Quantum key distribution (QKD), establishes highly secure keys between distant parties by using single photons to transmit each bit of the key. A unique aspect of quantum cryptography is that Heisenberg’s uncertainty principle ensures that any attempts to intercept and measure quantum transmissions, will introduce an anomalously high error rate in the transmissions between Alice and Bob, allowing them to detect the attempted eavesdropping. QKD is suitable for use in any key distribution application that has high security requirements including financial transactions, electoral communications, law enforcement, government, and military applications.
Currently Most Quantum Communication links are direct point-to-point links through telecom optical fibers and, ultimately limited to about 300-500 km due to losses in the fiber. Long distance fibre optic communications exploit the low loss of silica fibres in the 1.3 μm and 1.55 μm wavelength bands. In optical fibers, the wavelength of 1550 nm is very convenient, as it experiences the lowest absorption losses of the whole spectrum, and can be detected with Indium-Gallium-Arsenide avalanche photodiodes (InGaAs APD) in the single photon counting regime.
The next important milestone, is development of large scale QKD network to extend QKD from point-to-point configuration to multi-user and large-scale scenario. China has also operationalised the 2,000-km quantum communication main network between Beijing and Shanghai using quantum repeaters.
Another way to overcome distance limitation is by bringing quantum communication into space. An international team led by the Austrian physicist Anton Zeilinger has successfully transmitted quantum states between the two Canary Islands of La Palma and Tenerife, over a distance of 143 km.
Free space QKD channels based on Free space laser communication and have several advantages over the optical fiber. Firstly, the atmosphere is an almost non birefringent medium which guarantees the preservation of photon polarization. Secondly, there is a relatively low absorption loss in the atmosphere for certain wavelengths. This fact enables us to achieve a longer communication range. For free-space quantum channels, the transmission wavelength is usually chosen around 780 nm, which corresponds to the quantum efficiency peak of Silicon Avalanche Photodiodes (Si APD). Scientists have reported a successful free-space quantum key distribution (QKD) in daylight with the self-developed polarization encoding chip for the first time.
Recently, China launched a quantum science satellite and performing many quantum experiments with optical links between space and ground. In one of the experiment the Micius’ satellite used quantum key distribution for secure video chat between one ground station near Vienna, with one near Beijing. Heriot-Watt University is developing satellite to ground receivers for quantum communications in a move that could provide a leap forward in technology. The university said that the satellite to ground receiver would be compact, lightweight and affordable to allow for commercialisation and widespread use.
However, establishing global QKD networks would require combining satellite networks with fiber optic links and free space links all dissimilar in wavelength.
“Interfacing fundamentally different quantum systems is key to building future hybrid quantum networks. Such heterogeneous networks offer capabilities superior to those of their homogeneous counterparts, as they merge the individual advantages of disparate quantum nodes in a single network architecture. However, few investigations of optical hybrid interconnections have been carried out, owing to fundamental and technological challenges such as wavelength and bandwidth matching of the interfacing photons,” write authors in nature.
Entangling photons of different colors: Feb 2019
The optical components that store and process quantum information typically require visible-light photons (particles of light) to operate. However, only near-infrared photons—with wavelengths about 10 times longer—can transport that information over kilometers of optical fibers.
Now, researchers at the National Institute of Standards and Technology (NIST) have developed a novel way to solve this problem. For the first time, the team created quantum-correlated pairs made up of one visible and one near-infrared photon using chip-based optical components that can be mass-produced. These photon pairs combine the best of both worlds: The visible-light partners can interact with trapped atoms, ions, or other systems that serve as quantum versions of computer memory while the near-infrared members of each couple are free to propagate over long distances through the optical fiber.
The achievement promises to boost the ability of light-based circuits to securely transmit information to faraway locations. NIST researchers Xiyuan Lu, Kartik Srinivasan and their colleagues at the University of Maryland NanoCenter in College Park, demonstrated the quantum correlation, known as entanglement, using a specific pair of visible-light and near-infrared photons. However, the researchers’ design methods can be easily applied to create many other visible-light/near-infrared pairs tailored to match specific systems of interest. Moreover, the miniature optical components that created the entanglements are manufactured in large numbers.
Lu, Srinivasan and their colleagues recently described their work in Nature Physics.
