Quantum Sensing in Tweezer Arrays: Optical Magnetometry on an Individual-Atom Sensor Grid

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Quantum Sensing in Tweezer Arrays: Optical Magnetometry on an Individual-Atom Sensor Grid

Dominik Schäffner, Tobias Schreiber, Fabian Lenz, Malte Schlosser, and Gerhard Birkl

PRX Quantum 5, 010311 – Published 26 January 2024

Abstract

We implement a scalable platform for quantum sensing comprising hundreds of sites capable of holding individual laser-cooled atoms and demonstrate the applicability of this single-quantum-system sensor array to magnetic field mapping on a two-dimensional grid. With each atom being confined in an optical tweezer within an area of $0.5 μ m^{2}$ at mutual separations of $7.0 (2) μ m$ , we obtain micrometer-scale spatial resolution and highly parallelized operation. An additional steerable optical tweezer allows rearrangement of atoms within the grid and enables single-atom scanning microscopy with submicron resolution. This individual-atom sensor platform finds an immediate application in mapping an externally applied dc gradient magnetic field. In a Ramsey-type measurement, we obtain a field resolution of 98(29) nT. We estimate the sensitivity to be $25 μ T / \sqrt{Hz}$ .

Received 16 July 2023
Revised 27 November 2023
Accepted 3 January 2024

DOI:https://doi.org/10.1103/PRXQuantum.5.010311

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Quantum communication, protocols & technology Quantum control Quantum engineering Quantum metrology

Quantum Information, Science & Technology

Authors & Affiliations

Dominik Schäffner¹, Tobias Schreiber ¹, Fabian Lenz¹, Malte Schlosser ¹, and Gerhard Birkl ^1,2,*

¹Technische Universität Darmstadt, Institut für Angewandte Physik, Schlossgartenstraße 7, 64289 Darmstadt, Germany
²Helmholtz Forschungsakademie Hessen für FAIR, Campus Darmstadt, Schlossgartenstraße 2, 64289 Darmstadt, Germany

^*apqpub@physik.tu-darmstadt.de; https://www.iap.tu-darmstadt.de/apq

Popular Summary

Can a single atom be used as a sensitive measurement device? Can this sensor outperform a macroscopic sensor? The answer is “yes” to both questions when we consider the high potential of sensing devices relying on quantum physical principles as currently being developed in the field of quantum technology. Based on the fundamentally different behavior of quantum systems in relation to their classical counterparts, quantum physics is already outperforming classical physics in several applications and will do so in many more in the near future.

In the present work, we apply a two-dimensional grid of individual atoms, each atom serving as a separate sensor because its quantum-mechanical evolution is influenced by the interaction with its environment. This constitutes a two-dimensional sensor for magnetic fields, comparable to the camera in your mobile phone, which functions as a two-dimensional sensor for light.

In recent years, arrays of optical tweezers have grown to hold hundreds of single-atom quantum systems. The cooling and trapping of the atoms by laser light facilitates the realization of highly flexible platforms with strong emphasis on scalability, which becomes pivotal for future progress.

Devices based on trapped arrays of individual ultracold atoms have emerged as one of the leading technologies in quantum computing. In the present work, this platform is introduced to a different pillar of quantum technologies: quantum sensing. We expect that this experiment will have a deep impact on the development of quantum sensing.

You surely would miss the camera in your mobile phone, which transforms it into much more than just a phone. We expect that in the near future you also might miss our quantum sensor for magnetic fields in medical research such as for brain diagnostics or for magnetic resonance imaging.

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Vol. 5, Iss. 1 — January - March 2024

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Images

Figure 1
Micro-optical tweezer array for cold-atom magnetometry. (a) Experimental setup. A 270-site register of individual rubidium atoms at a wavelength of $797 nm$ is generated by a microlens array whose focal plane is reimaged into the vacuum chamber. Two-photon Ramsey spectroscopy is used to characterize the magnetic gradient test field induced by the two anti-Helmholtz coils with counterpropagating current $I_{t}$ . A pair of coils in the Helmholtz configuration provides a well-defined quantization axis via a homogeneous field generated by a current $I_{q}$ . (b) Averaged fluorescence of atoms in the $18 \times 15$ tweezer array. (c) Enhanced atom occupation in a predefined substructure with use of a steerable optical tweezer for atom transport. (d) With use solely of the steerable tweezer, atoms can be positioned independently of the array grid.
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Figure 2
Experimental details of individual-atom magnetometry. (a) Energy splitting $ℏ Δ_{| ↑ ⟩ | ↓ ⟩}$ of the two states $| ↑ ⟩$ and $| ↓ ⟩$ used for magnetic field sensing as a function of the magnetic field strength. For a fixed differential two-photon energy $ℏ Δ_{12}$ of the spectroscopy beam (dashed horizontal line), the effective detuning $δ_{eff}$ depends on the magnetic field experienced by the atom. (b) The inhomogeneous magnetic field $B_{t}$ generated by a pair of coils in the anti-Helmholtz configuration is used as a position-dependent test field. A homogeneous magnetic field $B_{q}$ defines the quantization axis. (c) A typical measurement cycle (see the main text for details; the time axis is not to scale). During the Ramsey sequence, consisting of state preparation, a $π / 2$ pulse, free precession, a $π / 2$ pulse, and state projection and selection, the trap depth $U$ is $k_{B} \times 0.2 mK$ , whereas for initial atom loading and the two imaging sequences $U = k_{B} \times 1 mK$ .
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Figure 3
(a) Ramsey signal for one of the central sensor grid positions and (b) Allan deviation for a measurement of the quantization field $B_{q}$ . The solid dots represent the mean value for the central $3 \times 3$ grid positions, the dashed line represents the inverse-square-root dependence expected for a shot-noise-limited measurement, and the triangle corresponds to the magnetic field resolution $δ B$ for the total averaging time (see the main text for details).
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Figure 4
Two-dimensional distribution of the Ramsey frequencies in the sensor plane without (gray) and with (color) an activated magnetic gradient test field $B_{t}$ . A linear variation of $ω_{R}$ along the $x$ direction is observed in the case of an activated test field, while only fluctuations resulting from the local tweezer light shift in $ω_{R, ref}$ occur for a deactivated test field.
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Figure 5
Measured shift $Δ B$ of the effective magnetic field strength. In the two-dimensional map plotted in (a), the results of the measurements for each of the 270 pixels equally distributed over the $119 μ m \times 98 μ m$ ( $18 \times 15$ pixel) sensor plane are given. The magnetic field gradient along the $x$ direction is clearly visible. The data obtained along the dashed line are plotted in (b). From an approximation of a linear function (red) to these data, a gradient of $77.3 (4) nT / μ m$ is determined.
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