Optical beam positioning and control for quantum computing

Abstract

Apparatus (20) for quantum computing includes an ion trap (24), which is configured to hold a first array of ions (40) in respective positions along an array axis (38). A radiation source (28) is configured to emit a second array of beams of coherent radiation, including first beams having respective first intensities (64) and having frequencies chosen to excite selected internal transitions of the ions and second beams having second intensities (66) at least ten times greater than any of the first intensities, and to switch respective locations of the first and second beams within the second array. Optics (82) focus the beams into the ion trap such that each beam in the second array is incident on a respective ion in the first array.

Description

[0001] OPTICAL BEAM POSITIONING AND CONTROL FOR QUANTUM COMPUTING

[0002] CROSS-REFERENCE TO RELATED APPLICATION

[0003] This application claims the benefit of U.S. Provisional Patent Application 63 / 595,349, filed November 2, 2023, which is incorporated herein by reference.

[0004] FIELD

[0005] The present disclosure pertains to the field of quantum computing, more specifically to the control and manipulation of quantum bits (qubits) in a quantum computer system.

[0006] BACKGROUND

[0007] Quantum computers apply principles of quantum physics in solving computational problems and have the potential to perform certain computations far more efficiently than existing digital computers. The basic building block of a quantum computer is the qubit. Quantum computers perform digital quantum computations using qubits and gates that operate the qubits, including single-qubit, two-qubit, and multi-qubit gates, as well as analog quantum simulations.

[0008] Trapped-ion systems, in which individual atomic ions serve as qubits, hold promise as a scalable, reliable platform for quantum computing and quantum simulations. In a trapped-ion system, the individual atomic ions are typically trapped by electromagnetic fields in an ultra-high vacuum and are cooled to their motional ground states. The internal electronic levels of the ions, as well as the motion of the ions in the trap, are controlled with high precision using lasers, microwaves, or radio-frequency (RF) fields. To perform digital quantum computations, gates are applied to the internal and motional states of the atomic ions by driving fields of the appropriate frequencies, amplitudes and duration.

[0009] In trapped-ion systems, entanglement gates are typically generated by driving the ions with electromagnetic fields that create phonon-mediated qubit-qubit interactions. Aspects of driving multi-qubit gates in a trapped ion array are described, for example, in PCT International Publication WO 2023 / 105434, whose disclosure is incorporated herein by reference.

[0010] The term “amplitude,” as used in the present description and the claims in reference to beams of radiation, refers to the complex amplitude, which includes the magnitude, frequency, and phase of the radiation.

[0011] SUMMARY

[0012] Embodiments of the present invention that are described hereinbelow provide improved systems and methods for quantum computing using arrays of trapped ions. There is therefore provided, in accordance with an embodiment of the invention, apparatus for quantum computing, including an ion trap, which is configured to hold a first array of ions in respective positions along an array axis. A radiation source is configured to emit a second array of beams of radiation, including first beams having respective first intensities and having frequencies chosen to excite selected internal transitions of the ions and second beams having second intensities at least ten times greater than any of the first intensities, and to switch respective locations of the first and second beams within the second array. Optics are configured to focus the beams into the ion trap such that each beam in the second array is incident on a respective ion in the first array.

[0013] In a disclosed embodiment, the second intensities are at least one hundred times greater than any of the first intensities.

[0014] In some embodiments, the second beams are configured to serve as optical tweezers to confine the ions on which the second beams are incident, while the ions on which the first beams are incident serve as parts of computational segments. In a disclosed embodiment, the second array of the beams is configured such that a subset of the second beams is incident on a corresponding set of ions, whereby the corresponding set is further confined by the optical tweezers.

[0015] Additionally or alternatively, the computational segments include single- or multi-qubit gates, each multi-qubit gate including a respective group of two or more of the ions within a computational segment on which the first beams are incident. In some embodiments, the first and second beams are arranged to define multiple single- or multi-qubit gates, on which the first beams are incident, wherein the multi-qubit gates are separated from one another by confined ions on which the second beams are incident. In a disclosed embodiment, the radiation source is configured to redefine the computational segments by switching the respective locations of the second beams and by applying single- or multi-qubit gate operations by the first beams, within the redefined computational segments.

[0016] In some of these embodiments, switching the respective locations of the second beams includes continuing to irradiate a first set of the ions with a first group of the second beams while initiating irradiation of a second set of the ions by a second group of the second beams. In one embodiment, the radiation source is configured to perform a mid-circuit ancilla measurement by applying one of the second beams in the second group to at least one of the ions in the first set. Additionally or alternatively, the radiation source is configured to perform a cooling operation and initialization to the at least one of the ions in this set. In another embodiment, the radiation source is configured to initiate the irradiation of the second set of the ions and terminate the irradiation of the ions in the first set by the second beams adiabatically.

[0017] In a disclosed embodiment, the apparatus includes a third array of detectors, which are configured to sense radiation emitted by detecting the state of the qubits, wherein the emitted radiation is indicative of respective states of the qubits, and wherein the optics are configured to direct the emitted radiation from the ion trap onto the detectors.

[0018] In some embodiments, the radiation source includes at least one laser, which is configured to output the coherent radiation, and a splitter, which is configured to divide the coherent radiation into the multiple beams. In one embodiment, the splitter includes a diffractive optical element (DOE). Additionally or alternatively, the splitter includes a spatial light modulator (SLM). In a disclosed embodiment, the SLM is configured to be divided into at least first and second segments, wherein each segment is configured to generate a different arrangement of the respective locations of the first and second beams within the second array, and wherein the radiation source includes an actuator, which is configured to switch the coherent radiation that is incident on the SLM between the first and second segments.

[0019] Further additionally or alternatively, the splitter includes a multi-beam acousto-optic deflector (AOD). In some embodiments, the apparatus includes a controller, which is configured to adjust the AOD to correct errors of alignment of the beams with the ions in the trap. In one embodiment, the apparatus includes a third array of detectors, which are configured to sense radiation emitted by the ions, wherein the controller is configured to detect the errors of alignment responsively to the sensed radiation.

