Session C07: General Quantum Computation

Performance of Grover-QAOA on 3-SAT: Quadratic Speedup, Fair-Sampling, and Parameter Clustering

The SAT problem stands as a quintessential NP-complete challenge with profound significance across various scientific and engineering disciplines; as such, it has long served as an essential benchmark for classical and quantum algorithms. This study reports numerical evidence for a quadratic speedup of the Grover Quantum Approximate Optimization Algorithm (G-QAOA) over brute-force searching for finding all solutions to 3-SAT problems. G-QAOA presents a reduction in resource demands and heightened adaptability for tackling 3-SAT challenges compared to Grover's algorithm, alongside its superior capacity for solution sampling over the conventional QAOA. This study elucidates these advantages through classical simulations of many-round G-QAOA on thousands of random 3-SAT instances. We also observe G-QAOA advantages on the IonQ Aria quantum computer for small instances, finding that current hardware suffices to determine and sample all solutions. A noteworthy finding is that imposing a single-angle pair constraint markedly reduces the classical computational overhead of optimizing G-QAOA angles, without detracting from its quadratic performance enhancement, and presents a clustering phenomenon of the angles. The single-angle pair constraints and parameter clustering significantly reduce obstacles to classical optimization of the G-QAOA angles, offering opportunities to solve problems beyond SAT.   

Universal Quantum Optimization with Cold Atoms in an Optical Cavity

Cold atoms in an optical cavity have been widely used for quantum simulations of many-body physics, where the quantum control capability has been advancing rapidly in recent years. Here, we show the atom cavity system is universal for quantum optimization with arbitrary connectivity. We consider a single-mode cavity and develop a Raman coupling scheme by which the engineered quantum Hamiltonian for atoms directly encodes number partition problems (NPPs). The programmability is introduced by placing the atoms at different positions in the cavity with optical tweezers. The NPP solution is encoded in the ground state of atomic qubits coupled through a photonic cavity mode, that can be reached by adiabatic quantum computing (AQC). We construct an explicit mapping for the 3-SAT and vertex cover problems to be efficiently encoded by the cavity system, which costs linear overhead in the number of atomic qubits. The atom cavity encoding is further extended to quadratic unconstrained binary optimization (QUBO) problems. The encoding protocol is optimal in the cost of atom number scaling with the number of binary degrees of freedom of the computation problem. Our theory implies the atom cavity system is a promising quantum optimization platform searching for practical quantum advantage.   

Demonstration of a Quantum Graph Coloring Game on a Trapped Ion Quantum Computer

In a graph coloring game, two players color vertices of a graph such that adjacent vertices are not given the same color. In the quantum version, the two players share an entangled state and pick colors by measuring their subspace in different bases. We demonstrate a quantum graph coloring game on a trapped ion computer with a graph where the optimal quantum strategy outperforms all classical strategies. We run 88 four-qubit circuits to obtain an experimental win rate of the optimal quantum strategy and compare with the theoretical quantum and classical win rate.   

Error-corrected simultaneous control at the quantum speed limit

Simultaneous control of two or more Qubits has typically been done by addressing the two qubits separately in frequency space or sequentially by having them make multiple revolutions. While effective, these approaches require relatively long pulses; executing state transitions as quickly as possible is a virtue in many quantum information applications.
Here we present an analytic result for simultaneous trajectories at the quantum speed limit. We demonstrate these pulses on a xenon-129 spin ensemble and further demonstrate error correction for this type of pulse as well. Our approach provides a way to achieve two key "wants" in quantum information that are often in tension: the ability to perform gates both quickly and robustly to errors.   

Benchmarking Quantinuum's Second-Generation Quantum Processor

One of the main challenges facing large-scale quantum computing is scaling systems to more qubits while maintaining high fidelity operations. In this talk, I will describe our efforts at Quantinuum in scaling trapped-ion quantum computers based on the quantum charge-coupled device architecture. We recently released our second-generation machine, which has a race-track shaped ion trap. The new system incorporates several technologies crucial to future scalability, including electrode broadcasting, multi-layer RF routing, and magneto-optical trap loading, while maintaining, and in some cases exceeding, the gate fidelities of our first-generation system. We initially released the system with 32 qubits, but future upgrades will allow for more. I will describe the thorough set of benchmarking experiments we performed to characterize the system, as well as present a selection of recent results of quantum circuits that have been run on the system.   

How to Wire a 1000-Qubit Trapped-Ion Quantum Computer

Scaling up quantum computers requires efficient signal delivery to the quantum processor (the "wiring" challenge). It is likely that integration of control electronics into the processor package will be necessary, but this process is heavily constrained by chip microfabrication and chip operation specifications. Here, we present our WISE (Wiring using Integrated Switching Electronics) architecture as an answer to the wiring question, where judicious integration of simple switching electronics into the ion trap chip is combined with parallel trap electrode control [1]. This significantly reduces the number of signal sources needed, such that a fully connected 1000-qubit trapped ion quantum computer might be operated using only ∼ 200 signal sources.

