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Quantum cryptography race creating new records in distance and data rate for building practical National Quantum Networks

Information security is becoming more and more of a critical issue not only for large companies, banks and defense enterprises, but even for small businesses and individual users. However, the data encryption algorithms we currently use for protecting our data are imperfect — in the long-term, their logic can be cracked. Cryptographic protection of sensitive information is  also under severe  threat from rapidly evolving quantum computers, which shall be able to break many cryptographic schemes.


The world faces a huge threat in the next few years when quantum computers become powerful enough to break the cryptography of conventional computers. Everything from government communications to the contents of Bitcoin wallets would become vulnerable to hackers. “We’re not talking in 10 years, it will be within 2 to 5 years,” says Markus Pflitsch, founder and CEO of Terra Quantum. Hackers and hostile governments are likely to be among the first to harness quantum computers for code-breaking and the computer industry is becoming acutely aware that it needs to find ways to protect itself.


Quantum cryptography is one of the best solutions — in theory, the laws of physics make a network linked by quantum connections unhackable. In practice, there is a big problem: quantum key distribution only works over relatively short distances. Photons, which are used for these transmissions, get scattered or absorbed along the route. Record distances for quantum key distribution so far have been 421km (in 2018), and a Chinese research group last year took this to 509km.


Quantum key distribution (QKD)

Contrary to algorithm-based encryption, systems that protect information by making use of the fundamental laws of quantum physics, can make data transmission completely immune to hacker attacks in the future. 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.


The original quantum cryptography system, built in 1989 by Charles Bennett, Gilles Brassard and John Smolin, sent a key over a distance of 36 centimeters [source: Scientific American]. 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. Other factors are high background noise of practical single-photon detectors, BER rates caused by microscopic impurities in the fiber and inefficient finite-key security analysis.


Experimentally, QKD has been implemented via optical means, achieving key rates of 1.26 megabits per second over 50 kilometers of standard optical fiber and of 1.16 bits per hour over 404 kilometres of ultralow-loss fibre in a measurement-device-independent configuration. Increasing the bit rate and range of QKD is a formidable, but important, challenge. A related target, which is currently considered to be unfeasible without quantum repeaters, is overcoming the fundamental rate-distance limit of QKD. This limit defines the maximum possible secret key rate that two parties can distil at a given distance using QKD and is quantified by the secret-key capacity of the quantum channel  that connects the parties.


With the aid of a quantum repeater, it would be possible to over-come this barrier. However, despite recent advances, such a device remains difficult to realize. One of the simplest versions, tailored for intercity distances, avoids using quantum memories and quantum error correction, but still requires non-demolition measurements, conditional optical switches and the multiplexing of a large number of single photon sources, all of which is far from trivial to implement.


Although a trusted-node network and the use of satellites can greatly extend the reach of quantum communications, they do not exceed the SKC barrier. In the former case, the information ceases to be quantum at each intermediate node. For the  latter, outer space provides a low-loss propagation medium, but the key rate per loss unit remains unchanged.


Quantum cryptography race creating new records in distance and data rate

The economic and military benefits of Quantum cryptography or QKD is driving countries towards new records in distance and data rate. For making quantum key distributions (QKDs) practical to be applied over national and regional networks, Researchers all over the world are achieving world record in distances over which secure keys can be generated over the fiber through advances in single-photon sources (SPSs), single-photon detectors, modulating schemes, and protocols. Researchers are also trying to integrate these systems into the installed optical fiber telecommunication network infrastructure.


China has also operationalised the 2,000-km quantum communication main network between Beijing and Shanghai using quantum repeaters. The quantum communication line 712 kilometers in length connecting Hefei, the capital of Anhui Province, and Shanghai, a coastal trade hub, has 11 trusted nodes along its length, Xinhua news agency reported .


China has also launched a quantum science satellite and performing many quantum experiments with optical links between space and ground. It spans 2600 km between two observatories – one east of Beijing and the other just a few hundred kilometres from China’s border with Kazakhstan.


