Scalar atomic magnetometers have many attractive features but their sensitivity has been relatively poor. We describe a Rb scalar gradiometer using two multi-pass optical cells. We use a pump-probe measurement scheme to suppress spin-exchange relaxation and two probe pulses to find the spin precession zero crossing times with a resolution of 1 psec. We realize magnetic field sensitivity of 0.54 fT/Hz 1/2 , which improves by an order of magnitude the best scalar magnetometer sensitivity and surpasses the quantum limit set by spin-exchange collisions for a scalar magnetometer with the same measurement volume operating in a continuous regime. PACS numbers: 07.55.Ge, 42.50.Lc, 32.30.Dx Alkali-metal magnetometers can surpass SQUIDs as the most sensitive detectors of magnetic field, reaching sensitivity below 1 fT/Hz 1/2 [1, 2], but only if they are operated near zero magnetic field to eliminate spin relaxation due to spin-exchange collisions [3,4]. Many magnetometer applications, such as searches for permanent electric dipole moments [5], detection of NMR signals [6], and low-field magnetic resonance imaging [7], require sensitive magnetic measurements in a finite magnetic field. In addition, scalar magnetometers measuring the Zeeman frequency are unique among magnetic sensors in being insensitive to the direction of the field, making them particularly suitable for geomagnetic mapping [8] and field measurements in space [9,10]. The sensitivity of scalar magnetometers has been relatively poor, as summarized recently in [11]. The best directly measured scalar magnetometer sensitivity is equal 7 fT/Hz 1/2 with a measurement volume of 1.5 cm 3 [12], while estimates of fundamental sensitivity per unit measurement volume for various types of scalar alkali-metal magnetometers range from several fT cm 3/2 /Hz 1/2 [13, 14] to about 1 fT cm 3/2 /Hz 1/2 [12]. Here we describe a new type of scalar atomic magnetometer using multi-pass vapor cells [15,16] and operating in a pulsed pump-probe mode [17] to achieve magnetic field sensitivity of 0.54 fT/Hz 1/2 with a measurement volume of 0.66 cm 3 in each multi-pass cell. The magnetometer sensitivity approaches, for the first time, the fundamental limit set by Rb-Rb collisions. We also develop here a quantitative method to analyze significant effects of atomic diffusion on the spectrum of the spin-projection noise in vapor cells with buffer gas using a spin time-correlation function.The sensitivity of an atomic magnetometer, as any other frequency measurement, is fundamentally limited by spin projection noise and spin relaxation [18]. For N spin-1/2 atoms with coherence time T 2 the sensitivity after a long measurement time t ≫ T 2 is given by δB = 2e/N T 2 t/γ, where γ is the gyromagnetic ratio. Spin squeezing techniques can reduce this uncertainty by a factor of √ e, but do not change the scaling with N [18-20]. The number of atoms can be increased until collisions between them start to limit T 2 . Writing T −1 2 = nσv, where n is the density of atoms, σ is the spin relaxation c...

Paramagnetic Faraday rotation is a powerful technique for atom sensing widely used in quantum nondemolition measurements, fundamental symmetry tests, and other precision measurements. We demonstrate the use of a multipass optical cell for Faraday rotation spectroscopy and observe polarization rotation in excess of 100 rad from spin-polarized Rb vapor. Unlike optical cavities, multipass cells have a deterministic number of light passes and can be used to measure large optical rotations. We also observe a tenfold suppression of transverse spin relaxation when Rb atoms are placed in a coherent superposition state immune to spin-exchange collisions.

An array of four 87 Rb vector magnetometers are used to detect nuclear quadrupole resonance (NQR) signals in an unshielded environment at 1 MHz. With a baseline of 25 cm, the length of the array, radio-frequency interference mitigation (RFIM) is also demonstrated; a radio-station signal is suppressed by a factor of 20 without degradation to the signal of interest. With these compact sensors, in which the probe beam passes through twice, the fundamental limit to detection sensitivity is found to be photon shot noise. More passes of the probe beam overcome this limitation. With a sensor of similar effective volume, 0.25 cm 3 , but 25 times more passes, the sensitivity is improved by an order of magnitude to 1.7 ± 0.2 fT/ √ Hz.

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We study nuclear spin frequency shifts in a 3 He-129 Xe comagnetometer caused by spin polarization of 3 He. We use stemless cylindrical cells to systematically vary the cell geometry and separately measure the cell shape-dependent and shape-independent frequency shifts. We find that a certain aspect ratio for a cylindrical cell cancels the dipolar effects of 3 He magnetization in the regime of fast spin diffusion. Using this control we observe a scalar 3 He-129 Xe collisional frequency shift characterized by an enhancement factor κHeXe = −0.011 ± 0.001 in excellent agreement with theoretical calculation. PACS numbers: 32.30.Dx, 06.30.Gv,39.90.+d Nuclear spin comagnetometers [1,2] are used in a number of precision fundamental physics experiments [3], such as searches for new long-range spin-dependent forces [4][5][6][7] and tests of CP, CPT and Lorentz symmetries [8][9][10]. They are also used for inertial rotation sensing [11][12][13][14][15], and magnetometry [16]. Measurements of nuclear spin precession frequencies allow nHz level frequency resolution because of long nuclear spin coherence times, as well as good accuracy and long-term stability [17,18].However, one unavoidable source of frequency shifts in nuclear magnetic resonance experiments is due to magnetic dipolar interactions between the spins. Local dipolar interactions are averaged to zero in a gas or liquid due to fast isotropic tumbling, but distant dipolar interactions can lead to complex spin dynamics [19][20][21][22][23]. Unlike most previous studies, we measure the dipolar frequency shifts in the regime where the time scale of atomic diffusion across the whole sample is much faster than both the time scale of long-range dipolar interactions and of the transverse spin relaxation. In this regime, used in most precision co-magnetometer experiments [18], the frequency shifts depend only on the shape of the cell containing the atoms. A similar fast-diffusion regime was previously studied by NMR in nanopores [24].Nuclear spin dipolar interactions cause systematic frequency shifts in comagnetometer precision measurements [25,26] and are subject of some controversy [27,28]. Here we use an anodic bonding batch fabrication process [29] to make a series of stemless cylindrical cells that contain 3 He and 129 Xe, as well as 87 Rb and N 2 . Well-defined cell shapes allow precision comparison with a simple theory for dipolar frequency shifts that we develop based on magnetometric demagnetizing factors [30]. We find that for a certain aspect ratio of the cylindrical cell the dipolar frequency shifts can be eliminated. An optimal and well-defined cylindrical geometry can improve the stability of nuclear-spin comagnetometers used for fundamen-tal physics experiments [4-7, 9, 10, 25]. It also can be used in NMR metrology applications instead of spherical cells that are hard to fabricate without stems [31].Excellent control of long-range dipolar fields also allows us to resolve a small scalar frequency shift between 3 He and 129 Xe nuclear spins mediated by sec...