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China Quantum Communiations Technology: Cryptography, Radar, Satellite, Teleportation, Network

Article | Published: 24 June 2019

Experimental quantum repeater without quantum memory
Nature Photonics (2019)

Abstract
Quantum repeaters—important components of a scalable quantum internet—enable entanglement to be distributed over long distances. The standard paradigm for a quantum repeater relies on the necessary, demanding requirement of quantum memory. Despite significant progress, the limited performance of quantum memory means that making practical quantum repeaters remains a challenge. Remarkably, a proposed all-photonic quantum repeater avoids the need for quantum memory by harnessing the graph states in the repeater nodes. Here we perform an experimental demonstration of an all-photonic quantum repeater. By manipulating a 12-photon interferometer, we implement a 2 × 2 parallel all-photonic quantum repeater, and observe an 89% enhancement of entanglement-generation rate over standard parallel entanglement swapping. These results provide a new approach to designing repeaters with efficient single-photon sources and photonic graph states, and suggest that the all-photonic scheme represents an alternative path—parallel to matter-memory-based schemes—towards realizing practical quantum repeaters.


Experimental quantum repeater without quantum memory | Nature Photonics
Scientists Firstly Realize All-photonic Quantum Repeater
Jul 12, 2019

Quantum repeaters, as the important components of a scalable quantum internet, enable the distribution of quantum states over long distances. The standard paradigm for a quantum repeater consists of three basic technologies, i.e., entanglement swapping, entanglement purification, and quantum memory. However, the limited performance of current quantum memories remains a major obstacle in realizing practical quantum repeaters.

Recently, the research team led by Prof. PAN Jianwei, Prof. CHEN Yuao and Prof. XU Feihu from University of Science and Technology of China of Chinese Academy of Sciences has demonstrated the all-photonic quantum repeater which eliminates the need for matter quantum memories, offering a new approach to construct the long-distance optical quantum internet. The study was published in Nature Photonics.

Scientists conducted the experiment which adopted a GHZ state and a passive scheme to realize the selective Bell measurement in the repeater nodes. By manipulating a 12-photon interferometer, they implemented a 2×2 parallel all-photonic quantum repeater, and observed an 89% enhancement of entanglement-generation rate over standard parallel entanglement swapping.

These results provided a new approach to design quantum repeaters with efficient single-photon sources and photonic graph states, and suggested that the all-photonic scheme represents an alternative path towards realizing practical quantum repeaters.

In the future, the research team will be devoted to combining the all-photonic scheme with the matter-memory-based scheme. These two schemes are important parallel research directions towards achieving a practical quantum repeater. By doing so, the repeater graph state (RGS) can relax the requirement of long coherent time of quantum memory, while a quantum memory can reduce the requirement of large size for the RGS.

The successful demonstration of all-photonic quantum repeater suggests that the quantum memory is no longer a necessary condition for building a quantum repeater, which opens up a new way for the research on long-distance quantum communications and networks.

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Experimental set-up. (Image by PAN Jianwei’s team)


Scientists Firstly Realize All-photonic Quantum Repeater---Chinese Academy of Sciences
 
Quantum teleportation moves into the third dimension – Physics World
07 Aug 2019


Physicists in China and Austria have shown for the first time they can teleport multi-dimensional states of photons. Carrying out experiments using photons encoded via three spatial states, they say their scheme can be extended to arbitrarily high numbers of dimensions and is a vital step in teleporting the entire quantum state of a particle. The work could also improve technology used in quantum communications and quantum computing.

Quantum mechanics forbids the quantum state of one particle from being copied precisely to another particle. But teleportation – the instantaneous transfer of a state between particles separated by a long distance – offers an alternative. The process involves no physical transfer of matter and erases the state of the particle to be copied.

