Demonstration of a superconducting nanowire microwave switch

Abstract

The functionality of a nanowire integrated into a superconducting transmission line acting as a single pole single throw switch is demonstrated. The switch has an instantaneous bandwidth from 2 to 8 GHz with more than 10 dB of isolation between the open and closed states. The switch consumes no power in the closed state and ≈ 15 nW in the open state. The rise and fall response time between open and closed states is approximately 370 ps. PACS numbers: #Quantum computing architectures employing as many as 72 qubits have recently been demonstrated. In order to scale such architectures further, miniaturized circulators, isolators, and switching networks that have low power consumption while operating at cryogenic temperatures will be necessary 1 . Several groups have demonstrated microwave switches and phase shifters based on Josephson junctions 2-6 , and novel semiconductor devices 1,7 . A cryogenic switch operating at DC, based on a cryotron has also been reported 8 . In this letter, we demonstrate a single pole single throw (SPST) switch fabricated from a nanowire integrated into a superconducting transmission line as an alternative low power, cryogenic microwave switch. Such nanowires are commonly used in the fabrication of superconducting single photon detectors 9-12 , and a three terminal variant of the device has been shown to operate as a transistor for digital logic applications 13 . We present two variants of the switch, one with a single nanowire and a second with two nanowires operated in tandem to provide improved isolation between the open and closed states of the switch. The single nanowire device is a small, w = 80 nm wide nanowire integrated into a superconducting transmission line with two on-chip inductors fabricated in a single metal layer. Surface mount capacitors are soldered off chip onto the transmission line feeding the device, creating a bias tee. Modulation of the switch is achieved by applying a low frequency signal to the inductive ports of the bias tee of sufficient power to exceed the critical current of the nanowire. The switch is in the closed state when the nanowire is superconducting and forms a lossless transmission line. Similarly the switch is open when the nanowire has been driven into the normal state. The switches are fabricated on a single layer of d = 8 nm thick NbN with a sheet resistance of ≈ 356 Ω/square. A wire only 3 squares long fabricated from this material will produce an impedance of ≈ 1 kΩ in the resistive state, enough to obtain more than 20 dB of RF isolation. In the tandem nanowire design a second nanowire increases isolation by shorting one RF port to ground while the first nanowire is simultaneously in the normal state. a) andrew.wagner@raytheon.com V S1 S2 2 µm (a) S1 S2 V 1 V 2 2 µm (b) FIG. 1. SEM micrograph images and circuit diagrams of the single nanowire switch (a) and tandem nanowire switch (b).The single nanowire switch is actuated by a voltage signal V that drives current through a resistor connected to the inductors of the bias tee. The tandem nanow...

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