IL49008A - Superconducting data-storage arrangement - Google Patents

Description

49008/2 Superconducting data-storage arrangement IHTERHATIOHAL BUSINESS HAOHISBS COHPORAHOS INTERNATIONAL BUSINESS MACHINES CORPORATION Armonk , New York 10504 , USA Superconducting Storage Arrangement The invention relates to a superconducting storage arrangement , comprising storage cells including at least a Josephson junction, which cells utilize magnetic flux quantization for data storage , further comprising means for write -in and read -out of information .

It is known that in superconducting circuits a so-called Josephson element can carry tunnel currents which are able to penetrate an extremely thin oxide layer between superconducting electrodes or an area comprising a weak link between superconductors . Across a Josephson junction either the pure Joseph-son current flows without any voltage drop across the oxide layer which is ba sei upon the tunnel effect of the Cooper pairs or a tunnel current flows of single electrons in addition causing a voltage drop between the superconducting electrodes . This voltage drop is correlated to the binding energy between the electrons of a Cooper pair, and it corresponds to the energy gap separating both kinds of carrier . The value of the maximum Josephson current can be influenced by the application of magnetic fields provided e .g . by currents in control lines . A Josephson junction switches from its superconducting state into the voltage state or so-called normal conducting state whenever the current through SZ 9 -74 -003 - 1 - the junction is greater than the maximum Josephson current . Therefore that switching can be caused either by decreasing the maximum Joseph so r¾e¾yrrent by means of a magnetic field e . g . by a control current or by raising the junction current or by both at the same time . A resetting of the Josephson junction into the superconducting state can be achieved by switching off the junction current or by utilizing other effects like the a . c . Josephson effect .

Superconducting storage arrangements are known comprising controlable Joseph-son junctions or Josephson gates which utilize the switching of the gates to control persistent ring currents in superconducting loops . The binary values to be stored are represented by the circulation direction of the switchable ring current . An example is described in US-patent 3 . 626 . 391 or its corresponding Swiss patent 486 . 095 . The relatively large storage cells comprise two Joseph-son junctions needed for switching the ring currents . The magnetic flux coupled to the persistent ring currents in the order of magnitude of several hundred flux quanta . is also relatively high .

Because the magnetic flux trapped in superconducting loops is quantized one can also use the quantization of magnetic flux for the storage of data . Storage arrangements comprising ring cells may be so designed that only a few flux quanta are coupled to the ring current . D. E . Mc Cumber proposed a storage cell with the binary values corresponding to states where only a single or no flux quantum is trapped in the ring cell . Refer to his paper "Tunneling and Weak-Link Superconductor Phenomena Having Potential Device Applications' on pages 2503 through 2508 of Volume 39 , No . 6 of Journal of Applied Physics , May 1968 . A technical description of such storage cells, can be found in US-patent 3 .705 . 393 or in IBM Technical Disclosure Bulletin Vol. 14 , Nr. 4 , SZ 9 -74 -003 - 2 - September 1971, page 1345. In this storage arrangement the cells are switched at the coin cidence of two currents which are applied to two current ccmducters magnetically coupled to the cell. The operation: method is based on ring currents being induced into the loop which are coupled to a magnetic flux. As soon as the sum of the currents exeeds the maximum Josephson current of the Josephson junction comprised in the loop a magnetic flux quantum penetrates the cell.

An essentially improved version of the storage arrangement is described in Swiss patent 539.919. There the basic theory is elucidated extensively.

However, magnetic flux quantization can be used.forthe storage of data not only in storage cells comprising superconducting loops. Flux quanta can also be trapped within a Josephson junction itself . , and there the form so-called vortex modes of different quantum numbers. The basic theoretical paper for this field has been published by C.S. Owen and D.J. Scalapino with the title "Vortex Structure and Critical Currents in Josephson Junctions" on pages 538 through 544 in Vol. 164, No. 2 of Physical Review on December 10, 1967. A Josephson junction storage has already been proposed which utilizes adjacent vortex modes for storage purposes (Swiss patent application 13.521/73 of September 20, 1973, .now Swiss patent 560.946).

All storage arrangements,known as jet or proposed, comprising Josephson junction utilizing the magnetic flux quantization for storage purposes are not able to read out the stored information non -destructively. Rather it is always necessary to rewrite the information which is erased* by read -out. In addition some of the storage arrangements demand a special preparatory memory cycle before information can be written in.

SZ 9-74-003 - 3 - It is an object of the invention to provide a superconducting storage arrange ment the essential parts of which can be produced in integrated circuji :ech- nology with high packing density .

It is a further object of the invention to provide storage cells where very little energy transfer is involved with the memory operations .