The basic idea is that Alice and Bob share a pair of entangled particles (in the terminology of quantum cryptography, Alice being the sender of a message and Bob the receiver). Then Alice interacts a third particle – in an unknown state – with her half of the entangled pair, measures the outcome of the interaction, and then tells Bob the result via a classical channel. Given that information and a measurement on his half of the entangled pair, Bob is able to work out the original unknown state – which is what has been teleported.

First proposed theoretically in 1993, quantum teleportation has since been demonstrated in many different guises. It has been carried out using two-level states of a single photon, a single atom and a trapped ion – among other quantum objects – and also using two photons. Then in 2015 Chaoyang Lu, Jian-Wei Pan and colleagues at the University of Science and Technology of China in Hefei demonstrated teleportation of two degrees of freedom – spin and orbital angular momentum – between single photons.

In the latest work, the same group, working with Anton Zeilinger and colleagues at the University of Vienna in Austria, has demonstrated teleportation of higher-dimensional states. As Lu explains, being able to transfer multiple degrees of freedom is only part of the challenge. That’s because particles in nature have properties that can take on many possible values, rather than the simple binary states (qubits) used in experiments to date. He says that even the simplest atom – hydrogen – can potentially exist in four different ground states and many excited states.

Measuring the Bell state
The chief difficulty in doing this, says Lu, is Alice’s initial measurement of the “Bell state” between the photon to be teleported and her half of the entangled pair. That measurement requires that the two photons interact with one another, but that interaction is extremely weak. There is a straightforward way around this problem when dealing with two dimensions, he says, but not for any higher number.

In two dimensions there are four possible Bell states, given that each photon can exist as a 1 or a 0: 00+11, 00-11, 01+10 and 01-10. Because three out of these four states are “symmetric” – meaning that switching the two particles leaves the combined wavefunction unchanged – the fourth “asymmetric” state can be unambiguously identified, allowing the state to be teleported successfully. However, in three dimensions there are nine possible states, three of which are antisymmetric, while the remaining six are neither symmetric nor antisymmetric.

Teleporting photons in three dimensions (Courtesy: C-Y Lu)

To overcome this problem, the researchers built a complex network of linear optical components linking multiple inputs with multiple outputs. The trick was to tap four different photons – the one to be teleported, the two being entangled and an extra one to enable successful Bell-state measurements. The photons were generated by a pulsed ultraviolet laser and then split along three different paths – to represent the three dimensions – with the three photons at Alice’s end then interfering with one another. The patterns of clicks from detectors recording the interferometers’ output revealed whether the three photons had been projected into a specific Bell state and could therefore be used for teleportation.

In a paper posted on arXiv, and accepted for publication in Physical Review Letters, Lu and colleagues report having successfully teleported photons 75% of the time – by preparing the input photons in a known, specific state and comparing them with the teleported photons (a standard procedure in teleportation experiments). This fidelity of 0.75, they point out, is well above the upper limit of 0.5 possible without entanglement, as well as the 0.66 that could conceivably be achieved with qubits only.

Towards higher dimensions
Lu says that the scheme could be scaled up quite easily to four, five or more dimensions mainly by adding a few more beam splitters, although he reckons that integrating the components on a photonic chip might be more practical for very large numbers of dimensions. More generally, his group is now looking to combine these higher dimensions with multiple degrees of freedom to try to teleport complete particles. “That is a necessary step if we ever want to teleport complex systems,” he says.

Technologically, Lu says that high-dimensional teleportation could be used to extend quantum communication networks – potentially providing higher bandwidth, more secure repeaters than could be achieved using qubits. It might also speed up logic operations inside quantum computers, he reckons. What’s more, he says, higher-dimensional Bell tests could provide greater scrunity of Einstein’s idea of local realism, since they would yield an even more extreme difference between classical and quantum measurements.

Lu and colleagues are not the only researchers to have demonstrated higher-dimensional teleportation. Guang-Can Guo and co-workers, also based at the University of Science and Technology of China in Hefei, have likewise reported teleporting photons in three-dimensions. Their scheme was quite similar to Lu’s but relied on two extra photons to carry out the Bell state measurement, rather than one. They also appear to have achieved a slightly lower fidelity – reporting a figure of “above 0.63”.
 
