Search

We introduce a Magnetic Particle Imaging Susceptometer (MPIS) that uses a high- sensitivity atomic magnetometer (AM) for recording the spatial distribution of fluid- suspended magnetic nanoparticles. We have evaluated the MPIS performance by one-dimensional scans of structured nanoparticle phantoms, demonstrating, in particular, resolutions of ≈2.5 mm prior to deconvolution and ≪1 mm after deconvolution. Our instrument conceptually follows the general principle of Magnetic Particle Imaging (MPI) for encoding spatial distributions into magnetic flux density variations. Conversely to previously demonstrated MPI methods, MPIS works in time- space by recording time series of the sample’s magnetic response including all Fourier components. The device deploys a specifically designed system of coils, a low-frequency excitation scheme, and a simple source localization algorithm. The difference of the AM’s frequency response with respect to the conventional receive coil detection allows us to work at much lower driving frequencies. We demonstrate operation at frequencies on the order of 100 Hz, enabling the beneficial use of larger nanoparticles. The spatial distribution encoded into the particles’ susceptibility needs a much lower excitation field amplitude compared to conventional MPI scanners. These two features make MPIS least harmful for biological samples and subjects compared to conventional MPI scanners. We also address performance characteristics and other possible applications of MPIS.

Results are reported from the first full-scale search for transient signals from exotic fields of astrophysical origin using data from a newly constructed Earth-scale detector: the Global Network of Optical Magnetometers for Exotic physics searches (GNOME). Data collected by the GNOME consist of correlated measurements from optical atomic magnetometers located in laboratories all over the world. GNOME data are searched for patterns of signals propagating through the network consistent with exotic fields composed of ultralight bosons such as axion-like particles (ALPs). Analysis of data from a continuous month-long operation of the GNOME finds no statistically significant signals consistent with those expected due to encounters with topological defects (axion domain walls), placing new experimental constraints on such dark matter scenarios.

Ultralight bosons such as axion-like particles are viable candidates for dark matter. They can form stable, macroscopic field configurations in the form of topological defects that could concentrate the dark matter density into many distinct, compact spatial regions that are small compared with the Galaxy but much larger than the Earth. Here we report the results of the search for transient signals from the domain walls of axion-like particles by using the global network of optical magnetometers for exotic (GNOME) physics searches. We search the data, consisting of correlated measurements from optical atomic magnetometers located in laboratories all over the world, for patterns of signals propagating through the network consistent with domain walls. The analysis of these data from a continuous month-long operation of GNOME finds no statistically significant signals, thus placing experimental constraints on such dark matter scenarios., A search for transient dark matter in the form of domain walls of axion-like particles finds no statistically significant signal. This places constraints on our theoretical understanding of such scenarios.

The European Physical Journal A, 57 (7), ISSN:1434-6001, ISSN:1434-601X

It has been proposed that there could be a mirror copy of the standard model particles, restoring the parity symmetry in the weak interaction on the global level. Oscillations between a neutral standard model particle, such as the neutron, and its mirror counterpart could potentially answer various standing issues in physics today. Astrophysical studies and terrestrial experiments led by ultracold neutron storage measurements have investigated neutron to mirror-neutron oscillations and imposed constraints on the theoretical parameters. Recently, further analysis of these ultracold neutron storage experiments has yielded statistically significant anomalous signals that may be interpreted as neutron to mirror-neutron oscillations, assuming nonzero mirror magnetic fields. The neutron electric dipole moment collaboration performed a dedicated search at the Paul Scherrer Institute and found no evidence of neutron to mirror-neutron oscillations. Thereby, the following new lower limits on the oscillation time were obtained: τnn′>352 s at B′=0 (95% C.L.), τnn′>6s for 0.4μT9s for 5.0μT

The Global Network of Optical Magnetometers to search for Exotic physics (GNOME) is a network of geographically separated, time-synchronized, optically pumped atomic magnetometers that is being used to search for correlated transient signals heralding exotic physics. The GNOME is sensitive to nuclear- and electron-spin couplings to exotic fields from astrophysical sources such as compact dark-matter objects (for example, axion stars and domain walls). Properties of the GNOME sensors such as sensitivity, bandwidth, and noise characteristics are studied in the present work, and features of the network's operation (e.g., data acquisition, format, storage, and diagnostics) are described. Characterization of the GNOME is a key prerequisite to searches for and identification of exotic physics signatures., Comment: 45 pages, 16 figures, 2 tables

