APPLICATIONS & RESEARCH
Researchers confirm ‘realistic’ answer to quantum network puzzle S cientists at the University of York’s (UK) Centre for Quantum Technology have keys in a completely secure fashion. Once these keys are shared by two remote
Optical Signals and from the Microstructured Fibre groups, in collaboration with the Time and Frequency Group at the National Physical Laboratory in Teddington, UK. It explores the development of a robust hollow core fibre suitable for use in demanding applications, such as the distribution of accurate time signals, that are very sensitive to environmental variation, for example to changes in temperature. Dr Radan Slavik, Principal Investigator for the project and leader of the Coherent Optical Signals group, says: “This represents a new and quite exciting research direction for my team. Optical fibre is a parties, they can communicate confidentially by encrypting and decrypting binary messages. The security of the scheme relies on one of the most fundamental laws of quantum physics, the uncertainty principle. Today’s classical communications by email or phone are vulnerable to eavesdroppers but quantum communications based on single particle levels (photons) can easily detect eavesdroppers because they invariably disrupt or perturb a quantum signal. By making quantum measurements, two remote parties can estimate how much information an eavesdropper is stealing from the channel and can apply suitable protocols of privacy amplification to negate the effects of the information loss. However, the problem with QKD protocols based on simple quantum systems, such as single-photon qubits, is their low key-rate, despite their effectiveness in working over long distances. This makes
discovered new evidence to support the development of scalable and secure high rate quantum networks. Earlier research with colleagues at the Technical University of Denmark, Massachusetts Institute of Technology, and the University of Toronto, saw the development of a protocol that used continuous- variable quantum systems to achieve key-rates at metropolitan distances at three orders-of- magnitude higher than previously. In a new study published in Nature Photonics, the researchers, led by Dr Stefano Pirandola, of the Department of Computer Science at York, say that a potential alternative using cryogenic devices and standard Quantum Key Distribution (QKD) is unlikely to approach the high rates achieved both theoretically and experimentally using a continuous variable quantum system. Standard protocols of Quantum Key Distribution (QKD) exploit random sequences of quantum bits (qubits) to distribute secret
Using quantum measurements, two remote parties can negate the effects of information loss
with the classical communication infrastructure. Continuous variable systems offer the best and cheapest technology for reaching high rates over metropolitan distances and they can work at room temperature. “On the other hand, the cryogenic devices needed to improve the bit rate on a system using standard qubit-based QKD would require a built-in facility that operated at temperatures close to zero kelvin (minus 273 degrees Celsius). This would be unrealistic from a cost perspective and would still not approach the rate of continuous- variable systems.”
them unsuitable for adaptation for use in metropolitan networks. The option of using continuous- variable quantum systems allows the parallel transmission of many qubits of information while retaining the quantum capability of detecting and defeating eavesdroppers. Dr Pirandola said: “We have compared the state of the art in continuous variable systems (optical modes) with the standard discrete variable systems (qubits). If you want to build a metropolitan network based on quantum cryptography you need a high-rate super-fast connection otherwise you can’t compete
ORC-led fibre research shows promise for ultra-stable applications A team from the Optoelectronics Research Centre (ORC), Southampton, UK, has great medium for guiding light, but there are still aspects of its performance that are far from ideal with current fibre with the silica glass at its core. These changes have a negligible impact for most fibre applications such as
telecommunications; however, they can be greatly detrimental in many others such as fibre-based interferometric experiments and devices. The paper shows that hollow core photonic bandgap fibres (HC-PBG) have a significantly smaller sensitivity to temperature variations than traditional solid core fibres. The researchers observed a reduction in thermal sensitivity by a factor of 18, making these fibres the most environmentally insensitive fibre technology available to date.
technology. One of them is its large temperature sensitivity – addressing this issue opens up a whole range of scientifically interesting and industrially relevant applications, and I am currently applying for further funding to research these”. Propagation time through an optical fibre changes with the environmental conditions occurring where the fibre is laid, since changes in the temperature alter both the fibre length (by a tiny but still significant fraction) and the refractive index associated
published its research into the development of an advanced fibre with zero-sensitivity to temperature changes. Published in Nature’s group Scientific Reports journal, the paper entitled; “Ultralow thermal sensitivity of phase and propagation delay in hollow core optical fibres”, reveals key developments in optical signal propagation time and frequency characteristics.
The research has been conducted by a team from the ORC’s Coherent
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ISSUE 6 | Q1 2016
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