arXiv subjects

R. P. Anderson

Publications and source records attributed to R. P. Anderson.

At least 19 recordsLinked to original sources

arXiv2023

Enhancing the sensitivity of atom-interferometric inertial sensors using robust control

Atom-interferometric quantum sensors could revolutionize navigation, civil engineering, and Earth observation. However, operation in real-world environments is challenging due to external interference, platform noise, and constraints on size, weight, and power. Here we experimentally demonstrate that tailored light pulses designed using robust control techniques mitigate significant error sources in an atom-interferometric accelerometer. To mimic the effect of unpredictable lateral platform motion, we apply laser-intensity noise that varies up to 20$\%$ from pulse-to-pulse. Our robust control solution maintains performant sensing, while the utility of conventional pulses collapses. By measuring local gravity, we show that our robust pulses preserve interferometer scale factor and improve measurement precision by 10$\times$ in the presence of this noise. We further validate these enhancements by measuring applied accelerations over a 200 $μg$ range up to 21$\times$ more precisely at the highest applied noise level. Our demonstration provides a pathway to improved atom-interferometric inertial sensing in real-world settings.

quant-ph↗

arXiv2023

Wavefront Curvature in Optical Atomic Beam Clocks

Atomic clocks provide a reproducible basis for our understanding of time and frequency. Recent demonstrations of compact optical clocks, employing thermal atomic beams, have achieved short-term fractional frequency instabilities in the $10^{-16}$, competitive with the best international frequency standards available. However, a serious challenge inherent in compact clocks is the necessarily smaller optical beams, which results in rapid variation in interrogating wavefronts. This can cause inhomogeneous excitation of the thermal beam leading to long term drifts in the output frequency. Here we develop a model for Ramsey-Bordé interferometery using optical fields with curved wavefronts and simulate the $^{40}$Ca beam clock experiment described in [Olson et al., Phys. Rev. Lett. 123, 073202 (2019)]. Olson et al.'s results had shown surprising and unexplained behaviour in the response of the atoms in the interrogation. Our model predicts signals consistent with experimental data and can account for the significant sensitivity to laser geometry that was reported. We find the signal-to-noise ratio is maximised when the laser is uncollimated at the interrogation zones to minimise inhomogeneity, and also identify an optimal waist size determined by both laser inhomogeneity and the velocity distribution of the atomic beam. We investigate the shifts and stability of the clock frequency, showing that the Gouy phase is the primary source of frequency variations arising from laser geometry.

quant-ph↗

arXiv2022

Spinsim: a GPU optimized python package for simulating spin-half and spin-one quantum systems

The Spinsim python package simulates spin-half and spin-one quantum mechanical systems following a time dependent Shroedinger equation. It makes use of numba.cuda, which is an LLVM (Low Level Virtual Machine) compiler for Nvidia Cuda compatible systems using GPU parallelization. Along with other optimizations, this allows for speed improvements from 3 to 4 orders of magnitude while staying just as accurate, compared to industry standard packages. It is available for installation on PyPI, and the source code is available on github. The initial use-case for the Spinsim will be to simulate quantum sensing-based ultracold atom experiments for the Monash University School of Physics \& Astronomy spinor Bose-Einstein condensate (spinor BEC) lab, but we anticipate it will be useful in simulating any range of spin-half or spin-one quantum systems with time dependent Hamiltonians that cannot be solved analytically. These appear in the fields of nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR) and magnetic resonance imaging (MRI) experiments and quantum sensing, and with the spin-one systems of nitrogen vacancy centres (NVCs), ultracold atoms, and BECs.

quant-ph↗

arXiv2019

Realization of a fractional period adiabatic superlattice

We propose and realize a deeply sub-wavelength optical lattice for ultracold neutral atoms using $N$ resonantly Raman-coupled internal degrees of freedom. Although counter-propagating lasers with wavelength $λ$ provided two-photon Raman coupling, the resultant lattice-period was $λ/2N$, an $N$-fold reduction as compared to the conventional $λ/2$ lattice period. We experimentally demonstrated this lattice built from the three $F=1$ Zeeman states of a $^{87}{\rm Rb}$ Bose-Einstein condensate, and generated a lattice with a $λ/6= 132\ {\rm nm}$ period from $λ=790 \ {\rm nm}$ lasers. Lastly, we show that adding an additional RF coupling field converts this lattice into a superlattice with $N$ wells uniformly spaced within the original $λ/2$ unit cell.

