# Quantum Information Science news

## 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.

## 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.

## Quantum biology paper published in J. Phys. Chem. Lett.

George, Luke, and Animesh have recently had a paper published in The Journal of Physical Chemistry Letters (DOI: 10.1021/acs.jpclett.7b00829) in collaboration with Patrick Rowe and Alessandro Troisi (both formerly Warwick Chemistry, now UCL and Liverpool respectively). In this latest work they explore the relationship between structure and energy transport on the nanoscale. In particular, they look at 50,000 different ways of arranging 6 bacteriochlorophyll molecules between a fixed input and output molecule. One such arrangement is the naturally occurring one found in the famous Fenna-Matthews-Olsen (FMO) light-harvesting complex — a prototypical component of photosynthesis. Whether the FMO has been adapted to support very efficient transport of a quantum of energy from where it is absorbed (elsewhere in the organism, eventually arriving at the 'input') to the reaction centre (the ‘output’, where it is stored as chemical energy) is a long-standing question. By looking at many alternative structures, one can gain some insight into this puzzle, and also try to identify which structural features of a general structure are important for optimising energy transport. Such insight would likely be very useful in designing artificial energy transport structures such as those found in solar cells.

## George published in NJP

With his latest publication in the New Journal of Physics (DOI: 10.1088/1367-2630/aa54ab), George has proposed experimental tests which can further constrain the extent to which the wavefunction can be epistemic (and thus representing only a lack of knowledge as to the true physical state of reality).

## Quantum metrology paper in PRA

Christos, Dominic and Animesh's paper on quantum-enhanced estimation of multiple phase parameters has been published in Physical Review A. It looks at fundamental bounds on the performance of Gaussian states for estimation of multiple phase parameters and the advantage gained over multiple individual estimation strategies. The results also suggest that the optimal states for single phase estimation do not necessarily have the same performance when generalised to the estimation of multiple similar parameters.

## Quantum optics paper in PRL

Paper on inferring quantum correlations from limited data outputs of quantum optics experiments is published in Physical Review Letters.

## Quantum metrology paper in PRL

Tillmann's paper on the quantum enhanced estiamtion of non-commuting phases is published in Physical Review Letters. It shows that the simultaneous estimation of all three components of a magnetic field can be better than estimating them indivdually. It presents the measurements that come close to attaining the quantum limit. Our study also reveals that too much quantum entanglement may be detrimental to attaining the Heisenberg scaling in the estimation of unitarily generated parameters.

Here is the link to a highlight on the Physics website.

## Paper on tomography of superconducting detectors published in New Journal of Physics

Latest paper with Ian Walmsley's group at Oxford, studying how photon numbers can be inferred from the continuous trace outputs of superconducting transition edge sensors.