It is particularly the problem of the invention to provide a superconducting storage arrangement of the species termed at the beginning without the mentioned drawbacks which store allows also a non -destructive read-out of the stored information .

According to the invention the storage arrangement is characterized in that the ' parameters determining the switching behavior of each storage cell are calculated such that the storage cell is underdamped .

Hitherto there wa s the endeavor to design superconducting storage arrangements comprising Josephson junctions to have always sufficiently damped storage cells to avoid possible oscillations with certainty . Every storage cell exhibits certain parameters determining the switching behavior of the cell like e . g . values of the equivalent inductivity L, of the equivalent capacity C , and the equivalent ohmic resistance R in an equivalent circuit . Underdamping means that the damping ratio y=L / 4 C R which is valid for a certain equivalent circuit is less than unity , i .e . the actual damping is lower than critical damping . Critical damping is present when a pulse perturbation just dies out aperi - odically . Weak underdamping causes a certain overshoot before the transient response is equalized . Namely the elements of the equivalent circuits form. a resonator with a certain resonant frequency . However, the damping factor SZ 9 -74 -003 - 4 - contains also nonlinearitie s , and moreover it depends upon the operating point It ha s been found out that the switching behavior of underdamped storage cells makes possible the non-destructive read-out of information .

The method for the operation of the storage arrangement according to the invention is characterized in that for storing binary data stable states of the storage cells are utilized , which states differ in the number of trapped flux quanta at least by unity , and that for non -destructive read -out of stored information such currents and pulse sequences are used for selection that due to its intrinsic underdamping the selected storage cell is transiently switched into a state following the state used , which switching in ca se of one out of the binary value stored results in trapping of at least two flux quanta what flux change can be detected as a voltage pulse induced , and which switching in ca se of the other binary value stored is not connected with trapping any flux quantum what can be interpreted by the lack of an induced voltage pulse .

The invention' will be described in more details by embodiments with the aid of the drawings . The figures show the following : Fig . 1 The equivalent circuit of an interferometer-type storage cell comprising two Josephson' junctions connected by an inductivity .

Fig . 2 An implementation of a storage cell after Fig . 1 .

Fig . 3 Part of a control characteristic of a storage cell after Fig . 1 , comprising the vortex modes with quantum numbers 0 , 1 and 2 .

SZ 9 -74 -003 - 5 - Fig . 4 A section of the control characteristic of a storage cell modified by an additional control current to allow write-in of inforfmation .

Fig . 5 Schematically a storage matrix arrangement which can be used a s read only store or as random access store .

Fig .. 6 An implementation of a storage cell with a shaped Josephson junction which can be used in the storage matrix after Fig . 5 .

Fig . 7 The circuit diagram of a -, first kind of sense circuit .

Fig . 8 The circuit diagram of a . second kind of sense circuit .

Fig . 9 The circuit diagram of a storage cell comprising a Josephson junction shunted by an inductivity where the control current, can be inductively coupled into the loop .

Fig . 10 An implementation of the storage cell after Fig . 9 .

Fig . 11 A diagram showing the maximum Josephson -current normalized to the junction current in the storage cell after Figs . 9 and 10 versus the pha se difference of the quantum mechanical wave functions normalized to the quantum of magnetic flux.

Storage cells needing possibly the lowest real estate on the chip are suitably operated by utilizing the vortex modes of flux quantization . Therefore single so-called long Josephson junctions can be used for instance whose virtual length equals at lea st twice the Josephson penetration depth . In operation SZ 9 -74 -003 - 6 - these junctions show characteristics of the kind of interferometer curves .

Especially pure interferometer cells exhibit these properties cells comprise two Josephson junctions in parallel . But interferometer characteristics can be achieved also with memory cells comprising a single shaped Josephson junction .

Fig . 1 shows the equivalent circuit of an interferometer-type storage cell with two point contacts as Josephson junctions which are connected by an inductivity L. The ohmic losses in the cell are represented by an equivalent resistance R . Either Josephson junction is able to carry a maximum Josephson current I m The junction current I supplied by a suitable source devides to both Josephson junctions . A control current I can inductively be coupled into the storage cell . c A possible implementation of an interferometer type storage cell is shown in Fig . 2 . In integrated circuit technology on a superconducting substrate (not shown) and isolated therefrom a metallization M2 is deposited forming the lower electrodes of both Josephson junctions and one of the terminals for the junction current.A thin oxide layer is deposited in the shaded area . Another metallization M3 forms the upper electrodes and the other terminal for the junction current which may be e . g . the word current I in a first coordinate direction w of a storage matrix . In the second coordinate direction an inductively coupled control line can supply the bit current I as control current . For reasons des- D cribed below likewise in bit direction a parallel auxiliary control line is running aux for supplying an auxiliary control current I . which line crosses the storage B cell in the area of both Josephson junctions . In all embodiments shown a storage matrix organisation has been chosen with this kind of selection method . But a storage matrix can also be organized in another way so that e .g . the junction current corresponds to the bit current of the storage cell , and the Josephson SZ 9 -74 -003 - 7 - junction control current corresponds to the word current of the storage cell .

