Chopped nonlinear magneto-optic rotation: A technique for precision measurements

We have developed a technique for precise measurement of small magnetic fields using nonlinear magneto-optic rotation (NMOR). The technique relies on the resonant laser beam being chopped. During the on time, the atoms are optically pumped into an aligned ground state (Δm= 2 coherence). During the off time, they freely precess around the magnetic field at the Larmor frequency. If the on-off modulation frequency matches (twice) the Larmor precession frequency, the rotation is resonantly enhanced in every cycle, thereby making the process like a repeated Ramsey measurement of the Larmor frequency. We study chopped-NMOR in a paraffincoated Cs vapor cell. The out-of-phase demodulated rotation shows a Lorentzian peak of linewidth 85μG, corresponding to a sensitivity of 0.15 nG/ √ Hz. We discuss the potential of this technique for the measurement of an atomic electric-dipole moment. Copyright c © EPLA, 2011 The well-known phenomenon of the rotation of the plane of polarization of near-resonant light passing through an atomic vapor in the presence of a longitudinal magnetic field, called magneto-optic rotation (MOR) or the Faraday effect, is understood to arise from the birefringence induced by the magnetic field. This phenomenon of MOR shows nonlinear effects when the light is sufficiently strong, as can be produced by a laser. In the simplest manifestation of nonlinear MOR (NMOR), the strong light field aligns the atom by inducing (through optical pumping) Δm= 2 coherences among the magnetic sublevels. The aligned atom undergoes Larmor precession about the longitudinal field, and causes additional rotation due to the precessed birefringence axis. The effect is nonlinear because the degree of optical alignment is dependent on the light intensity. NMOR in atomic vapor allows optical detection of zero-field level crossings in the shot noise limit, and has important applications in sensitive magnetometry [1,2], search for a permanent electric dipole moment (EDM) [3], and magnetic resonance imaging (MRI) [4]. In this work, we demonstrate a technique for measuring NMOR which is ideally suited for precision measurements. (a)E-mail: [email protected] The basic idea is similar to a repeated Ramsey separatedoscillatory-fields [5] measurement of the Larmor precession frequency. This is achieved by chopping the laser beam on and off. During its on time, the atoms are optically pumped into an aligned Δm= 2 coherence of the ground state. During the off time, they (freely) precess around the magnetic field at the Larmor frequency. If the onoff modulation frequency matches the Larmor precession frequency, the atoms are realigned exactly when the light field comes back on, thus resonantly enhancing the optical pumping process. The process continues again for the next on-off cycle, so this is like a repeated Ramsey method. Demodulation of the rotation at the on-off modulation frequency shows a narrow peak at the value of the field where the modulation frequency is equal to 2× the Larmor frequency. The factor of 2 appears because the atomic alignment (Δm= 2 coherence) has two-fold symmetry. As expected, the resonance width is limited by the decoherence time of the atomic alignment. Since the mean free path in a vapor cell at room temperature is several orders of magnitude larger than the cell size, the coherence can be destroyed by spin-exchange collisions with the walls. It has been shown that the use of paraffin coating on the walls reduces the ground-state depolarization rate and enhances the optical pumping signals [6]. We therefore use