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Physics - Synopsis: Quantum Teleportation Now Comes in 3D
August 15, 2019

The first experiment to teleport qutrits rather than qubits paves the way to teleporting the complete quantum state of a particle.

PhysRevLett.123.070505
R. Zhou/USTC

Quantum teleportation is the transfer of quantum information—for instance, a particle’s quantum state—between distant systems without moving a physical particle. All demonstrations of the phenomenon so far have transferred the state of a qubit—a simple two-level system. This is a far cry from teleporting the complete quantum state of a particle, involving multiple degrees of freedom, each with many possible values. A collaboration between groups at the University of Science and Technology of China, Hefei, and at the University of Vienna has taken a step towards that goal by demonstrating the transfer of a 3D quantum state, or qutrit.

The team shared a pair of entangled photons between the transmitter (Alice) and the receiver (Bob). Each photon could take one of three possible paths whose superposition yielded a 3D entangled state, a qutrit. An additional photon at Alice’s end—also in a 3D quantum state—provided the state to be teleported. The researchers made this third photon interfere with Alice’s half of the entangled pair and performed a state measurement on all three. This measurement resulted in the transfer of the 3D state to the photon held by Bob.

The scheme may be useful in high-speed quantum communications, since a qutrit can carry more information than a qubit. The approach could be generalized to teleport quantum states involving any degree of freedom with more than two levels (photon orbital angular momentum, for example). The authors also suggest that they could extend their scheme to an arbitrarily high number of dimensions by adding more paths for the photons. These features could eventually allow the complete state of a complex quantum particle to be transferred.

This research is published in Physical Review Letters.

–Marric Stephens
Marric Stephens is a freelance science writer based in Bristol, UK.

Quantum Teleportation in High Dimensions
Yi-Han Luo, Han-Sen Zhong, Manuel Erhard, Xi-Lin Wang, Li-Chao Peng, Mario Krenn, Xiao Jiang, Li Li, Nai-Le Liu, Chao-Yang Lu, Anton Zeilinger, and Jian-Wei Pan

Phys. Rev. Lett. 123, 070505 (2019)

Published August 15, 2019​
 
Physics - Synopsis: Long-Haul Quantum Key Distribution
September 5, 2019

Two independent studies demonstrate the practicality of twin-field quantum key distribution—a promising approach to performing quantum cryptography over long distances.
PhysRevLett.123.100505
Micius Saloon

Governments and corporations are investing in quantum key distribution (QKD)—a theoretically invulnerable encryption technology—but several barriers still hamper its application. In particular, light attenuation in the optical fibers carrying the quantum signals limits the range over which QKD can work. Recently, researchers have demonstrated an alternative approach—“twin-field” QKD—that could potentially extend QKD’s range by hundreds of kilometers. Its practicality, however, remains questionable. Now, two independent groups, led by Jian-Wei Pan at the University of Science and Technology of China in Shanghai and by Hoi-Kwong Lo at the University of Toronto, have brought the method’s application a step closer by solving a key practical problem.

In the twin-field technique, two distant parties (“Alice” and “Bob”) encode qubits in single photons, which are made to interfere at the detectors of an intermediary (“Charlie”). Theory shows that the better performance, compared to standard QKD, stems from the rate at which transmission encryption keys can be exchanged: this rate scales more favorably with distance in the twin-field version. The scheme is difficult to implement, however, because quantum interference requires that the single photons generated by remote sources maintain their phase over large distances.

The teams demonstrate two ways of addressing this challenge. Pan’s team uses a stable cavity to lock the phase of Alice and Bob’s independent lasers. With such sources, they provide the first demonstration of twin-field QKD in a real optical fiber, reaching distances of up to 300 km. Lo’s group uses instead an interferometric configuration in which light travels both ways in an optical fiber loop, automatically compensating for phase fluctuations. Their scheme simplifies the twin-field QKD setup by removing the need for complex active circuitry for phase stabilization. Both experiments show that twin-field QKD breaks fundamental rate-distance limits that apply to standard QKD.

