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Europhysics Letters (EPL)

Observation of nonequilibrium libration states in a single-junction superconducting quantum interferometer

Evgeni V. Il’ichev, M. V. Fistul, Boris A. Malomed, Hans E. Hoenig, H.‐G. Meyer

Published May 1, 2001• 2 citations
Publisher

Abstract

We report an experimental and theoretical study of various metastable states of a superconducting loop including a Josephson junction (a single-junction superconducting quantum interferometer). In the experiment, the metastable states manifest themselves by sharp drops in the magnetic-field dependence of the interferometer's response to an externally applied weak alternating bias current. At low temperatures we also observe satellite drops that appear at small values of ac bias. By making use of a mapping to the theory of nonlinear oscillator, we argue that this feature is due to the presence of nonequilibrium ac-induced libration states of the interferometer.

DOI
10.1209/epl/i2001-00277-9

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We report an experimental and theoretical study of various metastable states of a superconducting loop including a Josephson junction (a single-junction superconducting quantum interferometer). In the experiment, the metastable states manifest themselves by sharp drops in the magnetic-field dependence of the interferometer's response to an externally applied weak alternating bias current. At low temperatures we also observe satellite drops that appear at small values of ac bias. By making use of a mapping to the theory of nonlinear oscillator, we argue that this feature is due to the presence of nonequilibrium ac-induced libration states of the interferometer.
We implement the impedance measurement technique in order to characterize superconducting structures designed for applications in quantum computing. We report an experimental study of the circuit consisting of three small Josephson junctions incorporated in a low-inductance superconducting loop. Measurements of the circuit response to an applied weak ac bias as a function of the external magnetic flux φe yield complete information on the properties of the circuit. We found that the system displays two metastable states. From experimental data, we have determined the magnetic field dependent Josephson energy U(φe) of these states, the potential barrier ΔU between these states, as well as the difference of the critical currents of the junction in the loop.
We report on measurements of macroscopic quantum states in superconducting Josephson circuits using a highly sensitive hybrid quantum interferometer as the readout probe. The investigated Josephson circuit is one of the leading candidates as solid-state qubits (persistent current qubit), which are known to exhibit macroscopic quantum states with atomic-like properties. The readout device is a modified Andreev interferometer with semi-metallic normal segment in a “folded" geometry, and is designed to reduce the back action during measurement, as well as minimising the electromagnetic coupling between the circuit and the environment.\n A pulsed lock-in technique has been developed to perform continuous readout of the superconductor phase difference using pulse lengths down to 10 ns. The technique enables to control the energy of the probing quasiparticles in the normal segment of the interferometer, which in turn allows to control of the supercurrent owing in the SNS junction and prevents electron heating of the normal segment.\n An experimental set-up was designed and installed in a dilution fridge consisting of shielded wiring, magnetic screens, RF tight sample holder and printed circuit board to allow the injection of high frequency excitation signals, while minimising the environment effect on the qubit through careful electrical filtering. The effect of strong RF irradiation on Andreev interferometers allowed us to estimate the response time of the readout device to be less than 40 ps.\n The measurements show that two macroscopically distinct metastable states exist when the device is biased at the qubit degeneracy point, between which the system makes transitions that can be continuously monitored. Real time kinetics of the system has been investigated at different magnetic fluxes, pulse parameters, temperature and RF radiation. Based on statistical analysis of the transitions, we argue that the metastability is connected with macroscopic quantum tunnelling effects rather than thermal excitation. The experimental data support the hypothesis of a large low frequency noise causing low transition rates.
Gate-tunable Josephson junctions (JJs) are the backbone of superconducting classical and quantum computation. Typically, these systems exploit low-charge-concentration materials and present technological difficulties limiting their scalability. Surprisingly, electric field modulation of a supercurrent in metallic wires and JJs has been recently demonstrated. Here, we report the realization of titanium-based monolithic interferometers which allow tuning both JJs independently via voltage bias applied to capacitively coupled electrodes. Our experiments demonstrate full control of the amplitude of the switching current ( I s ) and of the superconducting phase across the single JJ in a wide range of temperatures. Astoundingly, by gate-biasing a single junction, the maximum achievable total I s is suppressed down to values much lower than the critical current of a single JJ. A theoretical model including gate-induced phase fluctuations on a single junction accounts for our experimental findings. This class of quantum interferometers could represent a breakthrough for several applications such as digital electronics, quantum computing, sensitive magnetometry, and single-photon detection.
The mechanisms of the decay of metastable current states in the superconducting loop of an rf SQUID in the hysteresis state are studied. Clamped point contacts are used as the Josephson weak link. The first derivative of the radio-frequency IVC dVT/dIRF, which gives the dependence of the decay probability density on the amplitude of the oscillations of the magnetic flux in the ring, was measured experimentally. Studies of the temperature dependence of the width of the probability distribution in the interval from 4.2 to 0.5 K show that for most high-resistance Nb–Nb, Nb–Ag–Nb contacts the quantum mechanisms of decay become dominant at a temperature of the order of 2 K. The experimentally measured parameters of the decay probability distribution in the quantum limit are compared with the theoretical values calculated based on the theory of macroscopic quantum tunneling in the limit of strong and weak dissipation. The experimental magnitude of the width of the probability density as well as the characteristic quantum temperature exceed the theoretical values, which can be explained by the deviation of the current-phase dependence of the contact from a sinusoidal dependence. In addition, some contacts apparently correspond to the case of an intermediate magnitude of dissipation. When the frequency of the rf oscillations changed from 30 to 6 MHz, the width of the distribution remained unchanged, which follows from the theory of quantum tunneling decay of the metastable current state in the loop in the limit of strong damping. At low temperatures (T ∼ 50 K) and for a sufficiently small damping coefficient, anomalous peaks were observed in the probability density with an amplitude of the rf oscillations significantly smaller than the critical magnitude of the magnetic flux in the loop.
Complex chaotic and stochastic phenomena in single-junction and double-junction superconducting quantum interferometers are studied with the help of specially designed high-speed analog circuit. In Contrast with the earlier versions of the Josephson junction analogs, the digital integrated circuits with the voltage-controlled time delay have been used as the basic elements of the circuit, which has enabled us to extend its frequency range up to 1OO KHz. Of the phenomena studied with the analog, two are discussed in detail: i. For the single-junction interferometer under the periodic external drive the region of parameters is found, where the device exhibits complex and chaotic behavior and thus cannot be used in parametric amplifiers and ac SQUIDs. The boundary of the chaotic behavior region is found to be very close to that predicted from a very simple criterion, ii. For the double-junction interferometer, we have measured the probability of the establishing the basic ("0-th") static state as a result of a slow decrease of the bias current, as a function of the interferometer parameters. The region is determined where the probability is very close to unity, and the interferometer can be used as an NDRO memory cell.