11 hours ago by Stuart Mason Dambrot Experimental concept, energy level diagram, and setup. (a) The memory protocol. A horizontally (H) polarized single photon (green, 723 nm) is written into the quantum memory with a vertically (V) polarized write pulse (red, 800 nm). After a delay , an H-polarized read pulse recalls a V-polarized photon. (b) Energy levels in the memory. The ground state j0i and the storage state |1>correspond to the crystal ground state and an optical phonon, respectively. The signal photon and the read-write pulses are in two-photon resonance with the optical phonon (40 THz) and are far detuned from the conduction band j2i. (c) The experimental setup. The laser output is split to pump the photon source and to produce the orthogonally polarized read and write beams. The photons are produced in pairs with one (signal) at 723 nm and the other (herald) at 895 nm. The signal photon is stored in, and recalled from, the quantum memory. The herald and signal photons are detected using APDs and correlations between them are measured using a coincidence logic unit. Credit: D. G. England, K. A.G. Fisher, J-P. W. MacLean, P. J. Bustard, R. Lausten, K. J. Resch, and B. J. Sussman, Storage and Retrieval of THz-Bandwidth Single Photons Using a Room-Temperature Diamond Quantum Memory, Phys. Rev. Lett. 114, 053602 (2015).
(Phys.org)Photonic quantum technologies including cryptography, enhanced measurement and information processing face a conundrum: They require single photons, but these are difficult to create, manipulate and measure. At the same time, quantum memories enable these technologies by acting as a photonic buffer. Therefore, an ideal part of the solution would be a single-photon on-demand read/write quantum memory. To date, however, development of a practical single-photon quantum memory has been stymied by (1) the need for high efficiency, (2) the read/write lasers used introducing noise that contaminates the quantum state, and (3) decoherence of the information stored in the memory.
Recently, scientists at National Research Council of Canada, Ottawa and Institute for Quantum Computing, University of Waterloo demonstrated storage and retrieval of terahertz-bandwidth single photons via a quantum memory in the optical phonons modes of a room-temperature bulk diamond. The researchers report that the quantum memory is low noise, high speed and broadly tunable, and therefore promises to be a versatile light-matter interface for local quantum processing applications. Moreover, unlike existing approaches, the novel device does not require cooling or optical preparation before storage, and is a few millimeters in size. The scientists conclude that diamond is a robust, convenient, and high-speed system extremely well-suited to evaluating operational memory parameters, studying the effects of noise, and developing quantum protocols.
Prof. Benjamin J. Sussman discussed the paper that he, Prof. Kevin Resch, Dr. Duncan G. England, and their colleagues published in Physical Review Letters. "The possibility of using single photons in quantum technologies offers a host of new opportunities in measurement and communications," Sussman tells Phys.org. "However, it's challenging to do so because the light we typically use that is, from the sun, light bulbs, or lasers contains tremendous numbers of photons." Therefore, much of the technology for manipulating and measuring light (including naturally-evolved light-detecting biological organs, such as our eye) have been designed to deal with larger numbers of photons and in addition, background noise from the faintest light source can mask these single photons.
"Creating a single photon is also a formidable problem," Sussman continues, adding that to generate single photons the scientists employ a low probability stochastic quantum optics process called spontaneous parametric down-conversion (SPDC). The method of generation is very effective, but the challenge is that being a probabilistic process a photon is generated not on demand, but unpredictably. "We have to wait for success and then perform an experiment, which means most of the time the experiment fails," Sussman explains. "However, quantum memories are very interesting because they act as photon buffers, and can convert a probabilistic process into a deterministic one. This effectively turns a repeat-until-success single-photon source into an on-demand source."
Sussman notes that the most difficult technical obstacle was verifying the non-classical photon statistics of the memory output. To determine whether single photons were actually retrieved from quantum memory, the scientists performed a so-called g(2) measurement (the degree of coherence between two fields) in which the output photon was coupled into a 50:50 beam splitter, and detectors placed at both output ports. "Because single photons are indivisible, one would never expect to measure coincident detection in both arms and this is what we were able to confirm. Nevertheless, experiments aren't perfect and where the single photon is even slightly contaminated by background noise, we very occasionally make a coincidence measurement. As a result, measuring enough of these coincidences in order to collect significant statistics required over 150 hours of continuous data acquisition." He adds that graduate students Kent Fisher and JP MacLean worked tirelessly to perform the experiment.
"A quantum memory is a conversion between quantum states of light and matter," Sussman tells Phys.org. "However, decoherence is constantly destroying the crucial quantum nature of the matter system, and thus the advantages of quantum technologies. Typically the narrow linewidths of the quantum levels involved limit the bandwidth of such memories to the gigahertz range or below. Our challenge was therefore to work with very short pulses of light to beat decoherence that is, to perform our operations before the system decays. Again, ultrafast Spontaneous Parametric Down-conversion is the most popular source of high purity single photons but with femtosecond oscillators it produces THz-bandwidth photons that can't fully be utilized in lower bandwidth systems. We were able to bridge this three orders of magnitude gap between light and matter by building an ultrafast capable quantum memory."
Since all quantum systems suffer from decoherence effects when they interact with an external environment, isolating the quantum system from its environment is a universal problem in quantum technology. "The key insight behind our experiment was that ultrafast lasers can avoid decoherence. Rather than try to isolate our memory from the environment, we address it on timescales that are fast compared to decoherence by using ultrafast laser pulses of ~200 femtoseconds duration."
Sussman notes that ultrafast lasers were developed to study picosecond and femtosecond dynamics in molecular and bulk phonon vibrations. "It's therefore not surprising that we'd employ these vibration or similar systems as substrates to operate at ultrafast speeds for quantum processing and Dr. England was able to leverage his expertise in these two areas to bridge the National Research Council and Institute for Quantum Computing teams and make the project a success."
The paper states that because the quantum memory is low noise, high speed and broadly tunable, it promises to be a versatile light-matter interface for local quantum processing applications. Sussman explains that the interface between light and matter is an important frontier for quantum information science, in that it combines the advantages of photonic qubits (which move fast and have long decoherence times) with those of matter qubits (stationary and with strong interactions). "The diamond memory is an important innovation because it provides a robust and convenient platform on which to investigate this interface," which he adds are due to its key advantages:
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Light, meet matter: Single-photon quantum memory in diamond optical phonons at room temperature