[1] M. Malinowski et al., PRX Quantum 4, 040313 (2023)   

Experimental tests of robust gate designs utilizing Rydberg dressing of neutral atoms

Neutral atoms in tweezer arrays have rapidly advanced as a platform for quantum computing in the last decade. As arrays have scaled up to hundreds of atoms and logical processing with early error correction has been demonstrated [1], the need for robust high-fidelity gates remains critical. Technical noise sources, including laser frequency and amplitude noise as well as electric field stability and atom motion, contribute to gate infidelity. We test the robustness of gates based on Rydberg-dressing approaches [2, 3] in the presence of laboratory noise and constraints. These schemes promise higher fidelity operations via robustness to technical noise, an important quality for practical quantum computers of the future.

 

Sandia National Laboratories is a multimission laboratory managed and operated by NTESS, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

 

1. D. Bluvstein, et al., Nature (2023): 1-3.

2. A. Mitra, et al., Physical Review A 101.3 (2020): 030301.

3. Y-Y. Jau, et al., Nature Physics 12.1 (2016): 71-74.   

All the advantages of quantum multiplexing

Quantum technologies will profoundly improve the approach to several problems in a variaty of fields, leading to faster computation, more secure communication between remote parties and more precise measurements. The future development of such technologies, however, is jeopardized by several erros that might occur in a real implementation. Device imperfections and losses in the channel, in fact, are very detrimental for the performance of such systems. Therefore they must strongly rely on efficient methods to detect and correct those errors. To address this goal, several quantum error correction codes (QECCs) and purification methods have been developed in the last few decades. However, an efficient realization of such QECCs is still far from being reached due to the enormous number of physical resources required. In a recent work the authors have shown that the technique of quantum multiplexing (QM) allows to reduce the number of quantum memories required in a purification protocol and in a simple error correction scheme. Here, we apply this technique to the redundancy QECC, the quantum Reed-Solomon code and the surface code in a quantum communication scenario, in which the channel loss is the main source of error. We show that for such codes, QM allows to reduce drastically the number of photons, qubits and two-qubit gates required for their encoding while maintaining the same performance of the traditional approach. We describe a possible realization of QM that is based on exploiting multiple degrees of freedom of a single photon (for instance time-bin and polarization) by using optical elements, such as beam splitters and optical switches.  We also show  a realistic implementation of QM using the state-of-the-art devices under a practical scenario. We believe that QM is a valid approach to make quantum technologies more feasible in the near future.    

Distributed Quantum Computing Architecture Utilizing Entangled Ion Qubit Shuttling

Scalability remains a significant challenge in quantum computing (QC), crucial for achieving fault-tolerant QC. In the trapped-ion platform, scaling approaches such as quantum charge-coupled device (QCCD) and photonic interconnect were proposed and their proof-of-concept experiments were demonstrated. In this presentation, we introduce another scaling approach, the shuttling-based distributed quantum computing (SDQC) architecture, which integrates shuttling and distributed quantum computing.SDQC comprises multiple ion chains with fixed data qubits where interchain entangling gates are mediated by entangled communication qubits transported by ion shuttling. Such architectures enable asynchronous operations between data qubit manipulation and shuttling of entangled qubits. We assessed the performance of SDQC in terms of the time cost and effective error of remote gates, comparing the result with other scaling approaches. The time cost is evaluated for operation run time and entanglement distribution time, while number of operations, shuttling loss, and coherence time were considered to estimate the effective error. Feasibility of hybrid architecture with photonic interconnect and prospects on large-scale systems are discussed as well.   

Distributed Quantum Computing with Photons and Atomic Memories

We propose a universal quantum computing scheme that harnesses photonic polarizations and atomic-ensemble ground-state quantum memories (QM). This photon-atom hybrid platform could allow modular quantum computing connectivity that circumvents probabilistic nature of entanglement with a deterministic one. Our approach achieves high storage-retrieval efficiency by converting photonic polarization states into efficient atomic-ensemble-based QM states. We present two CP gate configurations using Rydberg blockade approach: one employing dual QM atomic ensembles in close proximity with overlapping photon modes, and the other using a single ensemble accommodating two spatially overlapping photonic modes. Our analysis and simulations indicate that while QM loss primarily affects state generation efficiency, it has minimal impact on fidelity. The scalability of our method is demonstrated through its extension to the generation of N-photon Greenberger-Horne-Zeilinger (GHZ) states. We discuss the potential of our scheme for spatially and temporally distributed quantum computing, emphasizing its role as a quantum network interface that bridges flying photons with stationary atomic nodes.