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.  The quantum communication satellite that will eventually be connected to the Shanghai-Beijing line via a station in Beijing enabling the space-to-Earth quantum communication network.


In June 2021 it was reported, Toshiba breaks quantum communication record with 600 km of optical fibers. Toshiba is not the only company developing QKD. Telefonica, the Spanish multinational telecommunications provider, working with China’s Huawei and Spain’s Universidad Politecnica de Madrid (UPM), announced this May they had performed QKD field trials using commercial optical networks.


The Telefonica approach employs software-defined networking technology that enables the network to be centrally and intelligently controlled. Vicente Martin Ayuso, head of the Center for Computational Simulation at UPM, said at the announcement, “Now we have, for the first time, the capability to deploy quantum communications in an incremental way, avoiding large upfront costs and using the same infrastructure.”

In 2015, Japanese Researchers demonstrate Secure Quantum Key Distribution using true Single-Photon Emitter at World-Record Distance of 120 km

The collaboration involving the University of Tokyo, Fujitsu Laboratories, and NEC have achieved quantum key distribution at a world-record distance of 120 km developed over an optical fiber using system comprised of two key components. One is a high-purity quantum dot single-photon emitter operating in the 1.5μm band, which reduces the occurrence of simultaneous multi-photon emissions, one of the major limiting factors for long-distance QKD, to one in a million.


Their Single-Photon Emitter was developed by illuminating (exciting) a quantum dot placed in a so-called “optical horn structure”. Therefore they avoided the security vulnerability of attenuated pseudo single-photon emitter that has high probability of generating unwanted multiple photons. They also avoided the complexity of alternate method of artificially mixing optical pulses with different intensities (decoy states) to avoid eavesdropping.


The other is an optical-fiber-based QKD system optimized for use with single-photon emitters by employing superconducting single-photon detectors with ultra-low-noise characteristics. Secondly they employed superconducting single-photon detectors with ultra-low-noise characteristics due to which they were able to achieve secure key distribution at a world-record distance of 120 km, twice the previous longest distance.


At the distance of 100 km, we obtained the maximal secure key rate of 27.6 bps without using decoy states, which is at least threefold larger than the rate obtained in the previously reported 50-km-long QKD experiment. We also succeeded in transmitting secure keys at the rate of 0.307 bps over 120 km. This is the longest QKD distance yet reported by using known true SPSs


QKD System Optimized for Single-Photon Emitters Using Superconducting Single-Photon Detectors

Superconducting single-photon detector is an optical detector that uses the phenomenon of the destruction of electrical superconductivity by light absorption. They are far superior in performance to single-photon detectors using existing semiconductors, having sufficient sensitivity for single photons, and low noise (dark count rate), high quantum efficiency, and high temporal resolution.


Using a low-loss interference system optimized to a communications-wavelength band single-photon emitter that uses a planar lightwave circuit as a platform, which has good practicality proven in operation in the Tokyo QKD Network, the researchers built a practical single-photon QKD system that is insensitive to changes in temperature or tensile force that exist in actual optical fiber networks. Based on these results, the researchers will work on making the single-photon QKD system more compact and faster, with the aim of rolling out from 2020 highly secure communications for major urban centers.


QKD is implemented by using single photons as quantum information carrier, the researchers used phase-encoded photons as a qubit , instead of polarization qubit because it is generally difficult to preserve a polarization state in a long fiber. To send the key, pulses of photons are modulated with a key bit. Then, before they are launched into the fibre, they are heavily attenuated to have less than one photon per pulse on average – so some pulses have been completely absorbed.


Once in the fibre, remaining photons, which are still phase-modulated, either make it to the far end or is scattered away en-route. Detecting the phase of single photons at high speed is tricky to say the least. For these 1Gphoton/s receivers has been developed that uses semiconductor-based avalanche photodiodes on each of two output ports of an interferometer – one for each expected phase state.


