Session C08: Control and Measurement of Mesoscopic Systems

Graph optimisation on neutral atom array with local addressing

Neutral atoms have emerged as a powerful and scalable platform for quantum computing, offering the ability to generate large numbers of identical and high quality qubits in reconfigurable arrays. By coupling atoms to highly excited Rydberg states with strong, long-range dipole-dipole interactions this system can natively implement maximum independent set (MIS) graph problems on a unit disk graph, providing a route to performing analogue optimisation of real problems however with large systems required to reach a regime competitive against current classical optimisation protocols.

To extend this approach to explore optimisation of a wider class of problems including weighted graphs and quadratic unconstrained binary optimisation (QuBO), it is necessary to introduce locally addressed light-shifts to enable weighting of nodes during the optimisation process, enabling problems to be encoded with at worst a quadratic resource overhead. 

In this talk we present work to develop a large-scale system for quantum computing and annealing, and show preliminary results highlighting our ability to implement small-scale demonstrations of weighted graph optimisation using programmable local light-shifts across an atom array. We introduce a hybrid annealing process combining global addressing with ramped light shifts, and outline prospects for scaling this approach to larger graph problems as a potential pathway to quantum utility.   

Electron Temperatures and Atom Formation in near-threshold Photoionized Magnetized Ultracold Neutral Plasmas

In ultracold neutral plasmas (UNPs), three-body recombination is predicted to be the primary limitation to achieving the coldest possible electron temperatures. This limits the study of plasma physics in these systems, particularly with respect to studies of strong coupling and correlations in the electron component of UNPs. Magnetic fields are predicted to reduce the recombination rate which should in turn allow for lower electron temperatures. We have experimentally measured the atom population in UNPs formed through photoionization near the ionization threshold such that the initial kinetic energy imparted to the free electron component of the UNPs is small. From comparing these measurements to molecular dynamics simulations of these UNPs the electron temperature at multiple magnetic fields can be determined and compared to predicted limits.   

Multi-photonic microwave excitation of cold Rydberg atoms held inside a microwave cavity

We investigate the energy spectrum associated with the excitation of Rydberg states in cold 85Rb atoms confined within a standard magneto-optical trap (MOT) placed inside a microwave cavity. Rydberg atoms are generated through a two-step process. Initially, two optical photons at 780nm and 480nm connect the ground state 5S1/2 to the intermediate Rydberg state nS1/2 in a ladder configuration. Subsequently, microwave photons within the cavity's volume can drive multi-photonic microwave Rydberg transitions, either higher or lower in energy. Experimental characterization of the microwave cavity reveals a central resonance frequency peak of 13.053 GHz with a standard deviation of 5 MHz, precisely aligning with the coupling of the 67S1/2 Rydberg state to the nearby 66P3/2 excited state. Continuous monitoring of fluorescence from the cycling trap transition light (780nm) is achieved using sensitive photodetectors. The trap-loss spectrum is reconstructed by measuring the reduction in fluorescence resulting from the transfer of atoms from the MOT to the Rydberg state as a function of the 480nm laser frequency. Employing a timed sequence acquisition allows for the isolation of the atomic background vapor gas trap-loading dynamics. Our observations reveal robust multi-photonic microwave Rydberg excitation, demonstrating high excitation efficiencies across a range of energies. The results align closely with a multi-level Jaynes-Cumming formalism employed for modeling trap-loss spectroscopy, considering cavity imperfections.   

Simulating 2D-1D dimensional crossover with ultracold atoms

As a function of dimensionality, in quantum physics we have access to a broad range of phenomena. In this presentation, we describe the dimensional crossover along two integer dimensionalities, namely 2D to 1D, in strongly interacting ultracold atomic gases as a function of the transverse confining potential V_y. We showcase the analytical tools that can be used to describe the large V_y case where the system is well described by weakly-coupled chains, such as bosonization. In this regime, we use a mean-field approach to decouple the chains and study the temperature at which we expect the crossover to occur and it’s scaling as a function of V_y. We extend the analysis to a broader range of V_y by meaning of QMC simulations and study the superfluid fraction and the evolution of the one-body density matrix g₁(x) by comparing with experiment. These results allow us to define in the V_y - k_bT plane a phase diagram which ranges from BKT physics to 1D Luttinger liquid going through a distinct intermediate phase.   

