Quantum encryption using single photons is a promising technique for boosting the security of communication systems and data networks, but there are challenges in applying the method over large distances due to transmission losses. 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. Experimentally, QKD has been implemented via optical means, achieving key rates of 1.26 megabits per second over 50 kilometres of standard optical fibre and of 1.16 bits per hour over 404 kilometres of ultralow-loss fibre in a measurement-device-independent configuration.
Quantum internet—the quantum version of the current Internet—holds promise for accomplishing quantum teleportation, quantum key distribution (QKD) and precise synchronisation of atomic clocks among arbitrary clients all over the globe, as well as longer-baseline telescopes and possibly even simulation of quantum many-body systems. Developing the quantum network relies on two technologies, one is use of quantum satellite which can connect over large distances, China has already launched a quantum satellite.
Another method is to use optical amplifiers just as amplifiers are used to regenerate the signal in classical communication. Along the way, the signal passes through repeaters, where it is read, amplified and corrected for errors. Similar to its classical analogue, a quantum repeater is a device that can extend the range of quantum communication between sender and receiver. In contrast to classical network, however, quantum information cannot be detected or amplified without having its information converted back to classical information. Straightforward adoption of an optical amplifier in a quantum network therefore cannot work.
The whole process is at any point vulnerable to attacks. Secondly to realise it in a global scale for arbitrary users, it is reasonable to utilise also existing optical networks that have already been installed in the world. An indispensable building block for implementing such a quantum internet against photon loss of optical fibres is to use quantum repeaters over an optical network, irrespective of its topology. However, in the quantum regime this adds too much noise and destroys the coherence of the quantum states.
In China, extensive quantum networks have already been built use simple “trusted nodes” that measure and retransmit information about quantum states. But quantum repeaters at the present time are a long way from becoming standardized commercial products. NTT/NIST has utilized teleportation technique could be used make quantum repeaters. One significant limitation with current quantum repeater technology is that while it facilitates secure transactions through QKD, it may not itself be secure. “Quantum” repeaters today are actually hybrid systems and include classical computing devices that – as the Japanese scientists point out – are just as vulnerable as other classical systems to security violations. It is true that a quantum repeater can be physically secured, but quantum encryption based on repeaters that are not themselves secure, detracts from the business case for QKD and could also be a problem for those sharing of quantum computer resources over a cloud and/or a network.
How can you amplify and correct a signal if you can’t read it? The solution to this seemingly impossible task involves a so-called quantum repeater. Unlike classical repeaters, which amplify a signal through an existing network, quantum repeaters create a network of entangled particles through which a message can be transmitted. A quantum repeater has to achieve an effective “amplification” or restoration of the quantum information without resorting to a direct measurement of the laser light. The key technology for implementing quantum repeaters is quantum memories that allow the storage of quantum states.
If this stage in the evolution of the quantum repeater actually occurs, it will probably be a device that does not have extensive integrated memory requirements. At the present time, we are at a stage in quantum repeater development where storage is largely classical, raising the vulnerability issues mentioned above. The next stage will be quantum repeaters utilizing quantum memories. But NTT for one is researching quantum repeaters without quantum memories, says CIR report. Its optical quantum repeater, designed with some Canadian researchers, appears to eliminate the quantum memories in repeaters.
Quantum Repeater technologies
CQC2T aims to realize an operational quantum repeater that can be used in a quantum key distribution network for extending the communication range to beyond that of a passive network. Realization of an operation quantum repeater requires interfacing and integrating a number of quantum components into one complete system. In order to synchronize the various stages of the quantum communication line, light pulses need to be stored and retrieved on demand while preserving the embedded quantum information. At CQC2T our schemes are mainly based on the gradient echo memory (GEM) approach. Two complementary approaches adopted by us are (a) Rb gas cell Raman GEM and (b) rare-earth ion crystal GEM. These memories should have high efficiency and have long coherence time.
New quantum repeater paves the way for long-distance big quantum data transmission
Physicists have designed a new method for transmitting big quantum data across long distances that requires far fewer resources than previous methods, bringing the implementation of long-distance big quantum data transmission closer to reality. The results may lead to the development of future quantum networks, such as a global-scale quantum internet. In 2018, The researchers, Michael Zwerger and coauthors at the University of Innsbruck, Austria, have published a paper on the new long-range quantum communication method in a recent issue of Physical Review Letters.