[0020] In some embodiments, the radiation source includes at least one multi-channel acousto- optic modulator (mcAOM), which is configured to modulate respective amplitudes of at least the first beams. In one embodiment, at least one mcAOM is configured to apply different, respective frequency shifts to different ones of the first beams, optionally together with additional global or local beams, so as to drive respective Raman transitions of the ions on which the first beams are incident. Additionally or alternatively, the at least one mcAOM includes at least first and second mcAOMs, which are configured to modulate different, respective groups of the beams in the second array.

[0021] Further additionally or alternatively, the at least one mcAOM is configured to modulate the respective amplitudes of both the first and the second beams. In a disclosed embodiment, the at least one mcAOM is configured to attenuate the first beams so that the second intensities are at least ten times greater than the first intensities. In some embodiments, the first beams have respective first frequencies in a selected frequency range, and the second beams have respective second frequencies that are outside the selected frequency range. In a disclosed embodiment, the radiation source includes a dichroic beam combiner, which aligns the first beams and the second beams to form the second array.

[0022] There is also provided, in accordance with an embodiment of the invention, a method for quantum computing, which includes trapping a first array of ions in respective positions along an array axis in an ion trap. A second array of beams of coherent radiation is generated, including first beams having respective first intensities and having frequencies chosen to excite selected internal and motional transitions of the ions and second beams having second intensities at least ten times greater than any of the first intensities. The beams are focused into the ion trap such that each beam in the second array is incident on a respective ion in the first array. A first quantum computational operation is performed using a first configuration of the first and second beams in the first array. A second quantum computational operation is performed using a second configuration of the first and second beams in which respective locations of at least some of the first and second beams are switched within the second array.

[0023] There is additionally provided, in accordance with an embodiment of the invention, a quantum register, including a set of ions in respective positions along an array axis within a first array in an ion trap. The ions are configured to perform quantum operations under control of a radiation source, which emits a second array of beams of coherent radiation, including first beams having respective first intensities and having frequencies chosen to excite selected internal transitions of the ions and second beams having second intensities at least ten times greater than any of the first intensities, and to switch respective locations of the first and second beams within the second array, and optics focus the beams into the ion trap such that each beam in the second array is incident on a respective ion in the first array.

[0024] Typically, the first beams drive the ions to perform single- or multi-qubit operations.

[0025] The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Fig. 1 is a block diagram that schematically illustrates a quantum computing system, in accordance with an embodiment of the invention;

[0028] Fig. 2 is a block diagram that schematically illustrates an array of trapped ions configured as qubits in a quantum computer, in accordance with an embodiment of the invention; Figs. 3 A and 3B are schematic frontal views of an array of laser beams in two different configurations that are used to drive a number of single and multi-qubit gates in an array of trapped ions, in accordance with an embodiment of the invention;

[0029] Fig. 4 is a schematic side view of a multi-beam optical trapping and excitation subsystem used in a quantum computing system, in accordance with an embodiment of the invention;

[0030] Fig. 5 is a schematic detail view of a multi-beam generation and modulation module, in accordance with an embodiment of the invention;

[0031] Figs. 6A and 6B are schematic side and top views, respectively, of a multi-channel acoustooptic modulator used in the module of Fig. 5, in accordance with an embodiment of the invention;

[0032] Fig. 7 is a schematic detail view of a multi-beam generation and modulation module, in accordance with another embodiment of the invention; and

[0033] Fig. 8 is a schematic side view of a multi-beam optical trapping and excitation subsystem used in a quantum computing system, in accordance with an alternative embodiment of the invention.

[0034] DETAILED DESCRIPTION

[0035] OVERVIEW

[0036] Practical applications of quantum computing require large multi-qubit registers. Trapped ions are well suited for this purpose. A quantum register of hundreds or even thousands of qubits can be formed, for example, using an array of ions, which are preferably equally spaced, held along an array axis in a trap, such as a linear RF Paul trap, or multiple interconnected linear trap arrays. This sort of array is commonly referred to as an ion crystal.

[0037] There are still substantial difficulties to be overcome, however, in implementing this approach. As the number of ions in the array increases, the heating of the motional modes of the ions due to electric field noise increases rapidly. In addition, as the size of the array increases, the frequency spacing between adjacent motional modes becomes tighter. As a result, it becomes difficult to resolve and drive specific modes for high-fidelity qubit coupling.

[0038] Embodiments of the present invention that are described herein address these problems by segmenting the array into multiple groups of adjacent ions, in a reconfigurable manner, and while conserving connectivity between all ions in the crystal. Each group can then serve as a separate register for a respective single-qubit or multi-qubit gate. (It is also possible to isolate some ions to serve as ancilla-qubits for mid-circuit measurements in fault tolerant codes.) To prevent interaction among the ions in different groups, adjacent groups of ions are separated from one another by barriers comprising optically confined re-configurable set of ions. Any ions in the full crystal can be used as barrier ions in a re-configurable manner, in the course of the circuit operation, by using “optical tweezers”: high-intensity, focused optical fields, which strongly hold the barrier ions.

[0039] For sufficient confinement, the intensity of the laser beam that is used as the optical tweezers is typically at least ten times greater than that of the beams that are used to excite the ions in multi-qubit gates and may be at least one hundred times greater or more. The barrier ions thus isolate adjacent groups of ions in the array from one another by suppressing the transmission of vibrations from one group to the next. In some embodiments, for more effective separation between adjacent gates, the tweezers are applied to pairs of adjacent ions or even groups of three or more adjacent ions.

[0040] For flexible and efficient programming of a quantum computer using such a segmented ion array, it is desirable that the sizes and locations of the computational segments be variable, to enable different sorts of gate operations to be carried out in parallel, sequentially, or both. For this purpose, in embodiments of the present invention, the locations of the tweezer beams and the excitation beams may be changed, so that ions that were part of a group used in one computational segment in one stage of a computation become barrier ions in the next stage, and other ions that were barrier ions in one stage become parts of computational segments in another stage. A family of computational schemes implementing this sort of exchange between logic ions and barrier ions is described, for example, in U.S. Provisional Patent Application 63 / 490,007, filed March 14, 2023, whose disclosure is incorporated herein by reference.