A scalable local addressing system for optically addressable qubits using integrated photonics

Among the leading approaches to scalable quantum computers are systems made of optically addressable qubits, such as neutral atoms, trapped ions and solid-state emitters. In these platforms, quantum information is typically manipulated using optical fields, so that scaling to large systems demands a high channel-count, high-speed, and precise optical modulators at low incremental cost. Existing solutions are not practical beyond a few tens of channels, limiting the number of independent degrees of freedom in the quantum computer. Here we present a Photonic Control Unit (PCU) that enables scaling of the number of local addressing beams using integrated photonics. Using the PCU, we experimentally demonstrate high-speed, high-extinction modulation and multi-channel operation, supporting the visible to near-infrared wavelength range, therefore meeting the precision, power, and wavelength requirements of quantum computing applications. We outline the route to achieving hundreds to thousands of channels for scalable optical control of qubits.   

Quantum advantage and #P-hard complexity of many-body systems explained via eigen-squeeze modes as opposed to quasiparticles

        We show that the eigen-squeeze modes, not quasiparticles, are responsible for the computational #P-hardness of quantum many-body statistics in interacting systems [V.V. Kocharovsky et al., PRA 106, 063312 (2022), Entropy 25, 1584 (2023)]. We consider a generic example of the Bose-Einstein-condensed (BEC) gas. We suggest boson sampling of atom numbers in the noncondensed fraction of BEC gas as a new experimental platform for studying quantum advantage of many-body interacting systems over classical computers. In contrast to Gaussian boson sampling in linear interferometers, it does not require inputting squeezed bosons by sophisticated external sources. Besides, it overcomes the major limitation of optical boson sampling - an exponential growth of losses with an increasing number of modes. We present a new theoretical technique for analysis of quantum statistical properties which are #P-hard for computing. It is based on the newly found hafnian master theorem. We calculate atom sampling statistics and show its #P-hardness for computing. We explain the atom boson sampler as a quantum generator of random strings of excited-atom numbers based on a natural process of persistent equilibrium fluctuations as opposed to a quantum simulator of some input signal or controlled process.   

Diagnosing hardware and gate errors in trapped ions using reduced Choi-matrix tomography

Identifying and understanding hardware errors is crucial for assessing the performance of an experimental apparatus and enhancing the fidelity of the quantum circuits implemented within it. The initial step towards this goal involves the characterization of both the quantity and nature of error sources, specifically distinguishing between fluctuations and coherent errors from more intrinsic quantum errors. 

To accomplish this task, we introduce a benchmarking technique based on reduced Choi-matrix tomography [1, 2], the properties of which provide immediate and valulable insights into unknown quantum processes affecting the target unitary. Remarkably, this method eliminates the need for exhaustive knowledge of all noise sources affecting the system. 

We tested and validated the effectiveness of this technique on a single qubit gate implemented using a Calcium trapped ion. Furthermore, intentional noise injection served to confirm the method's reliability for error detection and identification.

This approach can be easily extended to cover the two-qubit gate scenario, and seamlessly integrated into the regular calibration and maintenance procedures for the experimental hardware. 

[1] B. H. Madhusudhana. Benchmarking multi-qubit gates – I: Metrological aspects, 2023, arXiv: 2210.04330.

[2] B. H. Madhusudhana. Benchmarking multi-qubit gates – II: Computational aspects, 2023, arXiv: 2301.07109.   

Characterization and mitigation of axial-motion induced noise on trapped ions using quantum noise spectroscopy

As quantum computing devices grow in capability, it is important to develop characterization techniques to study the noise on real machines at all scales. One such technique that is particularly well-suited to informing low-level hardware changes is quantum noise spectroscopy (QNS). In this work we use QNS to drive improvements to a trapped-ion quantum computer at the physical hardware calibration level. Using the ‘SineLobe’ spectroscopy technique (Maloney et al. 2022), we find that our leading source of noise stems from axial motion which causes the ion to experience the spatial inhomogeneity of the tightly-focused individual addressing beam. However, we demonstrate that by choosing the appropriate ion-beam position, we can strongly mitigate this noise source. We further use QNS to characterize the realized improvements on the Quantum Scientific Computing Open User Testbed (QSCOUT) device by using this updated calibration.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, 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. SAND2024-00483A