Qubit metrology and decoherence

Low-decoherence flux qubit

J. Q. You,1,2 Xuedong Hu,3 S. Ashhab,2 and Franco Nori2,4 1Department of Physics and Surface Physics Laboratory (National Key Laboratory), Fudan University, Shanghai 200433, China 2Frontier Research System, The Institute of Physical and Chemical Research (RIKEN), Wako-shi 351-0198, Japan 3Department of Physics, University at Buffalo, SUNY, Buffalo, New York 14260-1500, USA 4Center for Theoretic...

One , Two , and n Qubit Decoherence

Optimal dynamical decoherence control of a qubit.

We present a theory of dynamical control by modulation for optimal decoherence reduction. The theory is based on the non-Markovian Euler-Lagrange equation for the energy-constrained field that minimizes the average dephasing rate of a qubit for any given dephasing spectrum.

Decoherence of an n-qubit quantum memory.

We analyze decoherence of a quantum register in the absence of nonlocal operations, i.e., n noninteracting qubits coupled to an environment. The problem is solved in terms of a sum rule which implies linear scaling in the number of qubits. Each term involves a single qubit and its entanglement with the remaining ones. Two conditions are essential: first, decoherence must be small, and second, t...

Non-Markovian qubit decoherence during dispersive readout

We study qubit decoherence under generalized dispersive readout, i.e., we investigate a qubit coupled to a resonantly driven dissipative harmonic oscillator. We provide a complete picture by allowing for arbitrarily large qubit-oscillator detuning and by considering also a coupling to the square of the oscillator coordinate, which is relevant for flux qubits. Analytical results for the decohere...