Prof Andrew Steane

Abstract:

Robust qubit memory is essential for quantum computing, both for near-term devices operating without error correction, and for the long-term goal of a fault-tolerant processor. We directly measure the memory error ε_m for a ⁴³Ca⁺ trapped-ion qubit in the small-error regime and find ε_m<10⁻⁴ for storage times t ≲ 50  ms. This exceeds gate or measurement times by three orders of magnitude. Using randomized benchmarking, at t = 1  ms we measure ε_m=1.2(7)×10⁻⁶, around ten times smaller than that extrapolated from the T^∗₂ time, and limited by instability of the atomic clock reference used to benchmark the qubit.

Abstract:

We present the design and initial characterisation of a next-generation surface-electrode ion-trap designed for room-temperature or cryogenic operation, that will aim to improve both the fidelity and speed achieved in microwave-driven quantum gates.

Abstract:

This volume offers an in-depth presentation of the structure of science and the nature of the physical world, with a view to showing how it complements and does not replace other types of human activity, such as the arts and humanities, spirituality and religion. The aim is to better inform scientists, science educators, and the general public. Many think that science can and does establish that the natural world is a vast machine, and this is the whole truth of ourselves and our environment. This is wrong. In fact, scientific models employ a rich network of interconnecting concepts, and the overall picture suggests the full validity of further forms of truth-seeking and truth-speaking, such as art, jurisprudence, and the like. In fundamental physics, the equations that describe physical behaviour interact in a subtle symbiotic way with symmetry principles which describe overarching guidelines. The relationship between physics and biology is similar, and so is the relationship between biology and the humanities. Darwinian evolution is an exploratory mechanism which allows richer patterns and truths to come to be expressed; it does not negate or replace those truths. The area of values, of what can or should command our allegiance, requires a different kind of response, a response that is not completely captured by logical argument, but which is central to human life. Religion, when it is understood correctly and done well, is the engagement with the idea that we have a meaningful role to play, and much to learn.

Abstract:

Quantum bits (qubits) based on individual trapped atomic ions are a promising technology for building a quantum computer. The elementary operations necessary to do so have been achieved with the required precision for some error-correction schemes. However, the essential two-qubit logic gate that is used to generate quantum entanglement has hitherto always been performed in an adiabatic regime (in which the gate is slow compared with the characteristic motional frequencies of the ions in the trap), resulting in logic speeds of the order of 10 kilohertz. There have been numerous proposals of methods for performing gates faster than this natural 'speed limit' of the trap. Here we implement one such method, which uses amplitude-shaped laser pulses to drive the motion of the ions along trajectories designed so that the gate operation is insensitive to the optical phase of the pulses. This enables fast (megahertz-rate) quantum logic that is robust to fluctuations in the optical phase, which would otherwise be an important source of experimental error. We demonstrate entanglement generation for gate times as short as 480 nanoseconds-less than a single oscillation period of an ion in the trap and eight orders of magnitude shorter than the memory coherence time measured in similar calcium-43 hyperfine qubits. The power of the method is most evident at intermediate timescales, at which it yields a gate error more than ten times lower than can be attained using conventional techniques; for example, we achieve a 1.6-microsecond-duration gate with a fidelity of 99.8 per cent. Faster and higher-fidelity gates are possible at the cost of greater laser intensity. The method requires only a single amplitude-shaped pulse and one pair of beams derived from a continuous-wave laser. It offers the prospect of combining the unrivalled coherence properties, operation fidelities and optical connectivity of trapped-ion qubits with the submicrosecond logic speeds that are usually associated with solid-state devices.

Abstract:

We describe a simple approach to the problem of incorporating the response time of an atom or ion being Doppler-cooled into the theory of the cooling process. The system being cooled does not in general respond instantly to the changing laser frequencies it experiences in its rest frame, and this ‘dynamic effect’ can affect significantly the temperatures attainable. It is particularly important for trapped ions when there is a slow decay out of the cooling cycle requiring the use of a repumping beam. We treat the cases of trapped ions with two and three internal states, then apply the theory to ⁴⁰Ca⁺. For this ion experimental data exist showing the ion to be cold under conditions for which heating is predicted if the dynamic effect is neglected. The present theory accounts for the observed behaviour.