arXiv subjects

R. T. Sutherland

Publications and source records attributed to R. T. Sutherland.

18 recordsLinked to original sources

arXiv2025

Trapped-ion two-qubit gates with >99.99% fidelity without ground-state cooling

We introduce the 'smooth gate', an entangling method for trapped-ion qubits where residual spin-motion entanglement errors are adiabatically eliminated by ramping the gate detuning. We demonstrate electronically controlled two-qubit gates with an estimated error of $8.4(7)\times10^{-5}$ without ground-state cooling. We further show that the error remains $\lesssim 5\times10^{-4}$ for ions with average phonon occupation up to $\bar{n}=9.4(3)$ on the gate mode. These results indicate that trapped-ion quantum computation can achieve high fidelity at temperatures above the Doppler limit, which enables faster and simpler device operation.

quant-ph↗

arXiv2025

Subspace Leakage Error Randomized Benchmarking of M{\o}lmer-S{\o}rensen Gates

We demonstrate a new technique that adapts single-qubit randomized benchmarking to two-qubit M{\o}lmer-S{\o}rensen gates. We use the controllable gate phase to generate Cliffords that act on a two-state subspace, enabling benchmarking of two-qubit gates without single-qubit operations. In addition to quantifying the gate infidelity, the protocol provides valuable information about the type of error by distinguishing between those that conserve the two-state subspace and those that result in leakage out of it. We demonstrate the protocol for calibrating and validating all-electronic maximally entangling gates in a trapped-ion quantum computer, achieving a two-qubit gate error of $2.6 (2)\times10^{-4}$.

quant-ph↗

arXiv2024

Cooling trapped ions with phonon rapid adiabatic passage

In recent demonstrations of the quantum charge-coupled device (QCCD) computer architecture, circuit times are dominated by cooling. Some motional modes of multi-ion crystals take orders-of-magnitude longer to cool than others because of low coolant ion participation. Here we demonstrate a new technique, which we call phonon rapid adiabatic passage (phrap), that avoids this issue by coherently exchanging the thermal populations of selected modes on timescales short compared to direct cooling. Analogous to adiabatic rapid passage, we quasi-statically couple these slow-cooling modes with fast-cooling ones using DC electric fields. When the crystal is then adiabatically ramped through the resultant avoided crossing, nearly-complete phonon population exchange results. We demonstrate this on two-ion crystals, and show the indirect ground-state cooling of all radial modes--achieving an order of magnitude speedup compared to direct cooling. We also show the technique's insensitivity to trap potential and control field fluctuations, and find that it still achieves sub-quanta temperatures starting as high as n~200.

quant-ph↗

arXiv2022

Synthesizing a $\hatσ_z$ spin-dependent force for optical, metastable, and ground state trapped-ion qubits

A single bichromatic field near-resonant to a qubit transition is typically used for $\hatσ_x$ or $\hatσ_y$ Mølmer-Sørensen type interactions in trapped ion systems. Using this field configuration, it is also possible to synthesize a $\hatσ_z$ spin-dependent force by merely adjusting the beat-note frequency. Here, we expand on previous work and present a comprehensive theoretical and experimental investigation of this scheme with a laser near-resonant to a quadrupole transition in $^{88}$Sr$^+$. Further, we characterise its robustness to optical phase and qubit frequency offsets, and demonstrate its versatility by entangling optical, metastable, and ground state qubits.

quant-ph↗

arXiv2022

Individual addressing of trapped ion qubits with geometric phase gates

We propose a new scheme for individual addressing of trapped ion qubits, selecting them via their motional frequency. We show that geometric phase gates can perform single-qubit rotations using the coherent interference of spin-independent and (global) spin-dependent forces. The spin-independent forces, which can be generated via localised electric fields, increase the gate speed while reducing its sensitivity to motional decoherence, which we show analytically and numerically. While the scheme applies to most trapped ion experimental setups, we numerically simulate a specific laser-free implementation, showing cross-talk errors below $10^{-6}$ for reasonable parameters.

quant-ph↗

arXiv2021

Laser driven superradiant ensembles of two-level atoms near Dicke's regime

We report the experimental observation of superradiant emission emanating from an elongated dense ensemble of laser cooled two-level atoms, with a radial extent smaller than the transition wavelength. In the presence of a strong driving laser, we observe that the system is superradiant along its symmmetry axis. This occurs even though the driving laser is orthogonal to the superradiance direction. This superradiance modifies the spontaneous emission, and, resultantly, the Rabi oscillations. We also investigate Dicke superradiance in the emission of an almost fully-inverted system as a function of atom numnber. The experimental results are in qualitative agreement with ab-initio, beyond-mean-field calculations.

