arXiv subjects

Kevin C. Cox

Publications and source records attributed to Kevin C. Cox.

At least 19 recordsLinked to original sources

arXiv2026

Robust cavity metrology beyond the limits of Pound-Drever-Hall

Precise measurement of the resonance frequencies of microwave and optical cavities is a foundational capability across quantum technologies. One of the highest-performance methods currently available is Pound-Drever-Hall (PDH) spectroscopy, which relies on a three-tone interrogation and eliminates sensitivity to path-length fluctuations. However, PDH is known to suffer from systematic offsets due to residual amplitude modulation and demodulation-phase errors. Here we leverage an optimal linear combination of the phases of the three interrogation tones to implement a method for robust cavity metrology which eliminates significant systematic-error sensitivities of PDH. We experimentally demonstrate robust cavity measurements in both the optical and microwave frequency regime, showing insensitivity to sideband imbalance and demodulation phase, as well as demonstrating single-shot readout of a cavity-coupled superconducting qubit.

quant-ph↗

arXiv2026

Approximate simulation of complex quantum circuits using sparse tensors

The study of quantum circuit simulation using classical computers is a key research topic that helps define the boundary of verifiable quantum advantage, solve quantum many-body problems, and inform development of quantum hardware and software. Tensor networks have become forefront mathematical tools for these tasks. Here we introduce a method to approximately simulate quantum circuits using sparsely-populated tensors. We describe a sparse tensor data structure that can represent quantum states with no underlying symmetry, and outline algorithms to efficiently contract and truncate these tensors. We show that the data structure and contraction algorithm are efficient, leading to expected runtime scalings versus qubit number and circuit depth. Our results motivate future research in optimization of sparse tensor networks for quantum simulation.

quant-ph↗

arXiv2025

The Pound-Drever-Hall Method for Superconducting-Qubit Readout

Scaling quantum computers to large sizes requires the implementation of many parallel qubit readouts. Here we present an ultrastable superconducting-qubit readout method using the multi-tone self-phase-referenced Pound-Drever-Hall (PDH) technique, originally developed for use with optical cavities. In this work, we benchmark PDH readout of a single transmon qubit, using room-temperature heterodyne detection of all tones to reconstruct the PDH signal. We demonstrate that PDH qubit readout is insensitive to microwave phase drift, displaying $0.73^\circ$ phase stability over 2 hours, and capable of single-shot readout in the presence of phase errors exceeding the phase shift induced by the qubit state. We show that the PDH sideband tones do not cause unwanted measurement-induced state transitions for a transmon qubit, leading to a potential signal enhancement of at least $14$~dB.

quant-ph↗

arXiv2025

RydIQule Version 2: Enhancing graph-based modeling of Rydberg atoms

Rydberg atomic radio-frequency (rf) sensors are an emerging technology platform that relies on vaporous atoms, interrogated with laser beams and nearly ionized, to receive rf signals. Rydberg rf sensors have a number of interesting fundamental distinctions from traditional receiver technologies, such as those based on metallic antennas, since they are governed by the quantum physics of atom-light interactions. As Rydberg sensors quickly advance from laboratory experiments into fieldable devices, there is a need for a general software modeling tool that fully encompasses the internal physics of the sensor. The Rydberg Interactive Quantum Module (RydIQule) is a Python package designed to fill this need. The initial public release of RydIQule in late 2023 built the core functionality described above. Here we outline RydIQule's version 2 release which expands on its capabilities to more accurately model real-world atoms.

physics.atom-ph↗

arXiv2025

Electrically-Small Rydberg Sensor for Three-Dimensional Determination of rf k-Vectors

We present an electrically-small Rydberg atom electric field sensor with the ability to extract the three-dimensional $k$-vector of an elliptically polarized radio frequency (rf) field. In most mediums, the $k$-vector (or wave vector) provides the direction of propagation of an electromagnetic wave. Our method uses a field vector measurement at a single point in space and is thus compatible with a sensor volume that is arbitrarily small compared to the carrier wavelength. We measure the $k$-vector of a circularly polarized signal field with average absolute errors in the polar and azimuthal angles of $33~\text{mrad}$ and $43~\text{mrad}$ respectively, and statistical noise of $1.3~\text{mrad}/\sqrt{\text{Hz}}$ and $1.5~\text{mrad}/\sqrt{\text{Hz}}$. Additionally, we characterize the performance of the sensor as a function of the ellipticity of the input field and the size of the sensing region. We find that the sensor works over a broad range of ellipticities and validate that an electrically-small sensing region is optimal.

