From quantum repeaters and single-photon detectors to space satellites, photonics promises to bring quantum encryption to the mainstream.
Quantum communications has already been proved possible over short distances. Via fiber optics, quantum key distribution (QKD) has been demonstrated over 250 km; for free-space transmission, the longest demonstrated distance has been more than 144 km.
The problem is that experts don’t expect these distances to extend to more than a few hundred kilometers with current technologies. Essentially, the technology can go only so far before markedly better devices or newer, smarter techniques have to be developed. But interesting progress is being made.
One approach is the development of quantum repeaters, devices that will store and entangle quantum states in a way that allows entanglement to be distributed over much larger distances. But these devices are still at the stage of fundamental research.
Professor Nicolas Gisin of the Group of Applied Physics at the University of Geneva and a team of researchers from France, Germany, Sweden and Switzerland have made important steps toward the realization of quantum repeaters in the European Commission-funded QuReP (Quantum repeaters for long-distance fiber-based quantum communication) project, which ended in June.
In this experiment, conducted by Dr. Félix Bussières of the University of Geneva, a highly coherent green laser illuminates a nonlinear crystal (right corner), where the photon pairs are created. Photo courtesy of Professor Nicolas Gisin.
The QuReP project aimed to produce a quantum repeater similar to the repeaters used in standard communications today; their role is to boost an incoming signal and repeat it on the other side, so the signal does not lose its strength as it travels.
“Quantum repeaters will allow one to distribute quantum cryptographic keys over arbitrarily long distances,” Gisin said. “Contrary to classical communication, for quantum communication one can’t merely amplify the signal. One has to resort [to] the fascinating process of quantum teleportation. In brief, one segments the total line into several shorter links, establishes entanglement over these shorter links and eventually teleports a quantum state over this quantum network.”
QKD over quantum repeaters requires the combination of two photons coming from two entangled pairs. The problem is that these two photons are not likely to arrive exactly at the same time at the repeater, meaning one photon must be stored and released only when the second arrives.
These two crystals, illuminated by two lasers, act as quantum memories that could synchronize future quantum communications networks. They also demonstrate that centimeter-sized objects can be entangled to share a common nonlocal feature.
“This can be done thanks to quantum memories. The latter reversibly map the quantum state of a photon in and out of some atoms,” Gisin said. “In the case of QuReP, we used ensembles of atoms ‘frozen’ in some optical crystals.”
Daisy-chaining QKD systems
The main challenge to unlocking the commercial potential of QKD is distance, according to Gregoire Ribordy, co-founder and CEO of ID Quantique, a QKD specialist for financial, defense and government organizations. “The whole community is waiting for the development of a quantum repeater, but this will take time,” he said. “In the short term, there is renewed interest in daisy-chaining QKD systems using so-called trusted nodes to build long-range networks.”
ID Quantique, based in Geneva, is currently in collaboration with US independent R&D organization Battelle; they plan to perform a long-distance proof-of-concept demonstration in 2015 between Battelle’s headquarters in Columbus, Ohio, and its facilities in Washington, approximately 750 km away.
For Battelle, the day of supercomputers breaking keys that encrypt data is on the near horizon, and important data must be protected using new methods such as QKD. The businesses Battelle would first like to serve are the ones modern thieves prey on most often – banking and finance. But the fact is, almost all industries have data that must be protected.
ID Quantique and Battelle hope that their demonstration will mark an important milestone in bringing commercial off-the-shelf-based QKD products to the US market.
An yttrium oxyorthosilicate crystal doped with neodymium ions is illuminated by a yellow laser, demonstrating the ability to store and release a photon without perturbing its quantum state. Such crystal quantum memories will synchronize future quantum communications networks. Photo courtesy of Professor Nicolas Gisin.
Another challenge is the ability to multiplex QKD with conventional optical communications systems using WDM (wavelength-division multiplexing) technology, Ribordy pointed out. “Single-photon QKD is extremely sensitive to noise induced by Raman scattering. A potential approach to overcome this is to use continuous variable QKD, which is based on homodyne detection and has good noise rejection.”
