GB2482008A

GB2482008A - Squid

Info

Publication number: GB2482008A
Application number: GB1011855.2A
Authority: GB
Inventors: Victor Petrashov
Original assignee: Royal Holloway and Bedford New College
Current assignee: Royal Holloway University of London
Priority date: 2010-07-14
Filing date: 2010-07-14
Publication date: 2012-01-18
Anticipated expiration: 2030-07-14
Also published as: GB2482008B; GB201011855D0; WO2012007736A1

Abstract

A quantum non-demolition interference device for ultralow noise ultrasensitive vector magnetometry and read-out of superconducting quantum logic circuits comprises a superconducting loop 51 interrupted by a short segment 52 of normal nonsuperconducting material that has negligible electromagnetic coupling to and hence negligible back action from the measuring circuit that includes measuring leads, amplifiers and current/voltage sources, and an interferometer. One branch of the interferometer may be made of superconducting material in electrical contact with the normal segment. The other branch of the interferometer comprises normal conducting spurs 59 and superconducting leads. The device may have a folded configuration to minimise mutual inductance of the superconducting loop and the measuring circuit (figure 9). The normal conductors may be made of magnesium, bismuth, antimony, carbon nanotubes or graphene. The quantum non-demolition interference device may be provided with additional leads to form a superconducting quantum interference proximity transistor.

Description

QUANTUM INTERFERENCE DEVICE

100011 The present invention relates to quantum interference devices, and in particular to vector magnetometers and read-out devices based on quantum non-demolition interference devices.

100021 Superconducting Quantum Interference Devices (SQUIDs) are well known as extremely sensitive vector magnetometers and read-out devices for superconducting quantum systems. A basic form of a SQUID 10 is shown schematically in Fig. 1 of the accompanying drawings and consists of a superconducting loop II interrupted by quantum tunnelling barriers 12, known as Josephson junctions (JJs). Due to the Josephson effect and superconducting quantum interference, the current between points 13 and 13 at the onset of superconductivity is sensitive to the magnetic flux, ci', through the ioop, making a SQUID a sensitive magnetic flux-meter. This is described in T. Ryhänen, H. Seppä, R. Ilmoniemi, and J. Knutila, J.Low Temp. Phys., 16,287-386, (1989) and the references therein. SQUIDs work in two regimes: "underdamped" and "overdamped".

100031 Underdamped SQUIDs can be used for read-out of superconducting quantum states e.g. in quantum bits. However they have a number of disadvantages. First, to produce a read-out the SQUID is switched into a voltage state, a process that strongly disturbs both the measured quantum circuit and the SQUID itself and shows hysteresis. Bursts of non-equilibrium quasiparticles are created with energies exceeding the superconductor gap, thus "poisoning" the measured circuit. Second, due to the AC Josephson effect, the voltage across the SQUID produces a microwave voltage pulse that can drive neighbouring quantum circuits into their excited states. Finally, the values of the current that switches the SQUID to the normal (non-superconducting) state are random and high fidelity measurements require up to about I 0 switching events.

[0004J Overdamped SQUIDs contain small electrical resistors shunting the Josephson junctions. This helps to avoid hysteresis, however the shunting resistors create Johnson-Nyquist thermal noise that limits the sensitivity of the SQUID.

100051 Alternative methods using dispersive read-out schemes have also been explored, but these require expensive Radio-Frequency equipment.

100061 An unavoidable contribution to the noise level of a SQUID-based magnetometer is due to electromagnetic coupling of the magnetometer superconducting ioop to the measuring circuit which includes measuring leads, amplifiers and current/voltage sources via mutual segments (wires) and/or mutual inductance. The coupling is unavoidable as it is required by the measuring mechanism.

[0007] An alternative to a SQUID is a hybrid interferometer known as an Andreev probe.

