̽��ֱ�� of Cambridge - Richard Penty

Researchers demonstrate the UK’s first long-distance ultra-secure communication over a quantum network

sc604 — Mon, 07 Apr 2025 23:38:58 +0000

̽��ֱ��team, from the Universities of Bristol and Cambridge, created the network, which uses standard fibreoptic infrastructure, but relies on a variety of quantum phenomena to enable ultra-secure data transfer.

̽��ֱ��network uses two types of quantum key distribution (QKD) schemes: ‘unhackable’ encryption keys hidden inside particles of light; and distributed entanglement: a phenomenon that causes quantum particles to be intrinsically linked.

̽��ֱ��researchers demonstrated the capabilities of the network via a live, quantum-secure video conference link, the transfer of encrypted medical data, and secure remote access to a distributed data centre. ̽��ֱ��data was successfully transmitted between Bristol and Cambridge – a fibre distance of over 410 kilometres.

This is the first time that a long-distance network, encompassing different quantum-secure technologies such as entanglement distribution, has been successfully demonstrated. ̽��ֱ��researchers presented their results at the 2025 Optical Fiber Communications Conference (OFC) in San Francisco.

Quantum communications offer unparalleled security advantages compared to classical telecommunications solutions. These technologies are immune against future cyber-attacks, even with quantum computers, which – once fully developed – will have the potential to break through even the strongest cryptographic methods currently in use.

In the past few years, researchers have been working to build and use quantum communication networks. China recently set up a massive network that covers 4,600 kilometres by connecting five cities using both fibreoptics and satellites. In Madrid, researchers created a smaller network with nine connection points that use different types of QKD to securely share information.

In 2019, researchers at Cambridge and Toshiba demonstrated a metro-scale quantum network operating at record key rates of millions of key bits per second. And in 2020, researchers in Bristol built a network that could share entanglement between multiple users. Similar quantum network trials have been demonstrated in Singapore, Italy and the USA.

Despite this progress, no one has built a large, long-distance network that can handle both types of QKD, entanglement distribution, and regular data transmission all at once, until now.

̽��ֱ��experiment demonstrates the potential of quantum networks to accommodate different quantum-secure approaches simultaneously with classical communications infrastructure. It was carried out using the UK’s Quantum Network (UKQN), established over the last decade by the same team, supported by funding from the Engineering and Physical Sciences Research Council (EPSRC), and as part of the Quantum Communications Hub project.

“This is a crucial step toward building a quantum-secured future for our communities and society,” said co-author Dr Rui Wang, Lecturer for Future Optical Networks in the Smart Internet Lab's High Performance Network Research Group at the ̽��ֱ�� of Bristol. “More importantly, it lays the foundation for a large-scale quantum internet—connecting quantum nodes and devices through entanglement and teleportation on a global scale.”

“This marks the culmination of more than ten years of work to design and build the UK Quantum Network,” said co-author Adrian Wonfor from Cambridge’s Department of Engineering. “Not only does it demonstrate the use of multiple quantum communications technologies, but also the secure key management systems required to allow seamless end-to-end encryption between us.”

“This is a significant step in delivering quantum security for the communications we all rely upon in our daily lives at a national scale,” said co-author Professor Richard Penty, also from Cambridge and who headed the Quantum Networks work package in the Quantum Communications Hub. “It would not have been possible without the close collaboration of the two teams at Cambridge and Bristol, the support of our industrial partners Toshiba, BT, Adtran and Cisco, and our funders at UKRI.”

“This is an extraordinary achievement which highlights the UK’s world-class strengths in quantum networking technology,” said Gerald Buller, Director of the IQN Hub, based at Heriot-Watt ̽��ֱ��. “This exciting demonstration is precisely the kind of work the Integrated Quantum Networks Hub will support over the coming years, developing the technologies, protocols and standards which will establish a resilient, future-proof, national quantum communications infrastructure.”

̽��ֱ��current UKQN covers two metropolitan quantum networks around Bristol and Cambridge, which are connected via a ‘backbone’ of four long-distance optical fibre links spanning 410 kilometres with three intermediate nodes.