[0041] In the embodiments of the present invention that are described herein, the ions in the trap are addressed and segmented using a radiation source (comprising a single laser or multiple lasers), which emits an array of beams of coherent radiation. Optics focus the beams into the ion trap. In the embodiments that are described below, each beam in the beam array is incident on a respective ion in the ion array, although alternatively, some or all of the beams may be incident on respective groups of two or more adjacent ions. The beam array includes excitation beams, having amplitudes, including magnitudes, frequencies, and phases, chosen to excite selected internal and motional transitions of the ions, and tweezer beams having intensities at least ten times greater than the intensities of any of the excitation beams. The radiation source includes optical components that enable the respective locations of some or all of the excitation beams and the tweezer beams to be exchanged within the beam array and thus to change the configuration of the groups of ions that make up the computational segments in the whole ion array dynamically. In the present embodiments, in addition to controlling the beam intensities to create the excitation beams and the tweezer beams (as well as excitation beams operating on ancilla qubits for implementing mid-circuit measurements), the radiation source comprises a modulator, which modulates the respective amplitudes of the excitation beams. (As noted earlier, the term “amplitude” refers to the complex amplitude, which includes the magnitude, frequency, and phase of the beam in question.) In some embodiments, the modulator comprises at least one multichannel acousto-optic modulator (mcAOM), which applies different, respective frequency , amplitudes, or phases to some or all the excitation beams so as to drive respective internal transitions of the ions on which the beams are incident. In the example embodiments that are described herein below, the excitation beams drive Raman transitions of the ions, i.e., transitions between internal atomic levels (with or without vibrational sidebands) of an electronic transition of the ions, and the excitation beams are therefore referred to as Raman beams. Alternatively, some of the excitation beams may be applied to induce light shifts of the internal transitions to enhance the optical control used in implementing mid-circuit measurement operations within a register. Further alternatively, the principles of the present invention may be applied in driving other internal transitions and sidebands of an array of qubits.

[0042] In some embodiments, the mcAOM modulates the tweezer beams, as well, for example to shift their frequency relative to the excitation beams. The mcAOM may apply substantial attenuation to the excitation beams to engender the required difference in intensity between the tweezer beams and excitation beams, as noted above.

[0043] In one embodiment, a spatial light modulator (SLM) may be used to split a laser beam while modulating the beam amplitudes to create high-intensity tweezer beams and low-intensity excitation beams.

[0044] In some embodiments, errors of alignment of the beams with the ions in the trap can be corrected as follows: The misalignment can be detected using an array of detectors, which sense radiation emitted by the ions in the array, as described further hereinbelow. The alignment errors can then be corrected with an SLM or an acousto-optic deflector (AOD)

[0045] The embodiments that are described herein use certain particular types and arrangements of active and passive optical components in generating the array of beams that are input to the ion trap. Alternatively, other components and configurations that can be used in generating the beam array, including the excitation and tweezer beams, and in switching the locations of the beams in the array will be apparent to those skilled in the art after reading the present description and are considered to be within the scope of the present invention. Furthermore, the sorts of beam arrays that are described herein may be applied not only to arrays of trapped ions, but also to other sorts of quantum computing media, such as arrays of neutral atoms.

[0046] SYSTEM DESCRIPTION

[0047] Fig. l is a block diagram that schematically illustrates a quantum computing system 20, in accordance with an embodiment of the invention. System 20 is presented as a non-limiting example of an application environment in which arrays of excitation beams and tweezer beams can be used.

[0048] In this example embodiment, an atom source 22 injects a flow of neutral atoms, such as atoms of calcium, into a vacuum chamber 26 at ultra-high vacuum. A radiation source 28 directs several beams of radiation into vacuum chamber 26, including a beam that is tuned to ionize the atoms injected by source 22. (In the present example, as noted above, system 20 is assumed to be based on electronic transitions, and radiation source 28 is assumed to comprise lasers emitting beams of coherent radiation; but ionization detection and tweezer beams, for example, may alternatively be carried out using an incoherent beam.) The resulting atomic ions are captured in an ion trap 24, such as a Paul trap, which uses RF and DC fields to confine the ions along a specified axis within vacuum chamber 26. A magnetic field may also be applied to ion trap 24 to separate the different spin components of the electronic states of the ions into Zeeman levels.

[0049] An electronic qubit control and computation processor 32 drives radiation source 28 to direct additional beams toward the trapped ions in order to perform quantum computational operations and then read out the computational results. Typically, the results are read out by tuning a laser beam to an absorption line of one of the qubit states and then measuring the resulting fluorescent emission using an optical detector 30. Each step in the present example is defined by appropriate program code, using multiple gates in parallel, and may include redefining the groups of ions making up the registers on which the gates operate at each of the steps. Alternatively, the groups of ions may be defined and applied in quantum simulations.

[0050] Fig. 2 is a block diagram that schematically illustrates an array of trapped ions 40 configured as qubits in a quantum computer, such as in system 20, in accordance with an embodiment of the invention. Radiation source 28 (Fig. 1), provides several different laser beam inputs to ion trap 24 for different purposes. Ionization lasers 42 ionize the atoms output by atom source 22 to create ions 40, which are held in the trap. Additional cooling lasers 44 and repumping lasers 53 cool the ions to their electronic and motional ground states, by pumping appropriate state transitions of the ions while detuning the laser frequencies to engender mechanisms of Doppler cooling, sideband cooling, polarization gradient cooling, cooling by electrically-induced transparency (EIT), and / or other methods of cooling that are known in the art. Repumping lasers 53 are used to facilitate return of ions caught in a metastable state back to a state in which they can interact with the primary laser system.

[0051] Cooled ions 40 are held in a linear array along an axis 38 by the electromagnetic fields within trap 24. Coulomb repulsion between ions 40 and trapping fields determine the equilibrium distance between the ions, as well as the phonon frequencies v of the normal vibrational modes of motion of ions 40 in the array, including both transverse and longitudinal modes of vibration. These normal vibrational modes give rise to vibrational sidebands of the optical transition frequencies between the states of ions 40, which are used in quantum computations.

