GB2358932A - SQUID array readout - Google Patents

Abstract

A superconducting quantum interference device (SQUID) array 1 readout apparatus comprises a bias current source 14 and a sensing system 15 for the signals from the array of SQUIDs. Switches 10 and a switch control system 11 are arranged to selectively connect the current source 14 and the sensing system 15 to each of the SQUIDs in turn. Cooling arrangements may be provided to maintain the SQUID array 1 and switches 10 below 4.2 K and the sensing system 15 below 77 K. Only two switched connections 10A, 10B might be used to form the connections between the current source 14 and the sensing system 15 to a SQUID 1A and likewise for the other switches and SQUIDs. The sensing system 15 may include a bi-polar or field effect transistor amplifier. The readout apparatus may be used in a low temperature detector device for sub-millimetre and X-Ray astronomy experiments.

Description

2358932 1 SQUID ARRAY READOUT The present invention relates obtaining

signals from a SQUID array.

SQUIDs (Superconducting Quantum Interference Devices) are typically used to provide a readout from low temperature detectors, such as transition edge sensors. The SQUIDs operate such that for an applied bias current, the voltage generated across the SQUID will depend upon the magnetic flux applied to the SQUID. Accordingly, the signals output from the low temperature detectors are used to modulate a magnetic field which is then applied to the SQUID. This in turn causes modulation of the voltage readout obtained from the SQUID for a given applied bias current. As a result, the SQUID effectively amplifies the signals generated by the detectors.

As technology has improved it is now possible to build cryogenic detector arrays which can be used for obtaining multiple measurements in a number of applications. The arrays operate by having a respective SQUID associated with each detector in the array. Each SQUID therefore operates to amplify signals obtained from the respective detector. The amplified signals can then be used to generate data representative of events detected over a larger area than is possible with the use of a single detector.

Detector systems of this form are particularly useful for ground based and satellite based sub-millimeter and XRay astronomy experiments, in which measurements are made over an area, as opposed to a localized point.

However, in order to operate such a system, it is necessary to be able to apply a bias current to and measure the output voltage for each SQUID in the array. If each SQUID were biased and measured continuously, this would require a large number of connecting leads to couple the SQUIDs to a current source and a measuring device.

In order to function correctly, the SQUIDs must be cooled to a temperature of below 4.2 Kelvin, using to apparatus for 2 appropriate cryogenic cooling systems. In contrast, the measuring device which analyses the signals obtained from each SQUID is usually operated at a warmer temperature.

As a result, the presence of a large number of leads between the measuring device and the SQUIDs causes significant heat transfer from the warmer parts of the apparatus to the cooled SQUIDs. Accordingly, maintaining the SQUIDs at the desired operating temperature places an undue strain on the cryogenic cooling apparatus.

In order to overcome this it has been proposed to multiplex the readout from the SQUID array. This is described in a paper entitled "Superconducting Multiplexer For Arrays Of Transition Edge Sensors" by Chervenak et al, Applied Physics Letters, Vol. 74, No. 26, 28 June 1999.

This describes a system in which a first SQUID array is coupled to a respective number of detectors using appropriate input coils. The voltage output from these SQUIDs is then used to modulate SQUIDs in a second SQUID array which provides further amplification of the obtained signals. The voltages generated across the SQUIDs in the second array are then measured to obtain an indication of the detector outputs.

In this example, each SQUID in the first and second arrays are biased individually using an addressing system.

However, all the SQUIDs in a given array are connected in series to a load resistor and either a coil arrangement which forms the input to the next SQUID array, or to further electronics which process the obtained signals.

Accordingly, although a bias current is only applied to one SQUID at any one time, the output voltage will cause a current to flow through all the SQUIDs in the array, the load resistor and the coil or subsequent electronics, as appropriate. Unless the level of the current is controlled, it is therefore possible to bias all the SQUIDs in the array so that the detected voltage represents the sum of voltages generated by all the SQUIDs as opposed to one SQUID. This control is achieved by ensuring that the 3 load resistor has a sufficiently large resistance to prevent a voltage drop occurring across the unused SQUIDs which could result in the unused SQUIDs being biased.

As a result of this, the system loses significant resolution because a large proportion of the voltage output from the biased SQUID is dropped across the load resistor and is therefore not transferred via the coil to the second SQUID column, or to the measuring device. Furthermore, the presence of the load resistor results in increased noise 10 within the circuit.