̽��ֱ��network uses single-mode fibre over the EPSRC National Dark Fibre Facility (which provides dedicated fibre for research purposes), and low-loss optical switches allowing network reconfiguration of both classical and quantum signal traffic.

̽��ֱ��team will pursue this work further through a newly funded EPSRC project, the Integrated Quantum Networks Hub, whose vision is to establish quantum networks at all distance scales, from local networking of quantum processors to national-scale entanglement networks for quantum-safe communication, distributed computing and sensing, all the way to intercontinental networking via low-earth orbit satellites.

Reference:
R. Yang et al. ‘A UK Nationwide Heterogeneous Quantum Network.’ Paper presented at the 2025 Optical Fiber Communications Conference and Exhibition (OFC): https://www.ofcconference.org/en-us/home/schedule/

Researchers have successfully demonstrated the UK’s first long-distance ultra-secure transfer of data over a quantum communications network, including the UK’s first long-distance quantum-secured video call.

MR.Cole_Photographer via Getty Images

Abstract background

̽��ֱ��text in this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

Five hubs launched to ensure UK benefits from quantum future

sc604 — Fri, 26 Jul 2024 06:30:07 +0000

̽��ֱ��hub, called Q-BIOMED, is one of 5 quantum research hubs announced on 26 July by Peter Kyle MP, the Secretary of State for Science, Innovation and Technology, supported by £160 million in funding.

̽��ֱ��hub will exploit advances in quantum sensors capable of detecting cells and molecules, potentially orders of magnitude more sensitively than traditional diagnostic tests.

This includes developing quantum-enhanced blood tests to diagnose infectious diseases and cancer quickly and cheaply using portable instruments, and sensors measuring tiny changes to the magnetic fields in the brain that have the potential to detect early markers of Alzheimer’s disease before symptoms occur.

Other research will include quantum-enhanced MRI scans, heart scanners and surgical and treatment interventions for early-stage and hard-to-treat cancers.

“Quantum technologies harness quantum physics to achieve a functionality or a performance which is otherwise unattainable, deriving from science which cannot be explained by classical physics,” said Hub Co-Director Professor Mete Atatüre, Head of Cambridge’s Cavendish Laboratory. “Q-BIOMED will be delivered by an outstanding team of researchers from academia, the NHS, charities, government and industry to exploit quantum-enhanced advances for human health and societal good.”

“Our hub aims to grow a new quantum for health innovation ecosystem in the UK, and has already shaped the UK's new Quantum Mission for Health,” said Hub Co-Director Professor Rachel McKendry, from the London Centre for Nanotechnology and Division of Medicine at UCL. “Our long-term vision is to accelerate the entire innovation pipeline from discovery research, to translation, adoption and implementation within the NHS and global health systems, for the benefit of patients and societal good.”

“Quantum sensing allows us to gather information at cellular and molecular levels with unprecedented sensitivity to electric and magnetic fields," said Dr Ljiljana Fruk from the Department of Chemical Engineering and Biotechnology, a member of the Q-BIOMED team. "I look forward to learning from colleagues and engaging in challenging discussions to develop more sensitive, affordable tools for doctors and patients, advancing the future of healthcare.”

Cambridge researchers are also involved in three of the other newly-announced hubs:

