Quantum theory and design

Quantum technologies have the potential to revolutionise computing, communication, sensing and imaging. In addition to building these devices, it is important to develop the most efficient design of the technology and any supporting software tools that are needed to interface with the quantum system.

Quantum communication

Quantum communication and cryptography promises secure communication over long distances. It requires fast single photon sources and detectors, as well as ways to scale network architectures over continental distances using quantum repeaters.

We design network protocols that take into account the probabilistic aspects of quantum entanglement generation in order to minimise the resources and time needed to create the entangled network.

Our theory work in this area is closely aligned with our development of single photon sources in III-V materials for quantum communication.

Recent publications

Zixin Huang, Peter P. Rohde, Dominic W. Berry, Pieter Kok, J.P. Dowling, and Cosmo Lupo, Photonic quantum data locking. Quantum 5, 447 (2021). (arXiv:1905.03013) 


Boson sampling private-key quantum cryptography

We proved that the physics of Boson sampling (multiphoton interference) can be exploited to perform symmetric-key private cryptography. This protocol is shown to be optimal in the sense that it asymptotically encrypts all the information that passes through the interferometer using an exponentially smaller private key.
Quantum, 5, 447 (2021)

Generating maximal entanglement

Creating entanglement between quantum dots using photon interactions is very difficult, not least because each dot has a different transition energy and line width. We showed how we can circumvent this difficulty and create maximal entanglement between spectrally distinct solid state emitters.
Phys. Rev. Lett., 123, 023603 (2019)

Lead researcher

Pieter Kok

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Quantum computation

Quantum computing promises a dramatic speedup for certain important computational problems, such as the simulation of molecular dynamics. It requires a scalable architecture connecting the smallest information carriers, the qubits, with the macroscopic user interface. This requires precise control over the qubits along with robust error correction protocols that protect the computation from inevitable noise.

We are developing new algorithms, protocols and software tools to support more efficient fault-tolerant quantum computation.

Recent publications

Linear Programming Bounds for Approximate Quantum Error Correction Over Arbitrary Quantum Channels, Yingkai Ouyang; Ching-Yi Lai, IEEE Transactions on Information Theory, 68 (8), 2022, https://ieeexplore.ieee.org/abstract/document/9741787

Trade-offs on number and phase shift resilience in bosonic quantum codes, Y Ouyang, E Campbell, IEEE Transactions on Information Theory 67 (10), 6644-6652, 2021, https://ieeexplore.ieee.org/abstract/document/9508446/

Avoiding coherent errors with rotated concatenated stabilizer codes, Yingkai Ouyang, npj Quantum Information volume 7, Article number: 87 (2021), https://www.nature.com/articles/s41534-021-00429-8


Classical simulators of nearly-Clifford circuits

In collaboration with researchers at IBM and UCL, we developed a new set oftechniques for classically simulating quantum circuits that are “near-Clifford”. This allowed us to simulate some 50-60 qubits circuits on a laptop that were beyond the reach of supercomputers using standard simulation algorithms. We have released open source so you can run your own near-Clifford simulations on the IBM QISkit platform (see tutorial).
Quantum, 3, 181 (2019)

Circuit synthesis

Using evolutionary strategies for optimisation of quantum algorithms we improved the performance of the Hamiltonian simulation (arXiv1904.01336 and GECCO 2019). This was a collaboration between our groups in physics and computer science.

Quantum error correction in hyperbolic space

In collaboration with Barbara Terhal’s group in Aachen, we showed that embedding surface codes in hyperbolic space can provide denser storage, and a comparable threshold, to the conventional surface code in flat space.
Quantum Sci. Technol., 2 035007 (2017)

Magic state distillation

We developed a new family of magic state distillation protocols that prepare exotic resources that execute pre-synthesised circuit blocks.
Phys. Rev. Lett., 118 060501 (2017)

Lead researchers

Yingkai Ouyang, Earl Campbell, John Clark.

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Quantum sensing and imaging

Quantum sensing and imaging promises improvements in our ability to make precision measurements and create sharper images. It requires precise control over the creation of large entangled quantum states and their detection. 

We work on the theory of multiparameter quantum metrology and investigate bounds for the ultimate precision of quantum sensors that will estimate multiple potentially incompatible parameters using a variety of measurement strategies.  We also investigate how the ultimate precision bounds can be attained with theoretically optimal measurement operators, and also the sensing problem with realistic optical components.

In quantum imaging, we also consider the extraction of the maximum amount of information from classical light sources, as permitted by quantum mechanics.

We develop robust error correction protocols to maximize the promise of both quantum sensing and imaging in realistic settings.

Recent publications

Tight Cramér-Rao type bounds for multiparameter quantum metrology through conic programming, M Hayashi, Y Ouyang, Quantum 7, 1094, https://quantum-journal.org/papers/q-2023-08-29-1094/

Ugo Zanforlin, Cosmo Lupo, Peter Connolly, Pieter Kok, Gerald S. Buller, and Zixin Huang (2022) Optical quantum super-resolution imaging and hypothesis testing. Nature Communications (2022). (arXiv:2202.09406) 

Huang Z, Lupo C & Kok P (2021) Quantum-limited estimation of range and velocity. PRX Quantum, 2(3).  (arXiv:2011.07081)  

Sidhu JS, Ouyang Y, Campbell ET & Kok P (2021) Tight bounds on the simultaneous estimation of incompatible parameters. Physical Review X, 11(1). (arXiv:2011.07081)   

Lupo C, Huang Z & Kok P (2020) Quantum limits to incoherent imaging are achieved by linear interferometry. Physical Review Letters, 124(8). (arXiv:1909.09581)   


We showed how to create the optimal practical detection protocol for imaging distant classical objects, as well as measuring its temperature remotely.
Quantum, 1, 21 (2017)

This protocol was implemented recently using superconducting number-resolving detectors.
Phys. Rev. Lett., 123, 143604 (2019)

Reconstructing images via correlation measurements using this method gains an order of magnitude improvement over traditional measurements.

Lead researcher

Pieter Kok, Yingkai Ouyang

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Open systems

All quantum systems are in principal ‘open’, that is, a quantum system is influenced in some way by its external environment. In many cases this induces complex behaviour that cannot be captured using traditional theoretical methods. We develop novel tools and techniques capable of describing the behaviour of open quantum systems strongly coupled to their environment.

Using these tools we investigate the implications that such interactions have on realistic quantum systems, with relevance to a broad range of fields, including: quantum technologies, quantum biology, and quantum thermodynamics.


Strong coupling and non-equlibrium behaviour of open systems

We develop a formalism to rigorously describe the optical properties of a quantum emitter simultaneously coupled to both an electromagnetic and vibrational environment, showing that phenomenological treatments fail to even qualitatively capture the correct behaviour.
Phys. Rev. Lett., 123, 093601 (2019)

Noise enhanced quantum technologies

Coupling between a quantum system and a vibrational environment is often considered a detriment to quantum technologies, often leading to a suppression of quantum mechanical phenomena (eg coherence or entanglement). We show this is not always the case, and vibrational degrees of freedom can in fact be used to engineer states of light otherwise impossible in their absence. These states have exciting applications in quantum metrology.
Nature Communications, 10, 3034 (2019)

Solid-state single photon sources

Solid-state emitters are front-runner for generating single photons on-demand for use in quantum technologies, however, they naturally couple strongly to their external environment. This coupling leads to a trade-off between efficiency of a source and the indistinguishability of the photons that it may generate.
Nature Photonics, 11, 521–526 (2017)

Lead researcher

Jake Iles-Smith

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