The 1 -mode corresponds to the state of the storage cell where a sing rle flux quantum is trapped in the cell whereas with 0-mode no such flux quantum is trapped . Trapping a flux quantum can occur by trapping same in the loop inducti But with the vortex mode of a long Josephson junction a circulating super Vlty current is induced within the junction .itself : <„ .In this ca se that current flows along one of the electrodes and reverses along the other electrode thus forming a loop which is closed through the oxide layer of the junction . The loop encloses the bundle of flux lines corresponding approximately an elementary flux quantum 0 . When the inductivity is substantial enough it is possible that the overlapping area of both first modes contains the origin of the control characteristic diagram . In this case it is possible to store a single quantum of flux in the storage cell without needing standby currents .

The control characteristic of an interferometer-type storage cell consists of an number of overlapping branches attributed to respective modes . Fig . 3 shows a section of the control characteristic of a storage cell with an equiva lent circuit according to Fig ; 1 . The diagram shows the normalized junction current I /I versus the normalized control current I /I . Both currents are g mo c - mo normalized to the maximum Josephson current I occuring in the absence of mo a magnetic field . The control characteristic shape is practically determined by a dimensionless parameter X .which could be termed normalized inductivity .

It is λ = L I / ο . Therein L means the equivalent inductivity of the equi- m valent circuit and I means the Josephson current . Hence the LI product give s m h _1 5 m the maximum flux ^within the storage cell . 0 = 2· 10 Vsec is the elementary quantum of flux.

SZ 9 -74 -003 - 8 - The characteristic after Fig . 3 is valid in the ca se that the current I equals m the maximum current in the absence of a magnetic field (1 = 1 ) . The para - m mo * meter becomes Λ = 2 . Further the resonator quality factor is supposed to be Q= .4 . The maximum current density of the Josephson junctions may be 4 2 J = 10 A / cm . Only those characteristic branches are shown which max corre spond to the first three quantum states . The designated quantum numbers 0 , 1 and 2 signify that in the respective modes 0 , 1 or 2 quanta of flux are trapped in the storage cell . The control characteristic envelope is a more or less, sharply serrated wave form having its maxima essentially at the same level within the interesting region . At higher junction current values above that envelope the Josephson junction can exist only in the voltage state . The individual modes are partly overlapping . The boundarie s of the overlapping areas can be fo nd by test methods involving special test circuits .

In the overlapping areas the operating point can belong to either mode depending from the history . The stable point may lie in the superconducting area characterized by the number of trapped flux quanta . The overlapping area boundaries are drawn partly in full lines and partly in dotted lines . Namely more exact experiments have proven that the switching behaviour can be different when pa ssing the boundaries .

A suitable combination of junction currents and control currents is able to switch the storage cell from one mode to another mode thereby crossing at least a boundary of an overlapping area .

If a storage cell is sufficiently damped switching occurs between immediately adjacent modes , for instance from 0 to 1 , from 1 to 2 , and so forth . However, with underdamped storage cells a switching behaviour can be achieved where SZ 9 -74-003 - 9 - the cell is switched from one state to a following state which is not the immediately adjacent mode . For instance it is possible to switch directly from the 0-mode into the 2 -mode thereby skipping the 1 -mode which does not appear .

The condition that a system capable of free oscillations be underdamped can be expressed by demandingthat its damping factor y is le ss than unity . With respect to the storage cell with an equivalent circuit according to Fig . 1 the following is valid: C means the capacity of a Josephson junction .

The damping factor of a superconducting storage cell can also be defined with the aid of other parameters e . g . by the resonant . frequency , by the Josephson frequency of the a . c . Josephson effect , and by the resonator quality factor Q = R/,^> L. The thus defined damping factor reads: To estimate the needed resonator quality factor Q one starts from the above 4 2 mentioned supposed values of the current density J = 10 A/cm , and max of the parameter X of the normalized inductivity or of the flux, respectively , 1 - -2 "5T . With the materials used the energy gap voltage lies In this case the Josephson frequency amounts to = — The frequency ratio needed in the numerator of equation (2) can be derived from: /^ = f ■ io~ '/ λ. " (3) SZ 9 -74 -003 - 10 - To estimate the re sonator quality factor Q the damping factor of equation (2 ) is supposed to be unity , and the respective presumed values of current density and of the parameter Λ of the Josephson junctions are set in according to equation (3) . The quality factor results to be Q = . 122 . That means that the resonator quality factor of the Josephson junction must be 4 2 greater than . 12 for a current density of J = 10 A/cm and the maximum max flux ( λ = 2 ) in order that the storage cell be underdamped . Only in this ca se the storage cell exhibits, the wanted properties which allow the application of such a cell in a storage arrangement with non-destructive read -out .

