arXiv subjects

Kenneth R. Brown

Publications and source records attributed to Kenneth R. Brown.

At least 37 records · Page 2Linked to original sources

arXiv2025

Adaptive Estimation of Drifting Noise in Quantum Error Correction

Advancing quantum information processors and building fault-tolerant architectures rely on the ability to accurately characterize the noise sources and suppress their impact on quantum devices. In practice, noise often drifts over time, whereas conventional noise characterization and decoding methods typically assume stationarity or provide only a time-average behavior of the noise. This treatment can result in suboptimal decoding performance. In this work, we present a rigorous analytical framework to capture time-dependent Pauli noise, by exploiting the syndrome statistics of quantum error correction experiments. We propose a sliding-window estimation method which allows us to recover the frequency components of the noise, by using optimal window sizes that we derive analytically. We prove the noise-filtering behavior of sliding windows, linking window size to spectral cutoff frequencies, and provide an iterative algorithm that captures multiple drift frequencies. We further introduce an overlapping window algorithm that enables us to capture rapid multi-frequency noise drifts in a single-pass fashion. Simulations for both phenomenological and circuit-level noise models validate our framework, demonstrating robust tracking of multi-frequency drift. The logical error rate obtained from our estimated models consistently align with the ground-truth logical error rate, and we find suppression of logical errors compared to static error models. Our window-based estimation methods and adaptive decoding offer new insights into noise spectroscopy and decoder optimization under drift using only syndrome data.

quant-ph↗

arXiv2025

Estimating and decoding coherent errors of QEC experiments with detector error models

Decoders of quantum error correction (QEC) experiments make decisions based on detected errors and the expected rates of error events, which together comprise a detector error model. Here we show that the syndrome history of QEC experiments is sufficient to detect and estimate coherent errors, removing the need for prior device benchmarking experiments. Importantly, our method shows that experimentally determined detector error models work equally well for both stochastic and coherent noise regimes. We model fully-coherent or fully-stochastic noise for repetition and surface codes and for various phenomenological and circuit-level noise scenarios, by employing Majorana and Monte Carlo simulators. We capture the interference of coherent errors, which appears as enhanced or suppressed physical error rates compared to the stochastic case, and also observe hyperedges that do not appear in the corresponding Pauli-twirled models. Finally, we decode the detector error models undergoing coherent noise and find different thresholds compared to detector error models built based on the stochastic noise assumption.

quant-ph↗

arXiv2025

Stabilizer configuration interaction: Finding molecular subspaces with error detection properties

In this work, we explore a new approach to designing both algorithms and error detection codes for preparing approximate ground states of molecules. We propose a classical algorithm to find the optimal stabilizer state by using excitations of the Hartree-Fock state, followed by constructing quantum error-detection codes based on this stabilizer state using codeword-stabilized codes. Through various numerical experiments, we confirm that our method finds the best stabilizer approximations to the true ground states of molecules up to 36 qubits in size. Additionally, we construct generalized stabilizer states that offer a better approximation to the true ground states. Furthermore, for a simple noise model, we demonstrate that both the stabilizer and (some) generalized stabilizer states can be prepared with higher fidelity using the error-detection codes we construct. Our work represents a promising step toward designing algorithms for early fault-tolerant quantum computation.

quant-ph↗

arXiv2025

Leveraging commuting groups for an efficient variational Hamiltonian ansatz

Efficiently calculating the low-lying eigenvalues of Hamiltonians, written as sums of Pauli operators, is a fundamental challenge in quantum computing. While various methods have been proposed to reduce the complexity of quantum circuits for this task, there remains room for further improvement. In this article, we introduce a new circuit design using commuting groups within the Hamiltonian to further reduce the circuit complexity of Hamiltonian-based quantum circuits. Our approach involves partitioning the Pauli operators into mutually commuting clusters and finding Clifford unitaries that diagonalize each cluster. We then design an ansatz that uses these Clifford unitaries for efficient switching between the clusters, complemented by a layer of parameterized single qubit rotations for each individual cluster. By conducting numerical simulations, we demonstrate the effectiveness of our method in accurately determining the ground state energy of different quantum chemistry Hamiltonians. Our results highlight the applicability and potential of our approach for designing problem-inspired ansatz for various quantum computing applications.

quant-ph↗

arXiv2025

Towards a pulse-level intermediate representation for diverse quantum control systems