Psychological bias towards, or away from, prior measurements or theory predictions is an intrinsic threat to any data analysis. While various methods can be used to try to avoid such a bias, e.g. actively avoiding looking at the result, only data blinding is a traceable and trustworthy method that can circumvent the bias and convince a public audience that there is not even an accidental psychological bias. Data blinding is nowadays a standard practice in particle physics, but it is particularly difficult for experiments searching for the neutron electric dipole moment (nEDM), as several cross measurements, in particular of the magnetic field, create a self-consistent network into which it is hard to inject a false signal. We present an algorithm that modifies the data without influencing the experiment. Results of an automated analysis of the data are used to change the recorded spin state of a few neutrons within each measurement cycle. The flexible algorithm may be applied twice (or more) to the data, thus providing the option of sequentially applying various blinding offsets for separate analysis steps with independent teams. The subtle manner in which the data are modified allows one subsequently to adjust the algorithm and to produce a re-blinded data set without revealing the initial blinding offset. The method was designed for the 2015/2016 measurement campaign of the nEDM experiment at the Paul Scherrer Institute. However, it can be re-used with minor modification for the follow-up experiment n2EDM, and may be suitable for comparable projects elsewhere., The European Physical Journal A, 57 (4), ISSN:1434-6001, ISSN:1434-601X

We present the result of an experiment to measure the electric dipole moment (EDM) of the neutron at the Paul Scherrer Institute using Ramsey’s method of separated oscillating magnetic fields with ultracold neutrons. Our measurement stands in the long history of EDM experiments probing physics violating time-reversal invariance. The salient features of this experiment were the use of a 199Hg comagnetometer and an array of optically pumped cesium vapor magnetometers to cancel and correct for magnetic-field changes. The statistical analysis was performed on blinded datasets by two separate groups, while the estimation of systematic effects profited from an unprecedented knowledge of the magnetic field. The measured value of the neutron EDM is dn=(0.0±1.1stat±0.2sys)×10−26 e.cm., Physical Review Letters, 124 (8), ISSN:0031-9007, ISSN:1079-7114

An array of 16 laser-pumped scalar Cs magnetometers was part of the neutron electric dipole moment (nEDM) experiment taking data at the Paul Scherrer Institute in 2015 and 2016. It was deployed to measure the gradients of the experiment's magnetic field and to monitor their temporal evolution. The originality of the array lies in its compact design, in which a single near-infrared diode laser drives all magnetometers that are located in a high-vacuum chamber, with a selection of the sensors mounted on a high-voltage electrode. We describe details of the Cs sensors' construction and modes of operation, emphasizing the accuracy and sensitivity of the magnetic-field readout. We present two applications of the magnetometer array directly beneficial to the nEDM experiment: (i) the implementation of a strategy to correct for the drift of the vertical magnetic-field gradient and (ii) a procedure to homogenize the magnetic field. The first reduces the uncertainty of the nEDM result. The second enables transverse neutron spin relaxation times exceeding 1500 s, improving the statistical sensitivity of the nEDM experiment by about 35% and effectively increasing the rate of nEDM data taking by a factor of 1.8. An array of sixteen laser-pumped scalar Cs magnetometers was part of the neutron electric dipole moment (nEDM) experiment taking data at the Paul Scherrer Institute in 2015 and 2016. It was deployed to measure the gradients of the experiment's magnetic field and to monitor their temporal evolution. The originality of the array lies in its compact design, in which a single near-infrared diode laser drives all magnetometers that are located in a high-vacuum chamber, with a selection of the sensors mounted on a high-voltage electrode. We describe details of the Cs sensors' construction and modes of operation, emphasizing the accuracy and sensitivity of the magnetic field readout. We present two applications of the magnetometer array directly beneficial to the nEDM experiment: (i) the implementation of a strategy to correct for the drift of the vertical magnetic field gradient and (ii) a procedure to homogenize the magnetic field. The first reduces the uncertainty of the new nEDM result. The second enables transverse neutron spin relaxation times exceeding 1500 s, improving the statistical sensitivity of the nEDM experiment by about 35% and effectively increasing the rate of nEDM data taking by a factor of 1.8.

We propose a precision method for the in situ calibration of a three axis coil system based on the free induction decay of spin aligned atoms. In addition we present a simple and efficient method for measuring the three vector components of a residual magnetic field. © 2013 Springer Verlag Berlin Heidelberg.

