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arXiv:1105.5971 · Journal of Applied Physics

Current amplification with vertical Josephson interferometers

R. Monaco

Published Oct 1, 2011• 3 citations
arXivHTMLPDF

Abstract

It has long been recognized that a control current Ia injected into the section of a two- junction superconducting quantum interference device is able to produce a change of its critical current Ic, so that a current gain g = |dIc/dIa| can be identified. We investigate the circumstances under which large gains can be achieved by using vertical Josephson interferometers, which are characterized by small loop inductances. We discuss the theory of operation of such a device, its performances, and its advantages with respect to planar interferometers used in the previous works. Two potential applications are addressed.

DOI
10.1063/1.3646392

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It has long been recognized that a control current Ia injected into the section of a two- junction superconducting quantum interference device is able to produce a change of its critical current Ic, so that a current gain g = |dIc/dIa| can be identified. We investigate the circumstances under which large gains can be achieved by using vertical Josephson interferometers, which are characterized by small loop inductances. We discuss the theory of operation of such a device, its performances, and its advantages with respect to planar interferometers used in the previous works. Two potential applications are addressed.
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One can use a direct current (dc) superconducting quantum interference device (SQUID) as a current amplifier by injecting a signal current Is into part of the superconducting loop and detecting the resultant change in critical current Ic. The current gain max ‖dIc/dIs‖ can be increased by making the inductances of the two arms of the SQUID asymmetric, thereby skewing the transfer function Ic vs Is. Detailed simulations of the device performance include an analysis of the impact of inductance and junction asymmetry on the transfer function. At 77 K a maximum current gain of 5 should be achievable albeit for frequencies below 1 GHz. An increase in the operating speed to 2 GHz can be achieved by reducing the gain to 2. We have fabricated asymmetric YBa2Cu3O7−x dc SQUIDs on SrTiO3 bicrystal substrates and operated them at 77 K. Using a second, readout SQUID to monitor changes in the critical current, we have achieved a low-frequency current gain up to 2.3. By coupling two SQUIDs together, we have increased the gain to 8.5.
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.
A possible analog gain in Cooper-pair tunneling mode in a current injection Josephson network is theoretically investigated. The network consists of three Josephson junctions A, B, and C, and its gain can be widely controlled by varying the external shunt resistances of junctions B and C. Two different configurations of the planar structure of the network are suggested and their detailed principles of operation are explained. It is expected that the network will function at a higher speed than that possible in an amplifying device working in quasiparticle mode. A comparison between the present network and Faris QUITERON is also given.
Superconducting quantum interference devices (SQUIDs) that incorporate two superconductor/insulator/superconductor (SIS) Josephson junctions in a closed loop form the core of some of the most sensitive detectors of magnetic and electric fields currently available. SQUIDs in these applications are typically operated with a finite voltage which generates microwave radiation through the ac Josephson effect. This radiation may impact the system being measured. We describe here a SQUID in which the Josephson junctions are formed from strips of normal metal (N) in good electrical contact with the superconductor (S). Such SNS SQUIDs can be operated under a finite voltage bias with performance comparable or potentially better than conventional SIS SQUIDs. However, they also permit a mode of operation that is based on the unusual interplay of quasiparticle currents and supercurrents in the normal metal of the Josephson junction. The method allows measurements of the flux dependence of the critical current of the SNS SQUID without applying a finite voltage bias across the SNS junction, enabling sensitive flux detection without generating microwave radiation.