physics.atom-ph↗

arXiv2024

Complete three-dimensional vector polarimetry with a Rydberg atom rf electrometer

Radio frequency (rf) receivers using Rydberg atoms offer appealing features over classical sensors, such as their size, frequency tuning range, and lack of field absorption. In this work, we extend the application space by demonstrating a Rydberg atom rf polarimeter. Using rf heterodyne with three independent and orthogonal local oscillators, we are able to extract the polarization ellipse of the field in three dimensions, in addition to the total amplitude of the field. We use the relative phases and amplitudes of the three generated heterodyne signals to measure the field amplitudes and phases along three cardinal axes giving the full three-dimensional polarization. We demonstrate this polarization measurement for incoming fields at different angles around the sensor. Lastly, we investigate the reception of data symbols encoded in the horizontal and vertical signal field polarizations and the phase between them. Our measurements yield an amplitude noise of 57 $μ$V/m/$\sqrt{\text{Hz}}$ for horizontal polarization, 66 $μ$V/m/$\sqrt{\text{Hz}}$ for vertical polarization, and a standard deviation of 0.094 rad in the phase between the field components.

physics.atom-ph↗

arXiv2023

Quantum Sensing with Erasure Qubits

The dominant noise in an "erasure qubit" is an erasure -- a type of error whose occurrence and location can be detected. Erasure qubits have potential to reduce the overhead associated with fault tolerance. To date, research on erasure qubits has primarily focused on quantum computing and quantum networking applications. Here, we consider the applicability of erasure qubits to quantum sensing and metrology. We show theoretically that, for the same level of noise, an erasure qubit acts as a more precise sensor or clock compared to its non-erasure counterpart. We experimentally demonstrate this by artificially injecting either erasure errors (in the form of atom loss) or dephasing errors into a differential optical lattice clock comparison, and observe enhanced precision in the case of erasure errors for the same injected error rate. Similar benefits of erasure qubits to sensing can be realized in other quantum platforms like Rydberg atoms and superconducting qubits

quant-ph↗

arXiv2023

RydIQule: A Graph-based Paradigm for Modelling Rydberg and Atomic Systems

We describe a numerical technique and accompanying open-source Python software package called RydIQule. RydIQule uses a directional graph, relying on adjacency matrices and path-finding to generate a Hamiltonian for multi-level atomic systems. RydIQule then constructs semi-classical equations of motion (Bloch equations) into a tensor which can store an entire simulation consisting of varied system parameters. Using this framework, RydIQule returns solutions significantly faster than typical for interpreted programming languages. RydIQule extends beyond the capabilities of currently-available tools, facilitating rapid development in atomic and Rydberg spectroscopy. To demonstrate its utility, we use RydIQule to simulate a Doppler-broadened Rydberg atomic sensor that simultaneously demodulates five rf tones spanning from 1.7 to 116 GHz. Using RydIQule, this simulation can be solved in several hours on a commercial off-the-shelf desktop computer.

physics.atom-ph↗

arXiv2023

Spatiotemporal Multiplexed Rydberg Receiver

Rydberg states of alkali atoms, where the outer valence electron is excited to high principal quantum numbers, have large electric dipole moments allowing them to be used as sensitive, wideband, electric field sensors. These sensors use electromagnetically induced transparency (EIT) to measure incident electric fields. The characteristic timescale necessary to establish EIT determines the effective speed at which the atoms respond to time-varying RF radiation. Previous studies have predicted that this EIT relaxation rate causes a performance roll-off in EIT-based sensors beginning at a less than 10 MHz RF data symbol rate. Here, we propose an architecture for increasing the response speed of Rydberg sensors to greater than 100 MHz, through spatio-temporal multiplexing (STM) of the probe laser. We present experimental results validating the architecture's temporal multiplexing component using a pulsed laser. We benchmark a numerical model of the sensor to this experimental data and use the model to predict the STM sensor's performance as an RF communications receiver. For an on-off keyed (OOK) waveform, we use the numerical model to predict bit-error-ratios (BERs) as a function of RF power and data rates demonstrating feasibility of error free communications up to 100 Mbps with an STM Rydberg sensor.