̽��ֱ��UK Hub for Quantum Enabled Position, Navigation and Timing (QEPNT), led by the ̽��ֱ�� of Glasgow, will develop quantum technologies which will be key for national security and critical infrastructure and sectors such as aerospace, connected and autonomous vehicles (CAVs), finance, maritime and agriculture. Luca Sapienza (Engineering), Louise Hirst (Materials Science and Metallurgy/Cavendish Laboratory) and Dave Ellis (Cavendish Laboratory) are part of the QEPNT team.
QCI3: Hub for Quantum Computing via Integrated and Interconnected Implementations, led by the ̽��ֱ�� of Oxford, aims to develop the technologies needed for the UK to play a key role in the development of quantum computers, a market estimated to be worth $1.3 trillion by 2030. Ulrich Schneider (Cavendish Laboratory), Helena Knowles (Cavendish Laboratory), and Chander Velu (Institute for Manufacturing) are part of the QCI3 team.
̽��ֱ��Integrated Quantum Networks (IQN) Quantum Technology Research Hub, led by Heriot-Watt ̽��ֱ��, will undertake research towards the ultimate goal of a ‘quantum internet’, globally interlinked quantum networks connecting multiple quantum computers to produce enormous computational power. Richard Penty, Adrian Wonfor and Qixiang Cheng (Engineering), Atatüre and Dorian Gangloff (Cavendish Laboratory) are part of the IQN team.

̽��ֱ��fifth hub, UK Quantum Technology Hub in Sensing, Imaging and Timing (QuSIT), is led by the ̽��ֱ�� of Birmingham.

̽��ֱ��five hubs are delivered by the UKRI Engineering and Physical Sciences Research Council (EPSRC), with a £106 million investment from EPSRC, the UKRI Biotechnology and Biological Research Council, UKRI Medical Research Council, and the National Institute for Health and Care Research. Added to this are contributions from industry and other partners worth more than £54 million.

Peter Kyle, Secretary of State for Science, Innovation and Technology, said: “We want to see a future where cutting-edge science improves everyday lives. That is the vision behind our investment in these new quantum technology hubs, by supporting the deployment of technology that will mean faster diagnoses for diseases, critical infrastructure safe from hostile threats and cleaner energy for us all.

“This isn’t just about research; it’s about putting that research to work. These hubs will bridge the gap between brilliant ideas and practical solutions. They will not only transform sectors like healthcare and security, but also create a culture of accelerated innovation that helps to grow our economy.”

EPSRC Executive Chair Professor Charlotte Deane said: “Technologies harnessing quantum properties will provide unparalleled power and capacity for analysis at a molecular level, with truly revolutionary possibilities across everything from healthcare to infrastructure and computing.

“ ̽��ֱ��5 Quantum Technology Hubs announced today will harness the UK’s expertise to foster innovation, support growth and ensure that we capitalise on the profound opportunities of this transformative technology.”

A major new research hub led by the ̽��ֱ�� of Cambridge and UCL aims to harness quantum technology to improve early diagnosis and treatment of disease.

James Tye/UCL

L-R: Professor John Morton (UCL), Professor Rachel McKendry (UCL), Professor Mete Atatüre (Cambridge), Professor Eleni Nastouli (UCL)

Yes

Talking about a revolution: 25 years of BT and Cambridge

skbf2 — Fri, 14 Oct 2022 09:24:46 +0000

Cambridge and BT have been working together for more than 25 years developing new technologies, exploring human behaviour and considering how those two things come together to shape our world.

‘ ̽��ֱ��Next Leap Forward’ – four quantum technologies hubs to lead UK’s research drive

Anonymous — Fri, 12 Jul 2019 11:57:52 +0000

̽��ֱ��National Quantum Technologies Programme, which began in 2013, has now entered its second phase of funding, part of which will be a £94 million investment by the UK government, via UK Research and Innovation’s (UKRI) Engineering and Physical Sciences Research Council (EPSRC), in four Quantum Technologies Research Hubs.

̽��ֱ�� ̽��ֱ�� of Cambridge is a partner in the Quantum Communications Hub, led by the ̽��ֱ�� of York, which is pursuing quantum communications at all distance scales, to offer a range of applications and services and the potential for integration with existing infrastructure.

Through these Hubs, the UK’s world-leading quantum technologies research base will continue to drive the development of new technologies through their networks of academic and business partnerships.

“Harnessing the full potential of emerging technologies is vital as we strive to meet our Industrial Strategy ambition to be the most innovative economy in the world,” said Science Minister Chris Skidmore. “Our world-leading universities are pioneering ways to apply quantum technologies that could have serious commercial benefits for UK businesses. That’s why I am delighted to be announcing further investment in Quantum Technology Hubs that will bring academics and innovators together and make this once-futuristic technology applicable to our everyday lives.”

