Quantum information processing system

Abstract

A quantum information processing system is disclosed. The system comprises a charged particle trap (2) comprising a substrate (3, Fig. 2) and a set of electrodes (5,6,7; 15,25,26,27 Fig. 3; 15’,16,25,

Description

Field The present invention relates to a quantum information processing system. 5 Background Low-error quantum gates on trapped-ion systems can be performed using magnetic fields, as described in, for example, T. P. Harty et. al.: “High-fidelity preparation, gates, memory and readout of a trapped-ion quantum bit”: https: / / arxiv.org / abs / 1403.1524 10 (2014) and R. Srinivas et al.: “High-fidelity laser-free universal control of two trapped ion qubits”: https: / / arxiv.org / pdf / 2102.12533.pdf (2021). 26 09 24 26 09 24 Summary According to a first aspect of the present invention there is provided a quantum information processing system which comprises a charged particle trap comprising a substrate and a set of electrodes supported on the substrate, and a control system for 5 controlling the charged particle trap and for applying biases to the set of electrodes for trapping at least one charged particle and performing at least one quantum logic gate on the at least one charged particle. The electrodes include at least one currentcarrying electrode which is formed of a superconducting material, and the control system is configured to pass a current through the at least one current-carrying io electrode a peak current of at least i A. The at least one current-carrying electrode has a thickness t of at least 0.3 pm. This can help to reduce power dissipation when trapping charged particles and performing quantum information processing, such as quantum logic gates, on the 15 charged particles. The control system may be configured to pass the current through the at least one current-carrying electrode having a frequency less than or equal to 1 GHz. 20 The charged particles may be ions, such as calcium ions (such as +°Ca+ or 43Ca+). The charged particles may be atoms or molecules with net electric charge, or elementary charged particles, such as electrons or positrons. The thickness t may be at least 0.5 pm or may be at least 1 pm. 25 The superconducting material may be rare-earth barium copper oxide, ReBCO, niobium or an alloy comprising niobium. The electrodes may include at least one electrode formed of normal metal. 30 There may be at least two current-carrying electrodes, for example, three currentcarrying electrodes. The at least two current-carrying electrodes may have different widths. The control system may be configured to drive a respective current through each current-carrying electrode. The at least two currents may have different frequencies and / or different phases. The at least two currents may have different 35 current values. 26 09 24 The charged particle trap may comprise a first layer of co-planar electrodes and a second layer of co-planar electrodes. The first layer of co-planar electrodes may be interposed between the substrate and second layer of co-planar electrodes. 5 The first layer of co-planar electrodes may include the at least one current-carrying electrode. The second layer of co-planar electrodes may include electrodes formed of normal metal. The charged particle trap may further comprise a layer of dielectric material, such as io silicon dioxide, interposed between the first layer of co-planar electrodes and the second layer of co-planar electrodes. The set of electrodes may include a set of surface electrodes having a shielding effectiveness S = 20 logiO[Bo’ / Bi’] <20 dB or <1 dB where Bo’ would be the magnetic 15 field gradient at a charged particle when trapped by the charged particle trap in absence of the set of surface electrodes, and Bi’ would be the magnetic field gradient at a charged particle when trapped by the charged particle trap in the presence of the set of surface electrodes. 20 According to a second aspect of the present invention there is provided a method of operating the quantum information processing system. The method comprises trapping at least one charged particle in the trap, each charged particle providing a respective qubit, preparing initial qubit state(s) and applying a sequence of one or more gates to the qubit(s). Applying the sequence of one or more gates to the qubit(s) 25 includes driving current(s) through the, or each, of the at least one current-carrying electrodes, the or each respective current having a frequency less than or equal to 1 GHz and a peak current of at least 1 A. The method may further comprise reading out qubit state(s). 26 09 24 Brief Description of the Drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure i is a schematic view of a quantum information processing system comprising a 5 first surface-electrode trap and a control system; Figure 2 is a schematic cross-sectional view of the first surface-electrode trap shown in Figure 1 taken along the line A-A’; Figure 3 is a schematic cross-sectional view of a second surface-electrode trap; Figure 4 is a schematic cross-sectional view of a third surface-electrode trap; and io Figure 5 is a process flow diagram of a method of operating the quantum information processing system shown in Figure 1. Detailed Description of Certain Embodiments Introduction 15 T. P. Harty ibid, and R. Srinivas ibid, employ arrangements in which ions are trapped above a principal surface of a chip supporting conductive traces, and high currents (>1 A) are passed through the traces to generate strong near-field magnetic fields and magnetic field gradients. Although these arrangements have been used to perform ultra-low error single-qubit and two-qubit gates, power dissipation remains a 20 challenge. Although superconducting materials can be used in ion traps, they have only been used for electrodes which generate electric fields. Shannon X. Wang et al.: “Superconducting microfabricated ion traps” https: / / arxiv.org / pdf / 1010.6108.pdf 25 (2010), P. C. Holz et al.: “Electric field noise in a high-temperature superconducting surface ion trap” https: / / arxiv.org / pdf / 2106.03945.pdf (2021), and J. Chiaverini and J. M. Sage: “Insensitivity of the rate of ion motional heating to trap-electrode material over a large temperature range”, Physical Review A, volume 89, page 012317 (2014) describe superconducting ion traps. 