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Superconductor Science and Technology

Observation of the temporal evolution of an unstable macroscopic quantum system with a nanoseconds resolution

Yang Yu, Yu Zhang, Wei Qiu, Shaoxiong Li, Siyuan Han, Zhen Wang

Published Feb 22, 2002• 9 citations

Abstract

High-resolution (sub-microsecond to nanosecond) time-resolved measurements of the dynamics of superconducting electronic devices, such as Josephson tunnel junctions and SQUIDs, are indispensable for fundamental physics research, such as quantum mechanics of macroscopic variables. We describe the development of a time-resolved measurement technique that enables direct measurements, in the time domain, of the temporal evolution of Josephson junctions with a nanosecond resolution. The technique was applied to study escape dynamics of Josephson junctions, as macroscopic quantum systems, in the quantum tunnelling regime. The measured probability of junctions remaining in the initial metastable state—the survival probability—as a function of time decayed exponentially, agrees very well with the theoretical prediction of the incoherent tunnelling process.

DOI: 10.1088/0953-2048/15/4/312

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We determined the dissipation-induced decoherence time (DIDT) of a superconducting Josephson tunnel junction by time-resolved measurements of its escape dynamics. Double-exponential behavior of the time-dependent escape probability was observed, suggesting the occurrence of a two-level decay-tunneling process in which energy relaxation from the excited to the ground level significantly affects the escape dynamics of the system. The observation of temporal double-exponential dependence enables direct measurements of the DIDT, a property critical to the study of quantum dynamics and the realization of macroscopic quantum coherence and quantum computing. We found that the DIDT was tau(d) > 11 micros at T = 0.55 K, demonstrating good prospects for implementing quantum computing with Josephson devices.

We report on the first measurements of the intrinsic dissipation in a macroscopic system cooled at very low temperature (35 mK) and operating in the quantum regime. The system under study is an rf SQUID with a high quality Josephson junction. Below 50 mK the tunneling probability of escape from a metastable well vs applied flux presents a series of maxima due to energy level quantization. From the shape of the tunneling probability we can evaluate the intrinsic dissipation related to the overall system as well as the coherence time related to the Rabi oscillations in a future macroscopic quantum coherence experiment.

Experiments to investigate the quantum behavior of a macroscopic degree of freedom, namely the phase difference across a Josephson tunnel junction, are described. The experiments involve measurements of the escape rate of the junction from its zero voltage state. Low temperature measurements of the escape rate for junctions that are either nearly undamped or moderately damped agree very closely with predictions for macroscopic quantum tunneling, with no adjustable parameters. Microwave spectroscopy reveals quantized energy levels in the potential well of the junction in excellent agreement with quantum-mechanical calculations. The system can be regarded as a "macroscopic nucleus with wires."

We have developed a scheme for high resolution measurements of the switching current distribution of a current-biased Josephson tunnel junction using a timing technique. In the measurement setup digital control and read-out electronics are galvanically isolated from the analog sample bias electronics by an optical fiber link. We have successfully used this technique to investigate thermal activation and macroscopic quantum tunneling of the phase in a high-quality 5×5 μm2 Nb–Al/AlOx–Nb Josephson tunnel junction with a critical current of Ic≈325 μA. At temperatures above the cross over temperature of T★≈280 mK the escape is dominated by thermal activation. Due to the high quality factor of the junction (Q≈95), the escape temperature is noticeably affected by the thermal prefactor. At temperatures below T★, the value of which agrees well with the theoretical predictions, the escape of the phase by quantum mechanical tunneling is observed. The presented technique can be employed to characterize current-biased Josephson tunnel junctions for applications in quantum information processing.