A hybrid interferometer 20 is shown schematically in Fig. 2 of the accompanying drawings and consists of a normal (N) (non-superconducting) nanoscale conductor connected to a superconductor ioop 22. Such a device is described in V T Petrashov, V N Antonov, P Delsing, and T Claeson, Phys Rev Letters, 74, 52685271 (1995). The physics of the sensitivity of hybrid interferometers to magnetic flux is different from that of SQUIDs. The normal-superconducting interfaces at points 23 and 24 play the role of mirrors reflecting electrons via an unusual mechanism first described by Andreev in A.F. Andreev, Zh. Eksp.

Teor. Fiz. 46 1823 (1964) [Soy. Phys. JETP 19 1228 (1964)].

[0008] Quantum interference of Andreev reflected electrons results in high sensitivity to the magnetic flux ZJ of electrical resistance of hybrid interferometers that can be measured using a simple, inexpensive measurement setup. The operating voltages applied to contacts 25, 26 are smaller than the superconducting gap, hence the measurements do not create non-equilibrium quasiparticles, thus no "poisoning" the measured circuit. No microwave voltage pulses are produced. However, when measuring leads are connected to contacts 25, 26, fluctuations in the measuring circuit, which includes measuring leads, amplifiers and current/voltage sources, are transferred to the superconducting loop through parasitic inductances M thus limiting the sensitivity of the device.

100091 As shown in Fig. 3 of the accompanying drawings, extra current leads can be placed so that the corresponding superconducting phase difference between points 23, 24 can be controlled by the current between contacts 27 and 28. This converts the hybrid interferometer to a transistor 20 with the current across the interferometer (between contacts 25) controlled by bias current in the superconducting wire. Other parts in Figure 3 are the same as like numbered parts in Figure 2.

100101 Further developments of these devices are described in V. T. Petrashov, K. G. Chua, K.M. Marshall, R. Sh. Shaikhaidarov, and J. T. Nicholls, Phys Rev Letters 95, 147001 (2005).

As described therein, the interferometer structure is a symmetric structure to exclude parasitic potential differences, the critical current induced in the normal wires is zero and the length of the normal segment must be greater than the coherence length but less than the phase breaking length of electrons in the normal conductor. In this case the Johnson-Nyquist thermal noise in the normal conductor limits the sensitivity of the Andreev probe.

100111 Francesco Giazotto, Joonas T. Peltonen, Matthias Mescke and Jukka P. Pekola, Nature Physics, Vol 6 April 2010 disclose a further development of this device, referred to as a SQU[PT (Superconducting Quantum Interference Proximity Transistor) and shown schematically in Figure 4 of the accompanying drawings. SQUIPT 30 comprises a superconducting ioop 31 of aluminium interrupted by a normal metal (copper) section 32.

Superconducting contacts 33, 34 are connected to the superconducting loop opposite the normal metal section. Superconducting electrodes 35, 36 are connected to the nonnal metal section 32 via a tunnelling barrier 37 of AlO. This device exhibits low flux noise and dissipation, but requires high voltages for readout which can cause quasi-particle poisoning.

The device has an unavoidable contribution to the noise level resulting from electromagnetic coupling of the magnetometer superconducting ioop via mutual segments (wires) and mutual inductance to the measuring circuit that includes measuring leads, amplifiers and currentlvoltage sources.

[0012] It is an aim of the invention to provide an improved quantum interference device, and in particular such a device that does not require high voltages and has practically zero (negligible) back action and hence practically zero (negligible) intrinsic noise level.

[0013] According to an embodiment of the present invention, there is provided a quantum non-demolition interference device comprising: a superconducting loop interrupted by a normal conducting segment; and first and second normal conducting spurs connected to the normal conducting segment at the middle point to form a cross, wherein the first normal conducting spur is separated from the normal conducting segment by a tunnelling barrier.