The Global Network of Optical Magnetometers for Exotic physics searches (GNOME) is a network of time-synchronized, geographically separated, optically pumped atomic magnetometers that is being used to search for correlated transient signals heralding exotic physics. GNOME is sensitive to exotic couplings of atomic spins to certain classes of dark matter candidates, such as axions. This work presents a data analysis procedure to search for axion dark matter in the form of topological defects: specifically, walls separating domains of discrete degenerate vacua in the axion field. An axion domain wall crossing the Earth creates a distinctive signal pattern in the network that can be distinguished from random noise. The reliability of the analysis procedure and the sensitivity of the GNOME to domain-wall crossings are studied using simulated data.

We present the new spectrometer for the neutron electric dipole moment (nEDM) search at the Paul Scherrer Institute (PSI), called n2EDM. The setup is at room temperature in vacuum using ultracold neutrons. n2EDM features a large UCN double storage chamber design with neutron transport adapted to the PSI UCN source. The design builds on experience gained from the previous apparatus operated at PSI until 2017. An order of magnitude increase in sensitivity is calculated for the new baseline setup based on scalable results from the previous apparatus, and the UCN source performance achieved in 2016., Submitted as a web of conference proceedings paper

We report on the strategy used to optimize the sensitivity of our search for a neutron electric dipole moment at the Paul Scherrer Institute. Measurements were made upon ultracold neutrons stored within a single chamber at the heart of our apparatus. A mercury cohabiting magnetometer together with an array of cesium magnetometers were used to monitor the magnetic field, which was controlled and shaped by a series of precision field coils. In addition to details of the setup itself, we describe the chosen path to realize an appropriate balance between achieving the highest statistical sensitivity alongside the necessary control on systematic effects. The resulting irreducible sensitivity is better than 1 × 10−26e cm. This contribution summarizes in a single coherent picture the results of the most recent publications of the collaboration., EPJ Web of Conferences, 219, ISSN:2100-014X, ISSN:2101-6275

© 2019 American Physical Society. Magnetic-field uniformity is of the utmost importance in experiments to measure the electric dipole moment of the neutron. A general parametrization of the magnetic field in terms of harmonic polynomial modes is proposed, going beyond the linear-gradients approximation. We review the main undesirable effects of nonuniformities: depolarization of ultracold neutrons and Larmor frequency shifts of neutrons and mercury atoms. The theoretical predictions for these effects were verified by dedicated measurements with the single-chamber neutron electric-dipole-moment apparatus installed at the Paul Scherrer Institute. ispartof: Physical Review A vol:99 issue:4 status: published

We report magnetic resonance experiments with optical detection performed on cesium atoms trapped in a crystalline Hematrix. Multi-photon transitions, i.e., processes in which several radio-frequency photons are absorbed simultaneously in a given hyperfine Zeeman multiplet of the ground state, were the central topic of these studies. The long relaxation times of spin coherences of Cs in solid He allow such transitions to be spectrally resolved in fields as low as 1 mT. We observed all allowed multi-photon transitions up to the ΔM=8 transition in the F=4 state. We compare the experimental spectra with theoretical spectra obtained from numerical solutions of the Liouville equation that include optical pumping and the interaction with the static and oscillating fields. Multi-photon transitions may find applications in magnetometry, suppress systematic effects in EDM experiments, and allow the study of relaxation phenomena in doped He crystals. The demonstration of these features is still hindered by inhomogeneous line broadening.

Laser spectroscopy of atomic dopants can be used as a tool for the investigation of elementary excitations in quantum fluids and solids. Here we present results of a laser-spectroscopic study of transition-metal (Au, Cu) atoms in liquid and solid 4He. In particular we observe transitions of inner shell electrons that have not been studied before. Such transitions are weakly perturbed by the interaction with the helium matrix and display a characteristic structure composed of a sharp zero-phonon line and a relatively broad phonon wing.

We describe the design, applications, and performance of a simple light power controller. The device is built on a fiber-coupled electro-optic modulator with an active electronic feedback. It can be used to actively stabilize laser power or to impress an arbitrary waveform onto the power. The bandwidth of the device is ∼70 kHz.

