Some of the most sensitive methods of measuring magnetic fields utilize interactions of resonant light with atomic vapor. Recent developments in this vibrant field are improving magnetometers in many traditional areas such as measurement of geomagnetic anomalies and magnetic fields in space, and are opening the door to new ones, including, dynamical measurements of bio-magnetic fields, detection of nuclear magnetic resonance (NMR), magnetic-resonance imaging (MRI), inertialrotation sensing, magnetic microscopy with cold atoms, and tests of fundamental symmetries of Nature.

The magnetic field is one of the most fundamental and ubiquitous physical observables, carrying information about all electromagnetic phenomena. For the past 30 years, superconducting quantum interference devices (SQUIDs) operating at 4 K have been unchallenged as ultrahigh-sensitivity magnetic field detectors, with a sensitivity reaching down to 1 fT Hz(-1/2) (1 fT = 10(-15) T). They have enabled, for example, mapping of the magnetic fields produced by the brain, and localization of the underlying electrical activity (magnetoencephalography). Atomic magnetometers, based on detection of Larmor spin precession of optically pumped atoms, have approached similar levels of sensitivity using large measurement volumes, but have much lower sensitivity in the more compact designs required for magnetic imaging applications. Higher sensitivity and spatial resolution combined with non-cryogenic operation of atomic magnetometers would enable new applications, including the possibility of mapping non-invasively the cortical modules in the brain. Here we describe a new spin-exchange relaxation-free (SERF) atomic magnetometer, and demonstrate magnetic field sensitivity of 0.54 fT Hz(-1/2) with a measurement volume of only 0.3 cm3. Theoretical analysis shows that fundamental sensitivity limits of this device are below 0.01 fT Hz(-1/2). We also demonstrate simple multichannel operation of the magnetometer, and localization of magnetic field sources with a resolution of 2 mm.

Alkali-metal magnetometers compete with SQUID detectors as the most sensitive magnetic field sensors. Their sensitivity is limited by relaxation due to spin-exchange collisions. We demonstrate a K magnetometer in which spin-exchange relaxation is completely eliminated by operating at high K density and low magnetic field. Direct measurements of the signal-to-noise ratio give a magnetometer sensitivity of 10 fT Hz(-1/2), limited by magnetic noise produced by Johnson currents in the magnetic shields. We extend a previous theoretical analysis of spin exchange in low magnetic fields to arbitrary spin polarizations and estimate the shot-noise limit of the magnetometer to be 2x10(-18) T Hz(-1/2).

We report the results of a new experimental search for a permanent electric dipole moment of 199 Hg utilizing a stack of four vapor cells. We find d( 199 Hg) = (0.49±1.29stat ±0.76syst)×10 −29 e cm, and interpret this as a new upper bound, |d( 199 Hg)| < 3.1×10 −29 e cm (95% C.L.). This result improves our previous 199 Hg limit by a factor of 7, and can be used to set new constraints on CP violation in physics beyond the standard model. PACS numbers: 11.30.Er,32.10.Dk,32.80.Xx,24.80.+y The existence of a finite permanent electric dipole moment (EDM) of a particle or atom would violate time reversal symmetry (T ), and would also imply violation of the combined charge conjugation and parity symmetry (CP ) through the CP T theorem [1,2,3]. EDMs are suppressed in the standard model of particle physics (SM), lying many orders of magnitude below current experimental sensitivity. However, it is thought that additional sources of CP violation are needed to account for baryogenesis [4,5], and many theories beyond the SM, such as supersymmetry [6,7], naturally predict EDMs within experimental reach.Experimental searches for EDMs have so far yielded null results. The most precise and significant limits have been set on the EDM of the neutron [8], the electron [9], and the 199 Hg atom [10], leading to tight constraints on supersymmetric extensions of the SM [7]. Here we report the first result of a new mercury experiment, |d( 199 Hg)| < 3.1×10−29 e cm (95% C.L.), which improves our previous limit [10] by a factor of 7 and provides a yet more exacting probe of possible new sources of CP violation.199 Hg has a 1 S 0 electronic ground state and nuclear spin 1/2. An EDM of the ground state atom would point along the nuclear spin axis and arise mainly from CP violation in the nucleus. We measure the nuclear Larmor frequency ν given by hν = |2µB ± 2dE|, where µ and d are the 199 Hg magnetic and electric dipole moments, and B and E are the magnitudes of external magnetic and electric fields aligned parallel (+) or antiparallel (−) with each other. The signature for d = 0 is thus a shift in Larmor frequency when E is reversed relative to B.As shown in Fig. 1, our new apparatus uses a stack of four spin-polarized Hg vapor cells in a common B-field. The middle two cells have oppositely directed E-fields, resulting in EDM-sensitive Larmor shifts of opposite sign; the outer two cells, enclosed by the high voltage (HV) electrodes and thus placed at E = 0, are free of EDM effects and serve to cancel B-field gradient noise and provide checks for spurious HV-correlated B-field shifts.The vapor cells are constructed from high purity fused silica and contain isotopically enriched 199 Hg (92 %) at a density of 4 × 10 13 cm −3 , a paraffin wall coating, and 475 Torr of CO buffer gas. CO efficiently quenches excited state 199 Hg and thus reduces degradation of the wall coating [11]. Spin coherence times T 2 are 100 to 200 sec. A conductive SnO coating on the cell end-caps provides electric field plates separated by 11 mm. The average leakage...

