at least improvements in techniques, but which specific ones will come out of this, its difficult to predict. Usually, we push companies to make products that we need that they wouldnt otherwise make, and then a new field opens up for them in terms of applications for those products, and thats what often happens. The World Wide Web was invented, for example, because researchers like us needed to be able to exchange information in an efficient way across great distances, around the world, really, and thats how we have, well, web browsers, Zoom, Amazon and all these types of things today.

The other way we benefit is by educating young scientists some of whom will continue in the scientific and academic careers like myself but others will go on to different fields of endeavor in society. They will bring with them an expertise in very high-level techniques of measurement and analysis that arent normally found in many fields.

And then, finally, another outcome is intellectual betterment. One outcome of this work will be to help us better understand the universe we live in.

Q. Could we see more discoveries like this in the near future?

A. Yes, there is a whole class of experiments besides this one that look at highly precise tests of the Standard Model in a number of ways. Im always reminded of the old adage that if you lose your keys in the street late at night, you are first going to look for them under the street lamp, and thats what were doing. So everywhere theres a streetlight, were looking. This is one of those places and there are several others, well, I would say dozens of others, if you also include searches that are going on for subatomic particles like axions, dark matter candidates, exotic processes like double beta decay, and those kinds of things. One of these days, new things will be found.

We know that the Standard Model is incomplete. Its not wrong, insofar as it goes, but there are things outside of it that it does not incorporate, and we will find them.

Read the original post:

Q&A: Are We on the Brink of a New Age of Scientific Discovery? - University of Virginia