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EP-4552046-B1 - CHARGED PARTICLE TRAP OPERATION

EP4552046B1EP 4552046 B1EP4552046 B1EP 4552046B1EP-4552046-B1

Inventors

  • SRINIVAS, RAGHAVENDRA
  • ALLCOCK, DAVID
  • MALINOWSKI, MACIEJ
  • BALLANCE, Chris

Dates

Publication Date
20260506
Application Date
20230704

Claims (16)

  1. A method of operating a charged particle trap (3) which includes a set of trap electrodes (12), the method comprising: · trapping a first charged particle (1 1 ) at a first position (4 1 ), the first charged particle providing a first qubit (2 1 ) having a first transition frequency (f 1 ); · trapping a second charged particle (1 2 ) at a second position (4 2 ), the second charged particle having a second transition frequency (f 2 ); · applying (S2) a potential gradient (11) to the first and second charged particles wherein the first and second charged particles experience first and second magnitudes (g 1 , g 2 ) of potential gradient, respectively, and wherein the potential gradient oscillates at a given frequency (f G ) and is monochromatic; while applying the potential gradient: · applying (S3) a first oscillating potential to a first electrode (12 1 ) at a first frequency (f E1 ) so as to apply a first oscillating electric field (14 1 ) locally to the first charged particle; and · applying (S3) a second oscillating potential to a second electrode (12 2 ) at a second frequency (f E2 ) so as to apply a second oscillating electric field (14 2 ) locally to the second charged particle.
  2. The method of claim 1, further comprising: · the first oscillating electric field (14 1 ) has a first phase value (k 1 ); and/or · the second oscillating electric field (14 2 ) has a second, different phase value (k 2 ).
  3. The method of claim 1 or 2, wherein applying (S2) the potential gradient (11) comprises: · applying at least one magnetic field gradient (51) at the given frequency (f G ) to the first and second charged particles (1 1 , 1 2 ); · applying a laser field at the given frequency to the first and second charged particles; and/or · applying first and second laser fields to the first and second charged particles, wherein the first and second laser fields have first and second laser frequencies, respectively, and the difference between the first and second laser frequencies is equal to the given frequency.
  4. The method of any one of claims 1 to 3, further comprising: · applying a carrier drive to the first and second charged particles (1 1 , 1 2 ).
  5. The method of claim 3 or 4, wherein applying (S2) the potential gradient (11) comprises: · driving an oscillating current (I(t)) through an elongate conductive element (42) for generating the at least one magnetic field (50).
  6. The method of claim 5, wherein the elongate conductive element (42) includes first and second sections, wherein the first and second sections of the elongate conductive element are non-collinear.
  7. The method of claim 3 or 4, wherein the applying (S2) the at least one potential gradient (11) to the first and second charged particles (1 1 , 1 2 ) comprises: · driving a first oscillating current (I(t)) through a first elongate conductive element (42); and · driving a second oscillating current through a second elongate conductive element spaced apart from the first elongate conductive element.
  8. The method of any one of claims 1 to 7, wherein the charged particle trap (3) includes a substrate (80) having a principal surface (81), wherein at least a first set of the set of trap electrodes (84) are disposed on the principal surface of the substrate
  9. The method of claim 8, wherein a second set of the set of trap electrodes are supported on a different surface and are non-coplanar with the first set (84).
  10. The method of claim 8 or 9, wherein the charged particle trap (3) includes at least one elongate conductive element (42) for generating the at least one magnetic field (50).
  11. The method of claim 10, wherein the set of trap electrodes (44) includes first and second arrays of trap electrodes and the at least one elongate conductive element (42) is interposed between the first and second arrays of trap electrodes.
  12. The method of claim 10, wherein the at least one elongate conductive element (42) is supported on a different surface and is non-coplanar with the first set of trap electrodes (44).
  13. The method of any one of claims 1 to 12, wherein applying (S2) the potential gradient (11) to the first and second charged particles (1 1 , 1 2 ) comprises: · illuminating the first and second charged particles with at least one laser beam (88).
  14. The method of any one of claims 1 to 13, wherein the first charged particle (1 1 ) has a given mode of oscillation having a given direction of oscillation, the method comprising: · applying (S2) the potential gradient (11) such that the potential gradient (g 1 ) at the first charged particle has a component (E) which is not perpendicular to the given direction of oscillation and the potential gradient (g 2 ) at the second charged particle (1 2 ) has a component which is not perpendicular to the given direction of oscillation; · applying (S3) the first oscillating electric field (14 1 ) such that that first oscillating electric field is not perpendicular to the given direction of oscillation; and ·applying (S3) the second oscillating electric field (14 2 ) such that that second oscillating electric field is not perpendicular to the given direction of oscillation.
  15. The method of any one of claims 1 to 14, wherein the set of trap electrodes (12) includes the first and second electrodes (12 1 , 12 2 ).
  16. A system (8) comprising: a charged particle trap (3) which includes a set of trap electrodes (12); and a control system (34) for controlling the charged particle trap; the control system configured: · to trap a first charged particle (1 1 ) at a first position (4 1 ), the first charged particle providing a first qubit (2 1 ) having a first transition frequency (f 1 ); · to trap a second charged particle (1 2 ) at a second position (4 2 ), the second charged particle having a second transition frequency (f 2 ); · to apply (S2) a potential gradient (11) to the first and second charged particles, wherein the first and second charged particles experience first and second magnitudes (g 1 , g 2 ) of potential gradient, respectively, and wherein the potential gradient oscillates at a given frequency (f G ) and is monochromatic; while applying the potential gradient: · to apply (S3) a first oscillating potential to a first electrode (12 1 ) at a first frequency (f E1 ) so as to apply a first oscillating electric field (14 1 ) locally to the first charged particle; and · to apply a second oscillating potential to a second electrode (12 2 ) at a second frequency (f E2 ) so as to apply a second oscillating electric field (14 2 ) locally to the second charged particle.