[0052] For the purposes of defining and operating multi-qubit gates (including two, three, or larger numbers of qubits), as well as single-qubit gates, radiation source 28 (Fig. 1) comprises an excitation source 46, comprising one or more lasers, which directs beams of radiation at multiple different frequencies to impinge on ions 40. The beams may all be generated by the same laser, with appropriate amplitude, frequency, and phase modulation, or by multiple different lasers. Among these beams are Raman beams 48 and tweezer beams 50. Tweezer beams 50 are tuned to confine selected ions 40 using high-intensity optical fields and thus define multi-qubit registers between the confined ions. Raman beams 48, also referred to as excitation beams, are tuned to coherently excite and manipulate internal and motional transitions of individual ions 40 within these multi-qubit registers to drive gate operations for carrying out quantum computations. In addition, global Raman beams 49 propagate at an angle relative to Raman beams 48 for performing multiqubits gate operations. For example, a global Raman beam can counter-propagate relative to the individual Raman beams 48 to provide maximal photon momentum transfer to the ions in multiqubits gates.

[0053] Finally, readout beams 52 and shelving beams 51 are tuned to excite selectively the internal transitions of ions 40 for the purpose of reading out states of the gates. Shelving beams 51 selectively drive ions into a long-lived metastable state. When an ion is shelved in this metastable state, it will not emit photons in response to readout beams 52 during the readout process. Readout beams 52 cause ions 40 to fluoresce, with an intensity depending on the states of the corresponding gates. The resulting fluorescent emission is measured using optical detector 30, and the result of the computation is received by qubit control and computation processor 32 (Fig. 1). As part of the present scheme, an operational sequence of light pulses can also be used to light shift, shelve, store, and protect data qubits from error due to photon scattering during the measurement sequence. To operate multi-qubit gates in embodiments of the present invention, various coherent manipulation techniques can be used, such as quadrupole optical transitions used for optical qubits or Raman beams transitions used for hyperfine or Zeeman qubits. In the present example, Raman beams 48 coherently irradiate each group of ions 40 with a set of excitation frequencies oo±omcentered around a selected internal transition oo of the ions. Beams 48 are modulated, for example by a suitable mcAOM, as described below, to coherently include frequency components in multiple sidebands omof the internal transition frequency oo. For example, beams 48 output by a laser operating at around 400 nm may be modulated by the mcAOM to drive the ions with frequencies on sidebands of the Sl / 2 Zeeman split transition of calcium 40. The internal qubit level frequency, coo, is determined by an externally applied magnetic field B.

[0054] Raman beams 48 and 49 irradiate each of ions 40 with the selected frequency components at the optimal amplitudes for a gate time T to drive the multi-qubit gate from its initial state to an entangled target state. In one embodiment of the present invention, individual modulation of Raman beams 48 makes it possible to drive each ion 40 with its own vector of amplitudes, which typically differs from the amplitudes applied to the other ions. Other embodiments can also use global Raman beams or semi-global Raman beams (within each segment) to operate the needed entangling gates within each register segment or global beams enabling local operations by individually operated light shift.

[0055] After completion of a computational cycle, shelving beams 51 and readout beams 52 are directed toward ions 40 to read the states of the qubit registers defined by tweezer beams 50. Readout beams 52 are tuned to an absorption line of one of the states of the ions in the register. As mentioned above, absorption of the laser radiation by the ions in the appropriate state leads to fluorescence, which is measured by optical detector 30 (Fig. 1). Detector 30 measures the intensity of the fluorescent emissions and thus detects the final state of the gate.

[0056] Processor 32 typically comprises a general-purpose computer, with suitable interfaces to the other components of system 20. Processor 32 is driven by software to carry out the functions and computations that are described herein. The software may be stored on tangible, non- transitory computer-readable media, such as optical, magnetic, or electronic storage media.

[0057] ARRAYS OF RAMAN AND TWEEZER BEAMS

[0058] Figs. 3A and 3B are schematic frontal views of an array 60 of laser beams 62 in two different configurations, which are used to drive a number of multi-qubit gates in an array of trapped ions 40 in accordance with an embodiment of the invention. Beams 62 are numbered ordinally in the figures from left to right. Optics in system 20 (as shown in the figures that follow) focus each beam 62 onto a respective ion 40 in trap 24. Although for the sake of simplicity only twenty-five beams are shown in Figs. 3A / B, in practice radiation source 28 may output far larger numbers of beams simultaneously.

[0059] Beams 62 include Raman beams with low intensities 64 and tweezer beams with higher intensities 66. As noted earlier, intensities 66 are typically at least ten times greater than intensities 64 and may be at least one hundred times greater or even more than a thousand times greater. In an example implementation, each Raman beam delivers 50 pW of optical power to the vicinity of a respective ion in trap 24, while each tweezer beam delivers 300 mW of optical power. As described further hereinbelow, embodiments of the present invention use an mcAOM, together with one or more beam splitting components, to achieve this large dynamic range while still maintaining precise control of all the beams.

[0060] The ions that are excited by each group of Raman beams with low intensities 64 define a multi-qubit register 68, which can be used in single-qubit and multi-qubit gate operations. Registers 68 comprise groups of qubits that can be manipulated together to perform complex quantum computations. Conversely, beams 62 with high intensities 66 define barriers 70 between registers 68. These barriers 70 serve to limit heating rates within the ionic crystal, reduce spectral crowding due to the vibrational modes in crystal, and isolate mechanical vibrations in the registers from one another, ensuring that computations performed in one register do not interfere with computations in the neighboring registers. In the configurations shown in Figs. 3 A and 3B, one or more tweezer beams with high intensities 66 are incident on corresponding single ions or pairs or groups of the ions, whereby the ions are further locally confined by the optical tweezers. Barriers 70 can be created as shown in these figures by one or more tweezed ions. Alternatively, barriers can be created by few tweezed ions plus one or more un-tweezed ions between some of the tweezed ions, which accordingly weaken the overall mechanical movement crosstalk mediated by the effective Colomb force between adjacent segments, due to the extended separation.

[0061] Different registers 68 may comprise the same number of mutually adjacent ion qubits in each register or any different numbers of ions, including registers containing only one or two qubits. In the examples shown in Figs. 3 A and 3B, the tweezer beams are used to define the sizes of the multi-qubit registers. The Raman beams are arranged to define multiple single-qubit and multi-qubit gates within these registers, on which beams with low intensities 64 are incident, as well as to mitigate residual computational crosstalk between registers, and which are separated from one another by trapped ions on which beams with high intensities 66 are incident. In the transition between the configurations of Figs. 3 A and 3B, the sizes and locations of registers 68 are redefined at any needed time by exchanging some or all of the locations of low-intensity beams and high-intensity beams in array 60.

