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Physica B Condensed Matter

Correlation of flux states generated by optical switching of a superconducting circuit

C Cunningham, George S. Park, Blas Cabrera, M. E. Huber

Published Aug 1, 1990• 6 citations
Publisher

Abstract

We pulse a superconducting microbridge with light, changing the quantum flux state of a superconducting circuit. Long sequences of pulses are used to measure the degree of correlation between successive flux states. In a series of runs, the pulse length was changed over six decades from 6 ns to 10 ms. The correlations fit a simple Fokker-Planck conditional probability model.

DOI
10.1016/0921-4526(90)90448-4

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We pulse a superconducting microbridge with light, changing the quantum flux state of a superconducting circuit. Long sequences of pulses are used to measure the degree of correlation between successive flux states. In a series of runs, the pulse length was changed over six decades from 6 ns to 10 ms. The correlations fit a simple Fokker-Planck conditional probability model.
We have demonstrated experimentally that magnetic-flux quantization in a superconducting circuit is obeyed in an absolute sense, and have ruled out the possibility of a fractional quantum of magnetic flux. We use an optically switched superconducting microbridge to destroy and reestablish the phase coherence of the order parameter around a superconducting circuit. Changes in the vector potential while the global phase coherence is destroyed are still observed when the coherence is reestablished, even though the magnetic field is inacessible to the superconductor.
We study theoretically the behavior of a superconducting flux qubit subjected to a series of electromagnetic pulses. The possibility of controlling the system state via changing the parameters of the pulse is studied. We calculated the phase shift in the tank circuit weakly coupled to the qubit which can be measured by the impedance measurement technique. For the flux qubit we consider the possibility of estimating the relaxation rate from the impedance measurements by varying the delay time between the pulses.
The switching time and the optical control power of the NbN superconducting switch have been measured. The device is based on the ultrathin film 5-8 nm thick patterned as a structure of several narrow parallel strips (/spl sim/1 /spl mu/m wide) connected to wide current leads. The current-voltage characteristic of the switch at temperature 4.2 K demonstrated a hysteresis due to DC current self-heating. We studied the superconducting-to-resistive state transition induced by both optical and bias-current excitations. The optical pulse duration was /spl sim/20 ps and the rise time of the current step was determined to be less than 50 ps. The optical pulse was delivered to the switch by the semiconductor laser through an optical fiber. We found that the measured switching time is less than the duration of the optical excitation. The threshold optical power density does not exceed 3/spl middot/10/sup 3/ W/cm/sup 2/. The proposed device can be used in the fiber input of LTS rapid single flux quantum circuits.
We have realized and tested a fast stroboscopic detector for magnetic flux measurements. The key element of our detector is a hysteretic dc superconducting quantum interference device (SQUID). Stroboscopic read-out of the magnetic flux coupled with the SQUID is accomplished by biasing the SQUID with fast current pulses. The shorter these pulses, the more stroboscopic and less invasive the measurement we are performing. In order to reduce the duration of the current pulses, we take advantage of the superconducting-normal transition induced by laser light in thin superconducting films. The interaction of laser light with superconducting thin films has been investigated thoroughly in the past and many applications have been proposed which rely on the fast typical times with which superconductivity is broken and a resistive behavior arises. We have measured a threshold resolution of 6.9mΦ0 at 4.2 K, and this value corresponds to the thermodynamic limit of the SQUID. The detector has been accurately characterized: An improved and more sensitive version might prove useful for quantum mechanics and quantum computation experiments, for example, in detecting the state of flux qubits.
We experimentally demonstrate coherent oscillations of a tunable superconducting flux qubit by manipulating its energy potential with a nanosecond-long pulse of magnetic flux. The occupation probabilities of two persistent current states oscillate at a frequency ranging from 6 GHz to 21 GHz, tunable by changing the amplitude of the flux pulse. The demonstrated operation mode could allow quantum gates to be realized in less than 100 ps, which is much shorter than gate times attainable in other superconducting qubits. Another advantage of this type of qubit is its immunity to both thermal and magnetic field fluctuations.
We report on the direct observation of a single-flux-quantum (SFQ) pulse. The response of a metal–semiconductor–metal photodiode to a femtosecond laser pulse was used to switch Josephson junctions and to generate an SFQ voltage pulse on a superconducting microstrip line. The detailed shape of the pulse was measured optoelectronically, using a cryogenic electro-optic sampling system. The measured SFQ pulse had a width of 3.2 ps, an amplitude of 0.67 mV, and a total pulse content of 2.1±0.2 mV×ps, corresponding to the quantum of magnetic flux h/2e. With larger excitation, multiple SFQ pulses were observed. Numerical simulations are shown to be qualitatively similar to our experimental results.