“Some quantum key distribution (QKD) systems are based on entangled pairs of photons, but here, we’re using just single photons,” Dr Andrew Shields, assistant MD of Toshiba Research Europe, told IT Pro. “The benefit of using a single photon is we can achieve much higher bitrates and longer distances, although the security of the two techniques is the same,” he explained.


In 2016, Russian Scientists Develop Long-Range (250+ Kilometers) Secure Quantum Communication System

Researchers from the Quantum Information Centre of the International Institute of Photonics and Optical Information Technology at ITMO University along with colleagues from Heriot-Watt University in Edinburgh have devised a new way to effectively generate and distribute quantum bits. This is the first system in Russia, which can compete with the best existing analogues and makes it possible to share quantum signals via optical fiber across 250 kilometers in distance.

“To transmit quantum signals, we use the so-called side frequencies,” says Artur Gleim, head of the Quantum Information Centre at ITMO University, “This unique approach gives us a number of advantages, such as considerable simplification of the device architecture and large pass-through capacity of the quantum channel. In terms of bit rate and operating distance our system is comparable to absolute champions in the field of quantum communications.” The very possibility of stable transmission of quantum signals through fiber optical channels is instrumental to subsequent integration of quantum key distribution systems that will be used to secure the useful data.


According to Robert Collins, research associate at the Institute of Photonics and Quantum Sciences at Heriot-Watt University and one of the authors of the study, the work may become a big pivot point for the whole field of quantum communication and cryptography: “Down the track, this new approach can enable smooth coexistence of numerous data streams with different wavelengths in one single optical cable. On top of it, these quantum streams can be fed into the already existing fiber optic lines along with conventional communications.”


In order to encode quantum bits in the system, laser radiation is directed into a special device called the electro-optical phase modulator. Inside the modulator the central carrier wave emitted by the laser is split into several independent waves. After the signal is transmitted through the cable, the same splitting occurs on the receiver end. Depending on the relative phase shift of the waves generated by the sender and the receiver, the waves will either enhance or cancel each other. This pattern generated by overlapping wave phases is then converted into the combination of binary digits, 1 and 0, which serves to compile a quantum key.


Importantly, the scientists have achieved high stability of the relative phase shifts of the signal in the system. “All waves undergo random changes while passing through the fiber,” explains Oleg Bannik, one of the authors of the study and researcher at Quantum Information Centre, “But these changes are always identical and get smoothed over during the additional run through the receiver’s modulator. In the end, the receiver observes the same combination as the sender.”

Now the researchers are on the mission to create a full-fledged quantum cryptographic system, which will generate and distribute quantum keys and transmit useful data simultaneously.


“Using SCW QKD system a sifted bit rate of 800 bit/s was demonstrated in a quantum channel with 30 dB loss. This loss corresponded to, for example, a 150 km link of single mode fiber with 0.2 dB/km loss at a wavelength of 1550 nm.”


This technique can also be combined with traditional wavelength division multiplexing (WDM), which potentially allows significantly increased quantum channel bandwidth use in optical fibers (up to 40% compared to 2-4% in other QKD systems for 1 Gbit Ethernet), therefore making SCW QKD systems perfect candidates for building blocks in quantum networks

Chinese Quantum Cryptographers Set (400+ Kilometers) Distance Record in 2016

Quantum cryptography  is provably, perfectly secure, guaranteed by the laws of physics. In effect, it is the universe’s way of keeping secrets. At least in theory. In practice, things have turned out to be more tricky. The problem is that while the laws of physics guarantee perfect secrecy, the equipment used to perform this type of cryptography is flawed. For example, lasers that are supposed to send single photons one at a time sometimes send several unexpectedly, and this allows supposedly secret information to leak out. Hackers have already exploited these vulnerabilities to hack quantum cryptography systems.