“ ̽��ֱ��UK is leading the field in developing Quantum Technologies and this new investment will help us make the next leap forward in the drive to link discoveries to innovative applications. UKRI is committed to ensuring the best research and researchers are supported in this area,” said Professor Sir Mark Walport, Chief Executive of UKRI.

̽��ֱ��Quantum Communications Hub has already established the UK's first quantum network, the UKQN. They will be extending and enhancing the UKQN, adding function and capability, and introducing new Quantum Key Distribution (QKD) technologies - using quantum light analogous to that used in conventional communications, or using entanglement working towards even longer distance fibre communications.

“We will be extending the UKQN to a national scale, with links over the EPSRC National Dark Fibre Facility to London and Bristol, as well as a link to our industrial partner BT in Adastral Park in Ipswich,” said Professor Richard Penty from the Department of Engineering. “We will be using this network to trial more advanced quantum communications technologies, including quantum repeaters, quantum entanglement, continuous variable QKD and new algorithms.”

Although widely applicable, key-sharing does not provide a solution for all secure communication scenarios. ̽��ֱ��Hub will research other new quantum protocols and the incorporation of QKD into wider security solutions. Professor Adrian Kent from the Department of Applied Mathematics and Theoretical Physics is co-leading this work with other theorists in the Hub.

“We have been devising new applications of quantum communication which allow new secure cryptographic schemes, often also making use of the impossibility of faster-than-light signalling,” said Kent. “We have also been working with experimentalist colleagues in the Hub on the practical implementation of some of these schemes, for example over the UK Quantum Network.

“ ̽��ֱ��next phase of the Hub will allow us to extend our theoretical work and experimental collaborations, including work on space-based implementations via satellite links.”

̽��ֱ��Cambridge researchers will also be working on quantum communications on a chip, particularly for the networking aspects. “One of the barriers for take-up of quantum communications is that the transmitters and receivers are bespoke and made from discrete components,” said Penty. “Integrating many of the functions on the same chip will reduce the costs and speed up commercialisation.”

Given the changing landscape worldwide, it is becoming increasingly important for the UK to establish national capability, both in quantum communication technologies and their key components such as light sources and detectors. ̽��ֱ��Hub has assembled an excellent team to deliver this capability.

Adapted from a UKRI press release.

Technologies that will allow fire crews to see through smoke and dust, computers to solve previously unsolvable computational problems, construction projects to image unmapped voids like old mine workings, and cameras that will let vehicles ‘see’ around corners are just some of the developments already taking place in the UK.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

Yes

Quantum leap

sc604 — Wed, 06 Feb 2019 16:17:34 +0000

When buying an item online, we voluntarily hand over our credit card information. But how do we know that it’s safe? Most sensitive information sent over the internet is secured through encryption, a process that converts information into a code that can only be unlocked by those with the encryption key. Currently, encryption keys are essentially impossible to break with conventional computing equipment – it would simply take too long and too much computing power to do the mathematical calculations that could reveal the key.

But in the coming decades, all that could change. Google, IBM and many other companies are all working to build a quantum computer that would outperform contemporary computers by taking advantage of the ability of subatomic particles to exist in more than one state at a time. A quantum computer could enable us to make calculations and solve problems that are well out of reach of even the most powerful supercomputers, but in the wrong hands, they could also crack encryption keys with relative ease.

So how can individuals, corporations and governments keep information safe in the face of this potential threat?

A group of researchers in Cambridge’s Department of Engineering are working to defend against the security threats posed by quantum computers by developing ‘unhackable’ encryption keys hidden inside particles of light, or photons, and sent over optical fibres.

Quantum keys are generated randomly through quantum mechanics, taking advantage of a property of photons that prevents them being cloned. ̽��ֱ��real strength of quantum links, however, is that if an attacker attempts to intercept the key, the quantum state of the photons changes and they cannot be used as part of the key, rendering the information carried by the stolen photons worthless.

