Image Caption: The experiment's Alice and Bob communicated with entangled photons produced in this setup. Such apparatus could be miniaturized using techniques from integrated optics. Credit: IQC, University of Waterloo

Centre for Quantum Technologies

The way we secure digital transactions could soon change. An international team has demonstrated a form of quantum cryptography that can protect people doing business with others they may not know or trust a situation encountered often on the internet and in everyday life, for example at a banks ATM.

Having quantum cryptography to hand is a realistic prospect, I think. I expect that quantum technologies will gradually become integrated with existing devices such as smartphones, allowing us to do things like identify ourselves securely or generate encryption keys, says Stephanie Wehner, a Principal Investigator at the Centre for Quantum Technologies (CQT) at the National University of Singapore, and co-author on the paper.

In cryptography, the problem of providing a secure way for two mutually distrustful parties to interact is known as two-party secure computation. The new work, published in Nature Communications, describes the implementation using quantum technology of an important building block for such schemes.

CQT theorists Wehner and Nelly Ng teamed up with researchers at the Institute for Quantum Computing (IQC) at the University of Waterloo, Canada, for the demonstration.

Research partnerships such as this one between IQC and CQT are critical in moving the field forward, says Raymond Laflamme, Executive Director at the Institute for Quantum Computing. The infrastructure that weve built here at IQC is enabling exciting progress on quantum technologies.

CQT and IQC are two of the worlds largest, leading research centres in quantum technologies. Great things can happen when we combine our powers, says Artur Ekert, Director of CQT.

The experiments performed at IQC deployed quantum-entangled photons in such a way that one party, dubbed Alice, could share information with a second party, dubbed Bob, while meeting stringent restrictions. Specifically, Alice has two sets of information. Bob requests access to one or the other, and Alice must be able to send it to him without knowing which set hes asked for. Bob must also learn nothing about the unrequested set. This is a protocol known as 1-2 random oblivious transfer (ROT).

ROT is a starting point for more complicated schemes that have applications, for example, in secure identification. Oblivious transfer is a basic building block that you can stack together, like lego, to make something more fantastic, says Wehner.

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New Cryptography Scheme Secured By Quantum Physics

One of the recent revelations by Edward Snowden is that the U.S. National Security Agency is currently developing a quantum computer. Physicists aren't surprised by this news; such a computer could crack the encryption that is commonly used today in no time and would therefore be highly attractive for the NSA.

Professor Thomas Walther of the Institute of Applied Physics at the Technical University of Darmstadt is convinced that "Sooner or later, the quantum computer will arrive." Yet the quantum physicist is not worried. After all, he knows of an antidote: so-called quantum cryptography. This also uses the bizarre rules of quantum physics, but not to decrypt messages at a record pace. Quite the opposite -- to encrypt it in a way that can not be cracked by a quantum computer. To do this, a "key" that depends on the laws of quantum mechanics has to be exchanged between the communication partners; this then serves to encrypt the message. Physicists throughout the world are perfecting quantum cryptography to make it suitable for particularly security-sensitive applications, such as for banking transactions or tap-proof communications. Walther's Ph.D. student Sabine Euler is one of them.

As early as the 1980s, physicists Charles Bennett and Gilles Brassard thought about how quantum physics could help transfer keys while avoiding eavesdropping. Something similar to Morse code is used, consisting of a sequence of light signals from individual light particles (photons). The information is in the different polarizations of successive photons. Eavesdropping is impossible due to the quantum nature of photons. Any eavesdropper will inevitably be discovered because the eavesdropper needs to do measurements on the photons, and these measurements will always be noticed.

"That's the theory" says Walther. However, there are ways to listen without being noticed in practice. This has been demonstrated by hackers who specialize in quantum cryptography based on systems already available on the market. "Commercial systems have always relinquished a little bit of security in the past," says Walther. In order to make the protocol of Bennett and Brassard reality, you need, for example, light sources that are can be controlled so finely that they emit single photons in succession. Usually, a laser that is weakened so much that it emits single photons serves as the light source. "But sometimes two photons can come out simultaneously, which might help a potential eavesdropper to remain unnoticed" says Walther. The eavesdropper could intercept the second photon and transmit the first one.

Therefore, the team led by Sabine Euler uses a light source that transmits a signal when it sends a single photon; this signal can be used to select only the individually transmitted photons for communication. Nevertheless, there are still vulnerabilities. If the system changes the polarization of the light particles during coding, for example, the power consumption varies or the time interval of the pulses changes slightly. "An eavesdropper could tap this information and read the message without the sender and receiver noticing" explains Walther. Sabine Euler and her colleagues at the Institute of Applied Physics are trying to eliminate these vulnerabilities. "They are demonstrating a lot of creativity here" says Walther approvingly. Thanks to such research, it will be harder and harder for hackers to take advantage of vulnerabilities in quantum cryptography systems.

The TU Darmstadt quantum physicists want to make quantum cryptography not only more secure, but more manageable at the same time. "In a network in which many users wish to communicate securely with each other, the technology must be affordable," he says. Therefore, his team develops its systems in such a manner that they are as simple as possible and can be miniaturized.

The research team is part of the Center for Advanced Security Research Darmstadt (CASED), in which the TU Darmstadt, the Fraunhofer Institute for Secure Information Technology and the University of Darmstadt combine their expertise in current and future IT security issues. Over 200 scientists conduct research in CASED, funded by the State Initiative for Economic and Academic Excellence (LOEWE) of the Hessian Ministry for Science and the Arts. "We also exchange information with computer scientists, which is very exciting," says Walther.

After all, the computer science experts deal with many of the same issues as Walther's quantum physicists. For example, Johannes Buchmann of the department of Computer Science at the TU Darmstadt is also working on encryption methods that theoretically can not be cracked by a quantum computer. However, these are not based on quantum physics phenomena, but rather on an unsolvable math problem.

Therefore, it may well be that the answer to the first code-cracking quantum computer comes from Darmstadt.

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