“The greatest significance of our work is that we provide an efficient and scalable scheme for long-distance quantum communication,” Zwerger told Phys.org. “We believe that this will be an essential ingredient for a future quantum internet, where large amounts of quantum data will be transmitted. Most importantly, in contrast to previous proposals, the required resources (per transmitted qubit) at each repeater station do not scale with the distance, which makes the quantum data transmission more efficient.”
The new method relies on an alternative type of quantum repeater—a device that generates quantum entanglement at distant locations on a quantum network in order to combat signal loss, somewhat how an amplifier boosts the signal in classical communication networks. The biggest advantage of the new quantum repeater is that it can allow quantum data transmission to be scaled up to longer distances much more easily than with previous quantum repeaters. Typically, as the transmission distance increases, more resources (qubits) are needed at each repeater station. In previous schemes, the number of resources grows polylogarithmically or even polynomially at each repeater station with the distance.
Using the new quantum repeater, the number of resources per transmitted qubit remains constant at each repeater station; that is, it is entirely independent of the distance. This allows for quantum data to be transmitted over arbitrarily long distances using a relatively small amount of resources. In its current implementation, the method uses a few hundred qubits at each repeater station, and can reach intercontinental distances.
As the physicists explain, the key behind the new quantum repeater is an entanglement distillation protocol called hashing, which generates perfect pairs of entangled qubits. The researchers also used an optimized measurement-based implementation, which greatly reduces unwanted noise. These tools provide a high error tolerance and high transmission rates, allowing for quantum data transmission in realistically noisy scenarios, such as a quantum internet.
“Just think of the internet as it has grown over the years, where data transmission has increased dramatically,” Zwerger said. “One can envision a quantum internet, where rather than classical data quantum information is transmitted. Indeed, a number of very interesting applications of such quantum data transmission have been discussed, among them quantum cryptography, distributed quantum computing and distributed sensing. Truly secure transmission requires large keys, and hence also large quantum transmission rates. A similar thing can be said about the possibility of distributed quantum computation. In early proof-of-principle experiments, rates and overheads might not be a big deal, but this for sure will become highly relevant once one scales things up. This is where our proposal becomes relevant.”
In the future, the researchers plan to extend the new quantum repeater devices to work with larger networks. “The present proposal is for point-to-point communication between a sender and a receiver,” Zwerger said. “We plan to use similar ideas for multipartite quantum networks with many users. In addition, we are currently investigating novel schemes where we try to apply similar techniques on smaller scales—taking some of the ideas of the hashing protocol and design entanglement purification protocols and communication schemes that use only a few qubits. This might have an impact on a shorter timescale, when first prototype quantum communication systems will be built.”
Experimental demonstration of Quantum Repeater
In essence, a quantum repeater is a small, special-purpose quantum computer. At each stage of such a network, quantum repeaters must be able to catch and process quantum bits of quantum information to correct errors and store them long enough for the rest of the network to be ready. Until now, that has been impossible for two reasons: First, single photons are very difficult to catch. Second, quantum information is notoriously fragile, making it very challenging to process and store for long periods of time.
Lukin’s lab, in collaboration with Marko Loncar, the Tiantsai Lin Professor of Electrical Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), Hongkun Park, Mark Hyman Jr. Professor of Chemistry at the Harvard Faculty of Arts and Sciences (FAS), and Dirk Englund, Associate Professor of Electrical Engineering and Computer Science at Massachusetts Institute of Technology (MIT), has been working to harness a system that can perform both of these tasks well — silicon-vacancy color centers in diamonds. These centers are tiny defects in a diamond’s atomic structure that can absorb and radiate light, giving rise to a diamond’s brilliant colors. “Over the past several years, our labs have been working to understand and control individual silicon-vacancy color centers, particularly around how to use them as quantum memory devices for single photons,” said Mihir Bhaskar, a graduate student in the Lukin group.
The researchers integrated an individual color-center into a nanofabricated diamond cavity, which confines the information-bearing photons and forces them to interact with the single color-center. They then placed the device in a dilution refrigerator, which reaches temperatures close to absolute zero, and sent individual photons through fiber optic cables into the refrigerator, where they were efficiently caught and trapped by the color-center. The device can store the quantum information for milliseconds — long enough for information to be transported over thousands of kilometers. Electrodes embedded around the cavity were used to deliver control signals to process and preserve the information stored in the memory.
