"Quantum sensing" describes the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors, or atomic clocks More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits The field is expected to provide new opportunities - especially with regard to high sensitivity and precision - in applied physics and other areas of science In this review, we provide an introduction to the basic principles, methods and concepts of quantum sensing from the viewpoint of the interested experimentalist

Quantum sensing

This paper reviews the work on cavity quantum electrodynamics of free atoms. In recent years, cavity experiments have also been conducted on a variety of solid-state systems resulting in many interesting applications, of which microlasers, photon bandgap structures and quantum dot structures in cavities are outstanding examples. Although these phenomena and systems are very interesting, discussion is limited here to free atoms and mostly single atoms because these systems exhibit clean quantum phenomena and are not disturbed by a variety of other effects. At the centre of our review is the work on the one-atom maser, but we also give a survey of the entire field, using free atoms in order to show the large variety of problems dealt with. The cavity interaction can be separated into two main regimes: the weak coupling in cavity or cavity-like structures with low quality factors Q and the strong coupling when high-Q cavities are involved. The weak coupling leads to modification of spontaneous transitions and level shifts, whereas the strong coupling enables one to observe a periodic exchange of photons between atoms and the radiation field. In this case, atoms and photons are entangled, this being the basis for a variety of phenomena observed, some of them leading to interesting applications in quantum information processing. The cavity experiments with free atoms reached a new domain with the advent of experiments in the visible spectral region. A review on recent achievements in this area is also given.

https://iopscience.iop.org/article/10.1088/0034-4885/69/5/R02/pdf

Cavity quantum electrodynamics

Quantum technologies exploit entanglement to revolutionize computing, measurements, and communications. This has stimulated the research in different areas of physics to engineer and manipulate fragile many-particle entangled states. Progress has been particularly rapid for atoms. Thanks to the large and tunable nonlinearities and the well-developed techniques for trapping, controlling, and counting, many groundbreaking experiments have demonstrated the generation of entangled states of trapped ions, cold, and ultracold gases of neutral atoms. Moreover, atoms can strongly couple to external forces and fields, which makes them ideal for ultraprecise sensing and time keeping. All these factors call for generating nonclassical atomic states designed for phase estimation in atomic clocks and atom interferometers, exploiting many-body entanglement to increase the sensitivity of precision measurements. The goal of this article is to review and illustrate the theory and the experiments with atomic ensembles that have demonstrated many-particle entanglement and quantum-enhanced metrology.

Quantum metrology with nonclassical states of atomic ensembles

Trapping and manipulating ultracold atoms and degenerate quantum gases in magnetic micropotentials is reviewed. Starting with a comprehensive description of the basic concepts and fabrication techniques of microtraps together with early pioneering experiments, emphasis is placed on current experiments on degenerate quantum gases. This includes the loading of quantum gases in microtraps, coherent manipulation, and transport of condensates together with recently reported experiments on matter-wave interferometry on a chip. Theoretical approaches for describing atoms in waveguides and beam splitters are briefly summarized, and, finally, the interaction between atoms and the surface of microtraps is covered in some detail.

Magnetic microtraps for ultracold atoms

We investigate the stability of magnetically trapped atomic Bose-Einstein condensates and thermal clouds near the transition temperature at small distances $05\text{ }\ensuremath{\mu}\mathrm{m}\ensuremath{\le}d\ensuremath{\le}10\text{ }\ensuremath{\mu}\mathrm{m}$ from a microfabricated silicon chip For a $2\text{ }\ensuremath{\mu}\mathrm{m}$ thick copper film, the trap lifetime is limited by Johnson noise induced currents and falls below 1 s at a distance of $4\text{ }\ensuremath{\mu}\mathrm{m}$ A dielectric surface does not adversely affect the sample until the attractive Casimir-Polder potential significantly reduces the trap depth

https://arxiv.org/pdf/cond-mat/0308457.pdf

Impact of the Casimir-Polder potential and Johnson noise on Bose-Einstein condensate stability near surfaces.

We describe an experiment in which Bose-Einstein condensates and cold atom clouds are held by a microscopic magnetic trap near a room-temperature metal wire 500 microm in diameter The lifetime for atoms to remain in the microtrap is measured over a range of distances down to 27 microm from the surface of the metal We observe the loss of atoms from the microtrap due to spin flips These are induced by radio-frequency thermal fluctuations of the magnetic field near the surface, as predicted but not previously observed

https://researchbank.swinburne.edu.au/file/93ed02f1-be99-4270-883c-84094bf944f7/1/PDF (Published version).pdf

Spin coupling between cold atoms and the thermal fluctuations of a metal surface.

