Quantum cryptography is the ultimate secret message service. Now new research, to be presented at the 2012 AAAS Annual Meeting, shows it can counter even the ultimate paranoid scenario: when the equipment or even the operator is in the control of a malicious power.
Until now, quantum cryptography protocols have always assumed that an adversary would not have access to information about any choices that are made during the process of encryption. "We are challenging this assumption," says Artur Ekert, Director of the Centre for Quantum Technologies (CQT) at the National University of Singapore (NUS) and Professor at the University of Oxford, UK, who will present the research. "We are asking well, what if you are controlled?"
In a world of secrets, it pays to be paranoid. From ancient Rome to the modern age, most classical schemes for cryptography have relied on the 'decoding' step involving some problem that is hard to solve – but hard, rather than impossible. That has left cryptographic schemes, including those in wide use today, vulnerable to clever people or advances in technology.
Quantum cryptography, by contrast, offers security protected by the laws of physics. The technique provides a way for two parties to share a secret key – a random sequence of 1s and 0s – which can then be used to scramble a message. The security comes from quantum laws providing a built-in way to detect eavesdropping attempts. When the key is transmitted, using photons, say, any interception of the signal changes it in a way the legitimate parties can detect. Insecure keys can then be discarded.
But a "malicious manipulator" might have the ability to control the setup or influence the communicating parties' choice of settings in transmitting the key. The manipulation could even be something enshrined in fundamental physics – a limit on the amount of free will that humans can exercise.
It's a huge challenge to face, but the researchers believe quantum cryptography can still sometimes triumph. Ekert and his colleagues have worked out how to calculate, given the degree of manipulation, how much genuine 'randomness' remains in the key. This offers a measure of how much of the key has been left untouched and will, in turn, determine how much of the key can be guaranteed secret.
The breakthrough, which Ekert will present at AAAS on 18 February, builds on two recent twists that have given quantum cryptography a powerful boost against eavesdroppers.
The first came when researchers showed that one can design quantum cryptography setups such that devices of dubious provenance – such as those purchased from an untrusted supplier, or even an enemy – can still, with some care, be safely used for secure communication. This remarkable feat is known as 'device independent cryptography' and is on the edge of being technologically feasible.
The second twist was the realisation that device-independent schemes transcend the details of the underlying physics. Even if physicists discover new laws, such as a 'theory of everything' that replaces quantum mechanics, these schemes will continue to be secure.
The new work was led by Dax Koh, a scholar with Singapore's Agency for Science, Technology and Research (A*STAR) working at CQT, and Michael Hall of the Centre for Quantum Dynamics at Griffith University in Australia. Others involved in addition to Ekert are Valerio Scarani of CQT and NUS, Setiawan of NUS, Alastair Kay of CQT and the University of Oxford, and James Pope and Chiara Marletto of the University of Oxford. A paper is in preparation. A preprint is available on the physics preprint server arXiv.
At the AAAS Meeting in Vancouver from evening Friday 17 through Sunday 19 February.
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At the AAAS Meeting in Vancouver from Thursday 16 through Monday 20 February. Visit at the CQT booth (#525) in the Exhibition Hall (Fri-Sun) or contact as follows:
Notes for editors:
Artur Ekert will give the talk "A Powerful Twist on Quantum Cryptography" 2.25pm, Saturday 18 February at the AAAS Annual Meeting in Vancouver (Room 212, VCC West Building). The talk is part of the symposium "Quantum Information Science and Technology: A Global Perspective" which starts at 1.30pm. http://aaas.confex.com/aaas/2012/webprogram/Session4083.html
Artur Ekert is Lee Kong Chian Centennial Professor and Director at the Centre for Quantum Technologies at the National University of Singapore. He is also Professor of Quantum Physics at the Mathematical Institute, University of Oxford, UK. He is one of the pioneers of quantum cryptography. In his doctoral thesis (Oxford, 1991) he showed how quantum entanglement and non-locality can be used to distribute cryptographic keys with perfect security. In his non-academic life he is an avid scuba diver and a pilot. A recent newspaper profile: http://www.todayonline.com/Science/EDC120204-0000002/Thinker,-diver,-pilot,-code-breaker
About Centre for Quantum Technologies at the National University of Singapore
The Centre for Quantum Technologies (CQT) was established as Singapore's inaugural Research Centre of Excellence in December 2007. It brings together quantum physicists and computer scientists to explore the quantum nature of reality and quantum possibilities in information processing. CQT is funded by Singapore's National Research Foundation and Ministry of Education and is hosted by the National University of Singapore (NUS). More at http://www.quantumlah.org
About National University of Singapore
A leading global university centred in Asia, the National University of Singapore (NUS) is Singapore's flagship university which offers a global approach to education and research, with a focus on Asian perspectives and expertise. NUS has 16 faculties and schools across three campuses. Its transformative education includes a broad-based curriculum underscored by multi-disciplinary courses and cross-faculty enrichment. Over 36,000 students from 100 countries enrich the community with their diverse social and cultural perspectives. NUS has three Research Centres of Excellence (RCE) and 21 university-level research institutes and centres. It is also a partner in Singapore's 5th RCE. NUS shares a close affiliation with 16 national-level research institutes and centres. Research activities are strategic and robust, and NUS is well-known for its research strengths in engineering, life sciences and biomedicine, social sciences and natural sciences. It also strives to create a supportive and innovative environment to promote creative enterprise within its community. For more information, please visit http://www.nus.edu.sg