This research is published in Physical Review Letters.

–Matteo Rini
Matteo Rini is the Deputy Editor of Physics.

  1. Experimental Twin-Field Quantum Key Distribution through Sending or Not Sending
    Yang Liu, Zong-Wen Yu, Weijun Zhang, Jian-Yu Guan, Jiu-Peng Chen, Chi Zhang, Xiao-Long Hu, Hao Li, Cong Jiang, Jin Lin, Teng-Yun Chen, Lixing You, Zhen Wang, Xiang-Bin Wang, Qiang Zhang, and Jian-Wei Pan

    Phys. Rev. Lett. 123, 100505 (2019)

    Published September 5, 2019

  2. Proof-of-Principle Experimental Demonstration of Twin-Field Type Quantum Key Distribution
    Xiaoqing Zhong, Jianyong Hu, Marcos Curty, Li Qian, and Hoi-Kwong Lo

    Phys. Rev. Lett. 123, 100506 (2019)

    Published September 5, 2019
 
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Scientists Verify Feasibility of Long-distance Twin-field Quantum Key Distribution---Chinese Academy of Sciences
Sep 18, 2019

Quantum key distribution (QKD) can offer secure private communication. However, there are still some technical limitations on practical long distance quantum communication, among which channel loss and the detector noise are two most severe limitations, given that quantum signals cannot be amplified.

The new-found twin-field QKD (TFQKD) increases the relationship between code rate and distance from the linear relationship of general QKD to the square root level. Therefore, the code distance far beyond the general QKD scheme can be obtained, and theoretically the code rate far higher than the general QKD scheme can be obtained, which provides a new direction for long-distance, high-performance QKD. However, the demanding conditions for the experimental implementation of the TFQKD scheme are hard to realize.

Recently, a research team led by Prof. PAN Jianwei, Prof. ZHANG Qiang and Prof. LIU Yang from the University of Science and Technology of China of the Chinese Academy of Sciences (CAS), and the collaborators from Tsinghua University and Shanghai Institute of Microsystem and Information Technology of CAS, demonstrated TFQKD through the sending-or-not-sending protocol with a realistic phase drift over 300 km optical fiber spools on the basis of a single photon detector with a high detection rate.

The researchers realized the TFQKD on a 300-kilometer fiber channel with a sharp phase change in the real environment. Taking theoretical requirements such as statistical fluctuation and finite length analysis into consideration, they made the key generation rate reach at 300 km. The key generation rate is 50 times than that of the 2016 experiment and broke the theoretical limit of the highest rate of code for the general non-relay QKD scheme.

This study demonstrated the generation of secure keys at fiber distances of up to 300 km, yielding a higher key rate than the repeater less secret key capacity. The key rate calculation guaranteed the security in a practical situation. With existing technology and the results of theoretical simulations with practical parameters, the researchers expect that distribution distances of more than 500 km can be achieved in the near future.

Besides, this study verified the feasibility of the long-distance TFQKD scheme, and proved that the scheme has long-distance, high-code-rate performance and is very suitable for using in inter-city quantum key distribution backbone links.

Compared with the existing published TFQKD experiment, this study is the only one that considers the finite code length effect. In addition, the researchers also analyzed that the program can perform long-distance QKD over 700 kilometers under conditions such as improved detector performance.

The study, published in Physical Review Letters, was selected as the "Editor's Choice" by the Physical Review Letter, and featured highlights by Physics of American Physical Society.
 
QuantumCTek Gets Go-Ahead for China's First Quantum Coms IPO
TANG SHIHUA
DATE : NOV 14 2019/SOURCE : YICAI

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QuantumCTek Gets Go-Ahead for China's First Quantum Coms IPO

(Yicai Global) Nov. 14 -- QuantumCTek may become the first quantum communications company to get listed in China.

