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Applied Physics Letters

Variable transformer for controllable flux coupling

Maria Gabriella Castellano, F. Chiarello, Roberto Leoni, Daniela Simeone, G. Torrioli, C. Cosmelli, P. Carelli

Published Apr 1, 2005• 26 citations
Publisher

Abstract

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.

DOI
10.1063/1.1899746

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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.
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 present niobium-based Josephson devices as prototypes for applications to quantum computation with flux qubits. The most interesting feature of this device is the use of a Josephson vertical interferometer for a fine tuning of flux qubit and interquibit coupling. These devices allow a control of the off-diagonal Hamiltonian terms of flux qubit and of the flux transfer function of a superconducting transformer. In the vertical interferometer, the Josephson current is precisely modulated from a maximum to zero with fine control and precision by a small transversal magnetic field parallel to the radio-frequency superconducting loop plane and to the flux transformer.
We present the design and the performances of a vertical Josephson interferometer, which can be inserted within superconducting loops in order to obtain controllable Josephson devices. The most interesting feature of the vertical interferometer is that the Josephson current is precisely controlled by a small transversal magnetic field parallel to the superconducting loop plane and can be modulated from a maximum to zero with high precision. This system can be used to realize a switchable flux transformer to couple quantum bits when desired as well as to vary the critical current of a flux qubit in situ. In the latter application, the vertical interferometer is controlled by a magnetic field orthogonal to the one applied to tilt the qubit potential, thus minimizing the cross talking of the two magnetic fields.
We have fabricated and studied a promising kind of direct current superconducting quantum interference device (dc-SQUID) in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and π in the temperature range of 0.1–4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.
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.