cond-mat.quant-gas↗

arXiv2019

Unconventional topology with a Rashba spin-orbit coupled quantum gas

Topological order can be found in a wide range of physical systems, from crystalline solids, photonic meta-materials and even atmospheric waves to optomechanic, acoustic and atomic systems. Topological systems are a robust foundation for creating quantized channels for transporting electrical current, light, and atmospheric disturbances. These topological effects are quantified in terms of integer-valued invariants, such as the Chern number, applicable to the quantum Hall effect, or the $\mathbb{Z}_2$ invariant suitable for topological insulators. Here we engineered Rashba spin-orbit coupling for a cold atomic gas giving non-trivial topology, without the underlying crystalline structure that conventionally yields integer Chern numbers. We validated our procedure by spectroscopically measuring the full dispersion relation, that contained only a single Dirac point. We measured the quantum geometry underlying the dispersion relation and obtained the topological index using matter-wave interferometry. In contrast to crystalline materials, where topological indices take on integer values, our continuum system reveals an unconventional half-integer Chern number, potentially implying new forms of topological transport.

cond-mat.quant-gas↗

arXiv2017

Precise wavefunction engineering with magnetic resonance

Controlling quantum fluids at their fundamental length scale will yield superlative quantum simulators, precision sensors, and spintronic devices. This scale is typically below the optical diffraction limit, precluding precise wavefunction engineering using optical potentials alone. We present a protocol to rapidly control the phase and density of a quantum fluid down to the healing length scale using strong time-dependent coupling between internal states of the fluid in a magnetic field gradient. We demonstrate this protocol by simulating the creation of a single stationary soliton and double soliton states in a Bose-Einstein condensate with control over the individual soliton positions and trajectories, using experimentally feasible parameters. Such states are yet to be realized experimentally, and are a path towards engineering soliton gases and exotic topological excitations.

quant-ph↗

arXiv2017

Continuously observing a dynamically decoupled spin-1 quantum gas

We continuously observe dynamical decoupling in a spin-1 quantum gas using a weak optical measurement of spin precession. Continuous dynamical decoupling aims to dramatically modify the character and energy spectrum of spin states to render them insensitive to parasitic fluctuations. Continuous observation measures this new spectrum in a single-preparation of the quantum gas. The measured time-series contains seven tones, which spectrogram analysis parses as splittings, coherences, and coupling strengths between the decoupled states in real-time. With this we locate a regime where a transition between two states is decoupled from magnetic field instabilities up to fourth order, complementary to the parallel work at higher fields by Trypogeorgos et al. (arXiv:1706.07876). The decoupled microscale quantum gas offers magnetic sensitivity in a tunable band, persistent over many milliseconds: the length scales, frequencies, and durations relevant to many applications, including sensing biomagnetic phenomena such as neural spike trains.

cond-mat.quant-gas↗

arXiv2017

Magic-wavelength Faraday probe measures spin continuously and without light shifts

We describe a dispersive Faraday optical probe of atomic spin which performs a weak measurement of spin projection of a quantum gas continuously for more than one second. To date focusing bright far-off-resonance probes onto quantum gases has proved invasive, due to strong scalar and vector light shifts exerting dipole and Stern-Gerlach forces. We show that tuning the probe near the magic-zero wavelength at 790 nm between the fine-structure doublet of $^{87}$Rb cancels the scalar light shift, and careful control of polarization eliminates the vector light shift. Faraday rotations due to each fine-structure line reinforce at this wavelength, enhancing the signal-to-noise ratio for a fixed rate of probe-induced decoherence. Using this minimally-invasive spin probe we perform microscale atomic magnetometry at high temporal resolution. Spectrogram analysis of the Larmor precession signal of a single spinor Bose-Einstein condensate measures a time-varying magnetic field strength with 1 μG accuracy every 5 ms; or equivalently makes > 200 successive measurements each at $10\,\mathrm{pT/\sqrt{Hz}}$ sensitivity.

quant-ph↗

arXiv2016

Measurement and extinction of vector light shifts using interferometry of spinor condensates