“This means that we can send single photons over our networks and end up with keys at each end which are fundamentally secure,” says Professor Ian White, Head of the Photonics group in Cambridge’s Department of Engineering.

In June 2018, White and his colleagues Professor Richard Penty and Dr Adrian Wonfor started putting these ideas into practice with the launch of the UK’s first quantum network. ̽��ֱ��‘metro’ network provides secure quantum communications between the ̽��ֱ��’s Electrical Engineering Division in West Cambridge, the city centre and Toshiba Research Europe Ltd (TREL) on the Cambridge Science Park. It was built with corporate partners including ADVA and Toshiba.

̽��ֱ��network has since been extended and connected to other sites around the country, including BT’s research and development centre in Ipswich, and is currently being extended to the National Physical Laboratory in London and the ̽��ֱ�� of Bristol, creating the first UK quantum network.

̽��ֱ��quantum network is a project of the Quantum Communications Hub, a consortium of eight UK universities led by the ̽��ֱ�� of York, as well as private sector companies and public sector stakeholders. It’s funded by the Engineering and Physical Sciences Research Council (EPSRC) through the UK’s National Quantum Technologies Programme.

“This network provides us with a UK facility where we can test ideas that until now have been research-based, and to get users used to the concepts behind quantum communications so they can translate this technology into practice,” says Penty. “There’s a world of difference between transmitting quantum keys over a coil of fibre in the lab and actually putting it in the ground.”

̽��ֱ��network has the highest quantum key rate in the world. This secures a data network in Cambridge that runs at roughly five times the capacity of the entire ̽��ֱ�� network, and the link to BT in Ipswich operates at five times that again. ̽��ֱ��link to BT is comparable with the highest data capacity links in the UK, and has the possibility for scale-up in future.

“For us, it’s really important to get this right as it’s our first chance to start doing very detailed studies and see how these systems really work in the field,” says White. “This is only the start, however.”

In addition to the continued growth and development of the quantum network, the researchers are also investigating other ways that quantum technology could be used to secure information. For example, instead of counting individual photons, it could be possible to measure the amplitude and phase properties of pulses. “This way, you could use a type of hardware that’s not so different from conventional networks, so it would dramatically reduce the cost,” says Wonfor. “In theory, this would represent a huge step towards commercialising quantum technology, because it would effectively rely on technology that people are already used to.”

̽��ֱ��researchers are also looking at turning the entire concept on its head, and instead of relying on quantum mechanics for encryption key distribution, it could be used as a type of quantum alarm. In this scenario, the quantum signal would be in the background, buried inside a classical data signal, and would detect when an intruder attempts to break into the fibre.

“At the moment, it’s not easy to detect whether someone is tapping into the actual fibre, but with this kind of system working at the level of single photons, it would be much easier to do,” says Penty.

Another possibility is that of an entirely optical quantum-secured network. ̽��ֱ��Cambridge researchers have been developing optical switches that work with quantum signals so that everything stays in the optical domain. “Effectively, this would mean that quantum IP routers should be possible, a concept that is now testable thanks to the quantum network,” says Wonfor.

So where else might quantum encryption be used? According to White, it could go into space. At the moment, quantum keys can be distributed up to a maximum distance of approximately 100 km of fibre, which is why the quantum network is built on a series of nodes, with a new quantum key being generated at each node. This setup works well in urban areas with a high number of users but is not ideal for rural areas with few users. It also makes it impractical to send a quantum link across the Atlantic.

“An interesting movement within the field of quantum communications is to start involving satellites so that you could produce a quantum communications link for two remote sites,” says White. These satellites would work in parallel with fibre networks, sending quantum links to one of the trusted nodes within the network, where they could be managed, stored and distributed as needed.

̽��ֱ��Cambridge group, along with several other academic and industrial collaborators, have recently secured several parallel funding bids from Innovate UK to develop both lower cost terrestrial and space-based quantum communications.