Light (pseudo-)scalar fields are promising candidates to be the dark matter in the Universe. Under certain initial conditions in the early Universe and/or with certain types of self-interactions, they can form compact dark-matter objects such as axion stars or Q-balls. Direct encounters with such objects can be searched for by using a global network of atomic magnetometers. It is shown that for a range of masses and radii not ruled out by existing observations, the terrestrial encounter rate with axion stars or Q-balls can be sufficiently high (at least once per year) for a detection. Furthermore, it is shown that a global network of atomic magnetometers is sufficiently sensitive to pseudoscalar couplings to atomic spins so that a transit through an axion star or Q-ball could be detected over a broad range of unexplored parameter space., Comment: 8 pages, 3 figures

We demonstrate optical pumping on the four hyperfine components of the Cs D1 transition by unpolarized (UPL) resonant laser light. The evidence is based on the reduction of the absorption coefficients κ0 with increasing light power P in an uncoated Cs vapor cell with isotropic spin relaxation. For comparison we perform the same quantitative κ0(P) measurements with linearly-polarized light (LPL) and circularly-polarized light (CPL). We find that our previously published algebraic expressions give an excellent description of all experimentally recorded induced transparency signals. Based on this we can make reliable absolute predictions for the power dependence of the spin orientation and alignment produced by pumping with LPL, CPL and UPL.

We have performed quantitative measurements of the variation of the on-resonance absorption coefficients ${\ensuremath{\kappa}}_{0}$ of the four hyperfine components of the Cs ${D}_{1}$ transition as a function of laser power $P$, for pumping with linearly and with circularly polarized light. Sublevel populations derived from rate equations assuming isotropic population relaxation (at a rate ${\ensuremath{\gamma}}_{1}$) yield algebraic ${\ensuremath{\kappa}}_{0}(P)$ dependences that do not reproduce the experimental findings from Cs vapor in a paraffin-coated cell. However, numerical results that consider spin-exchange relaxation (at a rate ${\ensuremath{\gamma}}_{\mathrm{se}}$) and isotropic relaxation fit the experimental data perfectly well. The fit parameters, viz., the absolute value of ${\ensuremath{\kappa}}_{0}$, the optical pumping saturation power ${P}_{\mathrm{sat}}$, and the ratio ${\ensuremath{\gamma}}_{\mathrm{se}}/{\ensuremath{\gamma}}_{1}$, are well described by the experimental conditions and yield absolute values for ${\ensuremath{\gamma}}_{1}$ and ${\ensuremath{\gamma}}_{\mathrm{se}}$. The latter is consistent with the previously published Cs-Cs spin-exchange relaxation cross section.

International audience; We report on a laser based $^{199}$Hg co-magnetometer deployed in an experiment searching for a permanent electric dipole moment of the neutron. We demonstrate a more than five times increased signal to-noise-ratio in a direct comparison measurement with its $^{204}$Hg discharge bulb-based predecessor. An improved data model for the extraction of important system parameters such as the degrees of absorption and polarization is derived. Laser- and lamp-based data-sets can be consistently described by the improved model which permits to compare measurements using the two different light sources and to explain the increase in magnetometer performance. The laser-based magnetometer satisfies the magnetic field sensitivity requirements for the next generation nEDM experiments.

The discrimination of immobilised superparamagnetic iron oxide nanoparticles (SPIONs) against SPIONs in fluid environments via their magnetic relaxation behaviour is a powerful tool for bio-medical imaging. Here we demonstrate that a gradiometer of laser-pumped atomic magnetometers can be used to record accurate time series of the relaxing magnetic field produced by pre-polarised SPIONs. We have investigated dry in vitro maghemite nanoparticle samples with different size distributions (average radii ranging from 14 to 21 nm) and analysed their relaxation using the Néel–Brown formalism. Fitting our model function to the magnetorelaxation (MRX) data allows us to extract the anisotropy constant K and the saturation magnetisation MS of each sample. While the latter was found not to depend on the particle size, we observe that K is inversely proportional to the (time- and size-) averaged volume of the magnetised particle fraction. We have identified the range of SPION sizes that are best suited for MRX detection considering our specific experimental conditions and sample preparation technique.

We describe a method for recording the Rabi nutation of nuclear spin polarized 3He by optically pumped cesium magnetometers. The measurement is performed by detecting the time-dependent magnetic field produced by the 3He magnetization. The observed signals are compared to theoretical models and the results are used to precisely trace the evolution of the magnetization. This procedure represents a convenient way to control and measure the Rabi flip angle and the degree of spin polarization in experiments using 3He magnetometers. The method requires only very coarse knowledge of the applied magnetic field’s magnitude.

