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arXiv:0705.0841 · Japanese Journal of Applied Physics

Characteristics of Switchable Superconducting Flux Transformer with DC Superconducting Quantum Interference Device

Yoshihiro Shimazu, Takashi Niizeki

Published Apr 1, 2007• 1 citation
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Abstract

We have investigated the flux transfer characteristics of a switchable flux transformer comprising a superconducting loop and a DC superconducting quantum interference device (DC-SQUID). This system can be used to couple multiple flux qubits with a controllable coupling strength. Its characteristics were measured using a flux input coil and a DC-SQUID for readout coupled to the transformer loop in a dilution refrigerator. The observed characteristics are consistent with the calculation results. We have demonstrated the reversal of the slope of the characteristics and the complete switching off of the transformer, which are useful features for its application as a controllable coupler for flux qubits.

DOI
10.1143/jjap.46.1478

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We have investigated the flux transfer characteristics of a switchable flux transformer comprising a superconducting loop and a DC superconducting quantum interference device (DC-SQUID). This system can be used to couple multiple flux qubits with a controllable coupling strength. Its characteristics were measured using a flux input coil and a DC-SQUID for readout coupled to the transformer loop in a dilution refrigerator. The observed characteristics are consistent with the calculation results. We have demonstrated the reversal of the slope of the characteristics and the complete switching off of the transformer, which are useful features for its application as a controllable coupler for flux qubits.
We discuss and demonstrate a prototype of superconducting transformer with a flux transfer function that can be varied in a wide range, by acting on a control parameter. The device is realized by inserting a small hysteretic superconducting quantum interference device (dc-SQUID) with unshunted junctions, working as a Josephson junction with flux-controlled critical current, parallel to a superconducting transformer; by varying the magnetic flux coupled to the dc-SQUID, the transfer function for the flux coupled to the transformer can be varied. This feature can prove particularly appealing in the field of quantum computing, where it could be exploited to achieve a controllable magnetic coupling among flux-based qubits. Measurements carried out on a prototype at 4.2K show a reduction factor of about 30 between the “on” and the “off” states. We discuss the system characteristics and the experimental results.
We show a novel method for controlling the coupling of flux-based qubits by means of a superconducting transformer with variable flux transfer function. The device is realized by inserting a small hysteretic dc SQUID with unshunted junctions, working as a Josephson junction with flux controlled critical current, in parallel to a superconducting transformer; by varying the magnetic flux coupled to the dc-SQUID, the transfer function for the flux coupled to the transformer can be varied. Measurements carried out on a prototype at 4.2 K show a reduction factor of about 30 between the 'on' and the 'off' states. We discuss the system characteristics and the experimental results.
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A hysteretic dc-superconducting quantum interference device (SQUID) is used to trace the flux characteristic of a tunable rf-SQUID, the basic element for the realization of superconducting flux qubits. This allows important simplifications of circuitry and electronics in developing devices for quantum computing, by eliminating the necessity of more complex magnetometers. A hysteretic dc-SQUID is usually operated as a comparator, distinguishing only which one of two adjacent flux states is occupied by the rf-SQUID. The necessary sensitivity, moreover, is usually reached only at temperatures in the mK range. However, by exploiting the statistical properties of the current–voltage curve in the region where the switching from the zero-voltage state occurs, it is possible to obtain an accurate tracing of the input flux, even at a relatively high temperature (a few Kelvin). In our case, the input signal is given by the internal flux of a tunable rf-SQUID, the building block of a flux qubit.
We present a readout method for superconducting flux qubits. The qubit quantum flux state can be measured by determining the Josephson inductance of an inductively coupled dc superconducting quantum interference device (dc-SQUID). We determine the response function of the dc-SQUID and its back-action on the qubit during measurement. Due to driving, the qubit energy relaxation rate depends on the spectral density of the measurement circuit noise at sum and difference frequencies of the qubit Larmor frequency and SQUID driving frequency. The qubit dephasing rate is proportional to the spectral density of circuit noise at the SQUID driving frequency. These features of the back-action are qualitatively different from the case when the SQUID is used in the usual switching mode. For a particular type of readout circuit with feasible parameters we find that single shot readout of a superconducting flux qubit is possible.
We have designed, fabricated, and tested a Double Transformer (DT) coupled dc SQUID (Superconducting Quantum Interference Device) with low noise, an input inductance of 1μH and a smooth input-output characteristic. A transmission line model is presented to explain a resonance in the input-output characteristic of early versions of this device. Guided by the results of numerical simulations a new version of this device has been built and tested. Experimental results are presented that show that the resonance can be moved to a higher voltage by reducing the area of the SQUID loop. The voltage-external flux characteristic of some of these new devices agrees to within 10% with computer simulations. The minimum detectable energy per unit bandwidth (MDE) referred to the SQUID loop, is 10h, where h is Planck's constant. Computer simulations indicate an MDE of 6h.