QuantumCTek got the green light to go public on Shanghai's Star Market, the Shanghai Stock Exchange said in a statement yesterday. The Star Market is the eastern city's new Nasdaq-style tech board.

The firm plans to raise CNY304 million (USD43.3 million) by issuing 20 million shares, the Hefei-based developer of quantum information products said in its prospectus. More than four-fifths of that will be used for a network equipment project and the rest to build a research & development center.

Founded in 2009, QuantumCTek has independent intellectual property rights on core quantum cryptography technologies and its clients include related network builders and system integrators, according to its prospectus.

China has been exploring long-distance quantum communications for many years. In 2016, a pioneering Pan Jianwei-led team sent the Micius satellite, named after an ancient Chinese philosopher, to space to help develop safer and more efficient ways of transmitting information. Pan, an academician of the Chinese Academy of Sciences, has an 11 percent stake in QuantumCTek.

However, the controlling equity holder of QuantumCTek is the asset management unit of Hefei's University of Science and Technology of China, a leader in the scientific field, holding an 18 percent stake.

QuantumCTek posted CNY295.6 million revenue and CNY32.4 million in net profit in the first three quarters of this year, its filing shows.
 
我国首个可移动量子卫星地面站与“墨子号”成功实现空地握手
2019-12-31 17:16:00

12月30日晚,重量仅80多公斤的小型化可移动量子卫星地面站,通过卫星设备,成功与经过中国上空、距离地面500公里的“墨子号”量子卫星实现了对接。标志着我国在量子通信产业化、工程化领域迈出重要一步。

23时31分44秒,量子卫星地面站经过自动搜寻实现了与“墨子号”卫星的对接,完成对接后,地面站对“墨子号”分发的量子密匙,进行量子保密通信,整个对接过程持续了近8分钟。

China's First Portable/Mobile Quantum Satellite Ground Station and Micius Satellite Successfully Realize Air-to-ground Handshake
2019-12-31 17:16:00

On the evening of December 30, a miniaturized mobile quantum satellite ground station weighing only over 80 kilograms successfully connect through satellite equipment with the Micius or Mozi quantum satellite passing 500 kilometers above China. It indicates that China has taken an important step in the industrialization and engineering of quantum communication.

At 23:31:44, the quantum satellite ground station automatically searched and achieve linkage with the Mozi satellite. After the connection was completed, the ground station performed quantum encrypted communication with the quantum key distributed by Mozi. The communication process lasted nearly 8 minutes.

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China tests world's 1st mobile quantum satellite ground station
By Deng Xiaoci Source:Global Times Published: 2020/1/1 17:38:41

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Composite photo taken on Dec. 9, 2016 shows a satellite-to-earth link established between quantum satellite "Micius" and the quantum teleportation experiment platform in Ali, southwest China's Tibet Autonomous Region.(Xinhua/Jin Liwang)

China's Quantum Experiments at Space Scale, or better known as the Micius, have successfully conducted an eight-minute-long encrypted data transmission with a mobile ground station, marking a world's first.

A Global Times reporter learned from the project research team on Wednesday that the successful space-ground quantum communication experiment was conducted around midnight on Monday in Jinan, East China's Shandong Province. The project is led by Pan Jianwei, a quantum physicist from the University of Science and Technology of China (UTSC).

The mobile quantum satellite ground station weighing slightly over 80 kilograms and the size of a paint bucket, was jointly developed by the UTSC, QuantumC Tek, a leading manufacturer and provider of QIT-enabled ICT security products and services and the Jinan Institute of Quantum Technology. It is the first of its kind in the world.

Pan's team said the development of the ground station started in 2019, and was completed on December 24. The successful communication with the Micius satellite marked the completion of construction of China's first mobile quantum satellite ground station.

The transmission lasted some eight minutes, and a great amount of encrypted information was sent to the ground station, the team said.