A system-independent intermediate representation (IR) for pulse-level programming of quantum control systems is required to enable rapid development and reuse of quantum software across diverse platforms. In this work, we demonstrate the utility of pulselib as a candidate for such an IR. We implement graph-based IRs and transpilation pipelines for two unique frequency synthesizers and benchmark performance against existing IRs. Key elements of these pipelines are munchers and parametrizable pulse schedules. The former encodes target-specific constraints and allows translation of fundamentally system-agnostic pulse descriptions to arbitrary low-level representations, and the latter enables schedule reuse that produces savings in transpilation time relative to device-specific alternatives. Benchmarks reveal that pulselib provides performance comparable to fast, device-specific IRs while providing a speedup of up to 4.5x over existing IRs. For highly parametrized applications, pulselib provides favorable scaling of transpilation times with respect to the number of parameters and can exhibit speedups relative to existing IRs up to 69% larger than speedups provided by optimized, device-specific techniques.

quant-ph↗

arXiv2025

Tailored Quantum Device Calibration with Statistical Model Checking

Quantum devices require precisely calibrated analog signals, a process that is complex and time-consuming. Many calibration strategies exist, and all require careful analysis and tuning to optimize system availability. To enable rigorous statistical evaluation of quantum calibration procedures, we leverage statistical model checking (SMC), a technique used in fields that require statistical guarantees. SMC allows for probabilistic evaluation of properties of interest, such as a certain parameter's time to failure. We extend the SMC for Processor Analysis (SPA) framework, which uses SMC for evaluation of classical systems, to create SPA for Quantum calibration (SPAQ) enabling simplified tuning and analysis of quantum system calibration. We focus on a directed acyclic graph-based calibration optimization scheme and demonstrate how to craft properties of interest for its analysis. We show how to use SPAQ to find lower bounds of time to failure information, hidden node dependencies, and parameter threshold values and use that information to improve simulated quantum system availability through calibration scheme adjustments.

quant-ph↗

arXiv2025

Estimating decoding graphs and hypergraphs of memory QEC experiments

Characterizing the error sources of quantum devices is essential for building reliable large-scale quantum architectures and tailoring error correction codes to the noise profile of the devices. Tomography techniques can provide detailed information on the noise or quality of quantum states but are typically both computationally and experimentally intensive with respect to the system's size. For QEC experiments, however, the information captured by a detector error model is sufficient for extracting the success rate of the experiment, as well as some information about the underlying noise. In this work, we estimate Pauli noise on detector error models, including hypergraphs, using only the syndrome statistics. We apply this method to well-known codes such as the repetition, surface, and 2D color codes. Under bare-ancilla syndrome extraction, two-point correlations are enough to reconstruct the detector error model for repetition or surface codes. For color codes or repetition codes under Steane-style syndrome extraction, we show how to extend the estimation method to multi-point correlators and extract the error rates of the hypergraphs. Finally, we find an increase in logical error suppression when we calibrate the decoder to noise fluctuations typically present in experiments.

quant-ph↗

arXiv2025

Logical coherence in 2D compass codes

2D compass codes are a family of quantum error-correcting codes that contain the Bacon-Shor codes, the $X$-Shor and $Z$-Shor codes, and the rotated surface codes. Previous numerical results suggest that the surface code has a constant accuracy and coherence threshold under uniform coherent rotation. However, having analytical proof supporting a constant threshold is still an open problem. It is analytically proven that the toric code can exponentially suppress logical coherence in the code distance $L$. However, the current analytical lower bound on the threshold for the rotation angle $θ$ is $|\sin(θ)| < 1/L$, which linearly vanishes in $L$ instead of being constant. We show that this lower bound is achievable by the $Z$-Shor code which does not have a threshold under stochastic noise. Compass codes provide a promising direction to improve on the previous bounds. We analytically determine thresholds for two new compass code families that provide upper and lower bounds to the rotated surface code's numerically established infidelity threshold. Furthermore, using a Majorana mode-based simulator, we use random families of compass codes to smoothly interpolate between the $Z$-Shor codes and the $X$-Shor codes.

quant-ph↗

arXiv2025

Hamiltonian-based graph-state ansatz for variational quantum algorithms

One promising application of near-term quantum devices is to prepare trial wavefunctions using short circuits for solving different problems via variational algorithms. For this purpose, we introduce a new circuit design that combines graph-based diagonalization circuits with arbitrary single-qubit rotation gates to get Hamiltonian-based graph states ansätze (H-GSA). We test the accuracy of the proposed ansatz in estimating ground state energies of various molecules of size up to 12-qubits. Additionally, we compare the gate count and parameter number complexity of the proposed ansatz against previously proposed schemes and find an order magnitude reduction in gate count complexity with slight increase in the number of parameters. Our work represents a significant step towards constructing compact quantum circuits with good trainability and convergence properties and applications in solving chemistry and physics problems.

quant-ph↗

arXiv2024

Precise Determination of Excited State Rotational Constants and Black-Body Thermometry in Coulomb Crystals of Ca$^+$ and CaH$^+$