To create the entangled pairs, the team constructed a specially tailored optical “whispering gallery”—a nano-sized silicon nitride resonator that steers light around a tiny racetrack, similar to the way sound waves travel unimpeded around a curved wall such as the dome in St. Paul’s Cathedral in London. In such curved structures, known as acoustic whispering galleries, a person standing near one part of the wall easily hears a faint sound originating at any other part of the wall.
When a selected wavelength of laser light was directed into the resonator, entangled pairs of visible-light and near-infrared photons emerged. (The specific type of entanglement employed in the experiment, known as time-energy entanglement, links the energy of the photon pairs with the time at which they are generated.)
“We figured out how to engineer these whispering gallery resonators to produce large numbers of the pairs we wanted, with very little background noise and other extraneous light,” Lu said. The researchers confirmed that entanglement persisted even after the telecommunication photons traveled through several kilometers of optical fiber.
In the future, by combining two of the entangled pairs with two quantum memories, the entanglement inherent in the photon pairs can be transferred to the quantum memories. This technique, known as entanglement swapping, allows the memories to be entangled with each other over a much longer distance than would normally be possible. “Our contribution was to figure out how to make a quantum light source with the right properties that could enable such long-distance entanglement,” Srinivasan said.
Scientists exchanged quantum information on daylight in a free-space quantum key distribution: Dec 2018
The Electronics and Telecommunications Research Institute (ETRI) has reported a successful free-space quantum key distribution (QKD) in daylight with the self-developed polarization encoding chip for the first time. QKD is one of the most promising secure communication technologies, which encodes information into a single-photon, the smallest measurable unit of light. By using the quantum mechanical properties of the single-photon, quantum cryptography guarantees secure information exchange between the distant parties.
The report is particularly worthy of attention for the following reasons. First, ETRI’s free-space QKD system works successfully even during the daylight whereas most other systems have failed to operate properly due to substantial amount of noise photons from sunlight. By developing and adopting elaborate noise filtering technologies, ETRI’s QKD system achieved the secure key rate of 142.94 kbps with quantum bit error rate of 4.26% in daylight over the free-space distance of 275 m.
Second, ETRI’s QKD system is configured with the self-developed polarization encoding chip, which dramatically reduces the size of the system compared to conventional QKD systems. Miniaturizing key components is highly important to make QKD systems to be used for the secure communication solution of several applications requiring light-weight such as Unmanned Aircraft Vehicle (UAV) and automotive cars, whose security is one of the critical concerns. The chip-based QKD component of ETRI is considered as a core technology for the commercialization of QKD system in various fields.
ETRI is now applying their integrated-chip technologies to other optical components to realize miniaturized QKD transceiver modules. Also, ETRI is trying to conduct the free-space QKD experiments for the extended transmission distance in daylight.
Satellite study proves global quantum communication will be possible
Researchers in Italy have demonstrated the feasibility of quantum communications between high-orbiting global navigation satellites and a ground station, with an exchange at the single photon level over a distance of 20,000km. The milestone experiment proves the feasibility of secure quantum communications on a global scale, using the Global Navigation Satellite System (GNSS). It is reported in in the journal Quantum Science and Technology in Dec 2018.
The team’s results show the first exchange of a few photons per pulse between two different satellites in the Russian GLONASS constellation and the Space Geodesy Centre of the Italian Space Agency.
Co-lead author Professor Paolo Villoresi said: “”Our experiment used the passive retro-reflectors mounted on the satellites. By estimating the actual losses of the channel, we can evaluate the characteristics of both a dedicated quantum payload and a receiving ground station. “Our results prove the feasibility of QC from GNSS in terms of achievable signal-to-noise ratio and detection rate. Our work extends the limit of long-distance free-space single-photon exchange. The longest channel length previously demonstrated was around 7,000 km, in an experiment using a Medium-Earth-Orbit (MEO) satellite that we reported in 2016.”
Although high-orbit satellites pose a large technological challenge, due to losses from optical channels, Professor Villoresi explained the team’s reasoning for focussing on high-orbiting satellites in their study. He said: “The high orbital speed of low earth orbit (LEO) satellites is very effective for the global coverage but limits their visibility periods from a single ground station. On the contrary, using satellites at higher orbits can extend the communication time, reaching few hours in the case of GNSS.