We use differential Ramsey interferometry of ultracold atoms to characterize the vector light shift (VLS) from a far-off resonance optical dipole trap at $λ= 1064\,\mathrm{nm}$. The VLS manifests as a `fictitious' magnetic field, which we perceive as a change in the Larmor frequency of two spinor condensates exposed to different intensities of elliptically polarized light. We use our measurement scheme to diagnose the light-induced magnetic field and suppress it to $2.1(8)\times10^{-4}$ of its maximum value, by making the trapping light linearly polarized with a quarter-wave plate in each beam. Our sensitive measurement of the VLS-induced field demonstrates high-precision, in-vacuo interferometric polarimetry of dipole trapping light and can be adapted to measure vector shifts from other lasers, advancing the application of optically trapped atoms to precision metrology.

quant-ph↗

arXiv2015

Magnetic tensor gradiometry using Ramsey interferometry of spinor condensates

We have realized a magnetic tensor gradiometer by interferometrically measuring the relative phase between two spatially separated Bose-Einstein condensates (BECs). We perform simultaneous Ramsey interferometry of the proximate $^{87}$Rb spin-1 condensates in freefall and infer their relative Larmor phase -- and thus the differential magnetic field strength -- with a common-mode phase noise suppression exceeding $50\,\mathrm{dB}$. By appropriately biasing the magnetic field and separating the BECs along orthogonal directions, we measure the magnetic field gradient tensor of ambient and applied magnetic fields with a nominal precision of $30\,\mathrm{μG\,cm^{-1}}$ and a sensor volume of $2\times10^{-5}\,\mathrm{mm}^3$. We predict a spin-projection noise limited magnetic energy resolution of order $\hbar$ for typical Zeeman coherence times of trapped condensates with this scheme, even with the low measurement duty cycle inherent to BEC experiments.

quant-ph↗

arXiv2015

A Monte Carlo wavefunction method for semiclassical simulations of spin-position entanglement

We present a Monte Carlo wavefunction method for semiclassically modeling spin-$\frac12$ systems in a magnetic field gradient in one dimension. Our model resolves the conflict of determining what classical force an atom should be subjected to when it is in an arbitrary superposition of internal states. Spatial degrees of freedom are considered to be an environment, entanglement with which decoheres the internal states. Atoms follow classical trajectories through space, punctuated by probabilistic jumps between spin states. We modify the conventional Monte Carlo wavefunction method to jump between states when population transfer occurs, rather than when population is later discarded via exponential decay. This results in a spinor wavefunction that is continuous in time, and allows us to model the classical particle trajectories (evolution of the environment variables) more accurately. The model is not computationally demanding and it agrees well with simulations of the full spatial wavefunction of an atom.

quant-ph↗

arXiv2014

Self-induced Spatial Dynamics to Enhance Spin-Squeezing via One-Axis Twisting in a Two-Component Bose-Einstein Condensate

We theoretically investigate a scheme to enhance spin squeezing in a two-component Bose-Einstein condensate (BEC) by utilizing the inherent mean-field dynamics of the condensate. Due to the asymmetry in the scattering lengths, the two components exhibit density oscillations where they spatially separate and recombine. The effective non-linearity responsible for spin squeezing is increased by up to three orders of magnitude when the two components spatially separate \cite{treutlein2010}. We perform a multi-mode simulation of the system using the truncated Wigner method, and show that this method can be used to create significant spin squeezing in systems where the effective nonlinearity would ordinarily be too small, and that strong spatial dynamics resulting from large particle numbers aren't necessarily detrimental to generating spin-squeezing. We develop a simplified semi-analytic model that gives good agreement with our multi-mode simulation, and will be useful for predicting spin-squeezing in a range of different systems.

quant-ph↗

arXiv2013

A scripted control system for autonomous hardware timed experiments

We present the labscript suite, an open-source experiment control system for automating shot-based experiments and their analysis. Experiments are composed as Python code, which is used to produce low-level hardware instructions. They are queued up and executed on the hardware in real time, synchronized by a pseudoclock. Experiment parameters are manipulated graphically, and analysis routines are run as new data is acquired. With this system, we can easily automate exploration of parameter spaces, including closed-loop optimization.

cond-mat.quant-gas↗

arXiv2013

A versatile high resolution objective for imaging quantum gases

We present a high resolution objective lens made entirely from catalog singlets that has a numerical aperture of 0.36. It corrects for aberrations introduced by a glass window and has a long working distance of 35mm, making it suitable for imaging objects within a vacuum system. This offers simple high resolution imaging for many in the quantum gas community. The objective achieves a resolution of 1.3μm at the design wavelength of 780nm, and a diffraction-limited field of view of 360μm when imaging through a 5mm window. Images of a resolution target and a pinhole show quantitative agreement with the simulated lens performance. The objective is suitable for diffraction-limited imaging on the D2 line of all the alkalis by changing only the aperture diameter, retaining numerical apertures above 0.32. The design corrects for window thicknesses of up to 15mm if the singlet spacings are modified.