physics.atom-ph↗

arXiv2021

Universal hybrid quantum computing in trapped ions

Using discrete and continuous variable subsystems, hybrid approaches to quantum information could enable more quantum computational power for the same physical resources. Here, we propose a hybrid scheme that can be used to generate the necessary Gaussian and non-Gaussian operations for universal continuous variable quantum computing in trapped ions. This scheme utilizes two linear spin-motion interactions to generate a broad set of non-linear effective spin-motion interactions including one and two mode squeezing, beam splitter, and trisqueezing operations in trapped ion systems. We discuss possible experimental implementations using laser-based and laser-free approaches.

quant-ph↗

arXiv2021

Motional squeezing for trapped ion transport and separation

Transport, separation, and merging of trapped ion crystals are essential operations for most large-scale quantum computing architectures. In this work, we develop a theoretical framework that describes the dynamics of ions in time-varying potentials with a motional squeeze operator, followed by a motional displacement operator. Using this framework, we develop a new, general protocol for trapped ion transport, separation, and merging. We show that motional squeezing can prepare an ion wave packet to enable transfer from the ground state of one trapping potential to another. The framework and protocol are applicable if the potential is harmonic over the extent of the ion wave packets at all times. As illustrations, we discuss two specific operations: changing the strength of the confining potential for a single ion, and separating same-species ions with their mutual Coulomb force. Both of these operations are, ideally, free of residual motional excitation.

quant-ph↗

arXiv2021

High-fidelity laser-free universal control of two trapped ion qubits

Universal control of multiple qubits -- the ability to entangle qubits and to perform arbitrary individual qubit operations -- is a fundamental resource for quantum computation, simulation, and networking. Here, we implement a new laser-free scheme for universal control of trapped ion qubits based on microwave magnetic fields and radiofrequency magnetic field gradients. We demonstrate high-fidelity entanglement and individual control by creating symmetric and antisymmetric two-qubit maximally entangled states with fidelities in the intervals [0.9983, 1] and [0.9964, 0.9988], respectively, at 68% confidence, corrected for state initialization error. This technique is robust against multiple sources of decoherence, usable with essentially any trapped ion species, and has the potential to perform simultaneous entangling operations on many pairs of ions without increasing control signal power or complexity.

quant-ph↗

arXiv2020

Laser-free trapped-ion entangling gates with simultaneous insensitivity to qubit and motional decoherence

The dominant error sources for state-of-the-art laser-free trapped-ion entangling gates are decoherence of the qubit state and the ion motion. The effect of these decoherence mechanisms can be suppressed with additional control fields, or with techniques that have the disadvantage of reducing gate speed. Here, we propose using a near-motional-frequency magnetic field gradient to implement a laser-free gate that is simultaneously resilient to both types of decoherence, does not require additional control fields, and has a relatively small cost in gate speed.

quant-ph↗

arXiv2019

Analogue quantum simulation of superradiance and subradiance in trapped-ions

We discuss a protocol for the analogue quantum simulation of superradiance and subradiance using a linear chain of N trapped qubit ions with a single sympathetic cooling ion. We develop a simple analytic model that shows the dynamics of the qubit subspace converge to those of a cloud undergoing Dicke superradiance and subradiance. We provide numerical simulations of the full ion chain and show that they converge to the dynamics predicted by our analytic model with no fitting parameters. We also map out the parameter regime needed to reach this convergence.

physics.atom-ph↗

arXiv2019

Photon scattering from a cold, Gaussian atom cloud

We study the effect of a weakly driven atomic cloud's polarization distribution on its photon scattering lineshape. In doing this, we find three distinct polarization regimes. First, for dilute clouds, the polarization magnitude is relatively constant. Second, for denser clouds, polarization builds at the front of the cloud for near-resonant light. Third, when the cloud condenses to the point where its dimensions become comparable to the wavelength, light refocuses towards the back of the cloud for red detuning. For these regimes, we show which `mean-field' frameworks accurately describe the differing photon scattering lineshapes. Finally, for even denser clouds, mean field models become inaccurate and necessitate the full point dipole model that includes atom-atom correlations.

physics.atom-ph↗

arXiv2019

Trapped-ion spin-motion coupling with microwaves and a near-motional oscillating magnetic field gradient