In accordance with the present invention, we provide apparatus apparatus a.

is c.

d.

for obtaining signals from a SQUID array, the comprising: a current source for supplying a current to bias the SQUIDs; b. a sensing system for sensing signals obtained from the SQUIDs in the array; and, a number of switches for selectively coupling the processing system and the current source to the SQUIDs; and, a controller for controlling the switches so as to couple the sensing system and the current source to each SQUID in the array in turn.

Accordingly, by selectively applying the current source and the sensing system to each SQUID in the SQUID array separately, this overcomes the problem of providing additional load resistors to prevent voltage drops across other SQUIDs in the array. This results in a system of far greater sensitivity whilst maintaining a reduced amount of 30 wiring required between the sensing system and the SQUIDs.

Preferably the apparatus comprises a primary cooling system for maintaining the SQUID array with a temperature of below 4.2 Kelvin.

Preferably the primary cooling system is adapted to 35 maintain the switches at a temperature of below 4.2 Kelvin. However, it will be realized that the switches may be 4 cooled by an alternative cooling system and therefore may not be cooled to 4.2 Kelvin.

The sensing system and the current source are typically coupled to the switches in parallel so that two connections can be provided for coupling the sensing system and the current source to the switches. This allows the sensing system and the current source to use the same wires thereby reducing the number of wires which exit the primary cooling system. This in turn reduces the transfer of heat into the SQUID array.

The apparatus usually also comprises a secondary cooling system adapted to maintain the sensing system at a temperature below 77 Kelvin. However, this is not essential and the sensing system may operate at room is temperature, or alternatively be cooled by the primary cooling system and operate at a temperature of below 4.2 Kelvin. However, it is most energy efficient to use a secondary cooling system so as to avoid the need for cooling the sensing system to below 4.2 Kelvin, particularly as the sensing system will generally generate a large quantity of heat. Furthermore, better results are obtained if the sensing system is cooled rather than if it is used at room temperature.

Typically the sensing system comprises an amplifier for amplifying signals output from each SQUID in turn. The amplifier may be in the form of a bi-polar transistor amplifier as this provides good amplification of the received signals. However, an FET amplifier may alternatively be used.

The apparatus may further comprise a secondary SQUID array for coupling the switches to the sensing means. However, this is not essential in the present invention as the outputs obtained from the first SQUID array are sufficient to allow the received signals to be directly amplified by a bipolar transistor amplifier.

In this case, the secondary SQUID is also maintained at a temperature of below 4.2 Kelvin by the primary cooling system. Furthermore, the amplifier is typically an FET amplifier as this provides improved impedance matching with the the output of the secondary SQUID.

Examples of the present invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram of a first multiplexing system according to the present invention; Figure 2 is a schematic diagram of a modified version 10 of the multiplexing system of Figure 1; and, Figure 3 is a schematic diagram of a second multiplexing system in accordance with the present invention.

Figure 1 shows a SQUID array 1 including three SQUIDs is 1A,1B,1C which are positioned next to respective coils 2A, 2B, 2C of a coil array 2. Each coil 2A,2B,2C is connected to a voltage source 4 in series with a respective low temperature detector element 3A, 3B, 3C, as shown.

The detector elements 3A,3B,3C may be any form of suitable detector such as a transition edge sensor (TES).

Optional Nyquist filter coils SA,5B,5C may also be connected in series with the coils 2A,2B,2C and the detector elements 3A,3B,3C, as shown.

The SQUIDs 1A,1B,1C are coupled via a switch array 10 to a current source 14 and an amplifier 15. The switch array 10 includes four switches 10A, 10B,10C,10D which are used to connect the current source 14 and the amplifier 15 to each of the SQUIDs 1A, 1B, 1C, in turn, as will be described below. Operation of the switches is coordinated 30 by a controller 11, which is also coupled to the amplifier is.

In use, cooling apparatus (not shown) is used to maintain the SQUID array 1, the coil array 2, the detectors 3A,3B,3C and the switch array 10 at an operating temperature of below 4.2 Kelvin. In contrast to this, the current source 14, the amplifier 15 and the controller 11 6 can be maintained at a higher temperature, which in this example is below 77 Kelvin.

Operation of the system will now be described. When the signals output from the detectors are to be measured, the controller 11 causes the switches 10A,10B to close thereby connecting the current source 14 and the amplifier 15 to the SQUID 1A.

The current supplied by the current source 14 biases the SQUID 1A. The controller simultaneously sends a signal to the amplifier 15 which causes the amplifier to measure the voltage generated across the SQUID 1A.

