Prof Andrew Steane

Abstract:

Two techniques are described that simplify the experimental requirements for measuring and manipulating quantum information stored in trapped ions. The first is a new technique using electron shelving to measure the populations of the Zeeman sublevels of the ground state, in an ion for which no cycling transition exists from any of these sublevels. The second technique is laser cooling to the vibrational ground state, without the need for a trap operating in the Lamb-Dicke limit. This requires sideband cooling in a sub-recoil regime. We present a thorough analysis of sideband cooling on one or a pair of sidebands simultaneously. © 1998 The American Physical Society.

Abstract:

Quantum error correction methods use processing power to combat noise. The noise level which can be tolerated in a fault-tolerant method is therefore a function of the computational resources available, especially the size of computer and degree of parallelism. I present an analysis of error correction with block codes, made fault-tolerant through the use of prepared ancilla blocks. The preparation and verification of the ancillas is described in detail. It is shown that the ancillas need only be verified against a small set of errors. This, combined with previously known advantages, makes this 'ancilla factory' the best method to apply error correction, whether in concatenated or block coding. I then consider the resources required to achieve 2 · 1010 computational steps reliably in a computer of 2150 logical qubits, finding that the simplest [[n, 1, d]] block codes can tolerate more noise with smaller overheads than the 7L-bit concatenated code. The scaling is such that block codes remain the better choice for all computations one is likely to contemplate.