“ ̽��ֱ��main thrust of all of this work has been to develop technologies that can be commercialised and put into regular use,” says White. “Cybersecurity is such an important issue, and we think that the laws of physics can be used to make our data transmission as secure as possible.”

Cambridge researchers are devising new methods to keep sensitive information out of the hands of hackers. They launched the UK’s first ‘unhackable’ network – made safe by the “laws of physics” – in 2018.

It’s really important to get this right as it’s our first chance to start doing very detailed studies and see how these systems really work in the field

Ian White

Yes

Cambridge launches UK’s first quantum network

sc604 — Wed, 13 Jun 2018 10:46:23 +0000

̽��ֱ��‘metro’ network provides secure quantum communications between the Electrical Engineering Division at West Cambridge, the Department of Engineering in the city centre and Toshiba Research Europe Ltd (TREL) on the Cambridge Science Park.

Quantum links are so secure because they rely on particles of light, or photons, to transmit encryption keys through the optical fibre. Should an attacker attempt to intercept the communication, the key itself changes through the laws of quantum mechanics, rendering the stolen data useless.

Researchers have been testing the ultra-secure network for the last year, providing stable generation of quantum keys at rates between two and three megabits per second. These keys are used to securely encrypt data, both in transit and in storage. Performance has exceeded expectations, with the highest recorded sustained generation of keys in field trials that include encryption of data in multiple 100-gigabit channels.

̽��ֱ��Cambridge network is a project of the Quantum Communications Hub, a consortium of eight UK universities, as well as private sector companies and public sector stakeholders. ̽��ֱ��network was built by Hub partners including the ̽��ֱ��’s Electrical Engineering Division and TREL, who also supplied the Quantum Key Distribution (QKD) systems. Further input came from ADVA, who supplied the optical transmission equipment, and the ̽��ֱ��’s Granta Backbone Network, which provided the optical fibre.

̽��ֱ��UK Quantum Network is funded by the Engineering and Physical Sciences Research Council (EPSRC) through the UK’s National Quantum Technologies Programme. It brings together concentrations of research excellence and innovation, facilitating greater collaboration between the two in development of applications that exploit the unique formal guarantee of security provided by quantum physics.

“Through this network, we can further improve quantum communications technologies and interoperability, explore and develop applications and services, and also demonstrate these to potential end users and future customers,” said Professor Timothy Spiller of the ̽��ֱ�� of York, and Director of the Quantum Communications Hub.

“ ̽��ֱ��development of the UK Quantum Network has already led to a much greater understanding of the potential of this technology in secure applications in a range of fields, in addition to bringing new insights into the operation of the systems in practice,” said Professor Ian White from Cambridge’s Department of Engineering. “I have no doubt that the network will bring many benefits in the future to researchers, developers and users.”

“Working with the Quantum Communications Hub, Cambridge and ADVA has allowed us to develop an interface for delivering quantum keys to applications,” said Dr Andrew Shields, Assistant Director of Toshiba Research Europe Ltd. “In the coming years, the network will be an important resource for developing new applications and use cases.”

“Development of the network has brought together in the Quantum Communications Hub partnership many world-class researchers and facilities from both UK universities and industry,” said Dr Liam Blackwell, Head of Quantum Technologies at EPSRC. “This is a reflection of EPSRC’s commitment to investing in UK leadership in advanced research and innovation in quantum technologies.”

̽��ֱ��UK’s first quantum network was launched today in Cambridge, enabling ‘unhackable’ communications, made secure by the laws of physics, between three sites around the city.

̽��ֱ��development of the UK Quantum Network has already led to a much greater understanding of the potential of this technology in secure applications in a range of fields.

Ian White

Christopher Burns

Fiber Optic

Yes

Laser technique promises super-fast and super-secure quantum cryptography

sc604 — Mon, 04 Apr 2016 15:06:38 +0000

Researchers have developed a new method to overcome one of the main issues in implementing a quantum cryptography system, raising the prospect of a useable ‘unbreakable’ method for sending sensitive information hidden inside particles of light.