physics.optics↗

arXiv2012

Precision atomic gravimeter based on Bragg diffraction

We present a precision gravimeter based on coherent Bragg diffraction of freely falling cold atoms. Traditionally, atomic gravimeters have used stimulated Raman transitions to separate clouds in momentum space by driving transitions between two internal atomic states. Bragg interferometers utilize only a single internal state, and can therefore be less susceptible to environmental perturbations. Here we show that atoms extracted from a magneto-optical trap using an accelerating optical lattice are a suitable source for a Bragg atom interferometer, allowing efficient beamsplitting and subsequent separation of momentum states for detection. Despite the inherently multi-state nature of atom diffraction, we are able to build a Mach-Zehnder interferometer using Bragg scattering which achieves a sensitivity to the gravitational acceleration of $Δg/g = 2.7\times10^{-9}$ with an integration time of 1000s. The device can also be converted to a gravity gradiometer by a simple modification of the light pulse sequence.

quant-ph↗

arXiv2011

Cold atom gravimetry with a Bose-Einstein Condensate

We present a cold atom gravimeter operating with a sample of Bose-condensed Rubidium-87 atoms. Using a Mach-Zehnder configuration with the two arms separated by a two-photon Bragg transition, we observe interference fringes with a visibility of 83% at T=3 ms. We exploit large momentum transfer (LMT) beam splitting to increase the enclosed space-time area of the interferometer using higher-order Bragg transitions and Bloch oscillations. We also compare fringes from condensed and thermal sources, and observe a reduced visibility of 58% for the thermal source. We suspect the loss in visibility is caused partly by wavefront aberrations, to which the thermal source is more susceptible due to its larger transverse momentum spread. Finally, we discuss briefly the potential advantages of using a coherent atomic source for LMT, and present a simple mean-field model to demonstrate that with currently available experimental parameters, interaction-induced dephasing will not limit the sensitivity of inertial measurements using freely-falling, coherent atomic sources.

quant-ph↗

arXiv2011

Optically trapped atom interferometry using the clock transition of large Rb-87 Bose-Einstein condensates

We present a Ramsey-type atom interferometer operating with an optically trapped sample of 10^6 Bose-condensed Rb-87 atoms. The optical trap allows us to couple the |F =1, mF =0>\rightarrow |F =2, mF =0> clock states using a single photon 6.8GHz microwave transition, while state selective readout is achieved with absorption imaging. Interference fringes with contrast approaching 100% are observed for short evolution times. We analyse the process of absorption imaging and show that it is possible to observe atom number variance directly, with a signal-to-noise ratio ten times better than the atomic projection noise limit on 10^6 condensate atoms. We discuss the technical and fundamental noise sources that limit our current system, and outline the improvements that can be made. Our results indicate that, with further experimental refinements, it will be possible to produce and measure the output of a sub-shot-noise limited, large atom number BEC-based interferometer. In an addendum to the original paper, we attribute our inability to observe quantum projection noise to the stability of our microwave oscillator and background magnetic field. Numerical simulations of the Gross-Pitaevskii equations for our system show that dephasing due to spatial dynamics driven by interparticle interactions account for much of the observed decay in fringe visibility at long interrogation times. The simulations show good agreement with the experimental data when additional technical decoherence is accounted for, and suggest that the clock states are indeed immiscible. With smaller samples of 5 \times 10^4 atoms, we observe a coherence time of τ = (1.0+0.5-0.3) s.

quant-ph↗

arXiv2009

Radio-frequency dressing of multiple Feshbach resonances

We demonstrate and theoretically analyze the dressing of several proximate Feshbach resonances in Rb-87 using radio-frequency (rf) radiation. We present accurate measurements and characterizations of the resonances, and the dramatic changes in scattering properties that can arise through the rf dressing. Our scattering theory analysis yields quantitative agreement with the experimental data. We also present a simple interpretation of our results in terms of rf-coupled bound states interacting with the collision threshold.

cond-mat.quant-gas↗