But physicists have been fighting back. Soon after the first quantum hack in 2010, physicists devised a new quantum protocol for encrypting information that does not depend on a specific device to work. The trick is to use additional quantum states as decoys. So-called device-independent quantum cryptography suddenly made it possible to send information securely again.


Various teams have tried device-independent quantum cryptography and shown that it works. But there is a problem. It is slow—snail-blazingly, slothfully slow. The best demonstration so far sent information over a distance of 200 kilometers at a data rate of just 0.018 bits per second. At this rate, perfectly secure quantum cryptography would never be practical.


Physicists devised a new ultra-secure quantum protocol for encrypting information that does not depend on a specific device to work. The trick is to use additional quantum states as decoys. So-called device-independent quantum cryptography suddenly made it possible to send information securely again.


Hua-Lei and co have sent a key over a distance of more than 100 kilometeres at data rates measured in kilobits per second, and they have even managed distances of over 400 kilometers at lower data rates. “This is by far the longest distance reported for all kinds of quantum key distribution systems,” say the team.


And they’ve done this in a way that does not depend on the way the photons are detected. So there is no way a hacker can eavesdrop on the message by hacking into the photon detectors. But there are some caveats. The first is that the method is only partially device independent. The trick these guys have perfected is a way to make the cryptography independent of the photon detectors. They call it measurement-independent quantum cryptography. However, it is still possible for the transmitter to be hacked.



Toshiba registers new quantum record (600 Kms)  in 2021

Toshiba have demonstrated quantum communications sent over a record-breaking 600 km (373 miles) of optical fiber, reported in June 2021. The key was a new dual band stabilization technique they developed, which sends two optical reference signals along with the qubits themselves, which are encoded as a phase delay of a weak optical pulse. The first reference signal is at a wavelength designed to cancel out fluctuations from the environment, while the second one operates on the same wavelength as the qubits themselves and is used to precisely control the phase of the light.


Using this dual band technique, the Toshiba team was able to keep the quantum signal constant to within a few dozen nanometers. That in turn allowed them to transmit the data over 600 km of optical fibers, around six times farther than the previous record. It’s not the furthest ever though – satellite transmission holds the overall record of more than 1,200 km (746 miles), but a quantum internet would need a mix of both satellites and optical fibers.


“QKD has been used to secure metropolitan area networks in recent years,” says Andrew Shields, Head of the Quantum Technology Division at Toshiba Europe. “This latest advance extends the maximum span of a quantum link, so that it is possible to connect cities across countries and continents, without using trusted intermediate nodes. Implemented along with Satellite QKD, it will allow us to build a global network for quantum secured communications.” The breakthrough was described in a paper in the journal Nature Photonics.


Toshiba is leading in high-speed QKD and has been holding field trials in Japan and the United Kingdom for several years. In Sep 2018 it was reported that Toshiba and Tohoku Medical Megabank Organization (ToMMo) at Tohoku University  have achieved, for the first time, an average key distribution speed greater than 10 megabits per second over a one-month period. This is roughly five times as fast as the previous fastest QKD speed of 1.9 Mbps established by Toshiba Research Europe in 2016.


In this scheme, a QKD transmitter modulates a photon’s phase to randomly represent a zero or one. Modulated photons are transmitted to the QKD receiver. Based on the received photons, secure keys are generated at both ends. The keys are then fed into a one-time pad algorithm to encrypt and decrypt all other transmitted data. This combination of one-time pad and QKD ensures the transmitted data is fundamentally safe and secure from any known method of attack.


The speed gains are a result of Toshiba and Toshiba Research Europe using high-speed photon detectors and control electronics to register the signals, as well as an improved method for processing the signals into secure key data. Error correction and privacy amplification, until now a bottleneck in the system, have also been enhanced, which has greatly improved the system’s postprocessing speed.