[0062] For example, the transition between the configurations of Figs. 3A and 3B may be performed in sequence such that the initial locations of the tweezer beams are maintained, and a set of new tweezer beams are applied in addition to the existing beams. In this new configuration both old and new locations of tweezer beams are applied, to create a new (second) array of beams. The new tweezer beams create a further set of confined ions within each register 68, to enable further quantum operations, such as single operations of mid-circuit ancilla measurements performed within the second array. In addition, cooling operations can be performed by the redefined first beams within the second array and / or other cooling beams.

[0063] In this example the further step in the sequence of switching the respective locations of the second beams, the configuration of the multi-qubit ion segments is redefined by switching the initial respective locations of the second beams to the new beam location as depicted in Fig 3B. This transition may be performed in a controlled manner, such as phasing out the set of first tweezing beams and phasing in the set of a second tweezing beams in adiabatic transition, or by faster operation techniques.

[0064] OPTICAL SUBSYSTEM

[0065] Fig. 4 is a schematic side view of a multi-beam optical trapping and excitation subsystem 71 used in system 20 (Fig. 1), in accordance with an embodiment of the invention. Subsystem 71 forms and conveys beams 62 (as shown in Figs. 3 A / B) from excitation source 46 to ion trap 24.

[0066] Optical subsystem 71 comprises a splitter 72, which splits coherent radiation output by one or more lasers in excitation source 46 into multiple beams. Splitter 72 may comprise, for example, a diffractive optical element (DOE), a spatial light modulator (SLM), or a multi-beam deflector, such as an acousto-optic deflector (AOD) or micromirror array, or multiple single-channel deflectors. Splitter 72 divides the coherent radiation into multiple beams at different angles. Depending on the type of splitter, the beams may have equal (or roughly equal) intensities, or they may have substantially different intensities, depending on whether they are to serve as Raman beams or tweezer beams during a given time interval.

[0067] Following splitter 72, a lens 74 directs the array of beams onto a multi-channel acousto- optic modulator (mcAOM) 76, which modulates the amplitude, frequency, and phase of each of the beams. Specifically, mcAOM 76 applies different, respective frequency and phase shifts to different beams so as to drive respective Raman transitions of the ions on which the first beams are incident. Lens 74 may advantageously be configured as a Fourier transform lens, with splitter 72 at its front focal plane and mcAOM 76 at its rear focal plane. Lens 74 thus creates a spatial Fourier transform of the beams, in which the angle of deflection of each beam output by splitter 72 is converted to a transverse location on mcAOM 76 in an array of equally spaced, collimated beams.

[0068] The size of each collimated beam is scaled such that the downstream optics will create a focused beam on each ion, smaller than the distance between the ions. The mcAOM electrode design is such that its acoustic column is wider than each laser beam in its specific channel. This design enables adjusting the laser-beam location within the acoustic mcAOM column, for optimizing laser / ion interaction.

[0069] In addition, the transition between tweezer configurations may be further controlled by the mcAOM. For example, when a DOE is used to split the laser beam, active tweezer beam intensities are phased-out while the new tweezer beams are phased-in adiabatically. In all cases, the mcAOM enables fast and slow control without delay times or flickering due to reloading a new pattern into the mcAOM.

[0070] In the embodiment shown in Fig. 4, the same laser in excitation source 46 generates both the Raman beams and the tweezer beams, and mcAOM 76 modulates the amplitudes of both the Raman beams and the tweezer beams. (In other embodiments, as described hereinbelow, different lasers may be used to generate the Raman and tweezer beams.) When splitter 72 comprises a DOE or multi-beam deflector, the beams that are split out by the splitter may all have high intensities, sufficient so that any of beams 62 may be incident on ions 40 with high intensities 66 (as shown in Figs. 3A / B). Therefore, mcAOM 76 is controlled to apply substantial attenuation, by a factor of ten or even one hundred or more, to the high-intensity beams from splitter 72 that are to serve as Raman beams. Even when the splitter generates Raman and tweezer beams with different intensities, mcAOM 76 may apply additional attenuation to the Raman beams.

[0071] Alternatively, when splitter 72 comprises a multi-beam acousto-optic deflector (AOD), controller 32 (Fig. 1) may generate drive signals to adjust the respective intensities of the individual beams output by the AOD. This approach enables rapid switching between different configurations of tweezer and Raman beams, and the beams output by the AOD will have different frequency shifts, depending on the beam angles. A beam detector and feedback loop (not shown) may be used to control the beam intensities. Additionally or alternatively, the drive signals that are applied to mcAOM 76 can be adjusted to compensate for the intensity and frequency deviations of the beams that are output by the AOD.

[0072] In other embodiments, in which splitter 72 comprises an SLM, the SLM may be driven to modulate the intensities of the beams, so that the beams that are intended to serve as tweezer beams in each computing cycle have higher intensity than the beams intended to serve as Raman beams. For example, the SLM may comprise a liquid crystal on silicon (LCoS) device, such as LCoS devices made by HOLOEYE Photonics AG (Berlin, Germany). Controller 32 (Fig. 1) drives the SLM to generate a diffraction pattern, which splits the incident laser beam into multiple beams having the desired intensity distribution. The SLM may also be driven to correct for aberrations in the optics of subsystem 71.

[0073] SLM devices that are commercially available, such as the above-mentioned HOLOEYE devices, have frame rates of 100 Hz or less, meaning that switching between different configurations of the Raman and tweezer beams will typically take at least 10 ms. In one embodiment, to switch more rapidly between different configurations of the Raman and tweezer beams (for example, between the configurations of Figs. 3 A and 3B), the SLM is divided into at least two segments. Each segment of the SLM is driven to generate a different arrangement of the respective locations of the tweezer beams and Raman beams within array 60. A fast actuator, such as a rotating mirror or AOD, switches the beam of coherent radiation that is output by excitation source 46 between the first and second segments of the SLM. This arrangement makes it possible to shift beam configurations rapidly, for efficient execution of a sequence of quantum operations with high fidelity, notwithstanding the slow switching response of the SLM itself. Switching will take a few milliseconds when a rotating mirror is used or even less when an AOD is used.