The controller 11 then causes the switch 10A to open and the switch 10C to close thereby coupling the current source 14 and the amplifier 15 to the SQUID 1B. Again, a is measurement of the voltage across the SQUID will be sampled by the amplifier 15, following the receipt of a signal from the controller 11.

once this has been completed, the controller 11 causes the switch 10B to open and the switch 10B to close, thereby connecting the amplifier 15 and the current source 14 to the SQUID 1C. Again, the amplifier 15 will operate to sample the voltage across the SQUID 1C.

The output (not shown) of the amplifier 15 may be coupled to a display (not shown), or a data store (not shown) allowing the obtained results to be displayed or stored for subsequent retrieval, as will be understood by a person skilled in the art.

When biased in the manner described above, the SQUIDs have a low effective impedance. Accordingly, in this example, the amplifier 15 is preferably a bi-polar transistor amplifier 15 which has a low input impedance. matching the input impedance of the amplifier with the effective impedance of the SQUIDs ensures that none of the signal output from the SQUIDs is lost due to impedance mismatches in the circuit.

Figure 2 shows a modification of the apparatus of Figure 1. In this example, an additional feedback coil 7 array 16 is provided. This includes three coils 16A, 16B, 16C which are associated with the SQUIDs 1A, 1B, 1C respectively.

The feedback coils are used to provide an indication of the magnitude of the flux which has been applied to the SQUIDs 1A,1B,1C. An indication of this is then transferred to the amplifier 15 as shown, which is used to scale the output from the amplifier, as will be understood by a person skilled in the art.

A second example of the present invention is shown in Figure 3. In this example, the switches 10A, 10B, 10C, 10D in the switch array 10 are coupled to the current source 14 as in the previous example. However, instead of being coupled to the amplifier 15, the switches are coupled to a coil 13 is which is provided in series with the current source 14.

The coil 13 is used to apply a magnetic flux to an additional SQUID 12. The additional SQUID 12 is biased by an appropriate current source 17 and is connected in parallel to an amplifier 18, in the normal way.

Once again, the SQUID 12 would have to be cooled to appropriate operating temperatures by suitable cooling apparatus.

The additional SQUID 12 operates to amplify the signals output from the SQUIDs 1A,1B,1C. However, the output of the SQUID 12 has a much larger impedance that the output of the SQUIDs 1A,1B,1C. Accordingly, the amplifier 18 is usually in the form of an FET amplifier which has a higher input impedance that the bi-polar transistor amplifier of the first example.

It will be realized that in general the FET amplifier 18 will have worse performance characteristics than the bi polar transistor amplifier 15.

It will also be realized that the present invention may be applied to arrays including any number of detector elements and associated SQUIDs by using switching arrays including appropriate numbers of switches.

8

Claims (1)

1. Apparatus for obtaining signals from a SQUID array, the apparatus comprising: a current source for supplying a current to bias the SQUIDs; a sensing system for sensing signals obtained from the SQUIDs in the array; and, a number of switches for selectively coupling the processing system and the current source to the SQUIDs; and, d. a controller for controlling the switches so as to couple the sensing system and the current source to each SQUID in the array in turn.

2. Apparatus according to claim 1, the apparatus further comprising a primary cooling system for maintaining the SQUID array at a temperature of below 4.2K.

Apparatus according to claim 2, wherein the primary cooling system is adapted to maintain the switches at a temperature of below 4.2K. 4. Apparatus according to any of the preceding claims, the apparatus further comprising a secondary cooling system adapted to maintain the sensing system at a temperature of below 77K.

5. Apparatus according to any of the preceding claims, wherein two connections are provided for coupling the sensing system and the current source to the switches. 6. Apparatus according to any of the preceding claims, wherein the sensing system comprises an amplifier for amplifying signals output from each SQUID in turn. 7. Apparatus according to claim 6, wherein the amplifier is a bi-polar transistor amplifier. 8. Apparatus according to any of the preceding claims, the apparatus further comprising a secondary SQUID for coupling the switches to the sensing means. 9. Apparatus according to claim 8, when dependent on claim 2, wherein the primary cooling system is adapted to a.

c.

9 maintain the secondary SQUID at a temperature of below 4.2K.

10. Apparatus according to claim 8 or claim 9, when dependent on claim 6, wherein the amplifier is an PET amplif ier.

11. Apparatus according to any of the preceding claims, wherein the current source is a low temperature detector element.