We use collective oscillations of a two-component Bose-Einstein condensate (2CBEC) of \Rb atoms prepared in the internal states $\ket{1}\equiv\ket{F=1, m_F=-1}$ and $\ket{2}\equiv\ket{F=2, m_F=1}$ for the precision measurement of the interspecies scattering length $a_{12}$ with a relative uncertainty of $1.6\times 10^{-4}$. We show that in a cigar-shaped trap the three-dimensional (3D) dynamics of a component with a small relative population can be conveniently described by a one-dimensional (1D) Schr\"{o}dinger equation for an effective harmonic oscillator. The frequency of the collective oscillations is defined by the axial trap frequency and the ratio $a_{12}/a_{11}$, where $a_{11}$ is the intra-species scattering length of a highly populated component 1, and is largely decoupled from the scattering length $a_{22}$, the total atom number and loss terms. By fitting numerical simulations of the coupled Gross-Pitaevskii equations to the recorded temporal evolution of the axial width we obtain the value $a_{12}=98.006(16)\,a_0$, where $a_0$ is the Bohr radius. Our reported value is in a reasonable agreement with the theoretical prediction $a_{12}=98.13(10)\,a_0$ but deviates significantly from the previously measured value $a_{12}=97.66\,a_0$ \cite{Mertes07} which is commonly used in the characterisation of spin dynamics in degenerate \Rb atoms. Using Ramsey interferometry of the 2CBEC we measure the scattering length $a_{22}=95.44(7)\,a_0$ which also deviates from the previously reported value $a_{22}=95.0\,a_0$ \cite{Mertes07}. We characterise two-body losses for the component 2 and obtain the loss coefficients ${\gamma_{12}=1.51(18)\times10^{-14} \textrm{cm}^3/\textrm{s}}$ and ${\gamma_{22}=8.1(3)\times10^{-14} \textrm{cm}^3/\textrm{s}}$.

Measurement of s-wave scattering lengths in a two-component Bose-Einstein condensate

We have produced a Bose-Einstein condensate on a permanent-magnet atom chip based on periodically magnetized videotape We observe the expansion and dynamics of the condensate in one of the microscopic waveguides close to the surface The lifetime for atoms to remain trapped near this dielectric material is significantly longer than above a metal surface of the same thickness These results illustrate the suitability of microscopic permanent-magnet structures for quantum-coherent preparation and manipulation of cold atoms

/pdf/bose-einstein-condensation-on-a-permanent-magnet-atom-chip-1w0pq6zp7u.pdf

Bose-Einstein condensation on a permanent magnet atom chip

We report on the adiabatic splitting of a Bose-Einstein condensate of $^{87}\mathrm{Rb}$ atoms by an asymmetric double-well potential located above the edge of a perpendicularly magnetized TbGdFeCo film atom chip. By controlling the barrier height and double-well asymmetry, the sensitivity of the axial splitting process is investigated through observation of the fractional atom distribution between the left and right wells. This process constitutes a novel sensor for which we infer a single shot sensitivity to gravity fields of $\ensuremath{\delta}g/g\ensuremath{\approx}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. From a simple analytic model, we propose improvements to chip-based gravity detectors using this demonstrated methodology.

https://researchbank.swinburne.edu.au/file/474d8f76-beb7-48bc-b5d6-f8496166d01d/1/PDF (Published version).pdf

Condensate splitting in an asymmetric double well for atom chip based sensors.

We investigate the spatially dependent relative phase evolution of an elongated two-component Bose-Einstein condensate. The pseudospin-1/2 system is comprised of the |F=1,m_F=-1> and |F=2,m_F=+1> hyperfine ground states of 87Rb, which we magnetically trap and interrogate with radio-frequency and microwave fields. We probe the relative phase evolution with Ramsey interferometry and observe a temporal decay of the interferometric contrast well described by a mean-field formalism. Inhomogeneity of the collective relative phase dominates the loss of interferometric contrast, rather than decoherence or phase diffusion. We demonstrate a technique to simultaneously image each state, yielding subpercent variations in the measured relative number while preserving the spatial mode of each component. In addition, we propose a spatially sensitive interferometric technique to image the relative phase.

https://researchbank.swinburne.edu.au/file/069bb595-c1dc-44c9-adcc-a13123a6f6cc/1/PDF (Published version).pdf

B. V. Hall

Papers

Spin coupling between cold atoms and the thermal fluctuations of a metal surface.

Measurement of s-wave scattering lengths in a two-component Bose-Einstein condensate

Bose-Einstein condensation on a permanent magnet atom chip

Condensate splitting in an asymmetric double well for atom chip based sensors.

Spatially inhomogeneous phase evolution of a two-component Bose-Einstein condensate