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arXiv:cond-mat/0211246 · Physical review. B, Condensed matter

Decoherence of a superposition of macroscopic current states in a SQUID

Eugene M. Chudnovsky, Anatoly B. Kuklov

Published Feb 28, 2003• 11 citations
arXivHTMLPDF

Abstract

We show that fundamental conservation laws mandate parameter-free mechanisms of decoherence of quantum oscillations of the superconducting current between opposite directions in a superconducting quantum interference device---emission of phonons and photons at the oscillation frequency. The corresponding rates are computed and compared with experimental findings. The decohering effects of external mechanical and magnetic noises are investigated.

DOI
10.1103/physrevb.67.064515

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We show that fundamental conservation laws mandate parameter-free mechanisms of decoherence of quantum oscillations of the superconducting current between opposite directions in a superconducting quantum interference device---emission of phonons and photons at the oscillation frequency. The corresponding rates are computed and compared with experimental findings. The decohering effects of external mechanical and magnetic noises are investigated.
In this paper we explore the quantum behavior of a superconducting quantum-interference device (SQUID) ring which has a significant Josephson coupling energy. We show that the eigenfunctions of the Hamiltonian for the ring can be used to create macroscopic quantum superposition states of the ring. We also show that the ring potential may be utilized to squeeze coherent states. With the SQUID ring as a strong contender as a device for manipulating quantum information, such properties may be of great utility in the future. However, as with all candidate systems for quantum technologies, decoherence is a fundamental problem. In this paper we apply an open systems approach to model the effect of coupling a quantum-mechanical SQUID ring to a thermal bath. We use this model to demonstrate the manner in which decoherence affects the quantum states of the ring.
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A particular superconducting quantum interference device (SQUID) qubit, namely the double SQUID qubit, can be manipulated by rapidly modifying its potential with the application of fast flux pulses.In this system we observe coherent oscillations exhibiting non-exponential decay, indicating an unconventional decoherence mechanism.Moreover, via tuning the qubit in different conditions (different oscillation frequencies) by changing the pulse height, we observe a crossover between two distinct decoherence regimes and the existence of an 'optimal' point where the qubit is only weakly sensitive to intrinsic noise.We find that this behavior is in agreement with a model considering the decoherence caused essentially by low-frequency noise contributions, and we discuss the experimental results and possible issues.
During measurement, information is transferred from the measured quantum system to the detector via their coupling. The same coupling that extracts information from the quantum system transmits noise from the detector's environment to the system as well. In this paper, we derive the limit to the measurement efficiency of a superconducting persistent-current qubit measured by a dc superconducting quantum interference device (SQUID) and calculate the noise transmitted to the qubit from the environment of the dc SQUID. The method, with simple linear circuitry correspondence, can be applied to calculate the noise transferred to a quantum system from the environment of an arbitrary external system that interacts with it. The relaxation and decoherence induced by this noise are also estimated.
We present a study of the controllable nonlinear dynamics of a micromechanical beam coupled to a dc-SQUID (superconducting quantum interference device). The coupling between these systems places the modes of the beam in a highly nonlinear potential, whose shape can be altered by varying the bias current and applied flux of the SQUID. We detect the position of the beam by placing it in an optical cavity, which sets free the SQUID to be used solely for actuation. This enables us to probe the previously unexplored full parameter space of this device. We measure the frequency response of the beam and find that it displays a Duffing oscillator behavior which is periodic in the applied magnetic flux. To account for this, we develop a model based on the standard theory for SQUID dynamics. In addition, with the aim of understanding if the device can reach nonlinearity at the single phonon level, we use this model to show that the responsivity of the current circulating in the SQUID to the position of the beam can become divergent, with its magnitude limited only by noise. This suggests a direction for the generation of macroscopically distinguishable superposition states of the beam.