Session A53: Atomic Quantum Computing: Theoretical Investigations on Fault Tolerance and Algorithms

Dark-cat encoding in atomic levels - Part I

Neutral atoms become one of the most promising platforms for quantum information and simulation purposes. Finding hardware-efficient way to encode quantum information and performing error correction is still an important problem for this system. In our work, we show how decoherence-free qubits can be efficiently encoded in the large spin hyperfine ground state of lanthanide atoms. In particular, they are encoded in the dark states of a Raman-coupled hyperfine structure. This encoding resembles cat code structure in bosonic systems, which shares common features like autonomous stabilization and biased-noise structure, as bit-flip error is suppressed exponentially as the system size becomes larger. In the first part, we will go through the system setup, explain the analogy with bosonic cat, and uncover the mechanism behind autonomous stabilization as well as the biased noise structure.   

Dark-cat encoding in atomic levels - Part II

Neutral atoms become one of the most promising platforms for quantum information and simulation purposes. Finding hardware-efficient way to encode quantum information and performing error correction is still an important problem for this system. In our work, we show how decoherence-free qubits can be efficiently encoded in the large spin hyperfine ground state of lanthanide atoms. In particular, they are encoded in the dark states of a Raman-coupled hyperfine structure. This encoding resembles cat code structure in bosonic systems. For the encoded qubits, readily available laser coupling methods are used to construct bias-preserving single-qubit holonomic gates, while laser coupling to Rydberg states is employed to create bias-preserving entangling gates among qubits. The bias-preserving operation set is sufficient for universal quantum computing on the concatenated repetition code level. In the second part, we will focus on the detailed protocol on the single-qubit and entangling gate design and show how they can be constructed in a bias-preserving manner.   

A Flexible Toolbox for Hamiltonian Engineering with Driven Rydberg Atom Arrays

Neutral atom arrays have recently emerged as a powerful platform for programmable quantum simulation and computation. In this project, we introduce a technique for Hamiltonian engineering based on perturbing around time-reversed trajectories. In particular, we illustrate how a multitude of new interaction terms can be controllably engineered in Rydberg atom arrays using time-dependent global driving. We illustrate these techniques in two settings. First, we show how blockade-consistent exchange interactions can be engineered, enabling exploration of novel phases with emergent particle number conservation in near-term analog devices. Then, we show how higher-weight ring-exchange terms can be implemented, enabling studies of dynamics in 2D gauge theories. Extensions incorporating additional control, such as atom reconfiguration, are also discussed.   

Designing time-optimal dynamically corrected gates robust to finite blockade and laser errors using geometric space curves

Two qubit phase gates in neutral atom systems can be constructed through the so-called Rydberg blockade mechanism and typically suffer from errors produced by the finite strength of that interaction. In this talk, I will present a general technique to design control pulses that suppress noise due to the finite blockade strength. This is facilitated through the space curve quantum control formalism (SCQC) in which curves in Euclidean space map to robust control fields. We derive geometric constraints for canceling such errors and solve these to obtain time-optimal control fields robust to blockade errors, and additional types of laser noise plaguing the system.   

Demonstration of superadditive communication and nonlocality without entanglement with the Green Machine temporal mode sorter.

The ultimate limit of optical communications capacity entails superadditive communications, a higher bit rate than that of any symbol-by-symbol detection. It is a special case of the celebrated nonlocality without entanglement [1] and has yet to be realized. We propose a practical design of the Green Machine [2], a joint-detection receiver that can attain superadditive capacity with a binary-phase-shift-keying (BPSK) modulated Hadamard code [3, 4]. We demonstrate this receiver [5] and show that its achieved capacity, after backing out losses within the receiver, surpasses that of any symbol-by-symbol receiver permissible by quantum physics, in the low received photon-flux regime. In addition to reducing the transmitter peak power needed compared with the conventional pulse-position modulation used for deep-space laser communications, we show the Green Machine's self-referenced phase makes it far more immune to channel phase noise, e.g., due to atmospheric turbulence or platform vibrations, by orders of magnitude compared with other BPSK-compatible receivers. These advantages make the Green Machine a promising candidate for next-generation deep-space laser communications. 

