Neutral Atom Quantum Computer Research Articles

Benchmarking a neutral-atom quantum computer

In this study, we simulated the algorithmic performance of a small neutral atom quantum computer and compared its performance when operating with all-to-all versus nearest-neighbor connectivity. This comparison was made using a suite of algorithmic benchmarks developed by the Quantum Economic Development Consortium. Circuits were simulated with a noise model consistent with experimental data from [Nature 604, 457 (2022)]. We find that all-to-all connectivity improves simulated circuit fidelity by [Formula: see text]–[Formula: see text], compared to nearest-neighbor connectivity.

Timing-Aware Qubit Mapping and Gate Scheduling Adapted to Neutral Atom Quantum Computing

As a less developed but potential quantum technology, neutral atoms (NA) can provide advantages including higher qubit connectivity, longer-range interactions, and much more native multi-control gates than superconductivity. Long-range interactions, however, prevent parallelism of interacting qubit pairs with their surrounding restriction zones. Therefore, the quantum program cannot be run directly on a neutral atom quantum computer (NAQC) unless compiled. The recent compiling study of NAQC applies simple layer-by-layer scheduling and does not consider the difference in gate duration. To address the above issues, we focus on the qubit mapping and quantum gate scheduling () problem of quantum circuits to meet hardware constraints of superconductivity and even neutral atoms. Our goal is to shorten the execution time to mitigate decoherence noise. We propose a block-game-like abstraction mechanism which is an abstract model of the problem and aware of the duration difference of gates and several other NA characteristics. Based on the abstraction, we propose a heuristic greedy algorithm (HGA) to solve the problem efficiently. To further speed up the execution of the circuit, we embed HGA into a Monte Carlo tree search (MCTS) framework to solve the problem, which consumes more compiling time but achieves a better result. Comparing our MCTS algorithm with the only recent algorithm for NAQC, the average speedup ratio is 1.75× for several quantum circuits collected from RevLib, and 1.18× for circuits from Qiskit Lib.

High-fidelity parallel entangling gates on a neutral-atom quantum computer

The ability to perform entangling quantum operations with low error rates in a scalable fashion is a central element of useful quantum information processing1. Neutral-atom arrays have recently emerged as a promising quantum computing platform, featuring coherent control over hundreds of qubits2,3 and any-to-any gate connectivity in a flexible, dynamically reconfigurable architecture4. The main outstanding challenge has been to reduce errors in entangling operations mediated through Rydberg interactions5. Here we report the realization of two-qubit entangling gates with 99.5% fidelity on up to 60 atoms in parallel, surpassing the surface-code threshold for error correction6,7. Our method uses fast, single-pulse gates based on optimal control8, atomic dark states to reduce scattering9 and improvements to Rydberg excitation and atom cooling. We benchmark fidelity using several methods based on repeated gate applications10,11, characterize the physical error sources and outline future improvements. Finally, we generalize our method to design entangling gates involving a higher number of qubits, which we demonstrate by realizing low-error three-qubit gates12,13. By enabling high-fidelity operation in a scalable, highly connected system, these advances lay the groundwork for large-scale implementation of quantum algorithms14, error-corrected circuits7 and digital simulations15.

Hardware-Efficient, Fault-Tolerant Quantum Computation with Rydberg Atoms

Neutral-atom arrays have recently emerged as a promising platform for quantum information processing. One important remaining roadblock for the large-scale application of these systems is the ability to perform error-corrected quantum operations. To entangle the qubits in these systems, atoms are typically excited to Rydberg states, which could decay or give rise to various correlated errors that cannot be addressed directly through traditional methods of fault-tolerant quantum computation. In this work, we provide the first complete characterization of these sources of error in a neutral-atom quantum computer and propose hardware-efficient, fault-tolerant quantum computation schemes that mitigate them. Notably, we develop a novel and distinctly efficient method to address the most important errors associated with the decay of atomic qubits to states outside of the computational subspace. These advances allow us to significantly reduce the resource cost for fault-tolerant quantum computation compared to existing, general-purpose schemes. Our protocols can be implemented in the near term using state-of-the-art neutral-atom platforms with qubits encoded in both alkali and alkaline-earth atoms.9 MoreReceived 6 June 2021Revised 3 February 2022Accepted 12 April 2022DOI:https://doi.org/10.1103/PhysRevX.12.021049Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasOptical pumpingQuantum algorithmsQuantum computationQuantum gatesQuantum information processingQuantum protocolsSpontaneous emissionQuantum InformationAtomic, Molecular & Optical

Multi-qubit entanglement and algorithms on a neutral-atom quantum computer.

