��ݮ��Ƶ

Magneto-optical trapping of neutral atoms with light-induced effective magnetic fields���� ��

We introduce a novel method for producing ensembles of cold neutral atoms without the use of magnetic fields. The proposed laser cooling and trapping method is similar to the typical magneto-optical trap (MOT), but replaces the quadrupole magnetic field with an asymmetrical arrangement of additional laser beams that produce a fictitious magnetic field through state-dependent light shifts. This effective field induces a spatial dependence on the scattering force of the optical molasses beams, taking the place of the real magnetic field in a MOT. We show that this “opto-optical trap” can be applied to commonly trapped atomic species and discuss the unique advantages and potential applications of this approach to laser trapping of atoms.

A cryogenic single-ion Sr+ clock

Single-ion clocks are among the most accurate devices ever constructed. However, even the best clocks -- such as the Sr+ clock operated by the National Research Council of Canada -- are limited by systematic errors caused by blackbody radiation and background-gas collisions. To mitigate these two problems, we have designed and built a Sr+ clock that operates at 4 K: cryogenic operation suppresses both blackbody radiation and collisional shifts by many orders of magnitude. I will discuss the rationale for developing this device, present some unique aspects of its design, and discuss our efforts to characterize its performance.

Humpty-Dumpty is three-dimensional

Matter-wave Interferometry using Stern-Gerlach based beam-splitters has been described by Schwinger and co-workers [1] as a task as difficult as putting Humpty-Dumpty back together again! ��Nonetheless, such a Stern-Gerlach Interferometer (SGI) has recently been demonstrated [2]. ��

As shown by Comparat [3], simple models in one spatial dimension are insufficient to analyze SGIs, due to the three-dimensional nature of magnetic fields. I will discuss these effects on SGI using Rydberg atoms with electric field gradients, and the surprising prediction that many superficially similar SGI protocols are significantly different, with only some protocols demonstrating robust visibility in experimentally relevant regimes, and others infeasible. ��

Work done in collaboration with Danny Meng and Darren Chan.

[1] B.-G. Englert, J. Schwinger, and M. O. Scully, “Is spin coherence like Humpty-Dumpty? I. Simplified treatment”, en, Foundations of Physics 18, 1045–1056 (1988).

[2] Y. Margalit, O. Dobkowski, Z. Zhou, O. Amit, Y. Japha, S. Moukouri, D. Rohrlich, A. Mazumdar, S. Bose, C. Henkel, and R. Folman, “Realization of a complete Stern-Gerlach interferometer: Toward a test of quantum gravity”, Science Advances 7, Publisher: American Association for the Advancement of Science, eabg2879 (2021).

[3] D. Comparat, “Limitations for field-enhanced atom interferometry”, Physical Review A 101, Publisher: American Physical Society, 023606 (2020). "

Feedback Control Systems for the Stabilization of Traps in Large Rydberg Atom Arrays

Quantum processors have the potential to perform computations beyond the capabilities of classical processors. Realizing this potential would enable innovative applications in materials science, computational chemistry, and quantum many-body physics. Among the various platforms under development, Rydberg atom array quantum processors stand out for their scalability and controllability.

However, a critical challenge is generating large arrays of optical traps that are uniform in space and stable in time. Here, we introduce a closed-loop feedback system to create large, homogeneous arrays of optical traps whose power is stabilized in real time. The system tracks the power of more than a thousand traps on a camera, continuously updating polychromatic RF tones actuating a pair of acousto-optic deflectors. This enables the simultaneous optimization of processes such as trapping, cooling, and imaging of single neutral atoms in all traps.

We demonstrate a reduction in shot-to-shot fluctuations in loading efficiency and an increase in relative stability when performing adiabatic ramp-down experiments. This system can readily be deployed to implement atom-selective quantum gates across a large field of view and reduce atom loss during atom displacement.

Quantum Simulation Group

Metasurface-based cavities for quantum optical applications with atomic ensembles

Nano-Photonics and Quantum Optics Laboratory

Atomic ensembles coupled with cavity quantum electrodynamics (QED) offer a promising platform for quantum computing and networking, though with large challenges in implementation and scalability. We explore the use of metasurfaces and photonic crystals, sub-wavelegth nanostructues used to engineer optical wavefronts, to build exotic cavities for enhaced light-matter interaction. We present novel dichroic cavity designs and implementations tailored for free-space and fiber-integrated configurations.