For transient switching from e . g . the 0- mode into the 2- mode the selection currents to address the storage cell must be supplied in a predetermined manner and sequence . The boundary of the 0 -mode is allowed to be cros sed by junction currents only below a certain value which is designated a s point X in Fig . 3 between the fully drawn part and the dotted part of the line .

(When namely the boundary is crossed above that value an underdamped storage cell does not switch from the superconducting 0-state into the adjacent superconducting 1 -state rather it switches immediately into the voltage state . ) Switching from the 0-state to the 2 -state does occur with trapping two flux quanta . This flux change can be sensed by suitable sense circuits as induced voltage pulse . After switching off the selection currents or after ceasing of the corresponding pulses , respectively , the storage cell resets automatically again into the original state i . e . .into the 0-mode because it is underdamped .

The control characteristic example of Fig . 3 is valid for Q = .4 . The resonator quality is sufficiently high so that the storage cell is underdamped . With good approximation the boundaries of the vortex modes can be represented by straight lines . The state with no flux quantum trapped lies symmetrically to the ordinate SZ 9 -74 -003 - 1 1 - axis which designates the normalized junction current I A . Extending from g mo ^*: point 2 to the right the extending straight line reaches! the abscissa axi-s in a. point C at a positive value of the normalized control current I /I . The c mo left hand boundary of the 0-state runs symmetrically in the second quadrant . Further branches of the curve follow in the direction of positive control currents which branches are a s signed to higher vortex modes . The boundaries of the 1 -state reach the abscissa axis in points A and D . The left hand boundary of the 2 -mode reaches the control current axis at point B . For sake of clarity further, branches of the characteristic are not shown . The region shown is sufficient to explain the basic concept .

In the overlapping regions stable operating points of different adjacent vortex modes can exist . For example in the triangular region of the characteristic having the ba se points A and C at least the modes zero and one can be a ssumed , in the small partial triangle with base points B and C the 2 -mode is possible in addition . . .

Now an operating point is considered in the region which is left when the partial triangle with base points B and C is subtracted from the triangle with ba se points A and C . In the absence of the junction current a standby current I /I be present defining an operating point between A and B . Therefore co mo the storage cell can exist either in the 0-state or in 1 -state which vortex modes can be a ssigned e . g . the respective binary values " 0" or " 1 " . When the overlapping area of the 0-mode and of the 1 -mode is large enough so that it contains the coordinate origin a stable operating point can also be achieved without the application of a special standby current I /I co mo SZ 9 -74 -003 - 12 - The storage cell be originally in the 1 -state . For the selection of the cell first a certain control current I /I is supplied to the cell as bit ci rrent c mo ' I., , and thereafter a certain junction current I /I is applied as word current B g mo I . If these selection currents are applied in this sequence which is indicated w in Fig . 3 by respective arrows , then the storage cell remains in its original 1 - state although a -dotted- boundary ha s been crossed . Namely the operating poir remain always within the triangular region of the control characteristic with ba se points A and D during selection . But this is the region where the 1 - vorte mode can exist . No quantum transition takes place . The stored binary value " 1 " remains preserved. No output signal is generated , and this can be interpreted in the respective manner . The only condition which must be observed consists in that no such mode boundary of the control characteristic is allowed to be crossed where a transition of the storage cell into the voltage state could occur due to its underdamping . This is the fully drawn part of the boundary line in the figure . Thus the point designated with X must be circumvented so to say In this case it also achieved that the storage cell does not switch into the 2 - mode which could also be taken by the cell .

Now it be a ssumed that the storage cell is originally in its 0-state . Again the above described pulse sequence is applied for selection . However, now a transition takes place into a following quantum state . The storage cell switche into the next but one state with trapping two flux quanta i . e . into the 2 - vorte mode . This flux change can generate an output signal by inducing a voltage pulse which can be detected by a suitable sense circuit . After ceasing of the selection currents or current pulses , respectively , the underdamped storage cell automatically re sets 'into the original 0 - vortex mode . Therefore the stored binary value " 0" remains preserved also in this case .

