Description and control of decoherence in quantum bit systems

Narayan Ganesan

Henryk Gutmann

The description and control of decoherence of quantum bit systems have become a field of increasing interest during the last decade. We discuss different techniques to estimate and model decoherence sources of solid state quantum bit realizations. At first, we derive a microscopic, perturbation theoretical approach for Lindblad master equations of a spin-Boson model at low temperatures. A different sort of decoherence is investigate by means of the bistable fluctuator model. For this particular but nevertheless for solid state qubits relevant noise source, we present a suitably designed dynamical decoupling method (so-called quantum bang-bang). This works as a high-pass filter, suppressing low frequency parts of the noise most effectively and thus being a promising method to compensate the ubiquituous 1/f noise. Furthermore, we investigate the behaviour of a two coupled spin system exposed to collective and localized bath. For this dressed-spin system we receive by means of scaling-analysis in first order a quantum phase diagram. On that we can identify the various quantum dynamical and entanglement phases.

Tang Zhi-xiang, Deng-Yu Zhang

Using a two-level atom as a quantum bit, We have studied the decoherent properties of the single quantum bit model in a thermal reservoir and the intensity dependent coupling Jaynes-Cummings model by calculating the non-oposite angle elements of the reduced density matrix of the intraction system,respectively. We also work out the restriction equation that must be satisfied by the external controllable driving field when the decoherence is to be eliminated.

Piotr Marecki

P. Tombesi, David Vitali

陈月月, 刘呈普, 冯勋立 +3 more

Universal quantum computation overcoming influnce of bit-flip error is realized by suppressing collective decoherence with decoherence-free subspace method. The scheme is based on cavity QED system. A logical bit is encoded by two adjacent atoms trapped in a cavity, and a four-dimensional decoherence-free subspace is constructed to suppress the collective bit reversal error. By adjusting the coupling between the external optical field and cavity, two noncommutative single-qubit operations and a controlled phase gate are implemented in decoherence-free subspace. Universal quantum computation overcoming the collective bit reversal error is realized.