Magnetometers based on atomic spin have achieved great progress in recent years and magnetometers with different characteristics find important applications in various fields1,2,3. High sensitivity below has been demonstrated by Kominis et al.4 with a potassium SERF (spin exchange relaxation-free) magnetometer and by Budker et al.5 with NMOR (nonlinear magneto-optical rotation) magnetometers, making atomic magnetometers comparable to or even surpassing the state-of-the-art SQUIDs (superconducting quantum interference devices)6,7. In the field of ultrasensitive magnetic field detection, measurement of an external magnetic field can be realized by monitoring the Larmor precession of atomic spins8. However, the atomic spin will inevitably suffer from decoherence and decay to thermal equilibrium after a lifetime τ atom , which limits the precision of the measurement. Hence, extending the coherence time is the key in improving the performance of all coherent quantum systems9,10. Here, we investigate and report a novel, self-sustaining atomic spin magnetometer based on coherent optical pumping. By synchronously switching on the pumping light at the Larmor precession frequency triggered by non-destructive measurement of its phase, the atomic spins can be regenerated coherently against the relaxation and thus maintain its precession indefinitely. The phase coherence time of the Larmor precession can thus be extended to a scale much longer than τ atom . Consequently, the self-sustaining magnetometer utilizes the phase information, instead of merely the frequency of the Larmor precession to measure the magnetic field and shows the attractive feature that its measurement uncertainty averages down in time at a faster 1/τ rate.

The self-sustaining atomic magnetometer has several other advantages. It can self-oscillate from noise without requiring any initial preparation. Via the gyromagnetic ratio, it converts the measurement of magnetic field to that of frequency and time – which has the highest precision among all physical standards. Thus, this magnetometer is an absolute measurement device and can form a metrological standard. We note that spin precession driven by periodically modulated optical pumping was first studied by Bell and Bloom11 where persistent spin polarization was obtained. In the present work, we measure the precession spin signal’s phase via non-destructive measurement instead of by tracing the atoms’ absorption12,13,14, minimizing disruption to the precession itself. Finally, the continuous oscillating signal makes it much more convenient to apply further signal processing in many applications. For example, phase-locking of the precession signal to a reference local oscillator can produce a highly sensitive error signal to lock the magnetic field. The phase-locking stabilization of magnetic field can be very helpful when an ultra-stable magnetic field is needed8.

The magnetometer monitors the Larmor precession of the electron spin of an atom in the magnetic field. For a standard free precession process, the spin is first polarized along the propagation direction of a circularly polarized pump light, then the pump light is turned off and the spin starts precession around the external field . With the presence of decoherence, for example, caused by collision or by the atoms moving out of the detection region, the average value of the spin will relax to zero after a certain lifetime τ atom . For a time interval τ > τ atom the measurement can be repeated for a number of τ/τ atom times, resulting in a sensitivity of1

where SNR is the signal-to-noise ratio and γ is the gyromagnetic ratio. If the system is limited by the spin projection noise only, then SNR equals for an ensemble of N atoms. For a measurement time τ shorter than τ atom , let τ atom equal to τ. Then, the sensitivity will improve as 1/τ. For a measurement time τ much longer than τ atom , the sensitivity improves as due to the uncorrelated phase in each repeated measurement of time interval τ atom . The polarization preparation by pumping in every measurement cycle destroys any phase coherence beyond time τ atom , thus resulting in the rule.

The spin self-sustaining method replenishes the atomic spin coherently. We illustrate the method using 85Rb atoms in the experiment as shown in Fig. 1(a). Setting the -axis along the magnetic field, at t = 0 a circularly polarized light along the -axis pumps all the atoms into the magnetic sublevel state |m Fy = 3>. This pump light orients all atomic spins along the -direction. Then the pump is turned off and the atomic spins precess and relax in dark. At time t = 2π/ω L the population will evolve back to the initial |m Fy = 3> state. If the pumping light pulse is switched on right at this moment again, all atoms in |m Fy = 3> will remain unaffected while atoms in all other states due to relaxation will be pumped back to |m Fy = 3>. In this way, the spin is regenerated by the coherent pumping field and can maintain a very long lifetime.

Figure 1 Schematic diagram of the experimental apparatus. (a) The circularly polarized laser and the linearly polarized laser serving as the pumping and repumping light, respectively, go through the same AOM (acousto-optic modulator) so that they can be switched on and off simultaneously. The probe laser propagates through the Glan-Taylor polarizer, the cell and the Wollaston analyzer in sequence along to form the Faraday rotation measurement. Its beam size is about 2 mm and its frequency is red-detuned by Δ∼4 GHz from F = 3 → F′ of the D2 line. The three lasers come from three independent, tunable external cavity diode lasers and they are not phase locked to each other. The 20 mm long, 20 mm in diameter cylindrical cell used here is a self-made α-olefin coated cell containing natural abundance rubidium atoms. The cell is placed inside a five-layer μ-metal magnetic shield casing with a shielding factor of better than 105. A pair of Helmholtz coils controlled by a stable current source generates the uniform magnetic field B along for spin precession. The experiment is performed at room temperature. The signal goes through a band-pass filter and then to the high speed zero-crossing comparator based on chip MAX999 (Maxim Integrated Products). The PCB of the comparator is carefully designed to reduce the high frequency electrical noise. For simplicity the magnetic shielding, AOM and coils are not shown. (b) Energy levels of 85Rb atom. (c) Spin precession signals observed in self-sustaining setup and (d) “single-pump free precession” setup. The black line is the precession signal, the blue line is the control signal and the red line is a sine-decay fitting giving a decay time constant of approximately 30 ms. The optical pumping is on when the control signal is high. The duration of the pumping pulse for spin self-sustaining method is 10 μs and the timing uncertainty of the rising edge in the trigger is measured to be 1 μs. Full size image