"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

The structural properties of free nanoclusters are reviewed. Special attention is paid to the interplay of energetic, thermodynamic, and kinetic factors in the explanation of cluster structures that are actually observed in experiments. The review starts with a brief summary of the experimental methods for the production of free nanoclusters and then considers theoretical and simulation issues, always discussed in close connection with the experimental results. The energetic properties are treated first, along with methods for modeling elementary constituent interactions and for global optimization on the cluster potential-energy surface. After that, a section on cluster thermodynamics follows. The discussion includes the analysis of solid-solid structural transitions and of melting, with its size dependence. The last section is devoted to the growth kinetics of free nanoclusters and treats the growth of isolated clusters and their coalescence. Several specific systems are analyzed.

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Structural properties of nanoclusters: Energetic, thermodynamic, and kinetic effects

Experimental progress in controlling and manipulating trapped atomic ions has opened the door for a series of proof-of-principle quantum simulations. This article reviews these experiments, together with the methods and tools that have enabled them, and provides an outlook on future directions in the field.

Quantum simulations with trapped ions

This paper reviews the recent theoretical and experimental advances in the study of ultra-cold gases made of bosonic particles interacting via the long-range, anisotropic dipole–dipole interaction, in addition to the short-range and isotropic contact interaction usually at work in ultra-cold gases. The specific properties emerging from the dipolar interaction are emphasized, from the mean-field regime valid for dilute Bose–Einstein condensates, to the strongly correlated regimes reached for dipolar bosons in optical lattices. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0034-4885/72/12/126401/pdf

The physics of dipolar bosonic quantum gases

Squeezed states of the electromagnetic field have been generated by nondegenerate four-wave mixing due to Na atoms in an optical cavity. The optical noise in the cavity, comprised of primarily vacuum fluctuations and a small component of spontaneous emission from the pumped Na atoms, is amplified in one quadrature of the optical field and deamplified in the other quadrature. These quadrature components are measured with a balanced homodyne detector. The total noise level in the deamplified quadrature drops below the vacuum noise level.

Observation of squeezed states generated by four-wave mixing in an optical cavity

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

In this article, recent developments in helium nanodroplet isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. We discuss how molecular beam spectroscopy and matrix isolation spectroscopy can be usefully combined into a method that provides a unique tool to tackle physical and chemical problems which had been outside our experimental possibilities. Next, in reviewing the experimental methodology, we present design criteria for droplet beam formation and its seeding with the chromophore(s) of interest, followed by a discussion of the merits and shortcomings of radiation sources currently used in this type of spectroscopy. In a second, more conceptual part of the review, we discuss several HENDI issues which are understood by the community to a varied level of depth and precision. In this context, we show first how a superfluid helium cluster adopts the symmetry of the molecule or complex seeded in it and discuss the nature of the potential well (and its anisotropy) that acts on a solute inside a droplet, and of the energy levels that arise because of this confinement. Second, we treat the question of the homogeneous versus inhomogeneous broadening of the spectral profiles, moving after this to a discussion of the rotational dynamics of the molecules and of the surrounding superfluid medium. The change in rotational constants from their gas phase values, and their dependence on the angular velocity and vibrational quantum number are discussed. Finally, the spectral shifts generated by this very gentle matrix are analyzed and shown to be small because of a cancellation between the opposing action of the attractive and repulsive parts of the potential of interaction between molecules and their solvent. The review concludes with a discussion of three recent applications to (a) the synthesis of far-from-equilibrium molecular aggregates that could hardly be prepared in any other way, (b) the study of the influence of a simple and rather homogeneous solvent on large amplitude molecular motions, and (c) the study of mixed 3He/4He and other highly quantum clusters (e.g., H2 clusters) prepared inside helium droplets and interrogated by measuring the IR spectra of molecules embedded in them. In spite of the many open questions, we hope to convince the reader that HENDI has a great potential for the solution of several problems in modern chemistry and condensed matter physics, and that, even more interestingly, this unusual environment has the potential to generate new sets of issues which were not in our minds before its introduction.

Helium nanodroplet isolation rovibrational spectroscopy: Methods and recent results

In this article, recent developments in HElium NanoDroplet Isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. Topics discussed include experimental details, comparison of radiation sources, symmetry issues of the helium solvation structure, sources of line broadening, changes in spectroscopic constants upon solvation, and applications including formation of novel chemical structures.

Helium nanodroplet isolation ro-vibrational spectroscopy: methods and recent results

Characterizing many-body systems through the quantum correlations between their constituent particles is a major goal of quantum physics. Although entanglement is routinely observed in many systems, we report here the detection of stronger correlations—Bell correlations—between the spins of about 480 atoms in a Bose-Einstein condensate. We derive a Bell correlation witness from a many-particle Bell inequality involving only one- and two-body correlation functions. Our measurement on a spin-squeezed state exceeds the threshold for Bell correlations by 3.8 standard deviations. Our work shows that the strongest possible nonclassical correlations are experimentally accessible in many-body systems and that they can be revealed by collective measurements.

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Bell correlations in a Bose-Einstein condensate

Atom interferometers provide record precision in measurements of a broad range of physical quantities. Extending atom interferometry to micrometer spatial resolution would enable new applications in electromagnetic field sensing, surface science, and the search for fundamental short-range interactions. I present experiments where we use a small Bose-Einstein condensate on an atom chip as an interferometric scanning probe to map out a microwave field at distances down to $16$ micrometer from the chip surface with a few micrometers spatial resolution. By creating entanglement between the atoms, our interferometer overcomes the standard quantum limit of interferometry by 4 dB in variance and maintains enhanced performance for interrogation times up to 10 ms. This corresponds to a microwave magnetic field sensitivity of 77 pT/sqrt(Hz) in a probe volume of 20 cubic micrometer. High-resolution measurements of microwave near-fields, as demonstrated here, are important for the development of integrated microwave circuits for quantum information processing and applications in communication technology.

Quantum metrology with entangled atoms is particularly useful in measurements with high spatial resolution, since the atom number in the probe volume is limited by collisional losses. I analyze the effect of such density-dependent losses in high-resolution atom interferometry, and show that there is a strict upper limit on the useful number of atoms. Our experimental results indicate that even tighter limits on the particle number and interrogation time may arise from density-dependent dephasing, and provide a starting point for future studies towards the fundamental limits of coherence in Bose-Einstein condensates. Our experimental setup is ideally suited to experimentally address these questions, and provides a promising platform for further studies on quantum metrology and entanglement in many-particle atomic systems.

Roman Schmied

Papers

Quantum metrology with nonclassical states of atomic ensembles

Helium nanodroplet isolation rovibrational spectroscopy: Methods and recent results

Helium nanodroplet isolation ro-vibrational spectroscopy: methods and recent results

Bell correlations in a Bose-Einstein condensate

Quantum metrology with a scanning probe atom interferometer