Patent Publication Number: US-2023163859-A1

Title: Optical routing network-based quantum array control

Description:
BACKGROUND 
     Whereas classical digital computers manipulate units, e.g., bits, of classical information, quantum computers manipulate units, e.g., qubits, of quantum information. Both classical bits and quantum qubits can be represented physically using two-state carriers. Examples of two-state quantum carriers include an electron that can transition between a spin up and a spin down state, and an electron in an atom that can transition between hyperfine ground states. A classical two-state carrier assumes one of the two states (e.g., respectively representing a logic-0 and a logic-1) at any given time; a quantum two-state carrier can be in a coherent superposition of both states simultaneously. 
     Qubits can be represented by a variety of quantum-state carriers, including superconducting circuits, color centers in a solid-state host (e.g., nitrogen-vacuum centers in diamond), quantum dots, neutral atoms, ions, and molecules. Ions and neutral atoms of a given isotope are, by their nature, all identical and, so, have an advantage over manufactured alternatives. However, while closely spaced ions generally interact with their neighbors due to Coulomb forces, interactions between closely packed neutral atoms can be switched on and off, e.g., by making them enter and exit Rydberg states. Accordingly, cold atoms provide a favorable technology for implementing dense qubit arrays. 
     The quantum state of atoms can be controlled by electromagnetic radiation (EMR), for example, near infrared and visible light. Control of selected atoms within a dense atom array is possible by focusing laser light to a spot with radius smaller than the pitch, e.g., one to three microns, for atoms in an array. There are several technologies that have been used for steering and focusing laser light for quantum-state control. However, it has proven difficult to scale these technologies economically to keep up with demand for larger quantum arrays. What is needed is a more economically scalable approach to address atoms in a quantum array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic illustration of a quantum computing system that routes electromagnetic waves to quantum-state carriers using a multi-stage routing network of Mach-Zehnder interferometers (MZIs). 
         FIG.  2    is a network diagram of the MZI routing network of  FIG.  1   . 
         FIG.  3    is a diagram showing connections between a last stage of the multi-stage routing network of  FIG.  1    and an array of gradient couplers of the quantum computing system of  FIG.  1   . 
         FIG.  4    is a flow chart of an optical routing quantum-array control process implementable on the quantum computing system of  FIG.  1    and on other systems. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides for control of a quantum array over an optical routing network. The invention applies to arrays of quantum-state carriers (QSCs) with optically controllable quantum states such as neutral atoms, molecules, ions, quantum dots in a semiconductor host, and color centers in a solid-state host (e.g., nitrogen-vacuum centers in diamond). The optical routing network can be used to route electromagnetic (EM) waves to a selected array site to control the quantum state of the resident QSC (which, in turn, may affect one or more quantum states of respective neighboring QSCs). 
     Compared to steering-based approaches (e.g., using micro-mechanical machine (MEMS) mirrors, the present invention provides for cost-effective, accurate, and scalable control of quantum array states. These advantages arise in part due to the availability of optical routing networks in photonic integrated circuits (PICs), which leverage much of the technology and economies of scale developed over decades for electronic integrated circuits (ICs). Depending on the embodiment, the base material for the PICs can include silicon and silicon dioxide, lithium niobate (LiNbO3), gallium arsenide (GaAs), indium phosphide (InP), and others. 
     For example, a quantum system  100 , represented in  FIG.  1   , includes a quantum array  102 , an illumination (e.g., laser) system  104 , and a photonic integrated circuit (PIC)  106  for routing EM waves generated by illumination system  104  to respective QSCs  108  in array  102 . For expository purposes, quantum array  102  is depicted in  FIG.  1    as a four-by-four array of QSCs; in practice, much larger arrays are commonly used. 
     An optical coupling  110  (e.g., including one or more polarization-maintaining single-mode (PM/SM) optical fibers) guides EM waves output from illumination system  104  to PIC  106 . A Mach-Zehnder interferometer (MZI) routing network  112  on PIC  106  routes the received EM waves to respective EM radiators, in this case, grating couplers  114  of a radiator array  116 . Each grating coupler  114  coverts in coming EM waves into outgoing electromagnetic radiation (EMR) beams  118 , e.g., light beams, that propagate normal to the grating surfaces and are imaged onto the array of QSCs to (at least conditionally) alter QSC quantum states. An array coupling  120 , e.g., including a collimating lens  122  and a focusing lens  124 , focuses the EMR respective QSC sites of quantum array  102 . A network controller  130  provides for (e.g., electronic) control of EM-wave routing by MZI network routing network  112 . 
