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Physical Review A

Quantum measurements of coupled systems

Leonid Evgenevich Fedichkin, Michael Zalmanovich Shapiro, M. I. Dykman

Published Jul 27, 2009• 2 citations
Publisher

Abstract

We propose an approach to measuring coupled systems, which gives a parametrically smaller error than the conventional fast projective measurements. The measurement error is due to the excitations being not entirely localized on individual systems even where the excitation energies are different. Our approach combines spectral selectivity of the detector with temporal resolution and uses the ideas of the quantum diffusion theory. The results bear on quantum computing with perpetually coupled qubits.

DOI
10.1103/physreva.80.012114

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We propose an approach to measuring coupled systems, which gives a parametrically smaller error than the conventional fast projective measurements. The measurement error is due to the excitations being not entirely localized on individual systems even where the excitation energies are different. Our approach combines spectral selectivity of the detector with temporal resolution and uses the ideas of the quantum diffusion theory. The results bear on quantum computing with perpetually coupled qubits.
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We demonstrate the onset of strong on-site localization in a many-particle system, with effective localization length smaller than the intersite distance. The localization is obtained by constructing a bounded one-parameter sequence of on-site energies that eliminates resonant hopping between both nearest and remote sites. This sequence leads to quasiexponential decay of the single-particle transition amplitude. It also leads to on-site localization of stationary many-particle states in a finite-length chain. For an infinite many-particle system, we study the time during which all states remain on-site localized. We show that, for any number of particles, this time scales as a high power of the ratio of the bandwidth of on-site energies to the hopping integral. The proposed energy sequence is robust with respect to small errors. The formulation applies to fermions as well as perpetually coupled qubits. The results show viability of quantum computing with time-independent qubit coupling.