Einstein-Podolsky-Rosen (EPR) steering is often (implicitly or explicitly) taken to be evidence for spooky action-at-a-distance. An alternative perspective on steering – endorsed by EPR themselves – is that Alice has no causal influence on the physical state of Bob’s system; rather, Alice merely updates her knowledge of the state of Bob’s system by performing a measurement on a system correlated with his. In this work, we elaborate on this perspective and we are led to a resource-theoretic treatment of correlations in EPR scenarios.
The quantum circuit model allows gates between any pair of qubits, yet physical instantiations allow only limited interactions. We address this problem by providing an interaction graph together with an efficient method for compiling quantum circuits so that gates are applied only locally.
We show that quantum backaction of weak measurement can be used for tailoring long-range correlations of ultracold fermions, realising quantum states with spatial modulations of the density and magnetisation, thus overcoming usual requirement for a strong interatomic interactions. We propose detection schemes for implementing antiferromagnetic states and density waves. We demonstrate that such long-range correlations cannot be realised with local addressing.
In this paper, we give a universal completion of the ZX-calculus for the whole of pure qubit quantum mechanics. This proof is based on the completeness of another graphical language, the ZW-calculus, with direct translations between these two graphical systems.
We introduce a new architecture-agnostic methodology for mapping abstract quantum circuits to realistic quantum computing devices with restricted qubit connectivity, as implemented by TKET. We present empirical results showing the effectiveness of this method in terms of reducing two-qubit gate depth and two-qubit gate count compared to other implementations.
We propose an efficient method for simultaneously optimising both the structure and parameter values of quantum circuits with only a small computational overhead. Shallow circuits that use structure optimisation perform significantly better than circuits that use parameter updates alone, making this method particularly suitable for noisy intermediate-scale quantum computers.
In this paper, we give an overview of the circuit optimisation methods used by TKET. We focus on a novel technique based around phase gadgets presented in ZX-calculus, which makes it easy to reason about them. Taking advantage of this, we present an efficient method to translate the phase gadgets back to ∧X gates and single qubit operations suitable for execution on a quantum computer with significant reductions in gate count and circuit depth.
We present TKET, a quantum software development platform produced by Cambridge Quantum. The heart of TKET is a language-agnostic optimising compiler designed to generate code for a variety of NISQ devices, which has several features designed to minimise the influence of device error.
We describe techniques to reduce the T-count based on the effective application of “spider nest identities,” easily recognised products of parity-phase operations which are equivalent to the identity operation. We demonstrate the effectiveness of such techniques by obtaining improvements in the T-counts of a number of circuits in run-times, which are typically less than the time required to make a fresh cup of coffee.
The ability to transfer coherent quantum information between systems is a fundamental component of quantum technologies and leads to coherent correlations within the global quantum process. Motivated by recent techniques in randomised benchmarking, we develop a range of results for efficient estimation of correlations within a bipartite quantum channel.