SZ 9 -74 -003 - 13 - For non -de structive readrout of information the selection currents must be applied in the de scribed sequence . The bit current I should be greej?»r than the abscis sa value of point X, and it should be les s than the abscissa value of point C . The word current I should cross a -dotted- mode boundary , but w it should be le ss than a junction current which could cross a -fully drawn-mode boundary of the control characteristic . In Fig . 3 there is shown the possible operating region of the selection currents as shaded area .

Already a read only store can be built with the storage cell described a s yet since the information remains preserved during non -destructive read-out . But there is also provided an auxiliary control line capable of supplying an auxiliary control current I to have the capability of also loading the storage with D information . There is shown in Fig . 2 that this line crosses the storage cell in the area of the Josephson junctions . Therefore , one can decrease the maximum Josephson current by means of the auxiliary control current or by its magnetic field , respectively . In this manner the control characteristic of the storage cell is changed so that now write-in of information is possible by the application of suitable word and bit currents .

Fig . 4 is valid in the ca se that the maximum Josephson current is reduced to 80 % of its value without external magnetic field . Hence , it holds: I = .8 · I m m The parameter gets, so the new value of about λ = 5 which parameter strong ly influences the properties of a storage cell . The current values are normalized to the Josephson current without external magnetic field . The characteristic shows the junction current (word current) I /I versus the control current g mo (bit current)I /I . The achieved alteration of the storage cell properties cause c mo a transient increa se of the attenuation to overcritical damping values . Now write-in of information is possible because the storage cell can be switched into immediately adjacent vortex modes due to the alteration of the cell switch- SZ 9 -74 -003 - 14 - ing behavior.

Only the interesting branches are shown which relate to adja ■cent vor6tex mode zero and one . The operating point lies in the overlapping region of both modes between ba se points E and F . It is defined by the standby current I /I co mo To write a " 1 " first a bit current I is supplied with a value greater than the D abcissa of point X and less than that of point F . The immediately succeeding word pulse pa sses the dotted part of the boundary of the 0-mode region, and its value remains below the fully drawn 1 -mode boundary . To write a " 0" no such bit current pulse need be applied in this example . The word current puis of e . g . same amplitude as above now pas ses the dotted 1 -mode region bounda In both cases the storage cell takes the state_.or mode corresponding to the information written, and it remains in this state too when after ceasing of the cl UX word currents and bit currents the auxiliary control current I is switched D off , and thus the storage cell regains its damping properties necessary for non-destructive read-out .

In Fig . 5 there is schematicallyshown an implementation of a storage arrangement . Storage cells 10 are arranged in a matrix , and in one coordinate direction they are connected to bit drivers and decoding means generally marked by · 12 . ' . That circuits can deliver selectively the necessary control current pulses I via a first bit line per row and in addition also the auxiliary control current I B necessary to enable write-in which current runs along a second bit line per row. In the other coordinate direction word drivers 14 are connected to the column of storage cells at one side of the matrix to provide the junc tion current pulses I . The other ends of the word lines lead to sense ampli- w fiers 16 . Particulars of these circuits are not further described here . Implemen tation examples of sense amplifiers only are shown below .

SZ 9 -74 -003 - 15 - -The interferometer type storage cells can be designed according to Fig. 2. Fig. 6 shows a further embodiment. A shaped Josephson junction is 185 ilt in integrated circuit technology . The upper electrode covers by its metallization two shaded areas of a thin oxide layer which are connected by a narrow region. The inductively coupled control line for the bit current I runs iso- B lated above that narrow region with relatively raised inductivity. The auxiliary control line supplying the auxiliary control current I crosses both larger partial regions which can be regarded as a pair of Josephson junctions connected by an inductivity with regard to the operation method. Therefore that shaped Josephson junction storage cell exhibits an interferometer-type control characteristic. It can be used in the storage arrangement in exactly the same manner as described above by means of examples.

Flux quantization consists in that the storage cell stores a magnetic flux which virtually equals a multiple of the elementary quantum of flux = 2 * 10_15 Vs. The stored flux amounts to $ * ^T^ ¾ Λ"· . This shows that the maximum supercurrent I and the inductivitv L of the storage cell m are essential parameters with respect to the storage arrangement. This important LI -Product is. also an essential part of the above discussed characteris-tic parameter Ά = 2¾,-/Λ1/ ί of the. storage cell.

Each transition between adjacent vortex modes, is connected to an energy content change of the storage cell what enables the read-out of information. A flux change in the order of magnitude of a flux quantum of 2 · 10 ^ Vs causes -18 an energy change in the order of magnitude of about 10 Joules for currents in the miliampere region. The storage cell switching between modes occurs extremely fast, and it causes a very short voltage pulse which can be detected by extremely sensitive sense circuits.