[1] C. H. Bennett, et al., PRA 59, 1070 (1999).

[2] S. Guha, PRL 106, 240502 (2011).

[3] T. Rambo, Ph.D. Dissertation, Northwestern University (advisor: P. Kumar), (2016).

[4] K. Banaszek, et al., J. Light. Technol. 38, 2741 (2020).

[5] C. Cui, et al., arXiv: 2310.05889 (2023).    

Benchmarking cluster states built with QuEra neutral atoms

Measurement-based quantum computation proposes to leverage entangled cluster states to perform universal quantum computation. Large scale entanglement is particularly promising within the context of neutral atom systems because atoms have long quantum lifetimes and are readily addressable with currently available optical and microwave technology.  But implementation of large-scale, high fidelity cluster states via parallel operations remains an open challenge.  The QuEra neutral atom quantum device uses Rydberg interactions to entangle up to 256 atoms at once but, in contrast to digital quantum devices, the QuEra device is an analog quantum device.  Furthermore, the only local control is through atom location.  We construct a protocol to efficiently generate cluster states in parallel with always-on interactions available with the QuEra device.  We also construct a specific, non-local benchmarking procedure based on measurement-based teleportation that uses only global measurements available on the QuEra device.  We report  benchmarking results for QuEra cluster states that reveal the impact of noise sources, such as atom motion and laser fluctuations.  We also explore the role of long-range component of Rydberg interactions.   Our work lays the groundwork for scaling up the construction of cluster states using neutral atom systems.   

Counterdiabatic Driving for Efficient Quantum Solutions to the Maximum Independent Set Problem

This study explores the use of counterdiabatic driving in quantum annealing, focusing on its role in solving the maximum independent set (MIS) problem. We will show that counterdiabatic driving significantly enhances the overlap with the target solution. The work also highlights the potential of nested commutator, Krylov subspace, methods to approximate counterdiabatic driving, leading to more efficient computational strategies and implementations in experiments. These findings hold the promise of advancing quantum annealing capabilities and bringing the possibilities of applying these methods to a variety of other systems in the realm of quantum computing.   

A study of adiabaticity in electron transport by surface acoustic wave

Surface acoustic waves have been suggested as a carrier to enable remote transport of a single or multiple electrons from one quantum dot to another while maintaining quantum information encoded in the electron’s spin or orbital degrees of freedom. It is widely believed that the process where SAW picks up the electron from a fixed quantum dot is adiabatic, with the electron wavefunction following the instantaneous ground state centered at the collective potential minimum. In this work we explore the adiabaticity of this electron transfer from a static to a moving (or vice versa) dot as we vary system parameters such as relative dot sizes, bias potential, and speed of the moving dot.  In particular, we show that in a wide range of parameters, during the SAW pick-up process the electron could end up in a superposition state that includes many highly excited orbital states, gaining energy from the SAW potential. 

The high degree of excitation could negatively affect the electron spin decoherence, causing loss of quantum information encoded in the spin.    

Modern Quantum Information and the Impact of Quantum Computing on Cryptography

The advent of quantum computing represents a novel shift in computational power, heralding the transformation of various fields, such as cryptography. Quantum computing's ability to exploit the seemingly counterintuitive properties of quantum mechanics, such as superposition and entanglement, enables it to perform computations at speeds that no classical computers can match. With this unparalleled computational power, quantum computers have the capacity to break the widely-used cryptographic algorithms of the modern day, fundamentally altering the landscape of data security. This paper provides a deep exploration of the principles behind quantum computing and its implications for classical encryption methods, as well as the development of new strategies to secure digital communications and data. It highlights the vulnerability of current cryptographic techniques as well as the strength of our growing technological prowess. Ultimately, this paper seeks to provide a deeper understanding of the relationship between quantum mechanics, quantum computing, and cryptography, and to shed light on the urgent need of quantum-resistant cryptographic solutions to help secure sensitive data in the quantum era, ensuring that the digital world remains resilient to emerging challenges.   