[0074] When different segments of the SLM are used to generate different beam patterns, the angles of incidence of the beams on mcAOM 76 will vary. This variation can give rise to a mismatch between the angle of incidence and the Bragg angle of the acoustic grating that is formed in the mcAOM (as illustrated in Fig. 6A). To mitigate this problem, the drive frequencies that are applied to the mcAOM can be adjusted to match the different angles of incidences.

[0075] In addition to the amplitude modulation functions described above, mcAOM 76 may apply a frequency shift to the tweezer beams, relative to the Raman beams (or vice versa), so that the frequency range of the tweezer beams is outside the range of frequencies of the Raman beams. This frequency separation is useful in reducing the effect of crosstalk between the beams in the mcAOM. For this purpose, a detuning of about 80 MHz between the tweezer beams and the Raman beams should be sufficient. For example, channels of mcAOM 76 that are used in modulating the tweezer beams may be driven at a frequency of about 260 MHz, while the channels that are used in modulating the Raman beams are driven at about 160 MHz. Although this latter frequency may result in less efficient diffraction in the mcAOM, the loss of efficiency is insignificant considering the much lower intensity of the Raman beams relative to the tweezer beams.

[0076] Following mcAOM 76, a telescope 78 directs the modulated beams toward ion trap 24. Telescope 78 also enables adjustment of the beam spacing to match precisely the spacing between ions 40 in the trap. Additionally or alternatively, the drive signals applied to mcAOM 76 may be adjusted to compensate for deviations in shapes and focus of beams 62 on the respective ions 40.

[0077] Following telescope 78, the beams pass through a dichroic beamsplitter 80 and are then focused into ion trap 24 by objective optics 82. Objective optics 82 reduce the spot sizes of beams 62 that are incident on ions 40 to near the diffraction limit, i.e., approximately 1 pm or less for obj ective optics with NA=0.5, and reduce the spacing between the beams to the separation between ions 40 along axis 38, typically a few microns.

[0078] To read out interim and final states of registers 68, excitation source 46 and mcAOM 76 are adjusted to direct readout beams toward the qubits of interest in the registers. Absorption of photons in the readout beams causes ions 40 to emit fluorescent radiation that is indicative of respective states of the quantum gates. Objective optics 82 collect the fluorescent radiation that is emitted by ions 40 in the ion trap 24 and direct the emitted radiation toward dichroic beamsplitter 80, which reflects the emitted radiation onto a detector array 84. Detector array 84 measures the resulting fluorescent emission intensity and reads out the result to controller 32.

[0079] In some embodiments, controller 32 uses the signals output by detector array 84 in detecting errors of alignment between beams 62 and ions 40. For example, if the level of the fluorescent emission received from a given ion is lower than that of other ions that have been excited in a similar fashion, it can be concluded that the beam incident on the ion is misaligned. These alignment errors may include deviations in the locations of beams 62 relative to the corresponding ions 40, as well as defocus and other aberrations. When splitter 72 comprises an SLM or AOD, the drive signals applied to the splitter may be adjusted to correct errors of alignment, as well as certain errors of beam shape and focus.

[0080] Additionally or alternatively, controller 32 may modify the drive signals applied to mcAOM 76 in order to correct errors due to defocus and other aberrations.

[0081] Other specific excitation and measurement techniques may be applied in detecting errors of alignment. For example, the Rabi frequency of the transitions of ions 40 that are induced by the Raman beams may be measured, and ions exhibiting a low Rabi frequency may be identified as misaligned. As another example, the light shift produced by the spatial light field in a narrowband quadrupole transition of each of the ions in the array may be measured by scanning the laser light frequency that excites this transition. As the light shift is proportional to the intensity of the beam that is incident on each item, it can be used as an indicator of misalignment.

[0082] MULTI-CHANNEL ACOUSTO-OPTIC MODULATOR

[0083] Fig. 5 is a schematic detail view of a multi-beam generation and modulation module 87 in optical subsystem 71, in accordance with an embodiment of the invention. Excitation source 46 outputs a source beam 88, which is then divided into multiple input beams 90 by splitter 72. Fourier transform lens 74 directs beams 90 into an acousto-optic crystal 86 in mcAOM 76. As input beams 90 pass through acousto-optic crystal 86, they are diffracted by acoustic waves in the crystal, resulting in a change in the direction of the diffracted beams. These diffracted beams become an array of output beams 92. This diffraction process modulates output beams 92 so that each beam has its own intensity, frequency, and phase.

[0084] Figs. 6A and 6B are schematic side and top views, respectively, of mcAOM 76, in accordance with an embodiment of the invention. An array of piezoelectric transducers 94 is fixed along a face of acousto-optic crystal 86 that is transverse to the direction of beams 90. Electrical drive signals applied to piezoelectric transducers 94 cause the transducers to generate acoustic waves 95 that propagate through crystal 86 with magnitude, frequency, and phase that are determined by the drive signals. Acoustic waves 95 create a periodic variation in the refractive index of acousto-optic crystal 86, effectively forming a dynamic diffraction grating. As the input beams 90 pass through acousto-optic crystal 86, they are diffracted by this dynamic diffraction grating, generating an array of modulated output beams 92. Each output beam 92 in the array has distinct characteristics, such as intensity, phase and frequency, which are determined by the modulation applied by acousto-optic crystal 86.

[0085] Acoustic waves 95 are terminated by an absorber 96 on the opposite side of acousto-optic crystal 86. Additionally, any residual energy of input beams 90 that passes directly through the mcAOM is blocked so as not to impinge on the ions.

[0086] OPTICAL SUBSYSTEM USING MULTIPLE ACOUSTO-OPTIC MODULATORS

[0087] Fig. 7 is a schematic detail view of a multi-beam generation and modulation module 99, in accordance with another embodiment of the invention. Module 99 may be incorporated into optical subsystem 91. This module differs from the preceding embodiments in that it comprises two mcAOMs 76a and 76b, which modulate different, respective groups of input beams 90. Because the mcAOMs have limited resolution, each mcAOM can be used to modulate only a certain number of beams. Using two mcAOMs 76a and 76b, the number of modulated output beams 92 generated by module 99 can be doubled. Alternatively, a larger number of mcAOMs (with appropriate splitting and combining optics) can be used to generate an even larger number of modulated output beams.