We present high-resolution rovibronic spectroscopy of calcium monohydride molecular ions (CaH$^+$) co-trapped in a Coulomb crystal with calcium ions ($^{40}$Ca$^+$), focusing on rotational transitions in the $|X^1Σ^+, ν" = 0> \rightarrow |A^1Σ^+, ν' = 2>$ manifold. By resolving individual P and R branch transitions with record precision and using Fortrat analysis, we extract key spectroscopic constants for the excited state $|A^1Σ^+, ν' = 2>$, specifically, the band origin, the rotational constant, and the centrifugal correction. Additionally, we demonstrate the application of high-resolution rotational spectroscopy of CaH$^+$ presented here as an in-situ probe of local environmental temperature. We correlate the relative amplitudes of the observed transitions to the underlying thermalized ground-state rotational population distribution and extract the black-body radiation (BBR) temperature.

physics.atom-ph↗

arXiv2024

Experimental evidence for dipole-phonon quantum logic in a trapped calcium monoxide and calcium ion chain

Dipole-phonon quantum logic (DPQL) offers novel approaches for state preparation, measurement, and control of quantum information in molecular ion qubits. In this work, we demonstrate an experimental implementation of DPQL with a trapped calcium monoxide and calcium ion chain at room temperature. We present evidence for one DPQL signal in two hours of data collection. The signal rises clearly above the characterized noise level and has a lower bound on the statistical significance of 4.1$σ$. The rate of observation is limited by the low thermal population in the molecular ground rotational state.

quant-ph↗

arXiv2024

Qplacer: Frequency-Aware Component Placement for Superconducting Quantum Computers

Noisy Intermediate-Scale Quantum (NISQ) computers face a critical limitation in qubit numbers, hindering their progression towards large-scale and fault-tolerant quantum computing. A significant challenge impeding scaling is crosstalk, characterized by unwanted interactions among neighboring components on quantum chips, including qubits, resonators, and substrate. We motivate a general approach to systematically resolving multifaceted crosstalks in a limited substrate area. We propose Qplacer, a frequency-aware electrostatic-based placement framework tailored for superconducting quantum computers, to alleviate crosstalk by isolating these components in spatial and frequency domains alongside compact substrate design. Qplacer commences with a frequency assigner that ensures frequency domain isolation for qubits and resonators. It then incorporates a padding strategy and resonator partitioning for layout flexibility. Central to our approach is the conceptualization of quantum components as charged particles, enabling strategic spatial isolation through a 'frequency repulsive force' concept. Our results demonstrate that Qplacer carefully crafts the physical component layout in mitigating various crosstalk impacts while maintaining a compact substrate size. On various device topologies and NISQ benchmarks, Qplacer improves fidelity by an average of 36.7x and reduces spatial violations (susceptible to crosstalk) by an average of 12.76x, compared to classical placement engines. Regarding area optimization, compared to manual designs, Qplacer can reduce the required layout area by 2.14x on average

quant-ph↗

arXiv2024

Quantum Simulation of Spin-Boson Models with Structured Bath

The spin-boson model, involving spins interacting with a bath of quantum harmonic oscillators, is a widely used representation of open quantum systems. Trapped ions present a natural platform for simulating the quantum dynamics of such models, thanks to the presence of both high quality internal qubit states and the motional modes of the ions that can simulate the relevant quantum degrees of freedom. In our work, we extend the previous body of work that focused on coherent coupling of the spins and bosons to perform quantum simulations with structured dissipative baths using the motional states of trapped ions. We demonstrate the capability for adjusting the bath's temperature and continuous spectral density by adding randomness to fully programmable control parameters. Subsequently, we simulate the dynamics of various spin-boson models with noise spectral densities constructed from coupling to several dissipative harmonic oscillator modes. The experimental outcomes closely align with theoretical predictions, indicating successful simulation of open quantum systems using a trapped-ion system.

quant-ph↗

arXiv2024

Seeking a quantum advantage with trapped-ion quantum simulations of condensed-phase chemical dynamics

Simulating the quantum dynamics of molecules in the condensed phase represents a longstanding challenge in chemistry. Trapped-ion quantum systems may serve as a platform for the analog-quantum simulation of chemical dynamics that is beyond the reach of current classical-digital simulation. To identify a 'quantum advantage' for these simulations, performance analysis of both analog-quantum simulation on noisy hardware and classical-digital algorithms is needed. In this Review, we make a comparison between a noisy analog trapped-ion simulator and a few choice classical-digital methods on simulating the dynamics of a model molecular Hamiltonian with linear vibronic coupling. We describe several simple Hamiltonians that are commonly used to model molecular systems, which can be simulated with existing or emerging trapped-ion hardware. These Hamiltonians may serve as stepping stones toward the use of trapped-ion simulators for systems beyond the reach of classical-digital methods. Finally, we identify dynamical regimes where classical-digital simulations seem to have the weakest performance compared to analog-quantum simulations. These regimes may provide the lowest hanging fruit to exploit potential quantum advantages.

