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IEEE Workshop on Low Temperature Electronics (WOLTE)

Digital controlling of superconducting current steering switches

Oliver Brandel, J. Kunert, Torsten May, Thomas Ortlepp, Hannes Toepfer, Hans‐Georg Meyer

Published Jul 1, 2014• 0 citations
Publisher

Abstract

Large arrays of superconducting radiation sensors require a multiplexed readout. One implementation of the promising code division multiplexing technique uses current steering switches (CSSs) based on superconducting quantum interference devices (SQUIDs). We pursue the concept of utilizing rapid single flux quantum (RSFQ) electronics to control these switches, where each SQUID of a CSS is inductively coupled to the storing loop of a delay flip-flop. The internal state of this digital logic element is represented by the presence of a magnetic flux quantum, which is destined to control the CSS.

DOI
10.1109/wolte.2014.6881020

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Large arrays of superconducting radiation sensors require a multiplexed readout. One implementation of the promising code division multiplexing technique uses current steering switches (CSSs) based on superconducting quantum interference devices (SQUIDs). We pursue the concept of utilizing rapid single flux quantum (RSFQ) electronics to control these switches, where each SQUID of a CSS is inductively coupled to the storing loop of a delay flip-flop. The internal state of this digital logic element is represented by the presence of a magnetic flux quantum, which is destined to control the CSS.
Superconducting radiation sensors are of particular interest for imaging applications in the sub-mm wavelength band because of their extraordinary sensitivity. The rising number of sensors integrated in one array entails the requirement of multiplexing techniques in order to reduce the number of wires leading into the cryogenic stage and thus reduce the thermal losses. One kind of promising code division multiplexing technique is based on a current steering switch (CSS), which is composed of two identical superconducting quantum interference devices (SQUIDs) in parallel current paths. One of them is switched from the superconducting into the normal state controlled by the applied magnetic flux. In this way the signal path can be altered and they can act as a polarity switch for analogue signals. We pursue this concept to use rapid single flux quantum (RSFQ) electronics for controlling these switches. As a first step, the SQUIDs of the CSS are inductively coupled to the storing loops of two delay flip flops (DFFs). Thus, one is able to toggle the polarity of the analogue switch by controlling the state of the DFF by RSFQ control signals. The results of simulations and measurements and also margin analyses are discussed.
For spectroscopic and imaging applications in the submillimeter wavelength band, superconducting radiation sensors are widely used. Thereby, sensor arrays with a rising number of pixels require multiplexing techniques in order to reduce the number of wires leading to the cryogenic stage. A current steering switch (CSS) provides the basis for one kind of promising code division multiplexers. It is composed of two identical superconducting quantum interference devices (SQUIDs) in parallel current paths. Switching one of them from the superconducting into the normal state, which is controlled by the applied magnetic flux, alters the signal path; thus, they can act as a polarity switch for analog signals. In this paper, we describe a method that uses rapid single-flux quantum (RSFQ) electronics for controlling these switches. Therefore, their SQUIDs are inductively coupled each to the storing loop of an RSFQ delay flip-flop (DFF); hence, the state of the analog switch can be controlled by means of digital RSFQ signals. As a first step, we show the change in the critical current of the SQUIDs by the coupled operating digital circuit. For a wider operating range of the CSS, we developed a DFF, which is able to store a larger quantity of magnetic flux to make it possible to apply more flux to the coupled SQUID. For further improvements, we tested asymmetric SQUIDs to reduce the required magnetic flux. The results of the simulations and measurements are discussed.
Multiplexed superconducting quantum interference device (SQUID) amplifiers have recently enabled the deployment of kilopixel arrays of superconducting transition-edge sensor (TES) detectors on a variety of receivers for astrophysics. Existing multiplexing techniques for TES arrays, however, have constraints due to aliasing of SQUID noise, the size of the required filtering elements, or the complexity of the room-temperature electronics that make it difficult to scale to much larger arrays. We have developed a Walsh code-division SQUID multiplexer that has the potential to enable the multiplexing of larger arrays or pixels with faster thermal response times. The multiplexer uses superconducting switches to modulate the polarity of coupling between N individual TES detectors and a single output SQUID channel. The polarities of the detector signals are switched in the pattern of an N × N Walsh matrix, so a frame composed of N orthogonal samples can be used to reconstruct the detector signals without degradation. We present an analysis of the circuit architecture and preliminary results.
Abstract Time-division multiplexing constitutes a pivotal readout technique for arrays of transition edge sensors (TES). At the heart of constructing the multiplexing unit (Mux unit) lies a flux-actuated superconducting switch. The superconducting quantum interference device (SQUID) is ideally suited for this role due to its compatibility with the fabrication processes of the SQUID current amplifier, which is employed to read the TES signal. This compatibility facilitates the integration of the switches with the SQUID amplifiers on a single chip. Nevertheless, the critical current of a switch based on a conventional DC-SQUID exhibits abrupt changes with the control flux near the local minimum of the threshold curve, which is the typical operating point of the SQUID amplifier. Such abrupt changes compromise the stability of the SQUID amplifier’s operational point. To mitigate this issue, a Zappe interferometer-style switch design has been adopted, providing a threshold curve with a more pronounced flat region. In this work, we present the design and characterization of Zappe-style switches and demonstrate the operational efficacy of the Mux unit.
We present a novel multiplexer scheme for time-division multiplexing in superconducting quantum interference device (SQUID) readout configurations. The channels of our SQUID multiplexer (SQMUX) are dc SQUID sensors which are connected in series to form a column. A superconducting-to-normalconducting switch (SN-switch) in parallel with each channel is actuated in order to activate or deactivate an individual channel of the column. With a switch in the normalconducting state, its corresponding SQUID channel is active and sensitive to an input signal. When a switch is in the superconducting state its corresponding SQUID channel is short-circuited. It is then insensitive to input signals and does not generate noise. The addressing sequence is chosen such that only one SQUID channel is active at a time. We suggest employing SN-switches which are based on dc SQUIDs themselves. Doing so enables the addressing signals of the SN-switches to be applied inductively, i.e., galvanically isolated. The SQMUX operation is demonstrated in a configuration where both the SN-switches and the sensing SQUIDs are based on SQUID series arrays.
The ability to switch the coupling between quantum bits (qubits) on and off is essential for implementing many quantum-computing algorithms. We demonstrated such control with two flux qubits coupled together through their mutual inductances and through the dc superconducting quantum interference device (SQUID) that reads out their magnetic flux states. A bias current applied to the SQUID in the zero-voltage state induced a change in the dynamic inductance, reducing the coupling energy controllably to zero and reversing its sign.