rf-SQUID

A rf SQUID, consist of a superconducting loop of inductance L closed by a Josephson junction of critical current Ic. In the 70s and early 80s, it has been successfully employed as a high sensitive magnetometer or gradiometer. In this configuration, the rf SQUID is  inductively coupled to the inductance LT of an LC tank circuit. The tank circuit is driven by an rf current, and the resultant rf voltage is periodic in the flux applied to the SQUID loop with period Φ0. In the presence of an external flux Φe, the dynamic of a rf SQUID is described by a double-well potential. The external magnetic flux controls the energy difference between the minima, the symmetric situation being for Φe = Φ0. Each logical state is represented by a wave function localized in a distinct potential well, and corresponds to distinguishable flux states trapped in the ring with current flowing in opposite directions. For these reasons rf SQUID have been proposed as a qubit in quantum computing application.

rf-SQUID based device

We have developed a fully integrated Josephson device consisting of a rf- SQUID coupled to a readout system based on a dc SQUID sensor. The dc SQUID sensor is coupled to the probe via a superconducting flux transformer. Both dc SQUID and flux transformer are in a gradiometric configuration in order to reduce noise coming from the environment. In designing the device, special care has been devoted to obtain the optimum coupling between the probe and the readout system, in order to guarantee a good signal-to-noise ratio related to flux transitions, as well as to minimize the back action due to the dc SQUID readout into the quantum probe (this is an additional, often dominant source of decoherence). The energy associated to the various magnetic flux states in the rf loop can be controlled by an external current into an integrated filtered coil coupled to the probe. dc SQUID performances as well as on the decay rate from metastable flux states of the rf SQUID in the classical regime have been investigated.

In the figure the picture of the device and rf SQUID response measured by a dc SQUID as a function of the external flux Φx are reported. The quantized states in the loop, are characterized by a change of magnetic flux in the dc SQUID of about 3 mΦ0

Main references:

Josephson device for quantum experiments

C Granata, V Corato, L Longobardi, M Russo, B Ruggiero, P Silvestrini

Applied physics letters 80 (16), 2952-2954 (2002)

Measurement of the effective dissipation in an rf SQUID system

B Ruggiero, V Corato, C Granata, L Longobardi, S Rombetto, P Silvestrini

Physical Review B 67 (13), 132504 (2003)

Controllable rf SQUID based device

We developed a Josephson devices as prototypes for applications to quantum computation with flux qubits. In particular we have designed, fabricated, and characterized rf-SQUID including a vertical two Josephson junctions interferometer as a controllable element in the rf SQUID superconducting loop and in the superconducting flux transformer. In fact, such device can be used to control the inductance parameter of the rf SQUID and to realize a switchable and tunable superconducting flux transformer for interqubit coupling. The Josephson junction of the rf SQUID is in fact replaced by a double-junction interferometer, where the critical current is precisely controlled by a small transversal magnetic field parallel to superconducting loop plane and modulates from a maximum to zero with fine control and precision. Furthermore If the double junction interferometer is inserted in the superconducting flux transformer, it is possible to obtain a  a variable flux transformer. 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. Hence, 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 figure, the picture of the device and dc SQUID output response in FLL operation for a triangular-shaped test signal of a controllable superconducting transformer including VJI at T=4.2 K are reported

Main references:

Josephson devices for controllable flux qubit and interqubit coupling

C Granata, B Ruggiero, M Russo, A Vettoliere, V Corato, P Silvestrini

Applied Physics Letters 87 (17), 172507 (2005)

Vertical Josephson interferometers for quantum computation

B Ruggiero, C Granata, M Russo, V Corato, P Silvestrini

Physics Letters A 336 (1), 71-75 (2005)