Detector technology continues to advance
Crucial to quantum communications is sustained progress in detector technology – specifically, single-photon detectors, because information is embedded on single photons.
The Quantum Information Group at Toshiba Research Europe Ltd. in Cambridge, England, showed that semiconductor single-photon detectors could be operated at more than 1-GHz gating frequencies using a self-differencing technique that compares the output for successive gate periods. This boosted the secure bit rate by more than two orders of magnitude.
The self-differencer circuit improved the detector performance in several ways: It sped up the gating frequency to 2 GHz, which is faster than previous systems by two orders of magnitude; it has the shortest dead time for any type of single-photon detector; and the 1-GHz photon count rate is higher than those achieved by upconversion or superconducting detectors without any reduction in the detection efficiency.
Applied to QKD, these self-differencing thermoelectrically cooled semiconductor avalanche photodiodes have allowed the researchers to achieve the highest secure key rates in both laboratory and field environments.
Another important milestone was reached by scientists at NIST in 2010, when they developed an almost noise-free single-photon detector with around 99 percent efficiency.
Using essentially the same technology that permitted them to achieve 88 percent detection efficiency five years earlier, the team enhanced its ability to detect photons largely by improving the alignment of the detector and the optical fibers that guide photons into it.
The basic principle of the detector is to use a superconductor as an ultrasensitive thermometer: Each individual photon hitting the detector raises the temperature – and increases electrical resistance – by a minute amount, which the instrument registers as the presence of a photon.
Quantum physics links with space science
While most of the tough research to extend the commercial viability of quantum communications systems remains in the laboratory – at least for now – another interesting approach goes to the other extreme. The promising scheme is to deploy an orbiting satellite to act as a “node.” The satellite ensures that quantum communications through free-space optical links can be established at practically any point on the globe.
Satellites, even in low-Earth orbit, are several hundred kilometers away, and photons traveling parallel to the Earth’s surface must pass through the densest part of the atmosphere, increasing the likelihood of absorption of the photon and hence the loss of the entanglement information it carries. But pointing a beam straight up to a satellite avoids most of the Earth’s troublesome atmosphere. And once the data is there, the vacuum of space provides the ideal medium for quantum communications.
“The advantage, of course, is that there is no signal loss due to a transmission medium (like air or optical fiber) in space, so the signal-to-noise can remain high over much greater distances than terrestrial links,” said Dr. Brendon Higgins of the Institute for Quantum Computing in Waterloo, Ontario, Canada. “This is the approach that our group is pursuing, as are groups in the US, China and Europe.”
A model of the proposed QEYSSAT QKD demonstrator satellite currently under development by the Institute for Quantum Computing and partners from the Canadian Space Agency. Photo courtesy of Institute for Quantum Computing.
Higgins is part of a team headed by associate professor Thomas Jennewein that aims to establish secure global quantum communications networks via satellite. The group is currently working with the Canadian Space Agency in a project called Quantum Encryption and Science Satellite (QEYSSAT).
Commercial viability and the future
The two main scientific challenges are to realize quantum memories with longer memory times (from the current millisecond regime toward 1 second) and to significantly increase efficiency (from 10 to 90 percent), according to Gisin.
But even then, there will still be a huge technology challenge to make everything work together and with reasonable rates, Gisin noted.
According to Dr. Andrew Shields, assistant managing director at Toshiba Research Europe Ltd., the solution for longer distances in both conventional and quantum communications is to construct a network: Developing quantum repeaters is important, but not as important as network integration and reducing the cost.
While QKD is already commercially viable and suitable for high-security dedicated links, in order to make it accessible to a wider range of applications, the technology must be integrated with conventional data networks, and the cost must be reduced.
“From a commercial aspect, integration of components is an important factor,” Higgins said. “Whereas in the lab we can get away with large optical tables strewn with adjustable elements, that won’t fly when the customer wants a small turnkey box. Current work to develop integrated designs on, for example, waveguides will help that aspect.
“I think it’s pretty clear that we are now already at a stage where quantum communications devices are ready to begin moving out of the lab and into the real world. Certainly, the next few years will be very interesting, as the satellite approaches are engaged and quantum repeaters approach viability.”