We describe an ultra-sensitive atomic magnetometer using optically-pumped potassium atoms operating in spin-exchange relaxation free (SERF) regime. We demonstrate magnetic field sensitivity of 160 aT/Hz 1/2 in a gradiometer arrangement with a measurement volume of 0.45 cm 3 and energy resolution per unit time of 44h. As an example of a new application enabled by such a magnetometer we describe measurements of weak remnant rock magnetization as a function of temperature with a sensitivity on the order of 10 −10 emu/cm 3 /Hz 1/2 and temperatures up to 420 • C.High sensitivity magnetometery is used in many fields of science, including physics, biology, neuroscience, materials science and geology. Traditionally low-temperature SQUID magnetometers have been used for most demanding applications, but recent development of atomic magnetometers with sub-femtotesla sensitivity has opened new possibilities for ultra-sensitive magnetometery [1].Here we report new results of sensitive magnetic field measurements using a spin-exchange relaxation-free potassium magnetometer. By eliminating several sources of ambient magnetic field noise and optimizing operation of the magnetometer we achieve magnetic field sensitivity of 160 aT/Hz 1/2 at 40 Hz. The measurement volume used to obtain this sensitivity is 0.45 cm 3 , resulting in a magnetic field energy resolution of V B 2 /2µ 0 = 44h, a factor of 10 smaller than previously achieved with atomic magnetometers [2]. Energy resolution on the order ofh has been realized with SQUIDs at high frequency and milli-Kelvin temperatures with small input coils [3,4]. However for cm-sized SQUID sensors operating at 4.2 K the energy resolution at low frequency is typically several hundredsh [5,6,7] and the magnetic field sensitivity is about 1 fT/Hz 1/2 [8].When comparing various magnetometery techniques it is important to distinguish between applications requiring detection of smallest magnetic moments and those requiring detection of smallest magnetizations. For the former, it is usually advantageous to use the smallest possible sensor. For example, magnetic resonance force microscopy (MRFM) can detect a single electron spin [9]. On the other hand, for detection of very weak magnetization one needs a sensor with the highest magnetic field sensitivity, since B ∼ µ 0 M in the vicinity of the source. For example, recently developed magnetometers using a single nitrogen-vacancy (NV) center in diamond are promising for detection of single electron and nuclear spins because of their small size [10,11]. In diamond crystals with larger concentration of NV centers the magnetic field sensitivity is limited by dipolar interactions with other inactive color centers and has been optimistically projected at 10 −16 T/Hz 1/2 /cm 3/2 [12], which is the level already realized experimentally in this work.One of the well-developed magnetometry applications requiring high magnetization sensitivity is paleomagnetism [13]. Analysis of magnitude and direction of remnant magnetization in ancient rocks provides geologica...

