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.
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.
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.
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.
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
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.
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.
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.
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.
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).
Shibdas joins us from NUS where he was a postdoc in Mankei Tsang's group, having previously completed his PhD at the University of New South Wales, Australia. His primary research interest is in quantum measurement and control theory.
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.
Samuele and Jamie start their PhDs with us this week. Samuele joins us from the University of Turin. He will be working on quantum verification and is supported by the UK Quantum Technology hub NQIT. Jamie joins us from Aberystwyth University. He will be working on quantum-enhanced sensing, and is supported by the CDT on Diamond Technologies. Luke will continue his BSc project work in quantum biology on to an MSc with us.
`Quantum measurement' Physics Day on Sept. 28
Quantum optics paper in PRL
Paper on inferring quantum correlations from limited data outputs of quantum optics experiments is published in Physical Review Letters.
Theodoros worked on his PhD with Elham Kashefi at the University of Edinburgh's School of Informatics. He completed his PhD is early 2016, and his specialisation lies in the area of verification methods for quantum computation.