“The trial was conducted over 7-kilometer standard telecommunications single-mode fiber-optic lines connecting the two sites,” says Yoshimichi Tanizawa, senior research scientist at Toshiba’s Corporate Research & Development Center’s Network Systems Laboratory in Kawasaki, near Tokyo. “As the trial was conducted in a practical environment, it is an important step toward high-speed QKD commercialization.”


Toshiba says it has previously conducted successful field trials in the Tokyo area with single-span fiber-optic lines as long as 45 km, while lab tests in the U.K. have reached a distance of 240 km. Moreover, Toshiba announced in May that it has devised and is testing a new protocol called Twin-Field QKD that will extend the distribution distance to over 500 km of optical fiber. Whereas single photons are sent from one end of the fiber to the other in conventional QKD, with the Twin-Field protocol, photons are sent from both ends to a central location for detection. This effectively doubles the transmission distance.


Commenting on the trials, Alan Woodward, a computer scientist at the University of Surrey, in England, says, “Toshiba’s announcement is notable not so much for the speed achieved, as the fact that it appears to have been done over an extended period over already-installed fibers.” He added, however, that QKD’s widespread take-up will likely “depend not just on using existing installed fibers, but when you can multiplex QKD on fibers in some way with the data it’s there to protect.” In the Tohoku trial, Toshiba and ToMMo used separate fiber lines for the content and the key.


In addition to running these tests, Toshiba and ToMMo operated a wireless sensor network to continuously monitor the installed fiber optic lines using multisensor devices incorporating accelerometers and temperature sensors. The aim was to study how the fiber’s characteristics change with shifts in the weather and nearby vibrations, and how such changes impact the performance of the high-speed QKD. “The monitoring has confirmed the correlation between the stability of the high-speed QKD system and disturbances to the installed fiber,” says Tanizawa. “For example, we found wind-induced vibrations in the fiber affected stability. We are now working on improving the stability of the QKD system.”


One challenge that must still be overcome is standardizing features of the system. “For instance,” says Tanizawa, “we need to standardize the interface to deliver secure keys to any application, including those in health care, finance, and telecommunications.”And while he acknowledges more field testing is required to fine-tune the system, he says, “We assume it will be ready for commercialization in 2020.” Commenting on the growing competition to introduce QKD, Woodward said, “I’m not sure it’s about competing QKD systems but more about whether QKD will be seen as enough of a security advantage over postquantum crypto schemes to warrant the expense of the infrastructure.”

Terra Quantum announced 40,000 Km QKD record in July 2021

Terra Quantum, the Swiss quantum technology company,  announced a breakthrough in quantum cryptography with a technology that allows quantum cryptography keys to be transmitted over a distance of more than 40,000km — the circumference of the Earth. It blows all previous quantum cryptography distance records out of the water with an almost 100x improvement and potentially solves the biggest problem preventing quantum cryptography from becoming practically usable.


In order to send a secure quantum key between, say, London and New York (5,567km) to secure a stock market trade, you’d need to put multiple repeaters into the line to boost the signal. Each one of these repeaters becomes a potential point of vulnerability, where the signal can be intercepted. So quantum cryptography developers have been trying to find ways to do without these.


Some companies are trying to solve this by building new networks — UK-based Arqit, which recently announced plans to go public via a $1.4bn SPAC, for example, is planning to use satellites to distribute secure quantum cryptography keys. But this infrastructure will take time to put in place. One big advantage of Terra Quantum’s solution is that it can run inside a standard optical fibre line, already in use today in telecoms networks. The technology is based on measuring the loss of photons over a particular line and carefully controlling the transmitted signal so that the amount of signal that an eavesdropper on the line is never enough to be able to extract meaningful information.


“We can know exactly how much of the signal is being intercepted and can tweak it to make sure that the eavesdropper only gets a few photons — which would be obscured by quantum noise,” says Nikita Kirsanov, project lead at Terra Quantum. The technique is based around the second law of thermodynamics — the one that states that entropy always increases. There is a short video explainer of it here:



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