Entangled State Generation and Coherence in STM-ESR Geometries Using Quantum Master Equations

Experimental techniques using pulsed electron spin resonance (ESR) with scanning tunneling microscopy (STM) has uncovered a new method to generate entangled spin states using atomic sites prepared on a substrate [1]. In this work, we demonstrate the use of a newly developed NEGF-derived quantum master equation software (TimeESR) to model ESR-STM for realistic experimental conditions relevant to atomic-scale qubit systems. We predict changes in measured current on the STM tip within the time domain and correlate patterns in the current with the generation of a non-trivial entangled spin state. We also demonstrate the role of electron dynamics in the sequential regime on the coherence of the entangled state as a function of time.

[1] Yu Wang et al., Science 382, 87 (2023)   

Individual-atom control in array through phase modulation

Performing parallel-gate operations while retaining low crosstalk is an essential step to transform arrays of individual atoms into powerful quantum computers and simulators. Current methods based on using AC stark shift to differentiate individual qubits are challenging due to large laser power requirements. We tackle such a problem by introducing a method to engineer single qubit gates through phase-modulated continuous driving. Distinct qubits can be individually addressed to high accuracy by simply tuning the modulation parameters, which significantly suppresses crosstalk effects. When arranged in a lattice structure, individual control with optimal crosstalk suppression is achieved. With the assistance of additional addressing light or multiple modulation frequencies, we develop two efficient implementations of parallel-gate operations. Our results pave the way to scaling up atom array platforms with low-error parallel-gate operations, without requiring complicated wavefront design or high-power laser beams.   

Unambigous preparation of Bell pairs

The preparation of Bell pairs, i.e., entanglement concentration and distillation methods have been intensively studied in the past decades. Although these schemes have since been further optimized, they have remained rather impractical for current physical implementations. We present a practical, iterative LOCC protocol which is able to transform any unknown two-qubit pure input state into a perfect |Φ₊〉 Bell state in two iterations if the input state contains some initial entanglement, and is accessible in many copies. Separable input states never produce outputs after the second iteration, thus, if a successful output is realized, the parties can be sure that the |Φ₊〉 Bell state was produced. There is a zero measure set of input states for which the second iteration does not succeed even though they are entangled. We devise a method to distinguish these states from separable inputs and to rotate them by local operations so that they can also be transformed to a |Φ₊〉 Bell pair. We also show that three iterations of the protocol distill into an approximate |Φ₊〉 Bell pair with quadratically suppressed noise provided that the noise is small, but irrespective of its form.   

Machine Learning-Driven Modeling and Characterization of High-Fidelity Quantum Control in Rydberg Atom Systems

An optical tweezers-trapped array of Rydberg atoms provides an intriguing platform for realising quantum computing. Recent proposals and experimental demonstrations have shown high-fidelity gates within Rydberg atom systems. Despite these significant strides, achieving fault-tolerant computation demands further enhancements in gate fidelity. Various error sources, including phase and intensity fluctuations in lasers, alignment errors, optical potential variations, finite lifetime of Rydberg atoms, and significant atomic motion during gate operations, must be considered to boost fidelity.

To address this problem we have created a digital twin of the device in question, implementing a very comprehensive model of the system. We employ automatic differentiation, ensemble averaging, and adjoint equation-based approaches. These techniques enable us to develop protocols with state-of-the-art fidelities that are robust against the aforementioned sources of noise. Additionally, we characterise these noise sources to better understand their impact on fidelity and offer a comprehensive error budget for the optimised protocols, providing valuable guidance for their experimental implementation.

Furthermore, the versatile toolbox we have developed can readily be applied to analogous challenges in other qubit modalities, such as trapped ions.   

Towards Scalable GHz Spot Arrays for Atom Control

The packaging complexity of connecting modulators with external electronic control limits the scale of arrays of optical modulators. We propose a co-integrated electronic-photonic platform that may be tiled to produce an arbitrarily large GHz-speed visible light modulator array for individually controlling large-scale atomic systems. The platform comprises a set of complementary metal oxide semiconductor (CMOS) electronic chiplets bonded to an array of high-speed lithium niobate visible light modulators. Each electronic chiplet enables high precision voltage control of 30 electrical drivers with switching speeds exceeding 1 GHz, four of which can control a 120 optical channel device. We discuss the electrical innovations required to enable this platform and the prospects for scaling to thousands of optical channels.