SZ 9-74-003 - 16 - For reading information from a selected storage cell suitable selection currents like the bit current I„ and the word current I are applied in the a iove des- B w cribed manner so that point X of the control characteristic is safely circumvented. When the storage cell was in the 1-mode no flux quantum is trapped despite of the fact that a mode boundary is crossed. Hence no voltage signal is generated what is interpreted as "1" in this case. However, when the storage cell was in the 0-mode a transition takes place during selection into the 2-mode which is connected with trapping two flux quanta. A voltage pulse SV is generated so that fiV-et ¾ 2 φσ , whereby J -t means the pulse duration and 0α means the flux quantum. Half the voltage In Fig. 7 there is schematically shown a first implementation of suitable sense circuits. The word current I flows through a storage cell 10. In downward w direction the word line is connected to ground via a first inductivity 18 as low-pass filter. Suitably the word line is designed such that its characteristic impedance Zq is maintained. The upper end of the first inductivity 18 is connected to a resistence 20 leading to a first normally superconducting Josephson junction 22. As low - pass filter a second inductivity 24 is connected in parallel to that first Josephson junction 22. The first Josephson junction 22 is directly connected to earth. The second inductivity 24 leads to ground too but via a control line of a second Josephson junction 26.

Inductivities 18 and 24 are transparent for slow signals like word currents I , w but they block pulses generated during read-out of stored information. Hence these pulses run across resistance 20 and add to the biasing operating current SZ 9-74-003 - 17 - I, of the first Josephson junction 22 which is supplied by a suitable source bo (not shown) .

Operating current I is adjusted such that junction 22 is normally in its superconducting state . Only in case when the sense current peak adds to the operating current the second junction 26 maximum Josephson current is pa ssed over so that the junction is very rapidly switched into the normal conducting state . Thus an essential part of the operating current I is being bo transferred into the second inductivity 24 and flows therefore through the control line of the second Josephson junction 26 .

That sense junction 26 is supplied with an operating current I which nor- so mally keeps junction 26 in its superconducting state . Junction 26 is switched into its normal conducting state with increasing current in the control line .

In a conventional way the output information can be taken from the junction 26 An additional control line carrying the current I allows optimum adjustment cs of the- operating point of that sense junction to achieve maximum sensitivity .

In Fig . 8 there is schematically shown a second implementation of a suitable sense circuit . The word line traversing the selected storage cell 10 leads to ground through an inductivity 28 serving as low-pa ss filter . The sense pulse isolated towards ground runs through a resistance 30 and a control line of a first Josephson junction 32 to a second Josephson junction 34 . An operating current source 36 normally keeps the second junction 34 in its voltage state . The resistance 30 and the bia s voltage of the source 36 are chosen such that the operating point of the second junction 34 is close to the point where the junction spontaneously resets . That means the voltage drop across that Joseph son junction 34 is kept close to its voltage V where the junction resets to its superconducting state .

SZ 9 -74 -003 - 18 - The generated read-out voltage peak subtracts ^ from that bias voltage so that the voltage occuring across the Josephson junction gets momeritarily smaller than its reset voltage V . , whereupon the junction resets into its min superconducting state . That is why the current increa ses essentially in the control line of the first Josephson junction 32 . This current increase is used to switch the normally superconducting sense junction 32 into the normal conducting state in order to deliver the output signal . Also here an additional control line is provided to allow adjustment of the operating point for maximum sensitivity .

But also superconducting storage cells containing a Josephson junction shunted by an inductivity can be designed acording to the invention . That means they are so de signed that their damping is moderately undercritical which determines their switching behavior . In Fig . 9 there is shown a circuit diagram of such a storage cell . Fig . 10 shows a possible implementation in integrated circuit technology . Their operation method is analogous to that as described above in connection with the interferometer-type storage cell embodiments .

In a first coordinate direction a line contains the Josephson junction . In a storage matrix the junction current corresponds e . g . to the word current . An inductive loop L is connected in parallel to the Josephson junction . In a second coordinate direction bit current pulses I are supplied as control signals which are inductively coupled into the storage cell . The maximum Josephson current I can be decrea sed by an auxiliary control current I which is m B supplied through a respective control line running in the bit coordinate direction in such ; "a manner that its coupled magnetic field controls the Josephson junction .

SZ 9 -74 -003 - 19 - On the superconducting substrate (not shown) a first metallization M2 is deposited forming the lower electrode of Josephson junction 38 and f st part of the inductive loop . In the shaded area there is a thin oxide. layer .

