Tag Archives: entanglement-based quantum network

Secure quantum communication network with 15 users

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Things are moving quickly where quantum communication networks are concerned. Back in April 2021, Dutch scientists announced the first multi-node quantum network connecting three processors (see my July 8, 2021 posting with the news and an embedded video).

Less than six months later, Chinese scientists announced work on a 15-user quantum network. From a September 23, 2021 news item on phys.org,

Quantum secure direct communication (QSDC) based on entanglement can directly transmit confidential information. Scientist [sic] in China explored a QSDC network based on time-energy entanglement and sum-frequency generation. The results show that when any two users are performing QSDC over 40 kilometers of optical fiber, and the rate of information transmission can be maintained at 1Kbp/s. Our result lays the foundation for the realization of satellite-based long-distance and global QSDC in the future.

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A September 23, 2021 Chinese Academy of Sciences (CAS) press release on EurekAlert, which seems to have originated the news item, provides additional detail,

Quantum communication has presented a revolutionary step in secure communication due to its high security of the quantum information, and many communication protocols have been proposed, such as the quantum secure direct communication (QSDC) protocol. QSDC based on entanglement can directly transmit confidential information. Any attack of QSDC results to only random number, and cannot obtain any useful information from it. Therefore, QSDC has simple communication steps and reduces potential security loopholes, and offers high security guarantees, which guarantees the security and the value propositions of quantum communications in general. However, the inability to simultaneously distinguish the four sets of encoded orthogonal entangled states in entanglement-based QSDC protocols limits its practical application. Furthermore, it is important to construct quantum network in order to make wide applications of quantum secure direct communication. Experimental demonstration of QSDC is badly required.

In a new paper published in Light Science & Application, a team of scientists, led by Professor Xianfeng Chen from State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Physics and Astronomy, Shanghai Jiao Tong University, China and Professor Yuanhua Li from Department of Physics, Jiangxi Normal University, China have explored a QSDC network based on time-energy entanglement and sum-frequency generation (SFG). They present a fully connected entanglement-based QSDC network including five subnets, with 15 users. Using the frequency correlations of the fifteen photon pairs via time division multiplexing and dense wavelength division multiplexing (DWDM), they perform a 40-kilometer fiber QSDC experiment by implying two-step transmission between each user. In this process, the network processor divides the spectrum of the single-photon source into 30 International Telecommunication Union (ITU) channels. With these channels, there will be a coincidence event between each user by performing a Bell-state measurement based on the SFG. This allows the four sets of encoded entangled states to be identified simultaneously without post-selection.

It is well known that the security and reliability of the information transmission for QSDC is an essential part in the quantum network. Therefore, they implemented block transmission and step-by-step transmission methods in QSDC with estimating the secrecy capacity of the quantum channel. After confirming the security of the quantum channel, the legitimate user performs encoding or decoding operations within these schemes reliably.

These scientists summarize the experiment results of their network scheme:

“The results show that when any two users are performing QSDC over 40 kilometers of optical fiber, the fidelity of the entangled state shared by them is still greater than 95%, and the rate of information transmission can be maintained at 1 Kbp/s. Our result demonstrates the feasibility of a proposed QSDC network, and hence lays the foundation for the realization of satellite-based long-distance and global QSDC in the future.”

“With this scheme, each user interconnects with any others through shared pairs of entangled photons in different wavelength. Moreover, it is possible to improve the information transmission rate greater than 100 Kbp/s in the case of the high-performance detectors, as well as high-speed control in modulator being used” they added.

“It is worth noting the present-work, which offers long-distance point-to-point QSDC connection, combined with the recently proposed secure-repeater quantum network of QSDC, which offers secure end-to-end communication throughout the quantum Internet, will enable the construction of secure quantum network using present-day technology, realizing the great potential of QSDC in future communication.” the scientists forecast.

Here’s a link to and a citation for the paper,

A 15-user quantum secure direct communication network by Zhantong Qi, Yuanhua Li, Yiwen Huang, Juan Feng, Yuanlin Zheng & Xianfeng Chen. Light: Science & Applications volume 10, Article number: 183 (2021) DOI: https://doi.org/10.1038/s41377-021-00634-2 Published: 14 September 2021

This paper is open access.

For the profoundly curious, there is an earlier version of this paper on arXiv.org, the site run by Cornell University where it was posted after moderation but prior to peer-review for publication in a journal.

Entanglement-based quantum network courtesy of Dutch researchers

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Belated congratulations to the researchers at the Delft University of Technology! Very exciting news as an April 15, 2021 news item on ScienceDaily makes clear,

A team of researchers from QuTech in the Netherlands reports realization of the first multi-node quantum network, connecting three quantum processors. In addition, they achieved a proof-of-principle demonstration of key quantum network protocols. Their findings mark an important milestone towards the future quantum internet and have now been published in Science.

