We investigate the amount of noise required to turn a universal quantum gate set into one that can be efficiently modelled classically. This question is useful for providing upper bounds on fault tolerant thresholds, and for understanding the nature of the quantum/classical computational transition. We refine some previously known upper bounds using two different strategies. The first one involves the introduction of bi-entangling operations, a class of classically simulatable machines that can generate at most bipartite entanglement. Using this class we show that it is possible to sharpen previously obtained upper bounds in certain cases. As an example, we show that under depolarizing noise on the controlled-not gate, the previously known upper bound of 74% can be sharpened to around 67%. Another interesting consequence is that measurement based schemes cannot work using only 2-qubit non-degenerate projections. In the second strand of the work we utilize the Gottesman-Knill theorem on the classically efficient simulation of Clifford group operations. The bounds attained using this approach for the pi/8-gate can be as low as 15% for general single gate noise, and 30% for dephasing noise.
We present protocol that allows the generation of a maximally
entangled state between individual atoms held in spatially
separate cavities. Under ideal conditions, the scheme is
deterministic. In a realistic setting, when the the atom-cavity
interaction may be weak, and the detectors are imperfect, we show
that the scheme is robust against experimental inefficiencies and
yields probabilistic entanglement of very high fidelity.
Conference Committee Involvement (1)
Fluctuations and Noise in Photonics and Quantum Optics II
26 May 2004 | Maspalomas, Gran Canaria Island, Spain