We present the first results of a new search for a permanent electric dipole moment of the 199Hg atom using a UV laser. Our measurements give d(199Hg) = -(1.06+/-0.49+/-0.40)x10(-28)e cm. We interpret the result as an upper limit absolute value [d(199Hg)]<2.1x10(-28)e cm (95% C.L.), which sets new constraints on theta bar;(QCD), chromo-EDMs of the quarks, and CP violation in supersymmetric models.

A magnetometer using spin-polarized K and 3He atoms occupying the same volume is used to search for anomalous nuclear spin-dependent forces generated by a separate 3He spin source. We measure changes in the 3He spin precession frequency with a resolution of 18 pHz and constrain anomalous spin forces between neutrons to be less than 2x10(-8) of their magnetic or less than 2x10(-3) of their gravitational interactions on a length scale of 50 cm. We present new limits on neutron coupling to light pseudoscalar and vector particles, including torsion, and constraints on recently proposed models involving unparticles and spontaneous breaking of Lorentz symmetry.

We describe a nuclear spin gyroscope based on an alkali-metal-noble-gas co-magnetometer. An optically pumped alkali-metal vapor is used to polarize the noble gas atoms and detect their gyroscopic precession. Spin precession due to magnetic fields as well as their gradients and transients can be cancelled in this arrangement. The sensitivity is enhanced by using a high-density alkalimetal vapor in a spin-exchange relaxation free (SERF) regime. With a K-3 He co-magnetometer we demonstrate rotation sensitivity of 5×10 −7 rad/sec/Hz 1/2 . The rotation signal can be increased by a factor of 10 using 21 Ne due to its smaller magnetic moment and the fundamental rotation sensitivity limit for a 21 Ne gyroscope with a 10 cm 3 measurement volume is about 2 × 10 −10 rad/sec/Hz 1/2 .Sensitive gyroscopes find a wide range of applications, from inertial navigation to studies of Earth rotation and tests of general relativity [1]. A variety of physical principles have been utilized for rotation sensing, including mechanical sensing, the Sagnac effect for photons [1,2] and atoms [3,4], the Josephson effect in superfluid 4 He and 3 He [5] and nuclear spin precession [6]. While state-ofthe-art mechanical gyroscopes, such as those developed for Gravity Probe B [7], remain unchallenged in terms of sensitivity, their extremely high cost and difficulty of fabrication motivate the development of simpler, smaller and more robust rotation sensors.Here we describe a new gyroscope based on nuclear spin precession. Unlike the atom and photon interferometric gyroscopes based on the Sagnac effect, nuclear spin gyroscopes do not require a large area enclosed by the interferometer and can be made quite compact. Previous nuclear spin gyroscopes [6] have suffered from high sensitivity to magnetic fields. We show that a comagnetometer using spin-polarized noble gas and alkalimetal vapor can eliminate the sensitivity to magnetic fields, their gradients and transients. High short-term rotation sensitivity can be achieved with an alkali-metal magnetometer operating in the SERF regime [8]. For example, magnetic field sensitivity of 0.5 fT/Hz 1/2 that has been demonstrated in a K magnetometer [9] would result in a rotation sensitivity of 1 × 10 −8 rad/s/Hz 1/2 in a K-21 Ne gyroscope. The bandwidth and transient response of the gyroscope are also significantly improved compared with earlier spin gyroscopes by damping due to coupling between noble gas and alkali-metal spins. We describe an experimental implementation of the gyroscope using K and3 He atoms and demonstrate short term rotation sensitivity of 5 × 10 −7 rad/sec/Hz 1/2 with a sensing volume of only 0.5 cm 3 . We also present a theoretical analysis and experimental measurements of the gyroscope response to various perturbations, and derive fundamental limits for its performance.The co-magnetometer consists of a spherical glass cell containing an alkali metal, several atmospheres of noble gas and a small quantity of nitrogen. Alkali atoms are polarized by optical pumping and transfer the polariz...