# Quantum Information Science news

## Pan joins us

Pantita Palittapongarnpim joins us as a postdoc, having completed her PhD at the University of Calgary working with Barry Sanders.

## Dominic successfully defends

Dominic successfully defended his PhD thesis on Tuesday, passing with minor corrections. The examiners were Gavin Morley (Warwick) and Sougato Bose (UCL).

## Covert sensing paper published in PRA

Sensing has been in the centre of interest of the quantum information community in the last years. The main reason is that quantum mechanics allow for enhanced precision and the foremost focus has been to find optimal quantum probe states and measurements to attain the quantum enhanced precision.

In their recent work (DOI: 10.1103/PhysRevA.99.062321), Christos (Arizona, formerly at Warwick), Animesh, and colleagues from the University of Arizona, unlock another kind of feature: active covert sensing. The key element of covert sensing is that the sensing light can be hidden in the thermal environment. Specifically, it is shown that it is possible to sense a phase while an adversary remains unaware of the sensing process and they give the fundamental limit: The mean square error of any covert sensing task is lower bounded by the inverse square root of the probe's number of modes (or the number sensing attempts). Any attempt of the sensor to improve the precision necessarily leads to detection by the adversary

## Quantum errors paper published in PRA

Quantum computing is entering a new era of remotely-accessible quantum machines and, given their recent development, computation is more than likely accompanied by errors. One such error—quantum leakage—is an often-overlooked imperfection that amounts to quantum information escaping from the desired computational space and whose presence is rarely identified by a remote user. In work published in Physical Review A (DOI:https://doi.org/10.1103/PhysRevA.99.032328) Armands, Animesh, and George adapt one of dimension witness protocols designed for the purpose of a remote discovery of leakage and equip it with statistically robust, user-defined confidence levels before applying to a remotely accessed quantum processor. They find a circuit component "transmon" acting in a higher computational space than advertised.

## Quantum coherence paper published in PRA

Published this week in the journal *Physical Review A*, the paper "Subtleties of witnessing quantum coherence in nonisolated systems" (DOI: 10.1103/PhysRevA.98.052328) from George, Max, Luke, and Animesh could lead to experiments that help solve the debate on whether biological processes exploit quantum mechanics to their advantage, and whether evolution could provide us with a template for quantum technologies such as computers, sensors and energy sources.

## Francesco, Aiman and Andrew join us

Francesco Albarelli joins us as a postdoc having completed his PhD at Università degli Studi di Milano working on quantum metrology with Matteo Paris. Aiman Khan joins us as a PhD student following an Integrated Masters in physics at the Indian Institute of Technology-Roorkee on a Chancellor's International Scholarship. Andrew Jackson joins us doing research pending completion of his MPhys from the University of York.

## Quantum metrology paper published in PRL

Working in collaboration with Haixing Miao (University of Birmingham), Dominic and Animesh have published a paper on the fundamental quantum limits of optomechanical sensors in Physical Review Letters (DOI: 10.1103/PhysRevLett.121.110505). Being able to measure very weak forces is central to many applications, such as the direct detection of gravitational waves and monitoring subterranean movement of magma in volancially-active areas. The strength of a force can be inferred through its effect of displacing a mass: the displacement can be sensed by illuminating it with a laser and observing the reflected light, a case of optomechanical sensing. In this work, Dominic, Haixing and Animesh study the best precision attainable by optomechanical sensors when multi-coloured light is used.

## Quantum verification paper published in PRA

Samuele, Theodoros, and Animesh have published a paper on verification in Physical Review A (DOI: https://doi.org/10.1103/PhysRevA.98.022323) demonstrating an improvement on the existing requirements for schemes to verify quantum computations. Quantum computers are capable of solving certain problems whose scale lies outside that of classical computers. For some of these problems not even the solution can be efficiently checked with a classical computer. While schemes can verify an arbitrary quantum computation with a limited set of quantum operations, the minimum quantum resources to perform such a verification is an open question. In this work authors from the group demonstrate a verification scheme which works with a further reduced number of such quantum operations.

## Theodoros publishes review on quantum verification

Working with former Edinburgh colleagues Alexandru Gheorghiu and Elham Kashefi, Theodoros has published a review paper exploring existing techniques for the verification of quantum computation (DOI:10.1007/s00224-018-9872-3) in Theory of Computation.

Quantum computers offer the prospect of solving computational problems which would take an infeasible amount of time to solve with classical devices. For some of these, the solutions cannot be checked without solving the problem again - which would require use of and trust in a quantum computer. Through quantum verification techniques it becomes possible to test the performance of a quantum computer and even test whether a claimed quantum computer is genuine.

Along with an overview of the fundamental obstacle, a number of schemes which allow verification if the user has access to basic quantum apparatus, which allows them to confidently prepare a handful of simple states or to perform a few simple measurements, are discussed and compared in the review paper.

## George publishes in Science Advances

Working in collaboration with experimentalists at Heriot-Watt and Glasgow Universities, the paper entitled Attosecond-resolution Hong-Ou-Mandel interferometry (DOI:10.1126/sciadv.aap9416) has been published in Science Advances.

