New protocol for quantum technology unlocked

Multi-photon entanglement can be used to reveal the multiple parameters that describe optical processes with greater precision than traditional tomography

A new protocol for estimating unknown optical processes, called unitary operations, with precision enhanced by the unique properties of quantum mechanics has been demonstrated by scientists and engineers from the University of Bristol, UK, and the Centre for Quantum Technologies in Singapore.

The work, published in the June issue of Optica, could lead to both dramatically better sensors for medical research and new approaches to benchmark the performance of ultra-powerful quantum computers.

History tells us the ability to measure parameters and sense phenomena with increasing precision leads to dramatic advances in identifying new phenomena in science and improving the performance of technology: famous examples include X-ray imaging, magnetic resonance imaging (MRI), interferometry and the scanning-tunnelling microscope.

Scientists are understanding how to engineer and control quantum systems to vastly expand the limits of measurement and sensing is growing rapidly.  This area, known as quantum metrology, promises to open up radically alternative methods to the current state-of-the-art in sensing.

Members of the experimental team.
From left to right: Jonathan Matthews,
Rebecca Whittaker and Xiao-Qi Zhou

In this new study, the researchers re-directed the sensing power of quantum mechanics back on itself to characterise, with increased precision, unknown quantum processes that can include individual components used to build quantum computers.  This ability is becoming more and more important as quantum technologies move closer to real applications.

Dr Xiao-Qi Zhou of Bristol’s School of Physics said: “A really exciting problem is characterizing unknown quantum processes using a technique called quantum process tomography.  You can think of this as a problem where a quantum object, maybe a photonic circuit of optics or an atomic system, is locked in a box.  We can send quantum states in and we can measure the quantum states that come out.  Our challenge is to correctly identify what is in the box.  This is a difficult problem in quantum mechanics and it is a highly active area of research because its solution is needed to enable us to test quantum computers as they grow in size and complexity.”

One major shortcoming of quantum process tomography is that precision using standard techniques is limited by a type of noise known as ‘shot noise’.  By borrowing techniques from quantum metrology, the researchers were able to demonstrate precision beyond the shot noise limit.  They expect their protocol can also be applied to build more sophisticated sensors that identify molecules and chemicals more precisely by observing how they interact with quantum states of light.

Co-author Rebecca Whittaker, a PhD student in Bristol’s Centre for Quantum Photonics said: “The optical process we measured here can be used to manipulate quantum bits of information in a quantum computer but they can also occur in nature.  For example, our setup could be used to measure how the polarisation of light is rotated by a sample. We could then infer properties of that sample with better precision.

“Increasing measurement precision is particularly important for probing light-sensitive samples where we want to get as much information as we can before our probe light damages or causes alterations to the sample.  We feel this will have a big impact on the tools used in medical research.”

The researchers’ protocol relies on generating multiple photons in an entangled state and this study demonstrates that they can reconstruct rotations which act on the polarisation of light.

Paper

‘Quantum-enhanced tomography of unitary processes’ by Xiao-Qi Zhou, Hugo Cable, Rebecca Whittaker, Peter Shadbolt, Jeremy L O’Brien, Jonathan C. F. Matthews in Optica

Source: http://www.bris.ac.uk/news/2015/june/unitary-operations.html

Correlations of quantum particles help in distinguishing physical processes

Using quantum steering in the discrimination of physical processes.

Communication security and metrology could be enhanced through a study of the role of quantum correlations in the distinguishability of physical processes, by researchers at the Universities of Strathclyde and Waterloo.

The study involved analysing the impact of quantum steering, that is, the way through which a measurement performed on a particle can affect another distant particle. The study authors devised a method for both precisely quantifying steering’s impact and relating it to the task of distinguishing physical processes.

The research could have significant implications for quantum information processing.

The study was carried out by Dr Marco Piani, of Strathclyde’s Department of Physics, and Professor John Watrous, of Waterloo’s Institute for Quantum Computing (IQC) and David R. Cheriton School of Computer Science. Dr Piani was also at IQC at the time of the study.

Dr Piani said: “Quantum particles can be in a particular state known as `entangled’. Albert Einstein, with Boris Podolsky and Nathan Rosen, scrutinised quantum mechanics and specifically the entanglement of quantum particles. Faced with the perspective of the steering effect, they argued that quantum mechanics was still an incomplete theory, since it predicted what Einstein considered a `spooky action at a distance’ – indeed, two particles can be at opposite ends of a galaxy and still be entangled.

“We now know that steering is a crucial and real quantum effect; however, knowledge about what steering is actually useful for has remained limited. In our research, we related steering to the discrimination of physical processes, which seeks to answer questions about what happens in time to physical systems of interest, like microscopic particles. We were able to prove that the steering effect is the key to providing a specific advantage in this type of task.

“Our results, including the tools we introduced to quantify steering, could be applied to fields such as quantum cryptography, where secret keys are created between two parties so they can submit and encrypt messages to communicate privately – as it happens, for example, in online banking. Our results could also be useful in quantum metrology and in other areas of quantum information processing”.

Dr Watrous said: "Steering is an interesting phenomenon in quantum physics. Our work ties this concept in a new way to a specific information-theoretic task in which it functions as an essential resource. It is a hypothetical task that you won't find on your to-do list, but it is both natural and intuitive, and the connection offers a new insight into the nature of steering."

The research has been published in the journal Physical Review Letters and was presented by Dr Piani at the 18^th Conference on Quantum Information Processing, held in Sydney on 12-16 January. Their paper was one of around 40 accepted from more than 200 submissions.

The Research Excellence Framework 2014, the comprehensive rating of UK universities’ research, ranked the University of Strathclyde’s Physics research first in the UK, with 96% of output assessed as world-leading or internationally excellent.

https://uwaterloo.ca/institute-for-quantum-computing/news/correlations-quantum-particles-help-distinguishing-physical