A second metallization M3 forms the upper electrode of junction 38 and the remaining part of the conductor loop which is electrically connected to the first metalization M2 at a suitable position e . g . at 40 . On top and isolated therefrom line s are provided to carry the bit current I " which line runs above B a ux the inductivity, and to carry the auxiliary control current I which line B is arranged on top of Josephson junction 38 .

In Fig . 1 1 there is schematically shown a diagram which is suitable for the explanation of the operation method of that storage cell type according to Fig . 9 and 10 . It shows the junction current I /I normalized to the maxi- g mo mum Josephson current without magnetic field versus the pha se difference of the quantum mechanical wave functions , ψ = -2 ^ $ $o · In order to get only two stable operating points (P, Q) without biasing standby currents the characteristic parameter λσ should lie between certain limits: 3 ~Λ- < *a < 7 "* /z . The , Ae value can be chosen to a higher value when higher order modes can be admitted . In this case more than one flux quantum are involved at the memory operations .

The diagram of Fig . 11 represents the graphic solution of equation (5) of the current distribution of the normalized word current into that storage cell branch containing the Josephson junction and into that branch, representing the inductivity L . (4) SZ 9 -74 -003 - 20 - In the figure the slope of the straight line Γ corresponds to λσ = n / 2 . Operating point P corresponds to the 0-state of the storage cell with no flux quantum trapped within the inductivity . Operating point Q corresponds to the 1 -state with a trapped flux quantum .

Also by using this type of storage cell information write -in is done by a suitable combination of word and bit currents .Also here the Josephson junction maximum super current is decreased by the application of an auxiliary control current ID3UX to facilitate write-in of a " 1 " .

Non-destructive read -out of information occurs in a similar manner a s des cribed above in connection with interferometer type storage cell s . According to the invention the storage cell is underdamped . Selection occurs by the combination of suitable word and bit currents When such cells are in the 0-stai selection causes a reversible transition from the 0-state into the 2 -state with trapping two flux quanta . Thus the skipped 1 -state remains "invisible" .

In Fig . 11 there is illustrated the read-out of a " 0" so that during the transitior the operation point first follows the curve beginning at point P/ but then it circumvents the 1 -state due to oscillation overshoot - da sh dotted- and it ends the transition in the 2 -state at a point corresponding to the maximum current in the 0-state . After switching off the selection currents automatic resetting occurs to operation point P in the 0-state . Also this transition -da sh dotted- occurs by circumventing the 1 -state . Reading a " 1 " occurs without transition into another quantum state . In the diagram one can imagine the ope ration in a way that the selection currents are not able to shift the operation point from Q over the second maximum of the curve which is higher than the first one . Since now no flux quantum is trapped a flux change is lacking which SZ 9 -74 -003 - 21 - could be detected by sensing its induced voltage signal . Therefore the sense circuits should be so designed that they interpret a lacking sense signal after application of selection currents as read-out binary value " 1 " .

Similar considerations are valid in the ca se that higher values of can be admitted . For instance it is possible that the storage states can differ about more than one trapped flux quantum which states are a ssigned to binary value s zero and one . Also in this case a non -destructive read-out of information is possible if the storage cell is underdamped according to the invention . Now it is switched into a state following the state used with trapping more than two flux quanta . Also this operation is completely reversible due to the under-critical damping of the cell . This is valid not only for storage cells comprising a Josephson junction shunted by an inductivity but also for the first described interferometer-type storage cells . Further it is not necessary that one out of the selection signals is inductively coupled into the storage cell . Of course also currents can be used for the selection of the storage cell which control the maximum Josephson current of one or more junctions by their magnetic field . Also a so-called write junction can be provided . This is a Josephson junction which is only used for write-in of information into the storage cell . The essential principle of the invention is the undercritical damping of the storage cell which makes feasible the operation method for nondestructive read -out of information .

SZ 9 -74 -003 - 22 - 49008/2

Claims (17)

1. Superconducting storage arrangement, comprising storage cells including at least a Josephson junction, whirh cells utilize magnetic flux quantization for data storage, further comprising means for write-in and read-out o information, characterized in that the parameters determining the switching behaviour of each storage cell are calculated such that the storage cell is under-damped.

2. Storage arrangement according to Claim 1, characterized in that with respect to a storage cell whose equivalent circuit comprises two Josephson junctions 2 connected via an inductl lty, the damping ratio y » L/2CR is less than unity, whereby L means the equivalent induct!vity of th storage ceil, C means the equivalent capacit of one of both Josephson junctions, and R means the equivalent ohmic losses of the storage cell.

3. Storage arrangement according to Claim 1, characterized in that the damping ratio of a storage cell 2 2 · y - (ω^ω^) /4Q is less than unity, whereby means the resonant frequency, means the, Josephson frequency of the a*c. Josephso effect, and Q = R/ L means the quality-factor of the resonator.