Gate-model quantum computers promise to solve currently intractable computational problems if they can be operated at scale with long coherence times and high-fidelity logic. Neutral-atom hyperfine qubits provide inherent scalability owing to their identical characteristics, long coherence times and ability to be trapped in dense, multidimensional arrays1. Combined with the strong entangling interactions provided by Rydberg states2-4, all the necessary characteristics for quantum computation are available. Here we demonstrate several quantum algorithms on a programmable gate-model neutral-atom quantum computer in an architecture based on individual addressing of single atoms with tightly focused optical beams scanned across a two-dimensional array of qubits. Preparation of entangled Greenberger-Horne-Zeilinger (GHZ) states5 with up to six qubits, quantum phase estimation for a chemistry problem6 and the quantum approximate optimization algorithm (QAOA)7 for the maximum cut (MaxCut) graph problem are demonstrated. These results highlight the emergent capability of neutral-atom qubit arrays for universal, programmable quantum computation, as well as preparation of non-classical states of use for quantum-enhanced sensing.

Solving correlation clustering with QAOA and a Rydberg qudit system: a full-stack approach

We study the correlation clustering problem using the quantum approximate optimization algorithm (QAOA) and qudits, which constitute a natural platform for such non-binary problems. Specifically, we consider a neutral atom quantum computer and propose a full stack approach for correlation clustering, including Hamiltonian formulation of the algorithm, analysis of its performance, identification of a suitable level structure for 87Sr and specific gate design. We show the qudit implementation is superior to the qubit encoding as quantified by the gate count. For single layer QAOA, we also prove (conjecture) a lower bound of 0.6367 (0.6699) for the approximation ratio on 3-regular graphs. Our numerical studies evaluate the algorithm's performance by considering complete and Erdős-Rényi graphs of up to 7 vertices and clusters. We find that in all cases the QAOA surpasses the Swamy bound 0.7666 for the approximation ratio for QAOA depths p≥2. Finally, by analysing the effect of errors when solving complete graphs we find that their inclusion severely limits the algorithm's performance.

Two-Qubit Gate Operation on Selected Nearest-Neighbor Neutral Atom Qubits

We have previously discussed the design of a neutral atom quantum computer with an on-demand interaction [E. Hosseini Lapasar, et al., J. Phys. Soc. Jpn. 80, 114003 (2011)]. In this contribution, we propose an experimental method to demonstrate a selective two-qubit gate operation that is less demanding than our original proposal, although the gate operation is limited to act between two neighboring atoms. We evaluate numerically the process of a two-qubit gate operation that is applied to a selected pair of nearest-neighbor, trapped atoms and we estimate the upper bound of the gate operation time and corresponding gate fidelity. The proposed scheme is scalable and, though challenging, is feasible with current experimental capabilities.

Scalable Neutral Atom Quantum Computer with Interaction on Demand: Proposal for Selective Application of Two-Qubit Gate

We propose a scalable neutral atom quantum computer with an on-demand interaction through a selective two-qubit gate operation. Atoms are trapped by a lattice of near field Fresnel diffraction lights so that each trap captures a single atom. One-qubit gate operation is implemented by a gate control laser beam which is applied to an individual atom. Two-qubit gate operation between an arbitrary pair of atoms is implemented by sending these atoms to a state-dependent optical lattice and making them collide so that a particular two-qubit state acquires a dynamical phase. We give numerical evaluations corresponding to these processes, from which we estimate the upper bound of a two-qubit gate operation time and corresponding gate fidelity. Our proposal is feasible within currently available technology developed in cold atom gas, MEMS, nanolithography, and various areas in optics.