100141 The tunnelling barrier provided in an embodiment of the invention between one branch of the normal interferometer and the normal segment that interrupts the superconducting loop has the effect of decoupling the normal part of the interferometer from the environment, leading to reductions of the Johnson-Nyquist thermal noise Accordingly the device can provide a device having practically zero (negligible) back action and noise level (0015] According to a preferred embodiment of the invention, the quantum non-demolition interference device further comprises a pair of superconducting leads connected to the second normal conducting spur at a second end thereof.

10016] By using superconducting leads rather than normal leads to connect a branch of the interferometer to the environment, further reduction of the Johnson-Nyquist thermal noise is achieved.

100171 According to a preferred embodiment of the invention, the normal conducting segment and the first normal conducting spur are formed in first and second layers and the tunnelling barrier is formed between the first and second layers.

100181 According to a preferred embodiment of the invention the tunnelling barrier is formed by passivating a surface of the first normal conducting spur or the normal conducting segment.

100191 According to a preferred embodiment of the invention the interferometer branch and the second interferometer branch substantially overlap one another.

100201 According to another aspect of the invention, there is provided a quantum non-demolition interference device comprising: a superconducting loop interrupted by a normal conducting segment; first and second normal conducting spurs connected to the normal conducting segment at the middle point to form a cross; and a pair of superconducting leads connected to one of the normal conducting spurs at a second end thereof.

100211 According to another aspect of the invention, there is provided a quantum non-demolition interference device comprising: a superconducting loop interrupted by a normal conducting segment; and first and second normal conducting spurs connected to the normal conducting segment at the middle point to form a cross, wherein the second normal conducting spur is folded back over the first normal conducting spur.

100221 In an embodiment, an insulating spacer is provided between the first and second normal conducting spurs.

100231 The present invention thereby provides a device where the superconducting ioop has only a single common point and no mutual segments and mutual inductance with the measuring circuit that includes measuring leads, amplifiers and current/voltage sources.

[00241 In preferred embodiments of all aspects of the invention, the normal conducting segment and spurs are formed of a normal material that can be passivated such as Magnesium or Antimony. Other normal materials that can be used are Bismuth, Antimony alloys thereof, carbon nanotubes and Graphene.

[00251 Embodiments of the invention may further comprise a pair of superconducting current leads connected to the superconducting ioop either side of the normal conducting segment so that the electrical conductance of the normal conducting spurs is controlled by a bias current between the pair of superconducting current leads.

100261 In such an embodiment, the superconducting quantum interference device can be used as a superconducting quantum interference proximity transistor.

100271 The present invention also provides a vector magnetometer including a quantum non-demolition interference device as described above.

100281 The present invention also provides a computer system including at least one qubit and a quantum non-demolition interference device as described above acting as a readout device for the qubit.

[00291 The present invention will be described further below with reference to exemplary embodiments and the accompanying drawings, in which: 100301 Figures 1 to 4 are schematic diagrams of prior art quantum interference devices; 100311 Figure 5 is a schematic diagram of a quantum non-demolition interference device according to an embodiment of the invention; [0032] Figure 6 is a schematic diagram of a quantum non-demolition interference proximity transistor according to an embodiment of the invention; 100331 Figure 7 is a schematic diagram of a quantum non-demolition interference device according to an embodiment of the invention with an alternative arrangement of the branches of the interferometer; 100341 Figure 8 is a schematic diagram of a qubit according to an embodiment of the invention; [00351 Figure 9 is a schematic diagram of another quantum non-demolition interference device according to an embodiment of the invention; and 100361 Figure 10 is a schematic diagram of another quantum non-demolition interference device according to an embodiment of the invention.

100371 In the drawings, like parts are depicted by like reference numerals.

100381 A quantum non-demolition interference device (QNDID) 50 according to an embodiment of the invention is described below with reference to Figure 5, which is a schematic plan view of the device. QNDID 50 comprises a superconducting loop 51 interrupted by a normal segment 52 which connects to the superconducting loop 51 at junctions 53, 54. A two branch interferometer 55 is connected to the normal segment 52. The two branches 55a, 55b are connected to the midpoint of the normal segment 52 to form a cross.