We present first results on the imaging of the two dimensional magnetic field distributions using a recently developed magnetic field imaging camera (MFIC). The instrument is based on laser-pumped atomic magnetometry with an alkali vapour in a buffer gas. The device provides millimetre spatial and sub-second time resolution, and allows mapping the magnetic field from circa 2 cm2 surfaces. We apply the MFIC to image magnetic field patterns of deposited superparamagnetic nanoparticles (SPIONs) and quantify the achieved sensitivity. We address the applicability of the instrument to in-vitro and in-vivo imaging of SPIONs distributions in biological tissue and small animals.

The neutron gyromagnetic ratio has been measured relative to that of the 199Hg atom with an uncertainty of 0.8 ppm. We employed an apparatus where ultracold neutrons and mercury atoms are stored in the same volume and report the result γn/γHg=3.8424574(30)., Physics Letters B, 739, ISSN:0370-2693, ISSN:0031-9163, ISSN:1873-2445

The chapter gives a comprehensive account of the theory of atomic magnetometers deploying optically detected magnetic resonance (ODMR) in spin-polarized atomic ensembles, and of the practical realization of such magnetometers. We address single laser beam experiments throughout, but give explicit hints on how the results can be extended to pump-probe configurations. After a general introduction and the presentation of a classification of atomic magnetometer principles, we address the three major processes, viz., polarization creation, atom-field interaction, and optical detection that occur in the subclass of magnetic resonance-based magnetometers. The time-independent signals on which so-called Hanle magnetometers built are also reviewed for both spin-oriented and spin-aligned media. In the extended central part we derive an algebraic master expression (valid for all ODMR magnetometers) that expresses the signal, i.e., the detected time-dependent light power in terms of all system parameters. We then give explicit algebraic results for the absolute signals observed in the so-called Mz- and Mx-configurations for various geometries with arbitrary relative orientations of the static field, the oscillating field and the light propagation direction. Although the chapter’s main focus is on magnetic resonance processes driven by oscillating magnetic fields (we treat both spin-oriented and spin-aligned media), we also address magnetometers in which the magnetic resonance is driven by amplitude-, frequency-, or polarization-modulated light. The final section of the chapter gives a detailed account of the physical realization of an Mx-magnetometer array and the electronics used for its operation. We demonstrate that the observed resonance signals have the predicted spectral shapes and illustrate procedures for optimizing the magnetometric sensitivity.

20 Optically pumped atomic magnetometers measure the Larmor frequency νL at which the bulk magnetization of a spin-polarized alkali metal vapour in a glass cell precesses around the field of interest B → 0. The precession frequency is related to the modulus of the magnetic field by νL=(γ/2π)|B → 0|, where γ/2π≈3.5Hz/nT for 133Cs, so that the magnetic field measurement consists in a frequency measurement. In so-called ODMR magnetometers, νL is determined by Optically Detected Magnetic Resonance using laser radiation. The general principle of an ODMR magnetometer can be understood as follows: A bulk spin polarization is created in the atomic medium through optical pumping, a process introduced in the 1950s, by which angular momentum of polarized light, resonant with an atomic m partial view of sensor array in the 19-channel Fribourg magnetocardiography (MCG) device and (inset) subject positioned for MCG measurements (details in the text). Optically pumped alkali magnetometers for biomedical applications

The double-slit experiment has played an important role in physics, from supporting the wave theory of light, via the discussions of the wave–particle duality of light (and matter) to the foundations of modern quantum optics. Today it keeps playing an active role in the context of quantum optics experiments involving single photons. In this paper, we present a truly portable double-slit apparatus which demonstrates both the wave–particle duality of light and the phenomenon of quantum erasing. The device can be operated either with strong laser light and screen projection, or with individual photons, in which case quantum interference and quantum erasing are detected by a photomultiplier whose pulses are rendered acoustically by means of a loudspeaker. Alternatively, the phenomena can be displayed using multimedia projection of oscilloscope traces.