The previous ground station for the Micius satellite weighed more than 10 tons. Developers conducted hundreds of experiments in order to miniaturize the ground station.

The mobile version of the ground station can be installed on a vehicle, work anytime and anywhere, and its significantly reduced manufacturing cost paves the way for mass production in the future, the team said.

An experimental quantum communication network has already connected to the "Beijing-Shanghai Backbone" quantum communication link forming a national network, Pan's team said.

The project includes verifications and equipment based on key technologies used in long-distance quantum communications.
 
China is developing drones that use quantum physics to send unhackable messages
  • Particles can carry information securely because intercepting them would alter the message and alert the receiver or sender
  • Researchers in Nanjing have condensed the quantum equipment and packed it into a drone
Stephen Chen in Beijing
Published: 5:15am, 10 Jan, 2020

Chinese scientists say they have developed the world’s first fleet of drones equipped with quantum communication technology so that robots can share information securely with each other and human operators.

Researchers at Nanjing University, in eastern China, built drones able to generate pairs of “entangled” particles of light that could carry information in quantum states such as charges or polarisations representing 0 or 1, according to their paper published this month in the journal National Science Review.


....

China is developing drones that use quantum physics to send unhackable messages | South China Morning Post
 
Mixing Quantum States Boosts Fiber Communications
By Neil Savage
Posted 24 Jan 2020 | 19:00 GMT

MzU1NzgwMg.jpeg
Photo: Wits University

It’s difficult to send quantum information over the fiber-optic networks that carry most of the world’s data, but being able to would allow people to encrypt their messages with secret codes made unbreakable by the laws of physics. Now researchers have found a way to allow the transmission of such codes over long distances, by combining different quantum properties on the same photons.

Quantum communications works through a process called entanglement, which creates a pair of photons with complementary properties—one might be polarized so its waves move horizontally, while its twin is vertically polarized. Because the two are linked, determining the property of one automatically tells you the property of the other, no matter how far apart they are. It allows for secure communications, because if someone intercepts a photon and reads it, that action changes the photon, exposing the interception.

Another quantum property is orbital angular momentum (OAM), which looks like the spiral path around a corkscrew. OAM comes in an infinite set of patterns of electromagnetic waves, so it could encode vastly more information than the simple on/off of polarization states. The problem is that one type of fiber, single-mode, can only carry one pattern, or mode, at a time, limiting its capacity. The other type, multimode fiber, can carry many patterns, but over distance they tend to transfer energy among them, destroying the quantum information; the furthest entangled states have traveled over standard multimode fiber is about 1 meter.

Researchers Jian Wang of Huazhong University of Science and Technology in Wuhan, China, and Andrew Forbes of University of the Witwatersrand, South Africa, have combined polarization and OAM into a hybrid state on their entangled photons. In the latest issue of Science Advances, they describe how they first split one photon into two lower-energy photons, A and B, with different OAMs. They then pass photon A through a set of optics that gives it a particular polarization. Because the polarization state is just one mode, they can send it along a 250-meter-long single-mode fiber.

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Illustration: Wits University
Two photons are entangled in this illustration—one in polarization and the other in orbital angular momentum (or twisted light). By passing the first photon through fiber and keeping the other one in air, it’s possible to transport multiple dimensions of entanglement even over single-mode fiber.


They send the photons to separate detectors, and measure the polarization of photon A and the OAM of photon B. Because the two are entangled, information about photon B has been carried through the fiber by photon A. “The trick works because the photons don’t know what they are until we measure them, so the state is unaware that we have multiple patterns in the game,” Forbes says. “But by the time we measure them and indicate the patterns, the one photon has already passed through the fiber.”

The trick essentially expands the alphabet of quantum states that can encode information. Forbes says it’s similar to how having multiple quantum bits in a quantum computer quickly add up to extraordinary computing power; a machine with roughly 60 qubits can outperform a supercomputer.

