Quantum Engineering projects

1. Quantum metamaterials and applications

Project members: Dr Alexandre Zagoskin, Prof Sergei Saveliev, Dr Alexander Balanov, Dr Patrick Navez

Quantum coherent structures containing a large number of qubits constitute a new type of an artificial medium: quantum metamaterials. New features of these media, such as the ability to be in a superposition of states with different properties, strongly influence their response to the electromagnetic field and produce qualitatively new effects.

In particular, quantum metamaterials hold the promise of fast and sensitive quantum antennas and image processors with applications to both fundamental physics and medical imaging. The Department now collaborates with a consortium of European institutions on the development of such detectors for the detection of galactic axions, hypothetical particles – candidates for the role of the dark matter.

Papers:

A M Zagoskin, D Felbacq, E Rousseau, Quantum metamaterials in the microwave and optical ranges, EPJ Quantum Technology 3, 1 (2016) (review article)

A L Rakhmanov, A M Zagoskin, S Savel’ev, F Nori, Quantum metamaterials: Electromagnetic waves in a Josephson qubit line, Phys. Rev. B 77, 144507 (2008) (Editors’ Suggestion)

O Astafiev, A M Zagoskin, A A Abdumalikov, Jr, Yu A Pashkin, T Yamamoto, K Inomata, Y Nakamura, J S Tsai, Resonance Fluorescence of a Single Artificial Atom, Science 327, 840 (2010)

A M Zagoskin, R D Wilson, M J Everitt, S Savel’ev, D R Gulevich, J Allen, V K Dubrovich, E Il’ichev, Spatially resolved single photon detection with a quantum sensor array, Sci. Repts. 3, 3464 (2014)

S E Savel’ev, A M Zagoskin, Renninger’s Gedankenexperiment, the collapse of the wave function in a rigid quantum metamaterial and the reality of the quantum state vector. Sci. Reps. 8, 9608 (2018)

A P Sowa, A M Zagoskin, An exactly solvable quantum-metamaterial type model, J. Phys. A: Math. Theor. 52, 395304 (2019)

P Navez, A G Balanov, S E Savel’ev, A M Zagoskin, Towards the Heisenberg limit in microwave photon detection by a qubit array, arXiv:200911271 (2020)

Grants:

“Highly sensitive detection of single microwave photons with coherent quantum network of superconducting qubits for searching galactic axions” (European Commission 863313, 2020-2022)

“Testing quantumness: from artificial quantum arrays to lattice spin models and spin liquids” (EPSRC EP/M006581/1, 2015-2018)

Books:

A M Zagoskin, Quantum Engineering: Theory and Deign of Quantum Coherent Structures, Cambridge University Press (2011)

A M Zagoskin, Quantum mechanics: A complete Introduction, Hodder & Stoughton (2015)

 

2. Quantum circuit theory and quantum structural engineering

Project members: Dr Alexandre Zagoskin, Dr Alexander Balanov, Prof Sergei Saveliev, Dr Mark Greenaway

The design and realization of large artificial quantum structures poses a special challenge. Their behaviour cannot be extrapolated from the quantum properties of their unit elements by a direct simulation. Therefore, other methods are being devised, which concentrate on the transitions between qualitatively different regimes of operation of these systems, and the parameters controlling these transitions. Such transitions between regular, chaotic and hyperchaotic regimes in systems of qubits with pumping and dissipation are being currently investigated and may provide a useful tool for the structural engineering of various quantum devices, from quantum antennas to quantum computers.

Papers:

A V Andreev, A G Balanov, T M Fromhold, M T Greenaway, A E Hramov, W Li, V V Makarov, A M Zagoskin, Emergence and Control of Complex Behaviours in Driven Systems of Interacting Qubits with Dissipation, npj Quantum Information (2020; in press)

 

3. New ways to see quantum states

Project members: Dr Mark Everitt, Dr John Samson

"It is possible, therefore, that a closer study of the relation of classical and quantum theory might involve us in negative probabilities, and so it does." R P Feynman (1987)

Different approaches to a physical system, even though mathematically equivalent, can lead to different insights and different ways to solve problems. There are many approaches to quantum mechanics, from Heisenberg’s matrix mechanics and Schrödinger’s wave mechanics to Wigner’s phase space picture and Feynman’s path integrals, mathematically equivalent but all casting light from different directions.

Our interest is in the phase space picture, where we ask for the probability distribution that a particle is in a certain place and has a certain momentum. According to the Heisenberg uncertainty principle, this is a question that should not be asked. However, many years ago Wigner showed that such a distribution does indeed exist but what passes for a probability can turn out to be negative. Such quasiprobabilities (known as Wigner functions) have proved useful both as visualisation of the "quantumness" of states and as a computational tool, mathematically equivalent to standard quantum approaches.

Our group has been building on these ideas to develop a generalised framework that encompasses both such continuous systems and the discrete systems (quantum bits, or qubits) of increasing interest in quantum information technology. One application has been to investigate quantum algorithms for the graph isomorphism problem (whether two networks are the same if the points are reshuffled), a problem difficult for classical computers. Another application has been to atoms, giving a visualisation of the state of the electrons.

Papers:

- R. P. Rundle and M. J. Everitt, Overview of the phase space formulation of quantum mechanics with application to quantum technologies, https://arxiv.org/pdf/2102.11095.pdf

- R. P. Rundle, P. W. Mills, T. Tilma, J. H. Samson and M. J. Everitt, Simple procedure for phase-space measurement and entanglement validation, Phys Rev A 96, 022117 (2017), https://journals.aps.org/pra/abstract/10.1103/PhysRevA.96.022117

- B. I. Davies, R. P. Rundle, V. M. Dwyer, J. H. Samson, T. Tilma and M. J. Everitt, Visualizing spin degrees of freedom in atoms and molecules, Phys Rev A 100 042102 (2019), https://journals.aps.org/pra/abstract/10.1103/PhysRevA.100.042102