International audience; The measurement of the neutron electric dipole moment (nEDM) constrains the contribution of CP-violating terms within both the Standard Model and its extensions. The experiment uses ultracold neutrons (UCN) stored in vacuum at room temperature. This technique provided the last (and best) limit by the RAL/Sussex/ILL collaboration in 2006: dn < 2:9 × 10-26 e cm (90% C.L.). We aim to improve the experimental sensitivity by a factor of 5 within 2-3 years, using an upgrade of the same apparatus. We will take advantage of the increased ultracold neutron density at the Paul Scherrer Institute (PSI) and of a new concept including both, external magnetometers and a cohabiting magnetometer. In parallel, a next generation apparatus with two UCN storage chambers and an elaborate magnetic field control is being designed aiming to achieve another order of magnitude increase in sensitivity, allowing us to put a limit as tight as dn < 5 × 10-28 e cm (95% C.L.), if not establishing a finite value.

International audience; We describe the status of the new measurement of the neutron electric dipole moment (nEDM) to be performed at the strong source of ultra-cold neutrons at the Paul Scherrer Institut. The experimental technique is based on Ramsey's method of separated oscillatory fields, applied to UCN stored in vacuum in a chamber at room temperature. Our approach is performed in three phases: in phase one, new components have been developed and tested at the Institut Laue-Langevin. Phase two is being performed at PSI, where the apparatus was moved in 2009. Here, together with the optimization of the magnetic environment, the prospective UCN density of not, vert, similar 100 cm-3 should enable an improvement of the currently best limit by a factor of five within two years of data taking. In the third phase, a new spectrometer will then gain another order of magnitude in sensitivity. The improvements will be mainly due to (1) much higher UCN intensity, (2) improved magnetometry and magnetic field control, and (3) a double chamber configuration with opposite electric field directions.

In the conventional interpretation of quantum mechanics the interference of particles in a two-beam interferometer is closely related to the problem of which-way information. One of the mysteries of quantum mechanics relies on the assumption that the wavefunction of each photon propagates simultaneously along both classically allowed paths, and that interference arises as a consequence of the indistinguishability of those paths. Any attempt to obtain which-way information by putting individual labels on the photons in each pathway inevitably destroys interference. However, even in cases in which the photons carry which-way labels, it is possible to erase those labels after the particle has left the interferometer. The erasing process (partly or completely) destroys the which-way information, and thereby restores interference. This phenomenon is known as quantum erasing. Here we present a lecture demonstration experiment of quantum erasing based on a Mach–Zehnder interferometer operated with single photons.

The blackbody radiation shift, a manifestation of the Stark effect caused by blackbody radiation, contributes to the systematic uncertainty in Cs-based frequency standards at a level of 1 x 10(-15). Few measurements of the third-order scalar electric polarizability of the Cs ground states, mainly responsible for the ac Stark shift in atomic clocks, have given better than 10% accuracy. We report progress in the development of a fully optical Ramsey pump-probe measurement in a thermal atomic beam, based on coherent population trapping (CPT), for the measurement of the third-order scalar and tensor polarizabilities of the Cs ground states. We give details of the apparatus and measurement techniques as well as our first 500 Hz half-period Ramsey fringes.

High precision mapping of weak magnetic fields is of interest for several branches of pure and applied research. This paper presents a resume of the progress made at Fribourg in adapting laboratory style laser-pumped optical Cs magnetometers to multisensor field mapping problems. Progress has been made in sensor head design and manufacture, laser beam splitting and control, and signal treatment electronics. A summary of the technology and its applications are presented.

International audience; The effort towards a new measurement of the neutron electric dipole moment (nEDM) at the Paul Scherrer Institut's (PSI) new high intensity source of ultracold neutrons (UCN) is described. The experimental technique relies on Ramsey's method of separated oscillatory fields, using UCN in vacuum with the apparatus at ambient temperature. In the first phase, R&D towards the upgrade of the RAL/Sussex/ILL apparatus is being performed at the Institut Laue-Langevin (ILL). In the second phase the apparatus, moved from ILL to PSI, will allow an improvement in experimental sensitivity by a factor of 5. In the third phase, a new spectrometer should gain another order of magnitude in sensitivity. The improvements will be mainly due to (1) much higher UCN intensity, (2) improved magnetometry and magnetic field control, and (3) a double chamber configuration with opposite electric field directions.

The optical absorption and fluorescence spectra of alkali atoms isolated in liquid and solid He matrices depend on specific macroscopic matrix properties, such as their molar volume and (anisotropic) elasticity constants, and provide thus information about the quantum matrix. We apply optical spectroscopy to investigate the properties of a recently discovered impurity-stabilized doped He4 solid that exists in equilibrium with pressurized superfluid helium close to the solidification∕melting point of pure helium. The difference between the local He density around the implanted atoms obtained in the present experiment and the average density measured earlier suggests that the impurity-stabilized solid He is in fact a porous structure filled with liquid helium.