     As noted above, routing network  112  is a network of MZIs. A representative MZI  151  is shown in the detail of  FIG.  1   . MZI  151  includes an input  152  for receiving an EM wave, an optical coupling  154 , branches  156  and  158 , an optical coupling serving as a combiner  158  for interfering EM waves from branches  156  and  158 , and two outputs  160  and  162  for the interference results. Optical coupling distributes an incoming EM wave evenly between branches  156  and  158 , while the distribution at the outputs is controllable, as explained below. 
     Upper branch  156  includes a variable delay element  170 . Network controller  120  is coupled to delay element  170  so that it can control (e.g., electronically) the phase difference ϕ between branches  156  and  158  for MZI  151 , just as it controls phase differentials for all MZIs of routing system  112 . The EM waves in branches  156  and  158   interfere at optical coupling  160 . The phase difference determines the power distribution ϕ at output  162  which gets cos 2 (ϕ) and output  164  which gets sin 2 (ϕ). Thus, in the event that the EM waves in the branches are in phase, then ϕ=0 and the lower output signal is cancelled so that the entire input signal is output using upper output  162 . More generally, network controller  120  can control the output power distribution of MZI  151  so that EM power can be distributed losslessly in any desired manner between the outputs. 
     All other MZIs of routing network  112  operate as MZI  151  operates except that the output corresponding to output  164  is blocked for MZIs in the last stage of the illustrated MZI routing network as indicated in  FIG.  2   . In the latter case, the phase difference between branches is controlled to determine an amount of attenuation applied to the one (unblocked) output. 
     MZI routing network  112  is arranged in stages including a first stage  201 , intermediate stages  202 - 204 , and a final stage  205  as shown in  FIG.  2   . First stage  201  includes one MZ1  151  arranged to receive EM waves over illumination coupling  110 , as well as a phase control signal from network controller  130 . All first and intermediate stage MZIs provide outputs to an immediately succeeding stage, while all intermediate and final stage MZ1s receive inputs from MZIs of the immediately preceding state. First stage MZI  151  outputs a quadrature (?) pair of output signals, either can transmit none, some, or all of the power (depending on the EM wave input and phase settings) associated with the waves in the branches. 
     Second stage  202  includes MZIs  221  and  222 , each arranged to receive illumination from respective outputs from first stage MZI  151  as well as respective phase control signals from network controller  130 . Each of MZIs  221  and  222  outputs a quadrature pair of output signals for a total of four second stage  202  output signals. The four outputs of second stage  202  are respectively received by four MZIs  231 ,  232 ,  233 , and  234  of third stage  203 , yielding two outputs for each of the four MZIs for a total of eight third-stage outputs. The resulting eight outputs of third stage  203  are respectively received by eight MZIs  241 - 248  of the fourth switch network stage  204 . As with prior stages, the number of outputs can be double the number of MZIs so that the fourth stage  204  of MZI-routing network  112  has 16 outputs. 
     The 16 outputs of MZI routing network  112  are respectively received by 16 MZIs of fifth and final-stage MZI. Final stage  205  serves to equalize the outputs of MZI routing network  112  by making final attenuation adjustments, e.g., to balance the power outputs across the routing network outputs to radiator (gated couplings) array  114  ( FIG.  1   .). The attenuation adjustments are based on the phase shifts determined by phase control signals from network controller 130 . In an embodiment not requiring such balancing or power control or in an embodiment where such control is achieved in an alternative way, such a balancing/attenuating stage can be omitted. In such embodiments, the final stage can have MZIs with two outputs with each output coupled to a respective grating coupler. For example, each of the sixteen outputs of fourth stage  204  can include a variable attenuator if fifth stage  205  is omitted. 