Now it’s up to researchers to develop communications protocols that can take advantage of this new technique. And Forbes expects to reach greater transmission distances than the 250 meters of their demonstration. “It should be possible to get across 100 km of fiber, which would make it practical,” he says.


Mixing Quantum States Boosts Fiber Communications - IEEE Spectrum
 
Record-breaking quantum memory brings quantum internet one step closer
12 February 2020
By Leah Crane

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Optic fibres could be used to build a quantum internet
Federico Caputo / Alamy


Two clouds of atoms that store quantum information, called quantum memories, have been connected across a longer distance than ever before. They could prove useful for building a quantum version of the internet

Quantum communication relies on a phenomenon called entanglement. When a pair of particles or systems are quantum entangled, measuring one of them instantly influences the measured state of the other, regardless of the distance between them.

These connections can’t directly transfer information, because that would mean information is travelling faster than light, but entanglement can be used to create encrypted communications channels, secured against hacking by the laws of quantum physics.

Individual photons have been entangled across distances exceeding 1000 kilometres, but for larger systems of particles, which hold more information, maintaining this entanglement is harder. The maximum distance between a pair of entangled quantum memories so far is just 1.3 kilometres.

Xiao-Hui Bao at the University of Science and Technology of China and his colleagues have now smashed that record, entangling two quantum memories over 22 kilometres of fibre-optic cable installed underground.

Their quantum memories were each made of about 100 million extremely cold rubidium atoms in a vacuum chamber. The quantum state of each system of atoms was entangled with the state of a single photon, and the researchers sent those photons through the fibre-optic cables.

When a particular observation called a Bell measurement was performed on the two photons simultaneously, the quantum memories with which the photons were paired before the measurement became entangled to one another.

In a slightly different experiment using cables that weren’t installed underground but just coiled up in the lab, Bao and his team entangled quantum memories across 50 kilometres.

The end goal of this work is to create a quantum repeater that can receive and then retransmit quantum information so that it can be sent over long distances, eventually building up a secure internet of quantum information.

“Honestly, there is still a long way to go in order to see the quantum repeater working in real long-distance situations,” says Bao, but he thinks that building a small-scale prototype quantum network using quantum memories will be possible in the next few years.

Journal reference: Nature, DOI: 10.1038/s41586-020-1976-7



Record-breaking quantum memory brings quantum internet one step closer | New Scientist

Yong Yu, Fei Ma, Xi-Yu Luo, Bo Jing, Peng-Fei Sun, Ren-Zhou Fang, Chao-Wei Yang, Hui Liu, Ming-Yang Zheng, Xiu-Ping Xie, Wei-Jun Zhang, Li-Xing You, Zhen Wang, Teng-Yun Chen, Qiang Zhang, Xiao-Hui Bao & Jian-Wei Pan. Entanglement of two quantum memories via fibres over dozens of kilometres. Nature (2020). DOI: 10.1038/s41586-020-1976-7
 
Sending-or-Not-Sending with Independent Lasers: Secure Twin-Field Quantum Key Distribution over 509 km
Jiu-Peng Chen, Chi Zhang, Yang Liu, Cong Jiang, Weijun Zhang, Xiao-Long Hu, Jian-Yu Guan, Zong-Wen Yu, Hai Xu, Jin Lin, Ming-Jun Li, Hao Chen, Hao Li, Lixing You, Zhen Wang, Xiang-Bin Wang, Qiang Zhang, and Jian-Wei Pan

Phys. Rev. Lett. 124, 070501 – Published 20 February 2020

ABSTRACT
Twin-field (TF) quantum key distribution (QKD) promises high key rates over long distances to beat the rate-distance limit. Here, applying the sending-or-not-sending TF QKD protocol, we experimentally demonstrate a secure key distribution that breaks the absolute key-rate limit of repeaterless QKD over a 509-km-long ultralow loss optical fiber. Two independent lasers are used as sources with remote-frequency-locking technique over the 500-km fiber distance. Practical optical fibers are used as the optical path with appropriate noise filtering; and finite-key effects are considered in the key-rate analysis. The secure key rate obtained at 509 km is more than seven times higher than the relative bound of repeaterless QKD for the same detection loss. The achieved secure key rate is also higher than that of a traditional QKD protocol running with a perfect repeaterless QKD device, even for an infinite number of sent pulses. Our result shows that the protocol and technologies applied in this experiment enable TF QKD to achieve a high secure key rate over a long distribution distance, and is therefore practically useful for field implementation of intercity QKD.


