"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

(b) Write out the equations for the time dependence of ρ11, ρ22, ρ12 and ρ21 assuming that a light field interaction V = -μ12Ee iωt + c.c. couples only levels |1> and |2>, and that the excited levels exhibit spontaneous decay. (8 marks) (c) Under steady-state conditions, find the ratio of populations in states |2> and |3>. (3 marks) (d) Find the slowly varying amplitude ̃ ρ 12 of the polarization ρ12 = ̃ ρ 12e iωt . (6 marks) (e) In the limiting case that no decay is possible from intermediate level |3>, what is the ground state population ρ11(∞)? (2 marks) 2. (15 marks total) In a 2-level atom system subjected to a strong field, dressed states are created in the form |D1(n)> = sin θ |1,n> + cos θ |2,n-1> |D2(n)> = cos θ |1,n> sin θ |2,n-1>

The Quantum Theory of Light

Magnetic resonance imaging and optical microscopy are key technologies in the life sciences. For microbiological studies, especially of the inner workings of single cells, optical microscopy is normally used because it easily achieves resolution close to the optical wavelength. But in conventional microscopy, diffraction limits the resolution to about half the wavelength. Recently, it was shown that this limit can be partly overcome by nonlinear imaging techniques, but there is still a barrier to reaching the molecular scale. In contrast, in magnetic resonance imaging the spatial resolution is not determined by diffraction; rather, it is limited by magnetic field sensitivity, and so can in principle go well below the optical wavelength. The sensitivity of magnetic resonance imaging has recently been improved enough to image single cells, and magnetic resonance force microscopy has succeeded in detecting single electrons and small nuclear spin ensembles. However, this technique currently requires cryogenic temperatures, which limit most potential biological applications. Alternatively, single-electron spin states can be detected optically, even at room temperature in some systems. Here we show how magneto-optical spin detection can be used to determine the location of a spin associated with a single nitrogen-vacancy centre in diamond with nanometre resolution under ambient conditions. By placing these nitrogen-vacancy spins in functionalized diamond nanocrystals, biologically specific magnetofluorescent spin markers can be produced. Significantly, we show that this nanometre-scale resolution can be achieved without any probes located closer than typical cell dimensions. Furthermore, we demonstrate the use of a single diamond spin as a scanning probe magnetometer to map nanoscale magnetic field variations. The potential impact of single-spin imaging at room temperature is far-reaching. It could lead to the capability to probe biologically relevant spins in living cells.

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Nanoscale imaging magnetometry with diamond spins under ambient conditions

https://persianmof.weebly.com/uploads/4/8/8/3/48832931/chem._rev._2014,_114,_10575.pdf

Water stability and adsorption in metal-organic frameworks.

Magnetic resonance imaging (MRI) is well known as a powerful technique for visualizing subsurface structures with three-dimensional spatial resolution. Pushing the resolution below 1 micro m remains a major challenge, however, owing to the sensitivity limitations of conventional inductive detection techniques. Currently, the smallest volume elements in an image must contain at least 10(12) nuclear spins for MRI-based microscopy, or 10(7) electron spins for electron spin resonance microscopy. Magnetic resonance force microscopy (MRFM) was proposed as a means to improve detection sensitivity to the single-spin level, and thus enable three-dimensional imaging of macromolecules (for example, proteins) with atomic resolution. MRFM has also been proposed as a qubit readout device for spin-based quantum computers. Here we report the detection of an individual electron spin by MRFM. A spatial resolution of 25 nm in one dimension was obtained for an unpaired spin in silicon dioxide. The measured signal is consistent with a model in which the spin is aligned parallel or anti-parallel to the effective field, with a rotating-frame relaxation time of 760 ms. The long relaxation time suggests that the state of an individual spin can be monitored for extended periods of time, even while subjected to a complex set of manipulations that are part of the MRFM measurement protocol.

Single spin detection by magnetic resonance force microscopy

MAGNETIC resonance spectroscopy1 is a powerful tool for molecular characterization and structure determination. The sensitivity of conventional approaches is limited to about 1010 electron spins or 1016 nuclear spins; this sensitivity can be improved to about 105 spins by polarizing the spins via optical pumping and detecting optical rather than microwave photons2. Recently, fluorescence from single molecules was detected by tuning a single-frequency laser in the inhomogeneously broadened fluorescence excitation band of a dilute dispersion of pentacene in a host crystal of p-terphenyl3,4. Here we report that, by combining single-molecule fluorescence spectroscopy with optically detected magnetic resonance for the pentacene-doped p-terphenyl system, we can detect magnetic resonance in a single pentacene molecule. We observe two of the three possible transitions between sublevels of the metastable triplet state. The spectral lineshapes indicate that the proton nuclear spin states change during the measurement, leading to spectral diffusion within the magnetic resonance line.

Optical detection of magnetic resonance in a single molecule

We have measured the autocorrelation of the fluorescence of single aromatic molecules in polyethylene at T\ensuremath{\approxeq}2 K. Assuming intensity fluctuations to be caused by frequency jumps when two-level systems (TLS's) change states, we show that individual TLS's can be probed, allowing direct measurement of some of their properties, with minimal recourse to theory and no assumptions about the distribution of TLS parameters. Besides providing a simple confirmation of the tunneling theory of TLS's, this new method, by eliminating statistical uncertainties, proves that while some TLS's may be stable for several hours others are modified by transitions of their neighbors. Tunneling times of individual TLS's are measured directly, as are their temperature dependences, revealing three types of TLS-phonon coupling, of which one is new.

Probing individual two-level systems in a polymer by correlation of single molecule fluorescence.

Spectral diffusion and individual two-level systems probed by fluorescence of single terrylene molecules in a polyethylene matrix

The spectral trail of a single molecule is a 2D plot of many repeated spectra displayed as a function of time. Here, we present an analysis of the spectral trails of some 70 terrylene molecules in polymers. We compare the features of individual trails to the standard model of two-level systems (TLSs) in glasses at low temperatures, and we find a good overall agreement for a majority of molecules. However, more than 30% of the molecular trails cannot be reconciled with the standard model. For example, we found three-level systems, evidence for a strong coupling between some TLSs, and spectral drifts. Whereas our data confirm that the standard TLS model describes large ensembles of TLSs quite satisfactorily, this may result only from a cancellation of the deviations. Therefore, the application of the model to explain the spectral diffusion of individual molecules must be questioned in each particular case.

Are the spectral trails of single molecules consistent with the standard two-level system model of glasses at low temperatures?

Various time‐resolved experiments on optically detected magnetic resonance (ODMR) of single pentacene molecules in p‐terphenyl at 1.8 K are described and discussed with the help of a model based on the optical‐microwave Bloch equations. Intersystem crossing rates for this system are deduced from the ODMR spectra, from the fluorescence correlation function, which varies nonmonotonically with the microwave power, and from the analysis of the fluorescence recovery transients. We also discuss the enhancement of the ODMR effect on using fluorescence photons from single molecules as a time base to trigger the microwaves in fluorescence recovery experiments.

R. Brown

Papers

Optical detection of magnetic resonance in a single molecule

Probing individual two-level systems in a polymer by correlation of single molecule fluorescence.

Spectral diffusion and individual two-level systems probed by fluorescence of single terrylene molecules in a polyethylene matrix

Are the spectral trails of single molecules consistent with the standard two-level system model of glasses at low temperatures?

Kinetics of optically detected magnetic resonance of single molecules