By ‘seeding’ one laser beam inside another, the researchers, from the ̽��ֱ�� of Cambridge and Toshiba Research Europe, have demonstrated that it is possible to distribute encryption keys at rates between two and six orders of magnitude higher than earlier attempts at a real-world quantum cryptography system. ̽��ֱ��results are reported in the journal Nature Photonics.

Encryption is a vital part of modern life, enabling sensitive information to be shared securely. In conventional cryptography, the sender and receiver of a particular piece of information decide the encryption code, or key, up front, so that only those with the key can decrypt the information. But as computers get faster and more powerful, encryption codes get easier to break.

Quantum cryptography promises ‘unbreakable’ security by hiding information in particles of light, or photons, emitted from lasers. In this form of cryptography, quantum mechanics are used to randomly generate a key. ̽��ֱ��sender, who is normally designated as Alice, sends the key via polarised photons, which are sent in different directions. ̽��ֱ��receiver, normally designated as Bob, uses photon detectors to measure which direction the photons are polarised, and the detectors translate the photons into bits, which, assuming Bob has used the correct photon detectors in the correct order, will give him the key.

̽��ֱ��strength of quantum cryptography is that if an attacker tries to intercept Alice and Bob’s message, the key itself changes, due to the properties of quantum mechanics. Since it was first proposed in the 1980s, quantum cryptography has promised the possibility of unbreakable security. “In theory, the attacker could have all of the power possible under the laws of physics, but they still wouldn’t be able to crack the code,” said the paper’s first author Lucian Comandar, a PhD student at Cambridge’s Department of Engineering and Toshiba’s Cambridge Research Laboratory.

However, issues with quantum cryptography arise when trying to construct a useable system. In reality, it is a back and forth game: inventive attacks targeting different components of the system are constantly being developed, and countermeasures to foil attacks are constantly being developed in response.

̽��ֱ��components that are most frequently attacked by hackers are the photon detectors, due to their high sensitivity and complex design – it is usually the most complex components that are the most vulnerable. As a response to attacks on the detectors, researchers developed a new quantum cryptography protocol known as measurement-device-independent quantum key distribution (MDI-QKD).

In this method, instead of each having a detector, Alice and Bob send their photons to a central node, referred to as Charlie. Charlie lets the photons pass through a beam splitter and measures them. ̽��ֱ��results can disclose the correlation between the bits, but not disclose their values, which remain secret. In this set-up, even if Charlie tries to cheat, the information will remain secure.

MDI-QKD has been experimentally demonstrated, but the rates at which information can be sent are too slow for real-world application, mostly due to the difficulty in creating indistinguishable particles from different lasers. To make it work, the laser pulses sent through Charlie’s beam splitter need to be (relatively) long, restricting rates to a few hundred bits per second (bps) or less.

̽��ֱ��method developed by the Cambridge researchers overcomes the problem by using a technique known as pulsed laser seeding, in which one laser beam injects photons into another. This makes the laser pulses more visible to Charlie by reducing the amount of ‘time jitter’ in the pulses, so that much shorter pulses can be used. Pulsed laser seeding is also able to randomly change the phase of the laser beam at very high rates. ̽��ֱ��result of using this technique in a MDI-QKD setup would enable rates as high as 1 megabit per second, representing an improvement of two to six orders of magnitude over previous efforts.

“This protocol gives us the highest possible degree of security at very high clock rates,” said Comandar. “It could point the way to a practical implementation of quantum cryptography.”

Reference:
L.C. Comandar et al. ‘Quantum key distribution without detector vulnerabilities using optically seeded lasers.’ Nature Photonics (2016). DOI: 10.1038/nphoton.2016.50

A new method of implementing an ‘unbreakable’ quantum cryptographic system is able to transmit information at rates more than ten times faster than previous attempts.

This protocol gives us the highest possible degree of security at very high clock rates

Lucian Comandar

Depiction of indistinguishable photons leaving through the same output port of a beam splitter

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

Yes