Progress towards a barium-133 trapped ion quantum processor

We present our recent progress in developing a barium based trapped ion quantum processor using a microfabricated surface trap with 94 controllable DC electrode channels. Barium ions are among the most promising qubit candidates due to their long-lived quantum states and visible-wavelength optical transitions, allowing the use of commercial optics and waveguide-based modulators for individual qubit control.

Our trap is centrally mounted in the vacuum chamber, departing from conventional flange-mounted designs to maximize optical access while maintaining ultra-high vacuum conditions (3×10⁻¹¹ mbar). We highlight two major technical challenges: building a custom in-vacuum wire harness with 100 electrical connections to the trap, and implementing a multi-species barium atomic source that includes both radioactive barium-133 salt and rapidly oxidizing barium metal, each requiring specialized preparation and installation methods.

Laboratory for Quantum Information with Trapped Ions (QITI)

High-Dimensional Qudit Quantum Computing with Trapped 137Ba+ Ions

Authors: Nicholas CF Zutt, Gaurav A Tathed, Pei Jiang Low, Crystal Senko

The angular momentum eigenstates of the unpaired electron in trapped 137Ba+ ions offer a promising pathway toward high-fidelity qudit (d > 2) encoding. Due to the nonzero nuclear spin (I = 3/2), the 6S1/2 and 5D5/2 manifolds contain, respectively, 8 and 24 non-degenerate levels at intermediate magnetic fields (∼ few Gauss). Leveraging recent developments in state preparation techniques for trapped ion quantum computing, we demonstrate high-fidelity (> 99.5%) state preparation and measurement results over 25 basis states, the maximal measurable qudit encoding across the 6S1/2 and 5D5/2 manifolds in 137Ba+.

We demonstrate coherent control over this enlarged Hilbert space by performing Ramsey-type coherence probing measurements (generating many-state superpositions and probing phase-sensitive population recovery) and benchmark the scaling of decohering effects with increasing qudit dimension. We discuss the largest contributors to error in our system and the steps needed to maintain highdimensional coherence (as measured via the contrast of Ramsey-type measurements) in this qudit implementation.

Finally, we implement two-qubit algorithms (BernsteinVazirani and Grover’s search) on this single trapped ion qudit. This work establishes the feasibility of using trapped ions for large-qudit (d > 10) quantum computation, which is a promising alternative approach to expanding the Hilbert space in trapped ion quantum computing.

This research was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Canada Research Chairs.

Trapped Ion Quantum Control Lab

Dimer-association contact of a unitary Fermi gas���� ��

Interaction shifts of spectroscopic lines are a nuisance for atomic clocks, but also a signature of two-body correlations. We prepare a unitary Fermi gas of potassium-40, where atom-atom correlations manifest in the radiofrequency (rf) transfer spectra between internal states. In certain cases, the complete spectrum has a negative total clock shift, in seeming disagreement with the well studied positive high-frequency tail governed by the contact parameter.

We report that the “missing link” is a relatively deeply bound ac dimer. With a highly concentrated spectral weight, rf pulses on microsecond timescales can produce significant dimer-association rates, while Fourier broadening does not convolve the bare atomic resonance. We calibrate a Fourier-width-dominated dimer lineshape and observe that the integrated spectral weight and clock shift are directly proportional to correlations in the initial state extracted from the usual high- frequency tail.

The results are compared both to an analytic model with finite effective range and to a coupled-channels calculation. Our measurements inform the complete rf spectra of a unitary Fermi gas and provide a new tool for rapid observation of pair correlation dynamics.

Dynamical instability as a PT-symmetry breaking phase transition in a rotating Bose-Einstein condensate

We study a dilute gas of bosons in a rotating toroidal trap, focusing on a two-mode regime with a non-rotating mode and a rotating mode corresponding to a singly charged vortex. The system undergoes a symmetry-breaking transition as the ratio of interactions to disorder potential is varied, spontaneously selecting one mode, demonstrating macroscopic quantum self-trapping. The symmetry breaking is associated with dynamical instabilities driven by complex eigenvalues that can occur in a theoretical treatment as the Bogoliubov Hamiltonian is non-Hermitian, essentially because a U(1) symmetry is broken by choosing a phase for the BEC. A special feature of non-Hermitian quantum theory is that PT-symmetry replaces self-adjointness and we explore the connection between dynamical instability and PT-symmetry breaking phase transitions in this system.

O'Dell group