[0088] A splitter 97 divides input beams 90 between mcAOMs 76a and 76b, which are similar in design and operation to mcAOM 76 as described above. The two mcAOMs 76a and 76b operate in parallel, each modulating a subset of input beams 90 to generate a respective array of output beams 92. After the modulation process, the two arrays of output beams 92 are recombined into a single array by a beam combiner 98. This single array of output beams 92 is then directed towards the ion trap 24 for application to ions 40.

[0089] SEPARATION OF TWEEZER AND RAMAN BEAM SOURCES

[0090] Fig. 8 is a schematic side view of a multi-beam optical trapping and excitation subsystem 100, in accordance with an alternative embodiment of the invention. Subsystem 100 may be used in quantum computing system 20 in place of subsystem 71 (Fig. 4). Subsystem 100 differs from subsystem 71 in that in subsystem 100, Raman beams 48 and tweezer beams 50 are generated by separate, respective laser sources. This separation is helpful in making more efficient use of laser power, since Raman beams 48 have much lower intensity than tweezer beams 50, and the tweezer beams for each ion need not coherently match the other tweezer beams or the Raman beams, as explained above. Splitter 72 and / or mcAOM 76 may be driven to correct for any errors of alignment between the Raman beams and the tweezer beams, to make a single, uniformly spaced array of beams for input to ion trap 24.

[0091] Tweezers beams 50 can be offset in wavelength relative to Raman beams 48. Thus, the two sets of beams can be combined using a dichroic beam combiner 102, which aligns the tweezer and Raman beams to form array 60 for input to ion trap 24.

[0092] Alternatively or additionally, Raman beams 48 may have a polarization orthogonal to tweezer beams 50. In this case, dichroic beam combiner 102 may be replaced by a polarizing beamsplitter.

[0093] The embodiments described above are cited by way of example, and the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

Claims

CLAIMS1. Apparatus for quantum computing, comprising: an ion trap, which is configured to hold a first array of ions in respective positions along an array axis; a radiation source, which is configured to emit a second array of beams of coherent radiation, including first beams having respective first intensities and having frequencies chosen to excite selected internal transitions of the ions and second beams having second intensities at least ten times greater than any of the first intensities, and to switch respective locations of the first and second beams within the second array; and optics configured to focus the beams into the ion trap such that each beam in the second array is incident on a respective ion in the first array.

2. The apparatus according to claim 1, wherein the second intensities are at least one hundred times greater than any of the first intensities.

3. The apparatus according to claim 1, wherein the second beams are configured to serve as optical tweezers to confine the ions on which the second beams are incident, while the ions on which the first beams are incident serve as parts of computational segments.

4. The apparatus according to claim 3, wherein the second array of the beams is configured such that a subset of the second beams is incident on a corresponding set of ions, whereby the corresponding set is further confined by the optical tweezers.

5. The apparatus according to claim 3, wherein the computational segments comprise single- or multi-qubit gates, each multi-qubit gate comprising a respective group of two or more of the ions within a computational segment on which the first beams are incident.

6. The apparatus according to claim 5, wherein the first and second beams are arranged to define multiple single- or multi-qubit gates, on which the first beams are incident, wherein the multi-qubit gates are separated from one another by confined ions on which the second beams are incident.

7. The apparatus according to claim 6, wherein the radiation source is configured to redefine the computational segments by switching the respective locations of the second beams and by applying single- or multi-qubit gate operations by the first beams, within the redefined computational segments.

8. The apparatus according to claim 7, wherein switching the respective locations of the second beams comprises continuing to irradiate a first set of the ions with a first group of the second beams while initiating irradiation of a second set of the ions by a second group of the second beams.

9. The apparatus according to claim 8, wherein the radiation source is configured to perform a mid-circuit ancilla measurement by applying at least one of the second beams in the second group to at least one of the ions in the first set.

10. The apparatus according to claim 9, wherein the radiation source is configured to perform a cooling operation on and initialization of the at least one of the ions in the first set after completing the mid-circuit ancilla measurement.

11. The apparatus according to claim 8, wherein the radiation source is configured to initiate the irradiation of the second set of the ions and terminate the irradiation of the ions in the first set by the second beams adiabatically.

12. The apparatus according to claim 3, and comprising a third array of detectors, which are configured to sense radiation emitted by the qubits, wherein the emitted radiation is indicative of respective quantum state of the qubits, and wherein the optics are configured to direct the emitted radiation from the ion trap onto the detectors.

13. The apparatus according to any of claims 1-12, wherein the radiation source comprises at least one laser, which is configured to output the coherent radiation, and a splitter, which is configured to divide the coherent radiation into the multiple beams.

14. The apparatus according to claim 13, wherein the splitter comprises a diffractive optical element (DOE).

15. The apparatus according to claim 13, wherein the splitter comprises a spatial light modulator (SLM).

16. The apparatus according to claim 15, wherein the SLM is configured to be divided into at least first and second segments, wherein each segment is configured to generate a different arrangement of the respective locations of the first and second beams within the second array, and wherein the radiation source comprises an actuator, which is configured to switch the coherent radiation that is incident on the SLM between the first and second segments.

17. The apparatus according to claim 13, wherein the splitter comprises a multi-beam acoustooptic deflector (AOD).

18. The apparatus according to claim 17, and comprising a controller, which is configured to adjust the AOD to correct errors of alignment of the beams with the ions in the trap.

19. The apparatus according to claim 18, and comprising a third array of detectors, which are configured to sense radiation emitted by the ions, wherein the controller is configured to detect the errors of alignment responsively to the sensed radiation.

20. The apparatus according to any of claims 1-12, wherein the radiation source comprises at least one multi-channel acousto-optic modulator (mcAOM), which is configured to modulate respective amplitudes of at least the first beams.

21. The apparatus according to claim 20, wherein the at least one mcAOM is configured to apply different, respective frequency shifts to different ones of the first beams so as to drive respective Raman transitions of the ions on which the first beams are incident.

