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.] A sensitive magnetometer lies at the heart of the CPT experiment and our efforts in biomagnetic imaging. The magnetometer 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 magnetometer 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.]

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. With further optimization in the design and operation, we later improved this sensitivity by a factor of 3 in the rock magnetometer (see the section below).

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.

Unshielded magnetometer noise. 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.

 

The Rock Magnetometer Operated in the attoTesla Regime at Low Frequencies

As mentioned, the fundamental sensitivity of an atomic magnetometer due to atom shot noise is on the order of attoTesla. However, in practice atom shot noise is almost always dominated by technical noise (electronics noise, photon shot noise, laser noise etc.) or magnetic noise (from external environment or from parts of the system itself). The rock magnetometer is the most recently-built magnetometer in our lab. The motivations for this work is to test how close we can get to the fundamental limit by optimizing the signal and reducing magnetic/technical noise, and to implement magnetization measurements of actual objects, which in this case are geological samples.

A key improvement in the design of the rock magnetometer is the use of ferrite material for the innermost magnetic shielding layer. Having a high permeability, ferrite is comparable to mu-metal in terms of magnetic shielding performance (the shielding factor for the 10cm x 10cm cylindrical ferrite shield of thickness 1cm that we use is about 200). However, being a ceramic, ferrite has virtually no noise due to eddy current. The magnetization noise due to hysteresis loss is minimized by choosing the ferrite with the lowest measured loss factor. Our noise calculation predicts the magnetic noise of the ferrite shield to be below 1fT/Hz1/2 at above 10Hz, which can be further suppressed by doing a gradient measurement.


Another new feature in this system is the ability to load a sample very close to the cell for practical measurements. Samples of size as big as 1cm in diameter can be loaded through the quartz sample tube, which has vacuum-tight seals with the vacuum chamber, and which also serves as an oven that can be heated up to over 600°C, used for thermal demagnetization measurements. The sample can be rotated to modulate the signal at a desired frequency where the noise is low.

The ultimate sensitivity that we achieved with this magnetometer in gradiometer mode was 160 aT/Hz1/2 at tunable frequencies in the range of 30-40Hz. The composite noise spectrum shown in the figure consists of several noise spectra when the magnetometer was tuned to different resonant frequencies (note that SERF magnetometers have a very narrow bandwidth of only a few Hz). We demonstrated thermal demagnetization measurements of very weak rock samples. We were also able to measure the magnetization of 1mm-size single-crystal samples that have not been  measured before with commercial SQUID's, with signal to noise of one order of magnitude.

 

 

Relevant papers

Pictures

Magnetometer mock-up with lasers.
A mock-up of the magnetometer showing the active cell, the pump and probe beams, the beamsplitting analyzer and the segmented photodetector.
Cell containing K and N2.
Cell containing a droplet of K and some N2 buffer gas. The flat windows are good for imaging applications.
A double-walled oven.
The double-walled oven.
A double-walled oven.
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.