100391 A first branch 55a of the interferometer includes a tunnelling barrier 56 separating the normal leads 57, 58 from the normal segment 52. A second branch 55b of the interferometer comprises a normal spur 59 connecting to the normal segment 52 and superconducting leads 60, 61. When a current is passed across the interferometer 55, electrons are reflected from the normal superconducting interfaces 53, 54 (Andreev reflection). The flux through the superconducting loop 51 affects the phase difference between 53 and 54 and hence causes quantum interference between the electrons reflected by the two boundaries making the current I across the interferometer 55 sensitive to the flux i (00401 QNDID 50 has an increased sensitivity magnetic flux due to the introduction of the tunnelling barrier which sharpens V-flux dependence and decouples the normal part of interferometer from the environment. The latter leads to further reductions of the Johnson-Nyquist thermal noise.

(00411 One of the branches 55a of the normal interferometer is decoupled from the environment by the tunnelling barrier 56. The other branch 55b is decoupled by the replacement of the current-potential leads connecting it to the environment with superconducting material 60, 61. The superconducting leads 60, 61 act as reservoirs for the interferometer. This arrangement provides for a magnetic flux-dependent superconducting gap (tunnelling barrier 56) in the normal part of interferometer. As a result the Johnson-Nyquist thermal noise is strongly suppressed and the dependence of current through the interferometer on magnetic flux is much steeper than in the absence of the barrier.

Introduction of tunnelling barriers in both branches as well as replacement of both normal leads with superconductors is not advantageous since it leads to an increase in operating voltages thus strongly increasing back-action of the device. Threshold tunnelling voltage in a device with superconducting current leads is larger than in a device with normal current leads by the value of the superconducting gap in the lead.

100421 According to another embodiment of the invention, as shown in Figure 6, extra current leads 62, 63 are provided to convert the non-demolition interferometer to a transistor.

The electrical conductance across the interferometer is controlled by the bias current lb in the superconducting wire. Other parts in this embodiment are the same as like-numbered parts in the embodiment of Figure 5 and are not described in detail.

[00431 In an embodiment of the invention, the parts of the device can be formed by known thin film deposition techniques, using masking layers patterned by lithographic techniques, e.g. e-beam lithography.

(00441 The superconducting parts of an embodiment of the invention may be formed of aluminium for operation at temperatures below 1 Kelvin. For operation at higher temperature the superconducting part of an embodiment of the invention may be formed of a superconductor with higher critical temperature, e.g. Niobium. Normal parts may be formed of passivated normal materials such as Magnesium or Antimony or a semimetal with low carrier concentration, such as Bismuth, Antimony, alloys thereof, Carbon nanotubes and Graphene. The use of a semi-metal has the effect that Johnson-Nyquist thermal noise is strongly reduced. The tunnelling junction between one branch of the interferometer and the normal segment may be formed by passivating the layer forming either the normal segment or the interferometer branch.

[0045] An alternative geometry of a quantum non-demolition interference device according to an embodiment of the invention is shown in Figure 7, which is a schematic perspective view. As can be seen, in the embodiment of Figure 7 the two branches 55a and 55b of interferometer 55' are stacked on top of each other so as to overlap, rather than being on opposite sides of the normal segment 52. An insulating spacer 64 separates the two branches of the intereferometer, the insulating spacer is thicker than the tunnelling barrier.

Superconducting leads 60, 61 similar to those of the embodiment of Fig. 5 are provided in the second interferometer branch.

[0046] Functionally, quantum non-demolition interference device 50" is the same as the embodiment of Figure 5 Again, additional current leads can be provided to create a quantum non-demolition interference proximity transistor as in the embodiment of Figure 6.

10047] The folded geometry of the embodiment of Figure 7 minimises the mutual inductance between the measuring and superconducting circuits and so minimises the influence of fluctuations in the measurement device on the superconducting loop. In some embodiments, the mutual inductance can be eliminated entirely. Parasitic fluxes are avoided.

