Highly sensitive magnetometers capable of measuring magnetic fields below 1 pT impact areas as diverse as geophysical surveying, the detection of unexploded ordinance, space science, nuclear magnetic resonance (NMR), health care and perimeter and remote monitoring. Recently, it was shown that laboratory optical magnetometers, based on the precession of the spins of alkali atoms in the vapor phase, could achieve sensitivities in the femtotesla range [1] comparable to, or even exceeding, those of superconducting quantum interference devices (SQUIDs). We demonstrate here an atomic magnetometer based on a millimeter-scale microfabricated alkali vapor cell, with sensitivity below 70 fT/Hz1/2. The key physics that underlies several recent advances in optical magnetometry is the suppression of spin relaxation originating from spin-exchange collisions between the alkali atoms and the generation of a large ground-state atomic polarization at low magnetic field strengths. Operation of the magnetometer in this spin-exchange relaxation free (SERF) regime allows for spin relaxation times over 10 ms even at alkali atom densities above 1014 cm-3.
The vapor cell used in this experiment, shown in Figure 1(a), had interior dimensions of 3 mm × 2 mm × 1 mm and was fabricated with a MEMS process described here. The zero-field magnetic resonance was measured via optical absorption of a single circularly-polarized light field propagating in a direction perpendicular to the static magnetic field, B0 (see Figure 1(b)). The magnetic resonance, shown in Figure 1(c) has a full width at half maximum of 83 nT (corresponding to 580 Hz) and a transmission contrast of 40 %. The linewidth obtained by extrapolating to zero light intensity was around 15 nT (105 Hz). This linewidth is lower by a factor of 50 than the estimated spin-exchange limited linewidth at this alkali atom density and corresponds closely to the linewidth limited by alkali-buffer gas spin destruction. This clearly indicates that the magnetometer is operating in the SERF regime.
In order to guide designs for future MEMS-based instruments, we consider now how the sensitivity of such a magnetometer, and the power required to run it, scale with the size of the cell. The scaling of the atom-shot-noise-limited magnetometer sensitivity is plotted in Figure 2(a) as a function of cell size under the condition that the spin relaxation rate is equal to the combined relaxation rate due to wall and buffer-gas collisions and that the relaxation rates due to wall and buffer-gas collisions are themselves equal. We may evaluate the alkali density at which the relaxation rate due to spin-exchange collisions is equal to the combined relaxation rate due to atom diffusion and buffer-gas collisions. The corresponding cell temperature can be determined from standard vapor pressure curves. This cell temperature is plotted as a function of cell size in Figure 2(b). In well engineered MEMS-based atomic instruments, the power required to run the device, Pdiss, is expected to be dominated by that required to heat the cell to its operating temperature. To estimate this power, the results of reference [2] are scaled as V2/3, with the device temperature determined by Figure 2(b). Pdiss is plotted as a function of cell size in Figure 2(c). The results of Figure 2(a) are combined with those in Figure 2(c) to generate a plot of the expected power requirement as a function of desired sensitivity in Figure 2(d). Under SERF conditions, a sensitivity of 10 fT/Hz can in principle be achieved with under 10 mW of heating power for both 39K and 87Rb magnetometers with a MEMS design similar to that of reference [2].
References:
[1] Kominis, I. K., Kornack, T. W., Allred, J. C. & Romalis, M. V. A subfemtotesla multichannel atomic magnetometer. Nature 422, 596-599 (2003).
[2] Lutwak, R. et al. The chip-scale atomic clock - low-power physics package. Proceedings of the 36th Annual Precise Time and Time Interval (PTTI) Meeting, Washington, DC, 339-354 (2004).
Figure 1 Atomic magnetometry with a micromachined alkali vapor cell. a, Schematic of the measurement apparatus. The circularly polarized light beam passes through a microfabricated alkali vapor cell, and the transmitted power is detected with a photodiode (PD) as a function of transverse magnetic field, B0. b, Orientation and dynamics of the atomic spins (blue arrows) as a function of B0. For B0 ≠ 0, the spins are unpolarized in the plane perpendicular to B0, and the optical absorption is high. For B0 ≈ 0, the spins are optically pumped into a state polarized along the direction of propagation of the optical field and the optical absorption is reduced. c, A resonance is observed as the local field, B0, is scanned about zero. The red line indicates a Lorentzian fit with a full width at half maximum of 83 nT. d, Magnetic field sensitivity for the single-beam geometry (red trace) and the two-beam geometry (grey trace) at B0 = 0. The solid line indicates a sensitivity of 65 fT/Hz1/2, and the dashed lines indicates the estimated sensitivity due to photon shot noise (A: In single-beam geometry, B: In two-beam geometry).
Figure 2 Magnetometer sensitivity scaling under near-optimal conditions for linewidth for 133Cs (solid lines), 87Rb (dashed lines) and 39K (dotted lines) under spin-exchange-limited conditions (green) and SERF conditions (magenta). a, Sensitivity as a function of cell size. The solid blue line indicates the sensitivity if the detection signal-to-noise is fixed at 5x106Hz, corresponding to photon shot noise on 10 μW of detection power and a signal contrast of unity. b, Corresponding cell temperature assuming alkali diffusion and relaxation rates for a buffer gas of Ne and nuclear slow-down factors of 1/6 for 39K and 87Rb and of 1/22 for 133Cs.c, Corresponding cell heating power based on design of ref 27. d, Cell heating power required as a function of desired sensitivity.