quant-ph↗

arXiv2024

Error correction of transversal CNOT gates for scalable surface code computation

Recent experimental advances have made it possible to implement logical multi-qubit transversal gates on surface codes in a multitude of platforms. A transversal controlled-NOT (tCNOT) gate on two surface codes introduces correlated errors across the code blocks and thus requires modified decoding strategies compared to established methods of decoding surface code quantum memory (SCQM) or lattice surgery operations. In this work, we examine and benchmark the performance of three different decoding strategies for the tCNOT for scalable, fault-tolerant quantum computation. In particular, we present a low-complexity decoder based on minimum-weight perfect matching (MWPM) that achieves the same threshold as the SCQM MWPM decoder. We extend our analysis with a study of tailored decoding of a transversal teleportation circuit, along with a comparison between the performance of lattice surgery and transversal operations under Pauli and erasure noise models. Our investigation works towards systematic estimation of the cost of implementing large-scale quantum algorithms based on transversal gates in the surface code.

quant-ph↗

arXiv2024

Graph-Based Pulse Representation for Diverse Quantum Control Hardware

Pulse-level control of quantum systems is critical for enabling gate implementations, calibration procedures, and Hamiltonian evolution which fundamentally are not supported by the traditional circuit model. This level of control necessitates both efficient generation and representation. In this work, we propose pulselib - a graph-based pulse-level representation. A graph structure, with nodes consisting of parametrized fundamental waveforms, stores all the high-level pulse information while staying flexible for translation into hardware-specific inputs. We motivate pulselib by comparing its feature set and information flow through the pulse layer of the software stack with currently available pulse representations. We describe the architecture of this proposed representation that mimics the abstract syntax tree (AST) model from classical compilation pipelines. Finally, we outline applications like trapped-ion-specific gate and shelving pulse schemes whose constraints and implementation can be written and represented due to pulselib's graph-based architecture.

quant-ph↗

arXiv2024

One-Time Compilation of Device-Level Instructions for Quantum Subroutines

A large class of problems in the current era of quantum devices involve interfacing between the quantum and classical system. These include calibration procedures, characterization routines, and variational algorithms. The control in these routines iteratively switches between the classical and the quantum computer. This results in the repeated compilation of the program that runs on the quantum system, scaling directly with the number of circuits and iterations. The repeated compilation results in a significant overhead throughout the routine. In practice, the total runtime of the program (classical compilation plus quantum execution) has an additional cost proportional to the circuit count. At practical scales, this can dominate the round-trip CPU-QPU time, between 5% and 80%, depending on the proportion of quantum execution time. To avoid repeated device-level compilation, we identify that machine code can be parametrized corresponding to pulse/gate parameters which can be dynamically adjusted during execution. Therefore, we develop a device-level partial-compilation (DLPC) technique that reduces compilation overhead to nearly constant, by using cheap remote procedure calls (RPC) from the QPU control software to the CPU. We then demonstrate the performance speedup of this on optimal pulse calibration, system characterization using randomized benchmarking (RB), and variational algorithms. We execute this modified pipeline on real trapped-ion quantum computers and observe significant reductions in compilation time, as much as 2.7x speedup for small-scale VQE problems.

quant-ph↗

arXiv2024

Concatenated Steane code with single-flag syndrome checks

A fault-tolerant error correction (FTEC) protocol with a high error suppression rate and low overhead is very desirable for the near-term implementation of quantum computers. In this work, we develop a distance-preserving flag FTEC protocol for the [[49,1,9]] concatenated Steane code, which requires only two ancilla qubits per generator and can be implemented on a planar layout. We generalize the weight-parity error correction (WPEC) technique from [Phys. Rev. A 104, 042410 (2021)] and find a gate ordering of flag circuits for the concatenated Steane code which makes syndrome extraction with two ancilla qubits per generator possible. The FTEC protocol is constructed using the optimization tools for flag FTEC developed in [PRX Quantum 5, 020336 (2024)] and is simulated under the circuit-level noise model without idling noise. Our simulations give a pseudothreshold of $1.64 \times 10^{-3}$ for the [[49,1,9]] concatenated Steane code, which is better than a pseudothreshold of $1.43 \times 10^{-3}$ for the [[61,1,9]] 6.6.6 color code simulated under the same settings. This is in contrast to the code capacity model where the [[61,1,9]] code performs better.

quant-ph↗