quant-ph↗

arXiv2023

Simultaneous Multi-Band Demodulation Using a Rydberg Atomic Sensor

Electric field sensors based on Rydberg atoms offer unique capabilities, relative to traditional sensors, for detecting radio-frequency signals. In this work, we demonstrate simultaneous demodulation and detection of five rf tones spanning nearly two decades (6 octaves), from 1.7 GHz to 116 GHz. We show continuous recovery of the phase and amplitude of each tone and report on the system's sensitivity and bandwidth capabilities for multi-band detection. We use these capabilities to demonstrate a digital communication protocol, simultaneously receiving on-off-keyed binary data from four bands spanning over one decade of frequency.

physics.atom-ph↗

arXiv2022

Intra-Cavity Frequency-Doubled VECSEL System for Narrow Linewidth Rydberg EIT Spectroscopy

Vertical external-cavity surface-emitting lasers (VECSELs) augmented by intra-cavity nonlinear optical frequency conversion have emerged as an attractive light source of ultraviolet to visible light for demanding scientific applications, relative to other laser technologies. They offer high power, low phase noise, wide frequency tunability, and excellent beam quality in a simple and inexpensive system architecture. Here, we characterize the frequency stability of an intra-cavity frequency-doubled VECSEL with 690 mW of output power at 475 nm using the delayed self-heterodyne technique and direct comparison with a commercial external-cavity diode laser (ECDL). We measure the fundamental's Lorentzian linewidth to be $2π\times5.3(2)$ kHz, and the total linewidth to be $2π\times23(2)$ kHz. In addition, we perform Rydberg-state spectroscopy via electromagnetically induced transparency (EIT), observing narrow 3.5 MHz full-width half-maximum EIT. By doing so, we demonstrate that intra-cavity frequency-doubled VECSELs can perform precision spectroscopy at the MHz level, and are a promising tool for contemporary, and future, quantum technologies.

physics.atom-ph↗

arXiv2021

Optimal Atomic Quantum Sensing using EIT Readout

Quantum sensors offer the capability to reach unprecedented precision by operating at the standard quantum limit (SQL) or beyond by using quantum entanglement. But an emerging class of quantum sensors that use Rydberg electromagnetically-induced transparency (EIT) to detect rf electric fields have yet to reach the SQL. In this work we prove that this discrepancy is due to fundamental limitations in the EIT probing mechanism. We derive the optimum sensitivity of a three-level quantum sensor based on EIT, or more generally coherent spectroscopy, and compare this to the SQL. We apply a minimal set of assumptions, while allowing strong probing fields, thermal broadening, and large optical depth. We derive the optimal laser intensities and optical depth, providing specific guidelines for sensitive operation under common experimental conditions. Clear boundaries of performance are established, revealing that ladder-EIT can not achieve the SQL due to unavoidable absorption loss. The results may be applied to any EIT-based quantum sensor, but we particularly emphasize our results' importance to the growing field of Rydberg quantum sensing.

physics.atom-ph↗

arXiv2021

Linear and continuous variable spin-wave processing using a cavity-coupled atomic ensemble

Spin-wave excitations in ensembles of atoms are gaining attention as a quantum information resource. However, current techniques with atomic spin waves do not achieve universal quantum information processing. We conduct a theoretical analysis of methods to create a high-capacity universal quantum processor and network node using an ensemble of laser-cooled atoms, trapped in a one-dimensional periodic potential and coupled to a ring cavity. We describe how to establish linear quantum processing using a lambda-scheme in a rubidium-atom system, calculate the expected experimental operational fidelities. Second, we derive an efficient method to achieve linear controllability with a single ensemble of atoms, rather than two-ensembles as proposed in [K. C. Cox et al. Spin-Wave Quantum Computing with Atoms in a Single-Mode Cavity, preprint 2021]. Finally, we propose to use the spin-wave processor for continuous-variable quantum information processing and present a scheme to generate large dual-rail cluster states useful for deterministic computing.