     MZI routing network  112  thus permits an optical signal received at first stage MZI  151  to any selected one or more of the 16 final-stage MZIs  251 ,  252 ,  253 ,  254 ,  255 ,  256 ,  257 ,  258 ,  259 ,  25 A,  25 B,  25 C,  25 D,  25 E,  25 F, and  25 G. For example, phase control signals from network controller  120  can be used to route an optical signal from first stage MZI  151  along a route  265  through second stage MZI  221 , third stage MZI  232 , fourth stage MZI  243 , and fifth and final stage MZI  254 . In alternative embodiments, the first stage can have more than one MZI and there can be fewer or more than three intermediate stages. 
     Grating couplers  311 - 344  of radiator array  312  are arranged to receive EM waves from respective MZIs  251 - 25 G of the final stage  205  of routing network  112 . EM waves received by grating couplers  311 - 344  is radiated orthogonally out of the page of  FIG.  3   , and toward quantum array  102  ( FIG.  1   ). Each grating coupler  311 - 344  is optically coupled to a respective site of quantum array  102  so that only the coupled site receives the illumination transmitted from the respective grating coupler. Thus, PIC EMR router  106  ( FIG.  1   ) makes it possible to route EMR to zero, one, some, or all QSCs of quantum array  102  based on phase control signals from network controller  120 . 
     An optical routing quantum-array control process  400  is flow charted in  FIG.  4   . At  401 , a quantum computer program, e.g., a quantum circuit, is initiated. Typically, the program specifies operations and array addresses. At  402 , EM waves, e.g., laser beams, are generated based on the quantum computer program. At  403 , the EM waves are coupled to the PIC, e.g., including an electronically controlled MZI routing network and an array of EM radiators, e.g., grating couplers. 
     At  404 , the EM waves are routed to or toward a device that radiates EMR toward a QSC in the quantum array; enroute the EM waves can be attenuated by respective amounts, e.g., using a routing network final stage with single-output MZIs. For example, the attenuation can be selected to power balance the EM waves arriving at the EM radiators. Alternatively, the balancing can be omitted or implemented in an alternative approach. At  405 , EMR is radiated toward the respective QSC, e.g., using the EM radiators. At  406 , the radiated EMR is focused on the respective QSC, e.g., using collimating and focusing lenses or other optical elements. In a scenario, the switch network is used to map one input to one output. However, the routing network can route one input to two or more or even all outputs with equal power. 
     Quantum computer system  100  ( FIG.  1   ) can be modified in a number of ways to accommodate different objectives. For example, the first routing network stage can have more than one MZI so that multiple EMR inputs can be accepted, which may be of the same wavelength or different wavelengths so that illumination can be distributed among QSCs in different ways. Concurrent illumination signals can be time-aligned by adding delays in the outputs of some MZIs. For example, each switch network MZI can have a delay coupled to one or both of its outputs. 
     The illustrated quantum array is square and two dimensional. Alternative embodiments use rectangular and other shape arrays of QSCs and/or EM radiators (e.g., grating couplings) and provide for arrays of fewer (e.g., 1) or more (e.g., 3) dimensions. In various embodiments, the EM wavelengths of interest can range from 10 nm to 10,000 nm. In various embodiments, the QSCs can include atoms, molecules, ions, color centers in solid-state hosts, or quantum dots in a semiconductor host. 
     In the illustrated embodiment, the spatial mode of the beam exiting each grating coupler is a Gaussian with intensity profile I=exp(-2r 2 /w 2 ) where ris the distance from the center of the grating coupler and wis the beam waist. In alternative embodiments, the spatial mode is a higher order Gaussian, often denoted as a “super-Gaussian”, of the form I=exp(-2r n /w n ) with n&gt;2. Such a super-Gaussian profile is useful for achieving QSC control with high fidelity that is insensitive to errors in alignment of the optical beam onto the qubit. In other embodiments, the spatial mode approximates a uniform intensity that does not vary across the grating coupler. These different options can be implemented by design of the grating couplers. 
     In quantum system  100 , optical signals travel in only one direction, that is from the illumination system to the QSCs. In an alternative embodiment, quantum-state readout signals are routed in the reverse direction and coupled to a readout unit. In another embodiment, feedback signals are routed from the QSCs or a late stage of a network to an earlier stage of a network to adjust phase delay or some other parameter. 