Description

Field The present invention relates to operating a charged particle trap. Background A quantum computing architecture requires the ability to control individual qubits. Typically, this is achieved using spatially-varying control fields whereby each qubit experiences a different, locally-adjustable coupling. In trapped-ion architectures, two approaches are mainly used, namely focused laser beams and spatially-varying magnetic fields. Generating highly-controlled, localised laser or magnetic fields, at scale, is, however, challenging. Consequently, a common approach that is used to address single-qubit operations is to employ imperfectly localised fields and to manipulate ions, i.e., to move ions in and out of regions to which the fields are applied. In this so-called "shuttling-based" approach, a single-qubit operation consists of ion shuttling and applying laser or magnetic pulses. These approaches can have one or more drawbacks. For example, ion shuttling is slow due to the need to filter control electrodes, and this causes a bottleneck for performing single-qubit operations. Furthermore, simply shuttling ions within a field generated by a single source does not allow for individual phase control. Thus, to operate efficiently on N qubits in parallel, O(N) individually adjustable laser or magnetic field sources are usually needed. Integrating these sources into an ion-trap system is challenging and resource-intensive. R. T. Sutherland, R. Srinivas, and D. T. C. Allcock, "Individual addressing of trapped ion qubits with geometric phase gates", (14 June 2022) available from https://arxiv. org/pdf/2206. 06546. pdf describes a scheme for individual addressing of trapped ion qubits, selecting them via their motional frequency. In this scheme, one ion acts as a "target" qubit and another ion acts as "spectator" qubit. Both qubits are driven with a pair of bichromatic microwave fields and an rf B-field gradient, thereby resulting in a potential gradient which is trichromatic. Srinivas Raghu: "Laser-free trapped-ion quantum logic with a radiofrequency magnetic field gradient", 1 January 2020 (2020-01-01), pages 1-266, https://www.nist.gov/system/files/documents/2022/04/14/ RaghuSrinivas_2020.pdf, is a PhD thesis. This document describes that coupling the internal (spin) states of trapped ions to their shared motion is essential for applications in metrology, quantum simulation, and quantum information processing. This documents states that spin-motion coupling requires a state-dependent force and is typically performed with laser-based interactions, but that laser-based interactions can be limited by photon scattering, which is described as the leading error in the highest fidelity two-qubit gates demonstrated thus far. This document states that laser-free methods, which are not limited by photon scattering, have been proposed and demonstrated using either static magnetic field gradients, or magnetic field gradients close to the qubit frequency at gigahertz frequencies. The topic of the thesis is development of a laser-free method of spin-motion coupling that uses a radiofrequency magnetic field gradient. It is described that this technique is extended to two-qubit entangling gates that are intrinsically insensitive to qubit frequency errors, achieving a symmetric Bell-state fidelity of 0.999(1), which is described to be competitive with the highest-fidelity laser-based gates. The insensitivity to qubit frequency errors is described to enable laser-free individual addressing the employment of which is described to create anti-symmetric Bell states with fidelity 0.998(1). The described techniques are demonstrated in relation to a surface-electrode trap with integrated microwave and rf circuitry. Summary According to a first aspect of the present invention there is provided a method of operating a charged particle trap which includes a set of trap electrodes. The method comprises trapping a first charged particle at a first position, the first charged particle providing a first qubit having a first transition frequency, and trapping a second charged particle at a second position, the second charged particle having a second transition frequency. The method comprises applying a potential gradient to the first and second charged particles wherein the first and second charged particles experience first and second magnitudes of potential gradient, respectively, and the potential gradient oscillates at a given frequency and is substantially monochromatic. The method comprises, while applying the potential gradient, applying a first oscillating potential to a first electrode at a first frequency so as to apply a first oscillating electric field locally to the first charged particle, and applying a second oscillating potential to a second electrode at a second frequency so as to apply a second oscillating electric field locally to the second charged particle. The first and second transition frequencies may be different. The