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Chinese Physics B

Decoherence dynamics of a charge qubit coupled to the noise bath

Qin-Ying Yang, Bao‐Long Liang, Ji‐Suo Wang

Published Jul 1, 2013• 1 citation
Publisher

Abstract

By virtue of the canonical quantization method, we present a quantization scheme for a charge qubit based on the superconducting quantum interference device (SQUID), taking the self-inductance of the loop into account. Under reasonable short-time approximation, we study the effect of decoherence in the ohmic case by employing the response function and the norm. It is confirmed that the decoherence time, which depends on the parameters of the circuit components, the coupling strength, and the temperature, can be as low as several picoseconds, so there is enough time to record the information.

DOI
10.1088/1674-1056/22/7/070301

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By virtue of the canonical quantization method, we present a quantization scheme for a charge qubit based on the superconducting quantum interference device (SQUID), taking the self-inductance of the loop into account. Under reasonable short-time approximation, we study the effect of decoherence in the ohmic case by employing the response function and the norm. It is confirmed that the decoherence time, which depends on the parameters of the circuit components, the coupling strength, and the temperature, can be as low as several picoseconds, so there is enough time to record the information.
Decoherence properties of two Josephson charge qubits coupled via the ${\ensuremath{\sigma}}_{x}{\ensuremath{\sigma}}_{x}$ type are investigated. Considering the special structure of this new design, the dissipative effects arising from the circuit impedance providing the fluxes for the qubits' superconducting quantum interference device loops coupled to the ${\ensuremath{\sigma}}_{x}$ qubit variables are considered. The results show that the overall decoherence effects are significantly strong in this qubit design. It is found that the dissipative effects are stronger in the case of coupling to two uncorrelated baths than are found in the case of one common bath.
Decoherence properties of two Josephson charge qubits coupled via the sigma(x)sigma(x) type are investigated. Considering the special structure of this new design, the dissipative effects arising from the circuit impedance providing the fluxes for the qubits' superconducting quantum interference device loops coupled to the sigma(x) qubit variables are considered. The results show that the overall decoherence effects are significantly strong in this qubit design. It is found that the dissipative effects are stronger in the case of coupling to two uncorrelated baths than are found in the case of one common bath.
In two-level approximation, we investigate the influence of mutual inductive coupling in superconducting quantum circuits on the decoherence of flux qubits under sub-Ohmic thermal bath environment by utilizing Bloch-Redfield function. The investigation results show: (1) The memory effect existing in the solid-state environment is beneficial to prolong the decoherence time of the superconducting flux qubit, building sub-Ohmic thermal bath environments can improve the decoherence of the solid-state qubit. (2) When the quantum system and the thermal bath are in weak coupling, generally speaking, the mutual coupling effect between circuit elements will destroy the quantum coherence; but when the quantum system and the thermal bath are in strong coupling, it will help to enhance the decoherence time by controlling the mutual inductive coupling between the loop components.
We study decoherence effects in qubits coupled to environments that exhibit resonant frequencies in their spectral function. We model the coupling of the qubit to its environment via the Caldeira-Leggett formulation of quantum dissipation/coherence, and study the simplest example of decoherence effects in circuits with resonances such as a dc superconducting quantum interference device (SQUID) phase qubit in the presence of an isolation circuit. Using the dc SQUID phase qubit as an example, we obtain two quite general results. First, when the frequency of the qubit is at least two times larger than the resonance frequency of the environmental spectral density (the isolation circuit in the case of a dc SQUID phase qubit), we find that the decoherence time of the qubit is a few orders of magnitude larger than that of the typical Ohmic regime, where the frequency of the qubit is much smaller than the resonance frequency of the spectral density. Second, we show that, when the qubit frequency is nearly the same as the resonant frequency of the environmental spectral density, an oscillatory non-Markovian decay emerges, as the qubit and its environment self-generate Rabi oscillations of characteristic time scales shorter than the decoherence time.
For a mesoscopic radio frequency superconducting quantum interference device (rfSQUID), at a degeneracy point, the system reduces to a quantum two-state system which can be used as a flux qubit. When the noise environment is equivalent to a harmonic oscillators bath, by virtue of an operator-norm measure for the short time decoherence, this paper investigates the initial decoherence of the flux qubit operating in the ohmic noise environment and illustrates its property by means of the numerical evaluation.
To suppress decoherence of solid-state qubits induced by non-Markovian noises, we propose a strategy to couple the qubit with a chaotic device, whose broad power distribution in the high-frequency domain can be used to freeze the noises in the similar way of the dynamical decoupling control (DDC) method. Compared with the DDC, high-frequency components can be generated by the chaotic device even driven by a low-frequency field and thus the control fields do not need any optimization. Thus, our method is easier to be realized in experiments. As an application to superconducting circuits, we find that various noises in a broad band, including low-frequency 1/ƒ, high-frequency Ohmic, sub-Ohmic, and super-Ohmic noises, can be efficiently suppressed by coupling the qubit to a Duffing oscillator, and the decoherence rate of the qubit is efficiently decreased by about 100 times in magnitude. Using experimentally realizable parameters, the decoherence time of the superconducting charge qubit can reach 200 µs by our method, which is 10 ∼ 100 times longer in magnitude than that observed in experiments.