The studies of defects formed by impurity particles (atoms, molecules, exciplexes, clusters, free electrons, and positive ions) embedded in liquid and solid 4 He are reviewed. The properties of free electrons and neutral particles in condensed helium are described by the electron (atomic) bubble model, whereas for the positive ions a snowball structure is considered. We compare the properties of the defects in condensed helium with those of metal atoms isolated in heavier rare gas matrices.

The modification of melting temperatures and pressures by dissolved impurities is well known in classical fluids. However, to our knowledge such effects have never been studied in quantum solids because of the difficulties in introducing impurities into such crystals that exist only at cryogenic temperatures, and, in the case of 4He, at pressures exceeding 25 bar. Here, we present an effect that occurs during the melting of solid 4He doped with nanoscopic impurities (alkali atoms, clusters, ions and electrons): the doped part of the crystal remains solid under conditions at which pure helium is liquid. Using interferometry, we found that the density of the solid structure lies between the densities of pure liquid and pure solid helium. We tentatively interpret the solid structure as being an aggregation of positively charged particles and electron bubbles.

We have experimentally studied the selective reflection spectra of a circularly-polarized laser beam from a sub-micrometric Rb vapor cell with a thickness L around λ/2 (λ = 780 nm being the laser wavelength tuned to the D2line) in the presence of a longitudinal magnetic field. Based on the results of these studies, we propose a new method for the tunable locking of the diode laser frequency, which does not require frequency modulation nor complex electronics. The experimental realization of the technique has demonstrated its effectiveness and its competitiveness with the known DAVVL-type methods.

We report on extensive studies on the intrinsic sensitivity of a combined 3 He/Cs magnetometer. The magnetometer relies on the detection of the free spin precession of nuclear spin polarized 3 He by optically pumped cesium magnetometers. We characterize the relevant processes involved in the detection and quantify their impact on the total sensitivity of the magnetometer. An expression is derived that predicts the sensitivity of this magnetometer scheme and the results are compared to experiments. Excellent agreement is found between theory and experiments, and implications for an application of a 3 He/Cs magnetometer in an experiment searching for a permanent neutron electric dipole moment are discussed.

We describe a spin-echo method for ultracold neutrons (UCNs) confined in a precession chamber and exposed to a $|B_0|=1~\text{\mu T}$ magnetic field. We have demonstrated that the analysis of UCN spin-echo resonance signals in combination with knowledge of the ambient magnetic field provides an excellent method by which to reconstruct the energy spectrum of a confined ensemble of neutrons. The method takes advantage of the relative dephasing of spins arising from a gravitationally induced striation of stored UCN of different energies, and also permits an improved determination of the vertical magnetic-field gradient with an exceptional accuracy of $1.1~\text{pT/cm}$. This novel combination of a well-known nuclear resonance method and gravitationally induced vertical striation is unique in the realm of nuclear and particle physics and should prove to be invaluable for the assessment of systematic effects in precision experiments such as searches for an electric dipole moment of the neutron or the measurement of the neutron lifetime., Comment: 7 pages 5 figures, accepted by PRL, September, 08 2015

Existing Magnetic Particle Imaging (MPI) instruments detect the oscillating magnetic field produced by harmonically-excited magnetic nanoparticles (MNPs) with receive coils that pick up magnetic flux changes [1]. The direct measurement of MNP-produced magnetic flux was recently demonstrated [2] using fluxgate magnetometers as magnetic field sensors. Here we present an alternative approach to AC magnetic field sensing based on a laser-pumped atomic magnetometer operated as an rf field detector (atomic rf magnetometer, ARFM). We have developed an ARFM prototype and deploy it as field sensor in an AC magnetic susceptometer. Here we report on its current performance.