22. The apparatus according to claim 20, wherein the at least one mcAOM comprises at least first and second mcAOMs, which are configured to modulate different, respective groups of the beams in the second array.

23. The apparatus according to claim 20, wherein the at least one mcAOM is configured to modulate the respective amplitudes of both the first and the second beams.

24. The apparatus according to claim 23, wherein the at least one mcAOM is configured to attenuate the first beams so that the second intensities are at least ten times greater than the first intensities.

25. The apparatus according to any of claims 1-12, wherein the first beams have respective first frequencies in a selected frequency range, and the second beams have respective second frequencies that are outside the selected frequency range.

26. The apparatus according to claim 25, wherein the radiation source comprises a dichroic beam combiner, which aligns the first beams and the second beams to form the second array.

27. The apparatus according to any of claims 1-12, wherein the first beams have a first polarization, and the second beams have a second polarization orthogonal to the first polarization, and wherein the radiation source comprises a polarizing beam combiner, which aligns the first beams and the second beams to form the second array.

28. A method for quantum computing, comprising: trapping a first array of ions in respective positions along an array axis in an ion trap; generating a second array of beams of coherent radiation, including first beams having respective first intensities and having frequencies chosen to excite selected internal and motional transitions of the ions and second beams having second intensities at least ten times greater than any of the first intensities; focusing the beams into the ion trap such that each beam in the second array is incident on a respective ion in the first array; performing a first quantum computational operation using a first configuration of the first and second beams in the first array; and performing a second quantum computational operation using a second configuration of the first and second beams in which respective locations of at least some of the first and second beams are switched within the second array.

29. The method according to claim 28, wherein the second intensities are at least one hundred times greater than any of the first intensities.

30. The method according to claim 28, wherein the second beams are configured to serve as optical tweezers to further confine the ions on which the second beams are incident, while the ions on which the first beams are incident serve as parts of computational segments.

31. The method according to claim 30, wherein the computational segments are driven to carry out single- or multi-qubit gate operations, each multi-qubit gate operation carried out over a respective group of two or more of the ions within a computational segment on which the first beams are incident.

32. The method according to claim 31 , wherein performing the second quantum computational operation comprises redefining the computational segments by switching the respective locations of the first and second beams within the second array and applying single- or multi-qubit gate operations by the first beams, within the redefined computational segments.

33. The method according to claim 32, wherein switching the respective locations of the second beams comprises continuing to irradiate a first set of the ions with a first group of the second beams while initiating irradiation of a second set of the ions by a second group of the second beams.

34. The method according to claim 33, and comprising applying at least one of the second beams in the second group to at least one of the ions in the first set to perform a mid-circuit ancilla measurement.

35. The method according to claim 34, and comprising applying at least one beam to cool and initialize the at least one of the ions in the first set after completing the mid-circuit ancilla measurement.

36. The method according to claim 33, wherein switching the respective locations comprises initiating the irradiation of the second set of the ions and terminating the irradiation of the ions in the first set by the second beams adiabatically.

37. The method according to claim 30, wherein focusing the beams into the ion trap comprises applying optics to focus the beams onto the ions, and wherein the method comprises sensing radiation emitted by the qubits by directing the emitted radiation through the optics onto an array of detectors, wherein the emitted radiation is indicative of respective states of the qubits.

38. The method according to claim 28, wherein generating the second array of beams comprises splitting coherent radiation output by at least one laser into the multiple beams.

39. The method according to claim 38, wherein splitting the coherent radiation comprises applying a diffractive optical element (DOE) to the coherent radiation output by the at least one laser.

40. The method according to claim 38, wherein splitting the coherent radiation comprises applying a spatial light modulator (SLM) to the coherent radiation output by the at least one laser.

41. The method according to claim 40, wherein applying the SLM comprises dividing the SLM into at least first and second segments, wherein each segment is configured to generate a different arrangement of the respective locations of the first and second beams within the second array, and switching the coherent radiation that is incident on the SLM between the first and second segments.

42. The method according to claim 38, wherein splitting the coherent radiation comprises applying a multi-beam acousto-optic deflector (AOD) to the coherent radiation output by at least one laser.

43. The method according to claim 42, wherein modulating the respective amplitudes comprises controlling the AOD to correct errors of alignment of the beams with the ions in the trap.

44. The method according to claim 43, and comprising sensing radiation emitted by the ions, wherein controlling the AOD comprises detecting the errors of alignment responsively to the sensed radiation.

45. The method according to any of claims 28-44, wherein generating the second array of beams comprises modulating respective amplitudes of at least the first beams using at least one multi-channel acousto-optic modulator (mcAOM).

46. The method according to claim 45, wherein modulating the respective amplitudes comprises applying different, respective frequency shifts to different ones of the first beams so as to drive respective Raman transitions of the ions on which the first beams are incident.

47. The method according to claim 45, wherein modulating the respective amplitudes comprises applying at least first and second mcAOMs to modulate different, respective groups of the beams in the second array.

48. The method according to claim 45, and comprising modulating the respective amplitudes of the second beams using the at least one mcAOM.

49. The method according to claim 48, wherein modulating the respective amplitudes comprises attenuating the first beams so that the second intensities are at least ten times greater than the first intensities.

50. The method according to any of claims 28-44, wherein the first beams have respective first frequencies in a selected frequency range, and the second beams have respective second frequencies that are outside the selected frequency range.

51. The method according to claim 50, and comprising aligning the first beams and the second beams to form the second array using a dichroic beam combiner.

52. The method according to claim 28, wherein the first beams have a first polarization, and the second beams have a second polarization orthogonal to the first polarization, and wherein the method comprises aligning the first beams and the second beams to form the second array using a polarizing beam combiner.

53. A quantum register, comprising a set of ions in respective positions along an array axis within a first array in an ion trap, wherein the ions are configured to perform quantum operations under control of a radiation source, which emits a second array of beams of coherent radiation, including first beams havingrespective first intensities and having frequencies chosen to excite selected internal transitions of the ions and second beams having second intensities at least ten times greater than any of the first intensities, and to switch respective locations of the first and second beams within the second array, and optics focus the beams into the ion trap such that each beam in the second array is incident on a respective ion in the first array.

54. The quantum register according to claim 53, wherein the first beams drive the ions to perform single- or multi-qubit operations.

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