Phys. Rev. Lett. 124, 070501 (2020) - Sending-or-Not-Sending with Independent Lasers: Secure Twin-Field Quantum Key Distribution over 509 km
 
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MARCH 9, 2020
Study achieves a new record fiber QKD transmission distance of over 509 km
by Ingrid Fadelli , Phys.org

The sending-or-not-sending twin-field (SNS-TF) protocol has so far proved to be a highly promising strategy for achieving high rates over long distances in quantum key distribution (QKD) applications. In fact, by tolerating large misalignment errors, this protocol can surpass the repeaterless bound in more effective ways, which is a crucial factor in the realization of long-distance QKD.

Jian-Wei Pan, Qiang Zhang, Xiang-Bin Wang and other researchers at the University of Science and Technology of China and Tsinghua University have recently achieved an unprecedented QKD transmission distance using the SNS-TF protocol. Their paper, published in Physical Review Letters, reports QKD with a secure key distribution breaking the repeaterless bound over a 509-km-long optical fiber.


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https://phys.org/news/2020-03-fiber-qkd-transmission-distance-km.html
 
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One-kilometer Breakthrough Made in Quantum Field----Chinese Academy of Sciences
By ZHANG Nannan | Mar 20, 2020

A team led by Prof. GUO Guangcan from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) and collaborators first realized distribution of high-dimensional orbital angular momentum entanglement over a 1 km few-mode fiber. The result is published in Optica.

Increasing the channel capacity and tolerance to noise in quantum communications is a strong practical motivation for encoding quantum information in multilevel systems, qudits as opposed to qubits. From a foundational perspective, entanglement in higher dimensions exhibits more complex structures and stronger non-classical correlations. High-dimensional entanglement has demonstrated its potential for increasing channel capacity and resistance to noise in quantum information processing. Despite these benefits, the distribution of high-dimensional entanglement is relatively new and remains challenging.

The orbital angular momentum of photon is a high dimensional system which has been paid much attention to in recent years. However, orbital angular momentum entanglement is susceptible to atmospheric turbulence or mode crosstalk and mode dispersion in optical fibers. It can only transmit a few meters, and is limited to two-dimensional entanglement distribution.

In this work, researchers reported the first distribution of three-dimensional orbital angular momentum (OAM) entanglement via a 1-km-long few-mode optical fiber.

Using an actively stabilizing phase precompensation technique, they successfully transported one photon of a three-dimensional OAM entangled photon pair through the fiber. With their measures, they are able to certify three-dimensional entanglement via a fidelity to the three-dimensional maximally entangled state (MES) of 0.71, and a violation of a Collins–Gisin–Linden–Massar–Popescu (CGLMP) inequality.

In addition, they certified that the high-dimensional quantum entanglement survives the transportation by violating a generalized Bell inequality, obtaining a violation of ~ 3 standard deviations.

They showed that preserving the wavefront is possible with precompensation, potentially enabling further information processing after the fiber. The method developed can be extended to a higher OAM dimension and larger distances in principle.

Their work is a significant step forward for distributing high-dimensional entanglement in the transverse spatial modes of photons. In the future, they hope that together with recent results on the noise resilience exploiting higher dimensions, the work will motivate further experimental research into novel protocols that involve long-distance high-dimensional quantum communications through fiber.

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Schematic of the experimental setup for high-dimensional orbital angular momentum entanglement distribution. (Image by CAO Huan and other researchers)
 
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