Patent GB-1004963-A (GB)
Title: Superconductive devices
IPC: G11C11/44
Filed: 1963-07-30
Abstract: 1,004,963. Superconductive devices. INTERNATIONAL BUSINESS MACHINES CORPORATION. July 30, 1963 [July 30, 1962; Oct. 5, 1962], No. 30057/63. Heading H1K. [Also in Division H3] A superconductive arrangement comprises a hollow cylindrical member of superconducting material in which a quantised amount of magnetic flux is trapped by switching the member from the resistive to the superconducting state while it is threaded by an applied magnetic field, and means for directly sensing the amount of trapped flux or means for continuously raising the resultant magnetic field within the cylinder until it exceeds the critical field, thereby successively releasing quanta of trapped flux to produce current pulses in a sensing element which are counted to determine the total amount of flux trapped. An arrangement suitable for use in the latter manner, produced by thin film deposition techniques, is shown in Fig. 6. Along the axis of the cylindrical indium member 42<SP>11</SP> runs a copper strip 41<SP>11</SP> used as readout conductor. The sensing conductor is constituted by a layer 44<SP>11</SP> connected at both ends to superconductive shield 69 to form a coil which includes a neck portion 5111 forming the control conductor of a cryotron the gate of which is designated 56<SP>11</SP>. Operation is as follows: a magnetic flux parallel to the cylinder axis is established by passing current through member 55 while the cylinder is held resistive by means of a resultant field greater than the critical field. This may be the vector resultant of the field of member 55 and an auxiliary field generated either by current along conductor 4111 or by current along the cylinder itself. The auxiliary field is then cut off to render the cylinder superconducting whereupon a reaction current is generated in the cylinder sufficient to bring the flux within the cylinder parallel to the axis to an allowed quantised value. The external field is then removed whereupon the reaction current changes sufficiently to maintain the total flux at the quantised value. This flux may be read out by passing a current increasing as a ramp function axially along conductor 41 or cylinder 42. In either case the vectorial sun of the field thus generated and the quantised flux field increases until it exceeds the critical field. The cylinder then goes momentarily resistive and releases a quantum of stored flux to produce a current pulse in coil 44<SP>1</SP> sufficient to drive the cryotron gate resistive. When the quantum has been released the cylinder reverts to the superconductive state until the vectorial sum of the fields again exceeds the critical field, when a further pulse is produced, and so on. The pulses are counted to determine the amount of flux stored. A similar arrangement with a field coil replacing conductor 55 is also described (Fig. 5, not shown). In another arrangement (Fig. 4A, not shown) which lacks the built-in cryotron the pulses in the sensing coil are either counted using a cryogenic ring circuit or amplified and displayed on a cathode ray tube. The amplitude of the pulse increases and its width decreases as the number of quanta still stored increases and when a larger number of quanta is stored the time delay in switching from the superconducting to resistive state and back is such that several quanta are released simultaneously. A device enabling non-destructive readout, shown in Fig. 7 is maintained at a temperature such that under zero field conditions all the elements shown are with superconducting state. The device is connected in the circuit shown in Fig. 8. In this case quantised flux is trapped in tin cylinder 110 by holding it resistive by current in lead 115 while subjected to field generated by current in leads 113, 114, which are as broad as the cylinder is long and then terminating the current in lead 115. Since the critical current in indium conductor 112 is now dependent on the amount of flux trapped the latter may be sensed by increasing the current supplied to it as a ramp function. This current supplied by 151 divides between conductor 112 and a parallel lead including inductor 156 and one or more cryotron control elements 154. Initially as both paths are superconducting the greater inductance of the parallel path shows the rise of current therein but when the critical current is reached the current in conductor 112 stays substantially constant and the current through the cryotron control element begins to rise. At a certain point it drives the gate resistive, a condition which is sensed by periodic pulses from a strobe 63. The time elapsing before this happens is determined by the amount of stored flux. Several cryotron control conductors associated with differently biased gates may be placed in series in line 153. In this case the arrangement may be such that at the end of readout the number of gates in the resistive state corresponds to the number of quanta stored. A method of making a device constructionally similar to that of Fig. 5 by vapour deposition techniques is described. Dimensions of all the devices referred to above are given in the Specification. Specification 990,288 is referred to.