Polarization-dependent atomic dipole traps behind a circular aperture for neutral-atom quantum computing

The neutral-atom quantum computing community has successfully implemented almost all necessary steps for constructing a neutral-atom quantum computer. We present computational results of a study aimed at solving the remaining problem of creating a quantum memory with individually addressable sites for quantum computing. The basis of this quantum memory is the diffraction pattern formed by laser light incident on a circular aperture. Very close to the aperture, the diffraction pattern has localized bright and dark spots that can serve as red-detuned or blue-detuned atomic dipole traps. These traps are suitable for quantum computing even for moderate laser powers. In particular, for moderate laser intensities ($~100$ W/${\mathrm{cm}}^{2}$) and comparatively small detunings ($~1000$--10 000 linewidths), trap depths of $~1$ mK and trap frequencies of several to tens of kilohertz are achieved. Our results indicate that these dipole traps can be moved by tilting the incident laser beams without significantly changing the trap properties. We also explored the polarization dependence of these dipole traps. We developed a code that calculates the trapping potential energy for any magnetic substate of any hyperfine ground state of any alkali-metal atom for any laser detuning much smaller than the fine-structure splitting for any given electric field distribution. We describe details of our calculations and include a summary of different notations and conventions for the reduced matrix element and how to convert it to SI units. We applied this code to these traps and found a method for bringing two traps together and apart controllably without expelling the atoms from the trap and without significant tunneling probability between the traps. This approach can be scaled up to a two-dimensional array of many pinholes, forming a quantum memory with single-site addressability, in which pairs of atoms can be brought together and apart for two-qubit gates for quantum computing.

Universal quantum computation in a neutral-atom decoherence-free subspace

In this paper, we propose a way to achieve protected universal computation in a neutral atom quantum computer subject to collective dephasing. Our proposal relies on the existence of a Decoherence Free Subspace (DFS), resulting from symmetry properties of the errors. After briefly describing the physical system and the error model considered, we show how to encode information into the DFS and build a complete set of safe universal gates. Finally, we provide numerical simulations for the fidelity of the different gates in the presence of time-dependent phase errors and discuss their performance and practical feasibility.

Perfect pattern formation of neutral atoms in an addressable optical lattice

We propose a physical scheme for formation of an arbitrary pattern of neutral atoms in an addressable optical lattice. We focus specifically on the generation of a perfect optical lattice of simple orthorhombic structure with unit occupancy, as required for initialization of a neutral atom quantum computer. The scheme employs a compacting process that is accomplished by sequential application of two types of operations: a flip operator that changes the internal state of the atoms, and a shift operator that selectively moves the atoms in one internal state along the lattice principal axis. Realizations of these elementary operations and their physical limitations are analyzed. The complexity of the compacting scheme is analyzed and we show that this scales linearly with the number of lattice sites per row of the lattice. Neutral atoms trapped in an optical lattice constitute an attractive system for implementation of scalable quantum computation f1-3g, simulation of many-body systems f4g, and implementation of topological quantum computing f5-9g. In standard optical lattices, small lattice constants present a serious obstacle to implementing quantum compu- tation, since it is difficult to address individual qubits with an external field. An optical lattice with a large lattice constant is in principle addressable, and can allow for the quantum state manipulation of individual atoms by an optical field. High addressability and controllability and low decoherence make addressable lattices promising candidates for large- scale quantum computer implementation. The present work focuses on preparation of the initializa- tion of an addressable optical lattice for the purposes of quantum computing. The objective is a perfectly filled, regu- lar optical lattice, with each site occupied by a single atom in its motional ground state and in a specific internal state. We consider one-dimensional s1Dd, orthorhombic two- dimensional s2Dd, and three-dimensional s3Dd lattices. After loading and laser-cooling atoms in the optical lattice, half the sites have a single atom and half are vacant. In order to use this system for scalable quantum computation, a perfect lat- tice with each site occupied by a single atom is required. We propose here an efficient, feasible scheme forcompacting the optical lattice, i.e., for removing vacant sites to the edge of the lattice, thus creating a smaller lattice, but one more suit- able for quantum computation. The scheme presented here can as well be used to make arbitrary patterns of neutral atoms in an addressable optical lattice. These include lattices with fractional occupation, a specific translational and rotational lattice symmetry, a bro- ken symmetry, and heteroatomic patterns. Another important property of the scheme is that it can be applied recursively to reach any desired accuracy of the pattern formation. After a large number of elementary operations, the lattice can be cooled and imaged again. The remaining defects can be eliminated by repeating the compacting scheme. This recur- sion increases the total pattern formation time by the total duration of additional cooling and imaging cycles, but does not result in any increase in the scaling of the pattern forma- tion, i.e., the algorithmic complexity of the scheme is un- changed. The possibility of preparing any homoatomic or heteroatomic pattern of neutral atoms to an arbitrarily high degree of perfection makes this scheme attractive for initial- ization of quantum simulations of condensed phase systems, in addition to initialization of quantum computation.