[0048] Figure 8, which is a schematic plan view, shows a flux qubit device 70 according to an embodiment of the invention. The qubit device is formed as a superconducting loop 71 interrupted by a number of Josephson junctions 72. Although four are shown in the Figure more or fewer Josephson junctions 72 may be provided. Superconducting loop 51 interrupted by normal segment 52 and non-demolition interferometer 55 form the readout device.

Readout can be taken by measuring current or voltage between the ends of the two branches 55a, 55b of the interferometer 55.

100491 Figure 9 is a schematic perspective view of a quantum non-demolition interference device (QNDID) 80 according to another embodiment of the invention. QNDID 80 comprises a superconducting loop 81 interrupted by a normal segment 82 in electrical contact with the loop. Normal spurs 83, 84 contact the normal segment 82 at the middle point to form a cross. The cross-like geometry means that currents in orthogonal branches of the cross are independent: changes in one current do not lead to changes in the other. The two currents are orthogonal electric variables.

100501 The other end of normal spur 84 is in electrical contact with normal measuring leads 85. Normal segment 82, normal spurs 83, 84 and normal measuring leads 85 can be made of a semimetal -such as Bismuth, Antimony, alloys thereof, or Graphene -with low carrier concentration. Carbon Nanotubes can also be used. The use of a semimetal strongly reduces the Johnson-Nyquist thermal noise.

100511 A thin insulating spacer 86 overlaps normal segment 82, normal spurs 83, 84 and normal leads 85. Overlapping the insulating spacer 86 is a superconducting spur 87, which contacts normal spur 83 at one end and superconducting measuring leads 88 at the other.

Spacer 86 has two functions. Firstly, it insulates superconducting spur 87 from normal spurs 83, 84 and normal segment 82. Secondly, it minimises the magnetic flux produced by measuring currents. Superconducting spur 87 has the function of decoupling of the normal segment and making the superconducting gap in the normal segment 82, induced by the proximity effect, to have a final value when the superconducting phase difference between the ends 89 of the normal segment 82 is equal to r (180°). These ends, which contact the superconducting loop 81, function as superconducting mirrors for the interferometer.

[00521 In an embodiment it is desirable that the critical current induced in the normal wires is high. To ensure this, the length of the normal segment can be made less than the coherence length of the normal segment, N* This is the opposite approach to the prior art.

100531 Preliminary results show that this device has very promising figures of merit in a benchmark test for a flux measuring device: measurements with a superconducting flux quantum bit (qubit), which is known as an efficient noise spectrum analyser as described by R. J. Schoelkopf, A. A. Clerk, S. M. Girvin, K. W. Lehnert, M. H. Devoret in arXiv:cond-mat/0210247v1. The figure of merit of the device is the relaxation rate introduced by measuring device during the measurement process: t1r 2itM2(I)2f7hZ, (1) [0054] where M is the mutual inductance between the qubit and the measuring device, I, is the persistent current circulating in the qubit loop, f is the measuring frequency, h is Plank's constant and Z is the line impedance of the measuring device seen by the qubit. State-of-the-art measuring devices based on SQUIDs {A. Lupascu, S. Saito, T. Picot, P. C. de Groot, C. J. P. M. Harmans and J. E. Mooij, Nature Physics 3, 119-125 (2007)] have M>l0pH, so induce relaxation rates corresponding to qubit relaxation times up to trlO ts. To satisfy criteria enumerated in the DiVincenzo check-list for quantum computing ( D.P. DiVincenzo, Fortschr. Plzys. 48 771 (2000)) the relaxation time must be improved at least by two orders of magnitude.