A little more than a decade ago our team extended the field of defect spectroscopy in cryocrystals to solid He4 matrices, in both their body-centered cubic (bcc) and hexagonal close-packed (hcp) configurations. In this review paper we survey our pioneering activities in the field and compare our results to those obtained in the related fields of doped superfluid helium and doped helium nanodroplets, domains developed in parallel to our own efforts. We present experimental details of the sample preparation and the different spectroscopic techniques. Experimental results of purely optical spectroscopic studies in atoms, exciplexes, and dimers and their interpretation in terms of the so-called bubble model will be discussed. A large part of the paper is devoted to optically detected magnetic resonance (ODMR) processes in alkali atoms. The quantum nature of the helium matrix and the highly isotropic shape of the local trapping sites in the bcc phase make solid helium crystals ideal matrices for high-resolutio...

We discuss the design and performance of a laser-pumped cesium vapor magnetometer in the Mx configuration. The device will be employed in the control and stabilization of fluctuating magnetic fields and gradients in a new experiment searching for a permanent electric dipole moment of the neutron. We have determined the intrinsic sensitivity of the device to be 15 fT in a 1 Hz bandwidth, limited by technical laser noise. In the shot noise limit the magnetometer can reach a sensitivity of 10 fT in a 1 Hz bandwidth. We have used the device to study the fluctuations of a stable magnetic field in a multi-layer magnetic shield for integration times in the range of 2–100 seconds. The residual fluctuations for times up to a few minutes are traced back to the instability of the power supply used to generate the field.

We have performed a comparison of laser (LsOPM) and lamp (LpOPM) pumped cesium vapor magnetometers. Although the LsOPM operated 50% above its shot-noise limit we found an intrinsic sensitivity of 15 fT/\sqrt{Hz} and 25 fT/\sqrt{Hz} for the LsOPM and LpOPM respectively. Two modes of operation, viz.,the phase-stabilized and the self-oscillating mode were investigated and found to yield a similar performance. We have compared the performance of the LsOPM and the LpOPM directly by simultaneous measurements of field fluctuations of a 2 microtesla magnetic field inside a multilayer magnetic shield and have used one of the magnetometers for an active field stabilization. In the stabilized mode we found a gradient instability of 25 fT within an integration time of 100 s, which represents an upper limit of the long-term stability of the magnetometers., Comment: 11 pages, 13 figures, 1 table

This paper extends our previous work on near-degenerate magnetic resonance transitions in alkali ground states involving the simultaneous absorption of multiple radio-frequency quanta. New experimental results with an improved spectral resolution were obtained with cesium atoms trapped in the cubic phase of a helium crystal. The main objective of the paper is a theoretical study of the influence of stochastic perturbations of given multipole orders on the various multi-photon coherences. Algebraic and numerical results for perturbations of both dipolar and quadrupolar symmetry are presented. The present experimental resolution does not yet allow us to distinguish between these two most likely relaxation mechanisms. Nonetheless, the experimental spectra are very well described when allowing in the calculations for a magnetic field inhomogeneity of 2×10-5.

Magnetic fields produced by biological organisms contain valuable information on the underlying physiological processes and their pathologies. Currently, superconducting detectors cooled far below room temperature are required to measure these generally weak biomagnetic signals. We have developed a sensitive laser magnetometer based on optical pumping of cesium atoms that makes it possible to map the magnetic field produced by the beating human heart. A gradiometer formed by two identical sensors greatly reduces the influence of external stray magnetic fields. The magnetometer operates at room temperature and therefore opens the way to affordable and convenient monitoring of biomagnetic fields in research and medical diagnostics.

We report on the design and performance of a highly sensitive combined $^3$He/Cs magnetometer for the absolute measurement of magnetic fields. The magnetometer relies on the magnetometric detection of the free spin precession of nuclear spin polarized $^3$He gas by optically pumped cesium magnetometers. We plan to deploy this type of combined magnetometer in an experiment searching for a permanent electric dipole moment of ultracold neutrons at the Paul Scherrer Institute (Switzerland). A prototype magnetometer was built at the University of Fribourg (Switzerland) and tested at Physikalisch-Technische Bundesanstalt (Berlin, Germany). We demonstrate that the combined magnetometer allows Cram\'er-Rao- limited field determinations with recording times in the range of $\sim 500\mathrm{s}$, measurements above $500\mathrm{s}$ being limited by the stability of the applied magnetic field. % With a $100\mathrm{s}$ recording time we were able to perform an absolute measurement of a magnetic field of $\approx1\mathrm{\mu T}$ with a standard uncertainty of $\Delta B\sim60\mathrm{fT}$, corresponding to $\Delta B/B, Comment: The final publication is available at Springer via http://dx.doi.org/10.1140/epjd/e2015-60018-7