     Herein, a “system” is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A “process” is a system in which the elements are actions. “Quantum” characterizes a system as exhibiting or using quantum-mechanical phenomena such as eigenstates (solutions to Schrödinger’s time dependent or time independent wave equation), superposition, and entanglement. “Quantum states” include eigenstates and superpositions of eigenstates. A “quantum simulator” is a quantum system used to emulate another quantum system. Herein, a “quantum state carrier” (QSC) is any physical system that can assume alternative eigenstates and superpositions of those eigenstates. Examples of QSCs include superconducting circuits, quantum dots in semiconductor hosts, color centers in a solid-state host (e.g., nitrogen-vacuum centers in diamond,) and neutral and charged atoms and molecules. 
     A “quantum information-processing system” is a quantum system that uses quantum states to represent quantum information”. Herein, “information” is organized data. “Bits” are the smallest units of classical information and can assume two values such as logic-0 and logic-1. “Qubits” are the smallest unit of quantum information and can assume values corresponding to points on a unit circle in a complex plane, in other words, values of the form a + bi, wherein a and b are real numbers, iis the square root of negative one (-1), and a 2  + b 2  = 1. Qubits and larger units of quantum information (e.g., qutrits and other qudits), can be represented by quantum state carriers with a sufficient number of alternative eigenstates. A “quantum computer system” or “quantum computer” is a quantum information processing system that processes quantum information by manipulating quantum states in accordance with instructions. 
     Herein, an “array” is an ordered series or arrangement of elements. For example, atoms can be arranged in square array having four rows and four columns, with the rows and columns having the same inter-element spacing. A “quantum array” is an array in which the elements are quantum state carriers. 
     Herein, “electromagnetic waves” or “EM waves” encompasses ionizing radiation, ultraviolet light, visible light, infrared light, microwaves, and radio waves. Of interest herein, are wavelengths from 10 nanometers (nm) to 100,000 nm, corresponding to a frequency of range from very-low frequency 3 Kilohertz (kHz) to extreme ultraviolet 30 petahertz (30 PHZ), the visible and near-infrared light being most relevant to the illustrated embodiment. “Electromagnetic radiation” or “EMR” denotes EM waves propagating through space carrying electromagnetic radiant energy. Herein, a “waveguide” is a structure along which propagation of EM waves is confined. For example, a waveguide can be an elongated structure with a relatively high index of refraction bounded by material with a relatively low index of refraction such that internal reflections confine propagating wave to the elongated structure. Herein, an “EM radiator” is a device that converts EM waves propagating in a waveguide to electromagnetic radiation (EMR). For example, the radiator can be a diffraction grating. 
     Herein, a “network” is a set of devices connected to each other using a physical transmission medium. “Routing” is the process of selecting a path from a source (e.g., laser system) to a destination (e.g., an array site in which a target quantum state carrier resides). Herein, “a single output” implies fewer than two outputs, while having “plural outputs” implies at least two outputs. 
     Herein, a “delay element” is a structure that causes EM waves to arrive at a destination later than they would without the element. A “controllable” delay element allows the delay to be varied. For example, a waveguide can include a section of material with an electronically controlled index of refraction; the higher the index of refraction, the slower the propagation of the EM waves and the greater the resulting delay. The resulting delay can then result in a phase lag relative to an EM wave that is not delayed (e.g., because it is propagated along a parallel waveguide that lacks such a delay element). 
     Herein, a “photonic integrated circuit” or “PIC” is a device that integrates multiple (at least two) photonic functions (e.g., waveguides, power splitters, optical amplifiers, optical modulators, filters, lasers, and detectors). Accordingly, a PIC is analogous to an (electronic) integrated circuit and can share many of the benefits associated with integrated circuits, such as miniaturization, cost reductions, reliability, and scalability. Many of the processes (e.g., photolithography) and materials (silicon and gallium arsenide (GaAs) can be used to manufacture PICS with and without integrated electronics. However, some optical functions favor electro-optic crystals such as lithium niobate, while on-PIC semiconductor lasers can be made using GaAs and indium phosphate (InP). Of interest herein are integrated waveguides, optical gratings, and electrical conductors, all of which can be formed in PIC based on silicon, silica on silicon and silicon on insulator. 
     Herein, all art labeled “prior art”, if any, is admitted prior art; all art not labeled “prior art”, if any, is not admitted prior art. The illustrated embodiments, modifications thereto, and variations thereupon are provided for by the present invention, the scope of which is defined by the claims.