100551 Measurements with the device of Figure 9 showed relaxation times corresponding t=30 ms, that is 3000 times larger than state-of-the-art and that requested by the DiVincenzo criteria. This means that much faster magnetic measurements can be realised with much less time for averaging to achieve required signal-to-noise ratio. For example, the measuring time required to achieve high sensitivity of superconducting gyroscopes used in space applications can be reducedfrom about 1 week to less than 1 hour.. Another figure of merit, namely the noise equivalent flux (NEF) or "flux sensitivity" defined as NEF=<VN 2>'2ildV/d(11öLP2, where VN is the voltage noise within the frequency band Sv, CD is the magnetic flux. In our preliminary measurements the NEF was limited by the preamplifier noise and was found to be better than 1 0 CD Hz"2 and should be substantially higher than the QNDID intrinsic NEF, CD0 is the flux quantum. The measurement technique used with the device of Figure 9 can be much simpler than that of SQUID. This means that these devices have potential to replace SQUIDs in most applications.

100561 A variation of the device of Figure 9 is shown in Figure 10. This device 80' is essentially the same as device 80 of Figure 9 save for the addition of extra current leads 90 to convert the hybrid interferometer to a transistor. The electrical conductance across the interferometer is controlled by the bias current lb ifl the superconducting loop.

100571 Having described embodiments of the invention, it will be appreciated that the scope of the present invention is not limited by the above described embodiments but only by the terms of the appended claims. -10

Claims

CLAIMS1. A quantum non-demolition interference device comprising: a superconducting loop interrupted by a normal conducting segment; and first and second normal conducting spurs connected to the normal conducting segment at the middle point to form a cross, wherein the first normal conducting spur is separated from the normal conducting segment by a tunnelling barrier.
2. A device according to claim 1, further comprising a pair of superconducting leads connected to the second normal conducting spur at a second end thereof.
3. A device according to claims I or 2 wherein the normal conducting segment and the first normal conducting spur are formed in first and second layers and the tunnelling barrier is formed between the first and second layers.
4. A device according to claim 3 wherein the interferometer branch and the second interferometer branch substantially overlap one another.
5. A quantum non-demolition interference device comprising: a superconducting loop interrupted by a normal conducting segment; first and second normal conducting spurs connected to the normal conducting segment at the middle point to form a cross; and a pair of superconducting leads connected to one of the normal conducting spurs at a second end thereof.
6. A quantum non-demolition interference device comprising: a superconducting loop interrupted by a normal conducting segment; and first and second normal conducting spurs connected to the normal conducting segment at the middle point to form a cross, wherein the second normal conducting spur is folded back over the first normal conducting spur.
7. A device according to claim 6, further comprising an insulating spacer provided between the first and second normal conducting spurs.
8. A device according to any one of claims I to 7 wherein the normal conducting segment and spurs are formed of a passivated material such as Magnesium or Antimony or a material selected from the group consisting of Bismuth, Antimony, alloys thereof, carbon nanotubes and Graphene.
9. A quantum non-demolition interference device comprising: a superconducting loop interrupted by a normal conducting segment; and first and second normal conducting spurs connected to the normal conducting segment at the middle point to form a cross, wherein the normal conducting segment and spurs are formed of a passivated material such as Magnesium or Antimony or a material selected from the group consisting of Bismuth, Antimony, alloys thereof, carbon nanotubes and Graphene.
10. A device according to any one of the preceding claims, further comprising a pair of superconducting current leads connected to the superconducting loop either side of the normal conducting segment so that the electrical conductance of the normal conducting spurs is controlled by a bias current between the pair of superconducting current leads.
11. A device according to any one of the preceding claims wherein the length of the normal segment is less than the coherence length of the normal segment N.
12. A quantum non-demolition interference device constructed and arranged to operate as described hereinbefore with reference to and as illustrated in one of Figures 5, 6, 7, 9 or 10 of the accompanying drawings.
13. A vector magnetometer including a quantum non-demolition interference device according to any one of claims I to II.
14. A computer system including at least one qubit and a quantum non-demolition interference device according to any one of claims I to I I acting as a readout device for the qubit.