An April 15, 2021 Delft University of Technology (TU Delft) press release (also on EurekAlert), which originated the news item, describes the breakthrough in more detail, Note: QuTech is the research center for Quantum Computing and Quantum Internet, a collaboration between TU Delft and TNO is Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek (English: Netherlands Organisation for Applied Scientific Research), an independent research organization),

The quantum internet

The power of the Internet is that it allows any two computers on Earth to be connected with each other, enabling applications undreamt of at the time of its creation decades ago. Today, researchers in many labs around the world are working towards first versions of a quantum internet – a network that can connect any two quantum devices, such as quantum computers or sensors, over large distances. Whereas today’s Internet distributes information in bits (that can be either 0 or 1), a future quantum internet will make use of quantum bits that can be 0 and 1 at the same time. ‘A quantum internet will open up a range of novel applications, from unhackable communication and cloud computing with complete user privacy to high-precision time-keeping,’ says Matteo Pompili, PhD student and a member of the research team. ‘And like with the Internet 40 years ago, there are probably many applications we cannot foresee right now.’

Towards ubiquitous connectivity

The first steps towards a quantum internet were taken in the past decade by linking two quantum devices that shared a direct physical link. However, being able to pass on quantum information through intermediate nodes (analogous to routers in the classical internet) is essential for creating a scalable quantum network. In addition, many promising quantum internet applications rely on entangled quantum bits, to be distributed between multiple nodes. Entanglement is a phenomenon observed at the quantum scale, fundamentally connecting particles at small and even at large distances. It provides quantum computers their enormous computational power and it is the fundamental resource for sharing quantum information over the future quantum internet. By realizing their quantum network in the lab, a team of researchers at QuTech – a collaboration between Delft University of Technology and TNO – is the first to have connected two quantum processors through an intermediate node and to have established shared entanglement between multiple stand-alone quantum processors.

Operating the quantum network

The rudimentary quantum network consists of three quantum nodes, at some distance within the same building. To make these nodes operate as a true network, the researchers had to invent a novel architecture that enables scaling beyond a single link. The middle node (called Bob) has a physical connection to both outer nodes (called Alice and Charlie), allowing entanglement links with each of these nodes to be established. Bob is equipped with an additional quantum bit that can be used as memory, allowing a previously generated quantum link to be stored while a new link is being established. After establishing the quantum links Alice-Bob and Bob-Charlie, a set of quantum operations at Bob converts these links into a quantum link Alice-Charlie. Alternatively, by performing a different set of quantum operations at Bob, entanglement between all three nodes is established.

Ready for subsequent use

An important feature of the network is that it announces the successful completion of these (intrinsically probabilistic) protocols with a “flag” signal. Such heralding is crucial for scalability, as in a future quantum internet many of such protocols will need to be concatenated. ‘Once established, we were able to preserve the resulting entangled states, protecting them from noise,’ says Sophie Hermans, another member of the team. ‘It means that, in principle, we can use these states for quantum key distribution, a quantum computation or any other subsequent quantum protocol.’

Quantum Internet Demonstrator

This first entanglement-based quantum network provides the researchers with a unique testbed for developing and testing quantum internet hardware, software and protocols. ‘The future quantum internet will consist of countless quantum devices and intermediate nodes,’ says Ronald Hanson, who led the research team. ‘Colleagues at QuTech are already looking into future compatibility with existing data infrastructures.’ In due time, the current proof-of-principle approach will be tested outside the lab on existing telecom fibre – on QuTech’s Quantum Internet Demonstrator, of which the first metropolitan link is scheduled to be completed in 2022.

Higher-level layers

In the lab, the researchers will focus on adding more quantum bits to their three-node network and on adding higher level software and hardware layers. Pompili: ‘Once all the high-level control and interface layers for running the network have been developed, anybody will be able to write and run a network application without needing to understand how lasers and cryostats work. That is the end goal.’

This news has likely lit some competitive fires in the research community. I think this is the first time I’ve featured news about the quantum internet since 2016 when, as it turns out, it was research from the University of Calgary that piqued my interest. See Teleporting photons in Calgary (Canada) is a step towards a quantum internet (a September 21, 2016 posting).

Here’s a link to and a citation for this latest work

Realization of a multinode quantum network of remote solid-state qubits by M. Pompili, S. L. N. Hermans, S. Baier, H. K. C. Beukers, P. C. Humphreys, R. N. Schouten, R. F. L. Vermeulen, M. J. Tiggelman, L. dos Santos Martins, B. Dirkse, S. Wehner, R. Hanson. Science 16 Apr 2021: Vol. 372, Issue 6539, pp. 259-264 DOI: 10.1126/science.abg1919

This paper is behind a paywall.

There is a video which introduces the concept of a quantum internet,