We present a new method of spin-motion coupling for trapped ions using microwaves and a magnetic field gradient oscillating close to the ions' motional frequency. We demonstrate and characterize this coupling experimentally using a single ion in a surface-electrode trap that incorporates current-carrying electrodes to generate the microwave field and the oscillating magnetic field gradient. Using this method, we perform resolved-sideband cooling of a single motional mode to its ground state.

quant-ph↗

arXiv2019

Versatile laser-free trapped-ion entangling gates

We present a general theory for laser-free entangling gates with trapped-ion hyperfine qubits, using either static or oscillating magnetic-field gradients combined with a pair of uniform microwave fields symmetrically detuned about the qubit frequency. By transforming into a `bichromatic' interaction picture, we show that either ${\hatσ_ϕ\otimes\hatσ_ϕ}$ or ${\hatσ_{z}\otimes\hatσ_{z}}$ geometric phase gates can be performed. The gate basis is determined by selecting the microwave detuning. The driving parameters can be tuned to provide intrinsic dynamical decoupling from qubit frequency fluctuations. The ${\hatσ_{z}\otimes\hatσ_{z}}$ gates can be implemented in a novel manner which eases experimental constraints. We present numerical simulations of gate fidelities assuming realistic parameters.

quant-ph↗

arXiv2018

Gain measurement scheme for precise determination of atomic parity violation through two-pathway coherent control

Precision measurements of parity non-conserving (PNC) interactions in atoms, molecules and ions can lead to the discovery of new physics beyond the standard model and understanding of weak-force induced interactions in the nucleus. In this paper, we propose and analyze a novel atomic parity violation measurement scheme for a forbidden transition where we combine a two-pathway coherent control mechanism with probe gain techniques. We detail a feasible experimental geometry for $6S_{1/2}\rightarrow 7S_{1/2}$ transitions in a cesium vapor cell, and consider the statistical noise of such a measurement under reasonable laboratory conditions. We estimate the signal-to-noise ratio to be approaching $\sim2.3/\sqrt{Hz}$. This scheme, with low expected systematic errors, would allow for precise measurements in cesium and other heavy metal systems.

physics.atom-ph↗

arXiv2017

Degenerate Zeeman Ground States in the Single-Excitation Regime

In this work, we demonstrate the importance of considering correlations between degenerate Zeeman sublevels that develop in dense atomic ensembles. In order to do this, we develop a set of equations capable of simulating large numbers of atoms while still incorporating correlations between degenerate Zeeman sublevels. This set of equations is exact in the single-photon limit, and may be interpreted as a generalization of the coupled harmonic oscillator equations typically used the literature. Using these equations, we demonstrate that in sufficiently dense systems, correlations between Zeeman sublevels can cause non-trivial differences in the photon scattering lineshape in arrays and clouds of atoms.

physics.atom-ph↗

arXiv2016

Coherent Forward Broadening in Cold Atom Clouds

It is shown that homogeneous line-broadening in a diffuse cold atom cloud is proportional to the resonant optical depth of the cloud. Further, it is demonstrated how the strong directionality of the coherent interactions causes the cloud's spectra to depend strongly on its shape, even when the cloud is held at constant densities. These two numerical observations can be predicted analytically by extending the single photon wave-function model. Lastly, elongating a cloud along the line of laser propagation causes the excitation probability distribution to deviate from the exponential decay predicted by the Beer-Lambert law to the extent where the atoms in the back of the cloud are more excited than the atoms in the front. These calculations are conducted at low densities relevant to recent experiments.

physics.atom-ph↗

arXiv2016

Collective Dipole-Dipole Interactions in an Atomic Array

The coherent dipole-dipole interactions of atoms in an atomic array are studied. It is found that the excitation probability of an atom in an array parallel to the direction of laser propagation ($\boldsymbol{\hat{k}}$) will either grow or decay logarithmically along $\boldsymbol{\hat{k}}$, depending on the detuning of the laser. The symmetry of the system for atomic separations of $δr = jλ/2$, where $j$ is an integer, causes the excitation distribution and scattered radiation to abruptly become symmetric about the center of the array. For atomic separations of $δr < λ/2$, the appearance of a collection of extremely subradiant states ($Γ\sim 0$), disrupts the described trend. In order to interpret the results from a finite array of atoms, a band structure calculation in the $N\rightarrow \infty$ limit is conducted where the decay rates and the Collective Lamb Shifts of the eigenmodes along the Brillouin zone are shown. Finally, the band structure of an array strongly affects its scattered radiation, allowing one to manipulate the Collective Lamb Shift as well as the decay rate (from superradiant to subradiant) by changing the angle of the driving laser.

physics.atom-ph↗