4. Storage arrangement according to Claim 3, characterized in that the value of the resonator quality- ΐ factor Q is greater than 0.12. tt ;.

5. Storage arrangement according to Claim 1, characterized in that the storage cells are interferometer-type cells whose equivalent circuit is an inductive superconducting loop containing at least two Josephson junctions. 49008/2

6. Storage arrangement according to Claim 5, characterized in that in a storage matrix each storage cell is provided with leads for galvahically coupling a Josephso Junction current as selection current for the selection in a first coordinate direction, and that means are provided for inductively coupling a control narren^ as selection curren in a second coordinate direction.

7. Storage arrangement according to Claim 5, characterized iri that in one of the coordinate directions a control lie is provided to apply a current for the control of the maximum Josephson current of the Josephson junctions.

8. Si Storage arrangement according to Claim 5, characterized ih that the storage cell comprises a superconducting loop containing two Josephson junctions , that in a first coordinate direction two leads are connected galvanieally to said loo such that either loop half contains one of the Josephson junctions, that in a second coordinate direction a first control line is provided for inductively coupling a control current into the cell, and that a second control line is provided traversing the area of the Josephson junctions which can apply an auxiliary control current to control the maximum Josephson curren of the Josephson junctions.

9. Storage arrangement according to Claim 5, characterized i that the storage cell contains shaped Josephson junction -with two unction areas bridged by an inductance Ί*¾:/ta in a first coordinate direction two leads are providedHeading to the electrode of the Josephson junction, that in a second coordinate directio above said inductive bridge a first control1 line is provided for inductively coupling a control current into the cell and that a second control line is provided which traverses both junction areas and can apply an auxiliary control current to 49008/2

10. Storage arrangement according to Claim 1, characterized in that the storage cells comprise a Josephso junction shunted by an inductance.

11. Storage arrangement according to Claim 10, characterized i that the storage cell comprises an inductive superconducting loop containing a Josephson junction, that in a first coordinate direction two leads are provided leading to the Josephson junction electrodes, that in a second coordinate direction a first control line is provided to couple inductively a control current into said loo , and that a second control line is provided above the Josephson junction which can apply a auxiliary control current to control the maximum Josephson current of the Josephson junction.

12. Storage arrangement substantially as hereinbefore described by way of example and with reference to the accompanying drawings.

13. Method for the operation of the storage arrangemen according to Claim 1, characterized in that for storing binary data stable states of the storage cells are utilized, which state differ in the number of trapped flux quanta at least by unity, and that for non-destructive read-out of stored Information such currents and pulse sequences are Used for selection that due to its intrinsie underdamping the selected storage cell is transiently switched into a state following the stat used, which switching in case of one out of the binary values stored results in trapping of at least two flux quanta what flux change can be detected as a voltage pulse induced, and which switching in case of the other binary value stored is not connected with trapping any flux quantum what ca be interpreted by the lade of a induced voltage pulse* 49008/2 ■(

14. , Method according to Claim 13, characterized in that i order to read out a first binary value non-destructively by detecting a higher quantum state of the storage cell a first selection current is applied whose value with respect to the abscissa of the related control characteristic is greater than the abscissa of the final point of a mode boundary whose crossing would cause a switching of the cell into the voltage state, and ^which valiie Is les than the abscissa of the final point of the boundary separating adjacent but at the same time possible I modes , and that thereafter a second selection current is applied whose value traverses the ordinate of said adjacent mode boundary $ut remains belo a value which would traverse a mode boundary • leading into the voltage state, whereupon no further flux quantum is trapped in the storage cell so that the lack of an induced output signal as answer to said selection currents can be interpreted as first binary value read out by respective sense circuits*

15. Method according to Claim 13, characterized in that in order to read out a second binary value by detecting a lower quantum state of the storage cell a first selection current is applied whose value with respect to the abscissa of the related control characteristic is greater than the abscissa of the final point of a mode boundary whose crossing would cause a switching of the cell into the voltage stae, and which value is less tha the abscissa of the final point of the boundary separating adjacent but at the same time possible modes, and that thereafter a second selection current is applied whose value traverses the ordinate of said adjacent mode boundary but remain below a value which would traverse a mode boundary leading into the voltage state, whereupo two flux quanta are 49008/2

16. Method according to Claim 13 , characterized in that the damping of a cell is transiently increased when «* Information is to be written in an underdamped storage cell*

17. Method according to Claim 16, characterized in that in the storage cell the maximum Josephson current is transiently decreased by applying an auxiliary control current. ISscb