The team investigated an optical sensor that uses a type of interferometry based on the Hong-Ou-Mandel effect, whereby two identical photons deterministically bunch together at a balanced beam splitter. They were able to measure the optical thickness of an object by looking at the change in coincidence rate of a pair of photodetectors placed at the output of the beam splitter.

## George's work recognised by NJP

The New Journal of Physics has selected George Knee's paper "Towards optimal experimental tests on the reality of the quantum state" (DOI:10.1088/1367-2630/aa54ab) as one of their Highlights of 2017, recognising high-quality publications which have been well-received by the community.

The paper, which looks to find tests demonstrating the reality of the quantum state with ever stronger certainty, is one of just six quantum physics papers published in NJP last year to receive such recognition in the NJP's Highlights of 2017.

## Quantum Enhancements in Optical Microscopy workshop

Animesh, Dominic, and Evangelia along with Lijian Zhang (Nanjing University) are organising a workshop on quantum enhancements in optical microscopy on the 18th/19th April. For more information and to register please visit the workshop's webpage.

## Animesh recognised as an outstanding reviewer

Animesh has been recognised as an outstanding reviewer for the New Journal of Physics in the IOP's 2017 Reviewer Awards. The Outstanding Reviewer awards seek to recognise excellent reviewers for quality, quantity and timeliness of their reviews which contribute to the peer-review process.

## Theodoros to give talk at QCMC 2018

Theodoros has been accepted to give a contributed talk to the International Conference of Quantum Communication, Measurement and Computing 2018 in Baton Rouge, Louisiana. Theodoros' talk is on Thursday 15th March, where he will be presenting new results on Fault-tolerant quantum metrology.

## Paper selected for Editor's Suggestions in Phys. Rev. Letts.

A recent PRL on quantum metrology (DOI: 10.1103/PhysRevLett.119.130504), written by Animesh and external collaborators, has been selected as an Editor's Suggestion. The quantum Cramér-Rao bound is a lower bound on the attainable precision when estimating unknown properties of or parameters encoded in a quantum state. When estimating multiple parameters, it is not necessarily physically possible to construct an experiment capable of reaching the precision given by the quantum Cramér-Rao bound. In the letter they discuss the existence of a measurement which can be used to reach the precision of the quantum Cramér-Rao bound. Focusing on pure states being used to estimate a set of phases, a number of necessary and sufficient conditions are derived which projective measurements must satisfy in order to obtain the best possible precision.

## Evangelia Bisketzi joins as a PhD student

Evangelia has joined us after a masters at the University of Athens where she worked on quantum optics, to begin a PhD in quantum metrology which is funded by the QuantIC hub.

## Max Marcus joins as a postdoc

Max joins us to work on quantum biology and energy transport, having completed his DPhil in Physical and Theoretical Chemistry at the University of Oxford

## Publication in Linear Algebra and Its Applications

In collaboration with David Simmons and Justin Coon from Oxford Engineering, Animesh has had a paper published in Linear Algebra and Its Applications (DOI: 10.1016/j.laa.2017.06.038) which explores the relationship between the symmetric Laplacian of a graph (an extension of the graph Laplacian to irregular graphs) and the partial trace of an entangled pure state which can be associated with that graph. They show that the Von Neumann entropy of the graph can be a measure of bipartite entanglement in the corresponding pure state and explore the Renyi entropies of various graphs; demonstrating that the complete graph attains maximum entropy and showing extremal values for the k-regular and star graphs which contrasts with results obtained from analysing the ordinary graph Laplacians.

## Quantum metrology paper in Quantum Science and Technology

Magda, Tillmann, and Animesh have had a paper accepted into Quantum Science and Technology (https://iopscience.iop.org/article/10.1088/2058-9565/aa7fa9) which looks at the trade-offs faced when attempting to simultaneously estimate phase and phase diffusion. Looking at states which have a fixed total number of particles they have shown that a fixed number state can attain the maximum possible trade-off in the limit of large phase diffusion. In the case of small phase diffusion they specifically considered Holland-Burnett states, which are derived from inputting a fixed number of photons into each input port of a balanced beam splitter and known to perform well for phase estimation, and found that this was a factor of two below the maximum trade-off possible in the limit that phase diffusion approaches zero.

## Covert quantum sensing work presented at ISIT

Recent work on covert quantum sensing by Christos and Animesh is today being presented at the IEEE International Symposium on Information Theory in Aachen (ISIT 2017) by collaborator Boulat Bash. Typically parameter estimation benefits from the use of high energy probe states which use many photons to obtain a high-precision estimate of an unknown parameter. However such probes can easily be detected by an adversary who can recognise an attempt to probe this system by detecting these probe photons. In order to prevent the target itself or any third-parties from observing an attempt at sensing it is necessary to devise covert methods, hiding the probe state photons among thermal photons from the environment, which restrict the attainable precision. To quantify this restriction, a covertness constraint is derived which imposes a limitation on the probe state energy. While the mean square error scales, in general, with the number of repeated channel uses; these new results show that the improvement cannot exceed the square root of n without compromising the covertness of the measurement when using an n-mode state or making n uses of the channel.