“QC could also offer interesting solutions for GNSS security for both satellite-to-ground and inter-satellite links, which could provide novel and unconditionally secure protocols for the authentication, integrity and confidentiality of exchanged signals.” Space quantum communications (QC) represent a promising way to guarantee unconditional security for satellite-to-ground and inter-satellite optical links, by using quantum information protocols as quantum key distribution (QKD).”
Heriot-Watt University is leading the development and feasibility testing for next generation ground receiver technology that could help commercialise quantum communications
Current QKD systems are earth-based and limited in range to less than a few 100kms, unless specialist fibres, expensive detectors and often lab-based settings are used. This can make it inefficient, expensive and impractical to implement. QKD via low-Earth orbit satellites provides a more realistic option but while tests have been carried out in China and Japan, these have used expensive satellites and large telescopes, meaning there are still significant challenges to delivering a practical widespread network with global application.
While there have been major developments in QKD in space, the ground element has been relatively less explored. Dr Ross Donaldson from the Institute of Photonics and Quantum Sciences has been awarded a Royal Academy of Engineering Fellowship to lead the development and feasibility testing for next generation ground receiver technology, in collaboration with companies looking to commercialise QKD.
He explains: “Current ground receivers cost in the region of a million pounds, are specially built and generally need staff to run them. However, we are developing ground receiver technology with the aim of significantly reducing that cost. It will be fully autonomous so that its commercially viable and has widespread applications.
“We are investigating more efficient methods to achieve quantum communications, one of which involves developing a receiver that uses novel 2D spatial array detector technology that is sensitive to single particles of light. This will allow a reduction of the optical design complexity and will facilitate inline optical beaconing for fine pointing and tracking. “These features are important to industries looking to commercialise QC and will provide a research-led affordable system that will contribute to the development of a global market.
“Having an increase in ground network coverage will benefit the Scottish economy, enabling us to push for increased coverage in space with micro and nanosatellites. This will have a positive economic effect for local nanosatellite suppliers such as Clyde Space. The growth in the ground and space network will also push scientific experiments towards connecting quantum computers and developing a quantum internet.”
Dr Donaldson is funded through a RAEng Research Fellowship from The Royal Academy of Engineering and an additional grant from ESPRC. Heriot-Watt University is collaborating with the EPSRC Quantum Communications Hub, Arqit, BT, ID Quantique, the Knowledge Scientific and Technology Facilities, the University of Edinburgh, the University of Strathclyde, and the Scottish Centre of Excellence in Satellite Applications.
Quantum internet goes hybrid
In a recent study published in Nature, ICFO researchers led by ICREA Prof. Hugues de Riedmatten report an elementary “hybrid” quantum network link and demonstrate photonic quantum communication between two distinct quantum nodes placed in different laboratories, using a single photon as information carrier.
The ICFO researchers have developed a solution and solved the challenge of a reliable transfer of quantum states between different quantum nodes via single photons. A single photon needs to interact strongly and in a noise-free environment with the heterogeneous nodes or matter systems, which generally function at different wavelengths and bandwidths. As Nicolas Maring states “it’s like having nodes speaking in two different languages. In order for them to communicate, it is necessary to convert the single photon’s properties so it can efficiently transfer all the information between these different nodes.”
In their study, the ICFO researchers used two very distinct quantum nodes: the emitting node was a laser-cooled cloud of Rubidium atoms and the receiving node a crystal doped with Praseodymium ions. From the cold gas, they generated a quantum bit (qubit) encoded in a single photon with a very-narrow bandwidth and a wavelength of 780 nm. They then converted the photon to the wavelength of 1552 nm to demonstrate that this network could be completely compatible with the current telecom C-band range.
Subsequently, they sent it through an optical fiber from one lab to the other. Once in the second lab, the photon’s wavelength was converted to 606 nm in order to interact correctly and transfer the quantum state to the receiving doped crystal node. Upon interaction with the crystal, the photonic qubit was stored in the crystal for approximately 2.5 microseconds and retrieved with very high fidelity.
For this experiment the researchers used a photon encoding technique called time-bin encoding, which is very well suited to communicating qubits and preventing interference. Our results open up the prospect of optically connecting quantum nodes with different capabilities and represent an important step towards the realization of large-scale hybrid quantum networks.