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Superconductor Science and Technology

A SQUID multiplexer with superconducting-to-normalconducting switches

J. Beyer, D. Drung

Published Aug 21, 2008• 44 citations
Publisher

Abstract

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.

DOI
10.1088/0953-2048/21/10/105022

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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.
We present a MHz frequency-division direct-current superconducting quantum interference device (dc-SQUID) multiplexer that is based on flux ramp modulation and a series array of N identical current-sensing dc-SQUIDs with tightly coupled input coil. By running a periodic, sawtooth-shaped current signal through additional modulation coils being tightly but non-uniformly coupled to the individual SQUIDs, the voltage drop across the array changes according to the sum of the flux-to-voltage characteristics of the individual SQUIDs within each cycle of the modulation signal. In this mode of operation, an input signal injected in the input coil of one of the SQUIDs and being quasi-static within a time frame adds a constant flux offset and leads to a phase shift of the associated SQUID characteristics. The latter is proportional to the input signal and can be inferred by channelizing and down-converting the sampled array output voltage. Using a prototype multiplexer as well as custom readout electronics, we demonstrate the simultaneous readout of four signal sources with MHz bandwidth per channel.
We present a MHz frequency-division direct-current superconducting quantum interference device (dc-SQUID) multiplexer that is based on flux ramp modulation and a series array of N identical current-sensing dc-SQUIDs with tightly coupled input coil. By running a periodic, sawtooth-shaped current signal through additional modulation coils being tightly but non-uniformly coupled to the individual SQUIDs, the voltage drop across the array changes according to the sum of the flux-to-voltage characteristics of the individual SQUIDs within each cycle of the modulation signal. In this mode of operation, an input signal injected in the input coil of one of the SQUIDs and being quasi-static within a time frame adds a constant flux offset and leads to a phase shift of the associated SQUID characteristics. The latter is proportional to the input signal and can be inferred by channelizing and down-converting the sampled array output voltage. Using a prototype multiplexer as well as custom readout electronics, we demonstrate the simultaneous readout of four signal sources with MHz bandwidth per channel.
We have investigated the transient response of superconducting quantum interference devices (SQUIDs) used in time-division multiplexing schemes for the readout from arrays of superconducting transition-edge sensors (TES). In time-division multiplexing the per channel settling time is an important parameter since it limits the multiplexing frequency as well as the signal-to-noise ratio. Although SQUIDs in general are fast cryoelectronic devices operating at a nanosecond scale, experimental evidence shows that the settling time in bias-switched SQUID multiplexers can, in the case of a low impedance input circuit, reach several microseconds. We have confirmed this behavior through numerical transient analysis using a Josephson circuit simulator. Within the simulation we have found that the transient response can be reduced using an extra inductor in series with the input coil of the dc SQUID. This result in turn has been confirmed experimentally. We furthermore discuss the conflicting constraints that a desired high signal bandwidth places on the parameters of the multiplexer, the readout SQUIDs, and the TES.
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.
Recent and future astronomical instruments are based on a focal plane mapped by a large array of superconducting bolometers. Cryogenic analog multiplexing readout techniques, based on superconducting quantum interference devices (SQUIDs), are currently developed to achieve the readout of large arrays of this kind of low noise background-limited detectors. To effectively reduce the number of cryogenic wires (particularly, SQUID biasing), line/column addressing is currently used in time-division multiplexing, i.e., same biasing is applied to a few SQUIDs (on a line) of different columns. This technique should dramatically increase power consumption if parallel biasing is applied via resistors to isolate each column; the power budget is particularly limited on this kind of front-end cryogenic readout. A design with one transformer per SQUID is also used to read out SQUID biased in series with no excess of consumption and crosstalk. We propose here a new biasing technique using simple surface-mounted capacitors, which is easier to implement. These capacitors are used to parallel bias SQUIDs without additional Joule effect while minimizing crosstalk. However, capacitors do not allow dc biasing and need a current mean value equal to zero to avoid biasing source saturation. We have then tested square current biasing through capacitors on a commercial SQUID. This measurement shows that capacitors are able to proper bias SQUID and then to perform a nondissipative addressing for time-division SQUID multiplexing.
Superconducting switches are important components in multiplexers based on superconducting quantum interference devices (SQUIDs). In time-division multiplexing (TDM) circuits, the superconducting switches are mainly used to activate and control input SQUIDs. The superconducting switch has the advantages of low power consumption and easy control. Here, a superconducting switch based on weakly damped SQUIDs is proposed. The dynamic resistance of the superconducting switch can be increased by appropriately adjusting the Stewart-McCumber parameter βc\beta _{c}βc​ of the Josephson junction. This design is expected to reduce the number of SQUIDs in arrays and simplify the fabrication processes of switches.