physics.atom-ph↗

arXiv2021

Spin-Wave Quantum Computing with Atoms in a Single-Mode Cavity

We present a method for network-capable quantum computing that relies on holographic spin-wave excitations stored collectively in ensembles of qubits. We construct an orthogonal basis of spin waves in a one-dimensional array and show that high-fidelity universal linear controllability can be achieved using only phase shifts, applied in both momentum and position space. Neither single-site addressability nor high single-qubit cooperativity is required, and the spin waves can be read out with high efficiency into a single cavity mode for quantum computing and networking applications.

physics.atom-ph↗

arXiv2021

Waveguide-coupled Rydberg spectrum analyzer from 0 to 20 GHz

We demonstrate an atomic radio-frequency (RF) receiver and spectrum analyzer based on thermal Rydberg atoms coupled to a planar microwave waveguide. We use an off-resonant RF heterodyne technique to achieve continuous operation for carrier frequencies ranging from DC to 20 GHz. The system achieves an intrinsic sensitivity of up to -120(2) dBm/Hz, DC coupling, 4 MHz instantaneous bandwidth, and over 80 dB of linear dynamic range. By connecting through a low-noise preamplifier, we demonstrate high-performance spectrum analysis with peak sensitivity of better than -145 dBm/Hz. Attaching a standard rabbit-ears antenna, the spectrum analyzer detects weak ambient signals including FM radio, AM radio, Wi-Fi, and Bluetooth. We also demonstrate waveguide-readout of the thermal Rydberg ensemble by non-destructively probing waveguide-atom interactions. The system opens the door for small, room-temperature, ensemble-based Rydberg sensors that surpass the sensitivity, bandwidth, and precision limitations of standard RF sensors, receivers, and analyzers.

physics.atom-ph↗

arXiv2020

Assessment of Rydberg Atoms for Wideband Electric Field Sensing

Rydberg atoms have attracted significant interest recently as electric field sensors. In order to assess potential applications, detailed understanding of relevant figures of merit is necessary, particularly in relation to other, more mature, sensor technologies. Here we present a quantitative analysis of the Rydberg sensor's sensitivity to oscillating electric fields with frequencies between 1 kHz and 1 THz. Sensitivity is calculated using a combination of analytical and semi-classical Floquet models. Using these models, optimal sensitivity at arbitrary field frequency is determined. We validate the numeric Floquet model via experimental Rydberg sensor measurements over a range of 1-20 GHz. Using analytical models, we compare with two prominent electric field sensor technologies: electro-optic crystals and dipole antenna-coupled passive electronics.

physics.atom-ph↗

arXiv2019

Spin-Wave Multiplexed Atom-Cavity Electrodynamics

We introduce multiplexed atom-cavity quantum electrodynamics with an atomic ensemble coupled to a single optical cavity mode. Multiple Raman dressing beams establish cavity-coupled spin-wave excitations with distinctive spatial profiles. Experimentally, we demonstrate the concept by observing spin-wave vacuum Rabi splittings, selective superradiance, and interference in the cavity-mediated interactions of two spin waves. We highlight that the current experimental configuration allows rapid, interchangeable cavity-coupling to 4 profiles with an overlap parameter of less than 10%, enough to demonstrate, for example, a quantum repeater network simulation in the cavity. With further improvements to the optical multiplexing setup, we infer the ability to access more than $10^3$ independent spin-wave profiles.

physics.atom-ph↗

arXiv2019

Spatial Multiplexing in a Cavity-Enhanced Quantum Memory

We present a multiplexed quantum repeater protocol based on an ensemble of laser-cooled and trapped rubidium atoms inside an optical ring cavity. We have already demonstrated strong collective coupling in such a system and have constructed a multiplexing apparatus based on a two-dimensional acousto-optical deflector. Here, we show how this system could enable a multiplexed quantum repeater using collective excitations with non-trivial spatial phase profiles (spinwaves). Calculated entanglement generation rates over long distances reveal that such a multiplexed ensemble-cavity platform is a promising route towards long distance quantum entanglement and networking.

physics.atom-ph↗