30 Superconducting materials have also been used a single-photon detector co-fabricated with a surface-electrode ion trap. S. L. Todaro et al.:“ State Readout of a Trapped Ion Qubit Using a Trap-Integrated Superconducting Photon Detector”, Physical Review Letters, volume 126, 010501 (2021) describes a trap-integrated photon detector using a 35 superconducting nanowire single-photon detector. The trap electrodes are made of electroplated gold on an intrinsic silicon substrate. 26 09 24 Although some superconductors have DC critical current densities which might allow them to pass high currents (>1 A) while remaining in a superconducting state, this ability degrades with increasing frequency. Moreover, although superconductors exist 5 which exhibit high critical current densities, they tend to be harder to deposit and pattern, especially in a thick layer (>300 nm), and this can lead to divergence between the fabrication requirements of the trapping layer, and the fabrication requirements of high-critical current superconductors. 10 Quantum information processing system 1 Referring to Figure 1, a quantum information processing system 1 is shown which includes a first surface-electrode trap 2. The surface-electrode trap 2 comprises a substrate 3, for example, comprising silicon dioxide, having an upper surface 4 (or “principal surface”) supporting an arrangement of electrodes 5, 6, 7 which can be used 15 to trap and control one or more charged particles 8 providing respective qubits. The charged particle(s) 8 take the form of ion(s), such as calcium ions (for example, *°Ca+ or 43Ca+), although they may, however, take the form of atom(s) or molecule(s) with net electric charge, or elementary charged particle(s), such as electrons or 20 positrons. The surface-electrode trap 2 is housed in a cryogenic refrigerator (not shown) for cooling the surface-electrode trap 2 to a suitably low temperature T (e.g., below 77 K or 4.2 K). The surface-electrode trap 2 may be housed in a vacuum chamber (not shown), 25 which provides an ultra-high vacuum environment allowing individual charged particles to be isolated. The system 1 also comprises a control system 10 for applying DC and AC biases to the electrodes 5,6, 7 including a central electrode 5 through which current can be passed to generate a magnetic field gradient. A current-carrying electrode is preferably elongate (i.e., longer than it is wide) having first and second 30 ends. A current-carrying electrode may be referred to as a “conductive track”, “conductive trace” or simply “wire”. In the case the current-carrying electrode is formed from a superconducting material, the current-carrying electrode may also be referred to as a “superconducting track”, “superconducting trace” or simply “superconducting wire”. The first end may have or provide a first terminal (not shown) 35 connected to a bias source (not shown), such as current source or a voltage source, in the control system 10, and the second end may have or provide a second terminal (not 26 09 24 shown) connected to the bias source, to ground or to a terminal load allowing current to pass from the source through the wire. Examples of a quantum information processing system and surface-electrode traps can be found in WO 2021 / 205145 Ai which is incorporated herein by reference. 5 Referring also to Figure 2, the central electrode 5 and, optionally, some or all of the other electrodes 6, 7 are formed of a suitable superconducting material, such as niobium or Rare-earth barium copper oxide (ReBCO), and have a sufficient thickness, t, and width, w, to support DC or low-frequency AC currents (0 <f <1 GHz) having a 10 peak magnitude of at least 1A without exceeding critical current density, j<. For example, the ReBCO may be YBCO. The central electrode 5 and, optionally, the others electrodes 6, 7 has (have) a thickness t of at least 0.3 pm, preferably at least 0.5 pm and more preferably at least 1 pm. The central electrode 5 and, optionally, the RF electrodes 6, has (have) a width, w, of between 5 and 500 pm, preferably betw een io 15 and too pm transverse to the longitudinal axis 9. One or more layers of material, such as a metal, may be included under or on top of the layer of superconducting material for providing a better surface for wirebonds, for promoting adhesion, for providing a diffusion barrier and / or provide a protective 20 barrier layer. Examples of metals include gold, aluminium and chromium. For example, a layer of gold (e.g., having a thickness of 200 nm) may overlie the layer of superconducting material for aiding adhesion of wirebonds. A bilayer of aluminium and chromium may overlie a layer of niobium to provide a protective barrier against oxidation. 