The Spin Exchange Relaxation Free (SERF) Magnetometer
Ultra high sensitivity to and spatial resolution of magnetic fields.
![Schematic of the magnetometer. [Click to view vector format.]](./figures/MP2_schematic_array.png)
A sensitive magnetometer lies at the heart of the CPT experiment and our efforts in biomagnetic imagaging. The magetometer consists of a cell containing potassium vapor and a buffer gas. The unpaired electrons on the potassium atoms are spin-polarized by a pump laser. A perpendicular probe laser detects the precession of the electron spin in the presence of a magnetic field. By running the magetometer at low fields, it is potentially capable of sensitivities on the order of 10-18 Tesla, 1000 times more sensitive than a SQUID detector.
Potassium vapor is generated by heating a droplet of potassium inside a T-shaped glass cell. A high power laser is circularly polarized and is absorbed by the potassium electrons, putting them into a spin-polarized state with the electron spins pointing along the direction of circular polarization. A single frequency diode laser is used to detect the orientation of the electron spins as they precess in a magnetic field. This laser is detuned from the potassium resonance and as it passes through the polarized vapor, the laser polarization angle is rotated due to the circular dichroism of the vapor. The degree of rotation is proportional to degree to which spins are pointing along the probe beam. Two-point measurements or imaging of the magnetic field is done by focusing this probe beam onto an array of photodiodes. In this schematic, the probe laser is imaged onto a linear array of photodiodes.
Traditional atomic magnetometers are fundamentally limited by spin-exchange relaxation. When two polarized atoms collide, the electrons can transition into the other hyperfine state and precess in the opposite direction from the bulk of the ensemble, thereby causing decoherence and loss of signal. Spin-exchange relaxation is suppressed if the spin-exchange collisions happen fast enough in a sufficiently low magnetic field. In such a regime, the spins do not have enough time to precess and decohere between collisions. To achieve the required density, we heat the droplet of potassium in our cell to 180 °C. To reduce the precession frequency, the measurement cell containing the potassium is shielded from external magnetic fields by a factor of 106 using μ-metal magnetic shields. The name we give this new method, the Spin-Exchange Relaxation-Free (SERF) magnetometer, makes reference to the key feature of operating at high alkali densities. (And hapless squids sometimes wash up in the surf.)
![Sensitivity of the magnetometer for single point (dashed line) and differential (solid line) measurements. [Click to view vector format.]](./figures/MP2_sensitivity_simple.png)
This graph shows the measured sensitivity of the magnetometer as a function of frequency. The dashed line represents the noise from a single point measurement. The sensitivity is limited to 7 fT/Hz1/2 by magnetic noise produced by thermal currents flowing in the magnetic shields. A differential measurement, shown by the solid line, can subtract some of this magnetic noise to achieve a sensitivity of 0.5 fT/Hz1/2. To the best of our knowledge, this is the most sensitive measurement of a magnetic field.
We currently use faraday rotation techniques to precisely measure the polarization angle of our probe laser beam. As a reference for fellow researchers, a few calculations for designing a thick solenoid are freely available. We use this solenoid with a Tb-doped faraday rod to wiggle the polarization angle at several kHz.
The Unshielded Magnetometer
The unshielded SERF magnetometer uses Helmholtz coils, rather than magnetic shielding, to cancel out the magnetic field experienced by the potassium atoms. This leads to a loss of sensitivity over a shielded magnetometer, since the atoms become subject to ambient magnetic noise and gradients. On the other hand, the unshielded magnetometer gains portability, as well as the ability to measure the Earth's magnetic field and detect magnetic anomalies. Feedback from the magnetometer actively adjusts the current in the coils to cancel the ambient field, allowing the atoms to remain in the regime of spin-exchange suppression.
In the presence of small magnetic fields, to first order the magnetometer signal is linear in the field component that is orthogonal to both the pump and probe beams. By applying small field modulations along the other two directions and monitoring the signal with lock-in amplifiers, the magnetometer also becomes sensitive to the magnetic field components along those directions as well. This technique allows for operation as a three-axis vector magnetometer.
This graph shows the sensitivity of the unshielded SERF magnetometer as a function of frequency. Whereas a single point measurement has noise of about 10 pT/Hz1/2, a differential measurement has sensitivity on the order of 1 pT/Hz1/2, limited by ambient magnetic field gradients and 60-Hz noise. We are currently making improvements that should reduce these effects and enhance the magnetometer's sensitivity.
Relevant papers
- I. M. Savukov, S. J. Seltzer, M. V. Romalis and K. L. Sauer. "Tunable Atomic Magnetometer for Detection of Radio-Frequency Magnetic Fields." Phys. Rev. Lett. 95, 063004 (2005).
- I. M. Savukov, M. V. Romalis. "Effects of spin-exchange collisions in a high-density alkali-metal vapor in low magnetic fields." Phys. Rev. A 71, 023405 (2005).
- I. M. Savukov and M. V. Romalis. "NMR Detection with an Atomic Magnetometer." Phys. Rev. Lett. 94, 123001 (2005).
- S. J. Seltzer and M. V. Romalis, "Unshielded three-axis vector operation of a spin-exchange-relaxation-free atomic magnetometer." Appl. Phys. Lett. 85(20), 4804 (2004). [Copyright AIP]
- I. K. Kominis, T. W. Kornack, J. C. Allred & M. V. Romalis, "A subfemtotesla multichannel atomic magnetometer." Nature 422, 596 (2003).
- J. Allred, R. Lyman, T. Kornack, M. Romalis, "A high-sensitivity atomic magnetometer unaffected by spin-exchange relaxation." Phys. Rev. Lett. 89, 130801 (2002).
Pictures
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A mock-up of the magnetometer showing the active cell, the pump and probe beams, the beamsplitting analyzer and the segmented photodetector.
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Cell containing a droplet of K and some N2 buffer gas. The flat windows are good for imaging applications.
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The double-walled oven.
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Unshielded vector magnetometer, showing thick aluminum shields that attenuate high-frequency magnetic noise, particulary at 60 Hz. The dc-component of the ambient magnetic field remains unaffected and so can be measured. The smaller Helmholtz coils inside the shields are used to apply the field modulations necessary for vector operation.
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![Schematic of the magnetometer. [Click to view vector format.]](./figures/MP2_schematic_array.svg)
A sensitive magnetometer lies at the heart of the CPT experiment and our efforts in biomagnetic imagaging. The magetometer consists of a cell containing potassium vapor and a buffer gas. The unpaired electrons on the potassium atoms are spin-polarized by a pump laser. A perpendicular probe laser detects the precession of the electron spin in the presence of a magnetic field. By running the magetometer at low fields, it is potentially capable of sensitivities on the order of 10-18 Tesla, 1000 times more sensitive than a SQUID detector.![Sensitivity of the magnetometer for single point (dashed line) and differential (solid line) measurements. [Click to view vector format.]](./figures/MP2_sensitivity_simple.svg)