“This device combines the three most important elements of a quantum repeater — a long memory, the ability to efficiently catch information off photons, and a way to process it locally,” said Bart Machielse, a graduate student in the Laboratory for Nanoscale Optics. “Each of those challenges have been addressed separately but no one device has combined all three.” “Currently, we are working to extend this research by deploying our quantum memories in real, urban fiber-optic links,” said Ralf Riedinger, a postdoctoral candidate in the Lukin group. “We plan to create large networks of entangled quantum memories and explore the first applications of the quantum internet.
“This is the first system-level demonstration, combining major advances in nanofabrication, photonics and quantum control, that shows clear quantum advantage to communicating information using quantum repeater nodes. We look forward to starting to explore new, unique applications using these techniques,” said Lukin.
All-photonic quantum repeaters
For 17 years after the first proposal for quantum repeaters, it had widely been believed that their realisation needs demanding matter quantum memories or matter qubits. However, in 2015, this belief was disproved by a proposal of all-photonic quantum repeaters, which work without any matter quantum memories or matter qubits, that is, only with optical devices. A repeater for quantum communications has been prototyped that requires no costly cryogenics or elaborate ion traps—solely photons. Consequently, worldwide quantum cryptography and maybe the beginnings of a long-distance quantum “internet” could possibly be one step nearer to actuality.
Hoi-Kwong Lo, professor of physics and electrical and laptop engineering on the College of Toronto, agrees. He and a workforce of collaborators from the Japanese Universities of Osaka and Toyama and NTT Company in Japan have carried out the primary proof of precept experiment that establishes the viability of the all-photon quantum repeater expertise that they first proposed in 2015.
Relatively, it establishes the workability of maybe a very powerful hyperlink within the chain: the interpretation of the preliminary photon’s quantum state into an middleman swarm of photons that then convey the unique photon’s quantum state right into a receiving photon’s quantum state. “The ultimate objective is to send quantum communication over an arbitrarily long distance,” Lo says. “However, if we want to do a real quantum repeater, we need more than one component.” So the group is now engaged on rising the robustness of that intermediate swarm of quantum state-transmitting photons — performing a quantum triple axel jump maneuver known as the “time-reversed adaptive Bell measurement.”
“In particular, our TRA measurement—based only on optical devices without any quantum memories and any quantum error correction—passively but selectively performs the Bell measurement only on single photons that have successfully survived their lossy travel over optical channels. In fact, our experiment shows that only the survived single-photon state is faithfully teleported without the disturbance from the other lost photons, as the theory predicts.”
In particular, our scheme begins by entangling all the polarisation qubits at the node C initially, followed by connecting/disentangling the qubits dependently on the success/failure of the entanglement generation processes. This switching between connecting and disentangling is passively performed by adopting the type-II fusion gates as the implementation for the Bell measurement between optical pulses . The type-II fusion gate is composed of the polarising beam splitter (PBS), the half-wave plates and single-photon detectors
Thanks to its all-optical nature, this scheme has advantages that cannot be seen in conventional quantum repeaters necessitating matter quantum memories. For instance, first, the repetition rate of the protocol could be as high as one wants, similarly to a memory-function-less scheme, as it is determined only by the repetition rate of the optical devices, independently of the communication distance. This would lead to a future higher-bandwidth quantum internet.
Second, in principle, the scheme could work at room temperature and does not need any quantum interfaces among photons with different wavelengths, let alone between matter quantum memories and photons. Third, the scheme is an ultimate version of the all-optical network approach—which has already been identified as a promising infrastructure for fast and energy-efficient communication in the field of conventional communication. Thus, the scheme is an important target of the development of photonic networks, and in particular their implementations using integrated quantum circuits or frequency multiplexing. The scheme would also represent a step towards a future all-photonic quantum computer.
CIR’s latest forecasts of the quantum repeater market to be more than $800 million by 2026. The numbers come from “Quantum Networking Deployments, Components and Opportunities – 2017-2026,” a report that CIR published in October 2017. Based on these forecasts, CIR has concluded that in about five years, revenues generated from quantum repeaters will represent a significant and attractive business opportunity.