25 Other electrode arrangements Referring to Figure 3, a second surface-electrode trap 2’ is shown. The surface-electrode trap 2’ comprises a substrate 13, for example, formed of 30 magnesium oxide, having an upper surface 14 supporting a central superconducting strip 15, for instance formed of ReBCO, and a flanking layer 16 of dielectric material, such as silicon dioxide. The superconducting strip 15 and dielectric layer 16 are covered by a further layer 19 of 35 dielectric material, such as silicon dioxide, having an upper surface 20. The upper surface 20 of the dielectric material support an arrangement of electrodes 25, 26, 27 26 09 24 having the same configuration as the electrodes in the first surface-electrode trap 2 (Figure 1) described earlier, but formed of normal metal, such as gold, silver or aluminium (in a normal state, above its critical temperature). The superconducting strip 15 runs under the central electrode 25 and inner portions of the RF electrodes 26. 5 In other words, the superconducting strip 15 is wider than the central electrode 25. Referring to Figure 4, a third surface-electrode trap 2” is shown. The third surface-electrode trap 2” is the same as the second surface-electrode trap 2’ 10 except that the single, wide superconducting strip 15 (Figure 3) is replaced by three, narrower superconducting strips 15’, 16 substantially coterminous (when viewed in from above) with the central electrode 25 and RF electrodes 26. Surface electrodes shielding 15 The surface-electrode traps hereinbefore described may include one or more antennas (not shown) disposed under the surface electrodes, in other words, interposed between the substrate and surface electrodes. An antenna may take the form of a currentcarrying conductive track (or “wire”) which generates a magnetic field which can act on the trapped charged particles. A dielectric layer, such as silicon dioxide, having a 20 thickness of, for example, between 1 pm and 10 pm, may be deposited over the antenna(s) so as to electrically isolate the antenna(s) and the surface electrodes, such that the dielectric layer is interposed between the antenna(s) and the surface electrodes. 25 Shielding effectiveness S in dB is defined as: S = 20 logio(Bo’ / Bi’) (1) where Bo’ is the magnetic field gradient at the charged particle in absence of the 30 shielding structure, and Bi’ is the magnetic field gradient at the charged particle in the presence of the shielding structure. Shielding effectiveness can be divided into three regimes, namely: 35 - “Low shielding”: S <1 dB (i.e., Bl’ >0.9 Bo’) - “Intermediate shielding”: 20 dB >S >1 dB (i.e., 0.1 Bo’ <Bl’s 0.9 Bo’) 26 09 24 - “High shielding”: S >20 dB (i.e., Bl’ <o.i Bo’) The surface electrodes are preferably arranged so as to not shield the charged particles from the antenna(s), in other words, to be low shielding. Intermediate shielding may, 5 however, be acceptable. Low shielding can be achieved by (a) choosing a suitable material for the electrodes and a thickness for the electrodes that is much smaller than the skin depth 8 of the material (i.e., t <<8) and (b) configuring the electrode layout to have, for example, slots and / or io gaps. Shielding effectiveness may be estimated by modelling the layer above the antenna as a solid ground plane of thickness t and electrical conductivity o. For example, a layer of copper at room temperature having a thickness of 500 nm (t = 500 nm and 0 = 5.96 x 15 to7 S / m) is considered to be a low shielding layer at frequency f = 10 MHz since S <1 dB. A layer of copper with residual resistivity ratio RRR = 100 at 4 Kelvin having a thickness of 5 pm (t = 5 pm and 0 = 5.96 x 107 S / m) is a high-shielding layer at a frequency f = 300 MHz since S >20 dB. 20 Shielding effectiveness S is modified by electrode geometry, especially the presence of slots and cut-outs. Shielding effectiveness for a sheet without slots or cur-outs can, however, be used as approximate value. Operation 25 Referring to Figure 5, a method of operating the quantum information processing system will now be described. One or more charged particles 8 is (are) confined in the trap 2 (step Si) and initial qubit state(s) is (are) prepared (step S2). A sequence of one or more gates are applied to the 30 qubit(s), for example, in the form of Pauli gate followed by a Clifford gate (step S3), and final state(s) are measured (step S4). Examples of suitable operations can be found in T. P. Harty ibid, and R. Srinivas ibid. which are incorporated herein by reference. Reference is also made to H. Haffner, C. 35 Roos and R. Blatt “Quantum computing with trapped ions”: https: / / arxiv.org / pdf / 0809.4368.pdf (2008) which is incorporated herein by reference. 26 09 24 During trapping, initialization and / or gate operation, a current I of at least 1A is passed through one or more of the electrodes 5, 6. 5 Referring also to Figures 3 and 4, when performing a quantum gate (step S3), a high current (>1 A) oscillating at low frequency (<1 GHz) is applied to one or more of the three superconducting electrodes 15,15’, 16. Simultaneously, a low current (<1 A) may be applied to one or more of the normal electrodes 25, 26 and / or to one or more of the superconducting electrodes 15,15’, 16. 10 Modifications It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of charged particle traps 15 and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. The traps herein described have one layer of electrodes or two layers of electrodes (in 20 which one of the layers of electrodes is buried). Traps with three or more layers (which include at least two buried layers of electrodes) may be used. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present 25 invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features 30 and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 26 09 24

Claims

1. A quantum information processing system comprising:• a charged particle trap comprising a substrate and a set of electrodes supported5 on the substrate; and• a control system for controlling the charged particle trap, for applying biases to the set of electrodes for trapping at least one charged particle and for performing at least one quantum logic gate on the at least one charged particle;wherein the electrodes include at least one current-carrying w ire, which is formed io of a superconducting material, having first and second ends, wherein the first end is connected to a bias source and the second end is connected to the bias source, to ground or a terminal load, and wherein the control system is configured to pass a current through the at least one current-carrying wire having a peak current of at least 1 A during trapping, initialization and / or gate operation,15 wherein the at least one current-carrying wire has a thickness t of at least 0.3 pm.

2. The quantum information processing system of claim 1, wherein the control system is configured to pass the current having a frequency less than or equal to 1 GHz.20 3. The quantum information processing system of claim 1 or 2, wherein thesuperconducting material is rare-earth barium copper oxide, ReBCO.

4. The quantum information processing system of any one of claims 1 to 3, wherein the charged particle trap comprises a first layer of co-planar electrodes and a second 25 layer of co-planar electrodes, wherein the first layer of co-planar electrodes isinterposed between the substrate and second layer of co-planar electrodes.

5. The quantum information processing system of claim 4, wherein the first layer of co-planar electrodes includes the at least one current-carrying wire.

306. The quantum information processing system of claim 4 or 5, wherein the second layer of co-planar electrodes includes electrodes formed of normal material.

7. The quantum information processing system of any one of claims 4 to 6, wherein 35 the charged particle trap further comprise a layer of dielectric material interposed26 09 24between the first layer of co-planar electrodes and the second layer of co-planar electrodes.

8. The quantum information processing system of any one of claims 1 to 7, wherein 5 the set of electrodes include a set of surface electrodes having a shielding effectiveness S= 20 logio[Bo’ / Bi’] <20 dB where Bo’ would be the magnetic field gradient at a charged particle when trapped by the charged particle trap in absence of the set of surface electrodes, and Bl’ would be the magnetic field gradient at a charged particle when trapped by the charged particle trap in the presence of the set of surface10 electrodes.

9. A method of operating the quantum information processing system of any one of claims 1 to 8, the method comprising:• trapping at least one charged particle in the trap, each charged particle15 providing a respective qubit;• preparing initial qubit state(s); and• applying a sequence of one or more gates to the qubit(s):wherein applying the sequence of one or more gates to the qubit(s) includes driving current(s) through the, or each, of the at least one current-carrying wire, the or each 20 respective current having a peak current of at least 1 A.

10. The method of claim 9, wherein the or each respective current has a frequency less than or equal to 1 GHz.25 11. The method of claim 9 or 10, wherein applying the sequence of one or more gatesincludes:- manipulating the qubit(s) using low-frequency radiation.

12. The method of any one of claims 9 to 11, wherein applying the sequence of one or 30 more gates includes:- performing a ZZ gate.

13. The method of any one of claims 9 to 12, wherein applying the sequence of one or more gates includes:35 - performing a MS gate.14- The method of any one of claims 9 to 13, further comprising: ■ reading out qubit state(s).26 09 24

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