CPT Violation Experiment

CPT is a combined transformation of charge conjugation, space reversal, and time reversal. Conventional field theories, such as the Standard Model, are always symmetric under CPT transformation. However, string theories and other theories of quantum gravity can violate CPT symmetry. So, a violation of CPT symmetry would give a background-free signal of the effects of quantum gravity.

This experiment is motivated by a recently developed theoretical formalism that describes CPT violation. The presence of CPT violation results in the appearance of an effective magnetic field in interactions with spins. Compared to normal magnetic fields, this CPT violating field presemably interacts with different atomic spins differently. A sensitive co-magnetometer with two atomic species (with different spins) can differentiate CPT violating fields from magnetic fields.

In this experiment, we will use K and ³He atoms in a co-magnetometer. The K atoms are used to measure magnetic fields. They provide unpaired electrons, which are spin-polarized by pumping with circularly polarized light. The spins precess in the presence of a magnetic field and a probe beam orthogonal to the pump beam detects this precession. We have demonstrated the operation of this magnetometer at sensitivities surpassing even low temperature SQUIDs by building a SERF magnetometer. The ³He atoms compensate for magnetic fields. The ³He nuclear spins are polarized by spin-exchange collisions with the K electrons and are entrained on a magnetic field B_z that is coaxial with the spin polarization. The ³He magnetic moment is opposite to the spin and cancels the applied B_z field locally. The system can be calibrated so that the Helium magnetization cancels all external fields and the local magnetic field experienced by the K is close to zero. This configuration is illustrated in (a) below:

Operation of the compensated magnetometer. [Click for vector format.]

The field cancellation effect endures slow fluctuations of the external magnetic field. The ³He spins adiabatically track a slowly fluctating magnetic field and the ³He magnetization, which is always antiparallel to its spin, maintains the magnetic field cancellation. The ³He component of the co-magnetometer will shield the K from magnetic noise.

Frequency response of the coupled K-3He system showing compensation at low frequencies. [Click for vector format.]

In this graph, the K response is plotted as a function of excitation frequency. For slow field fluctuations, the ³He compensates for the applied field and the K does not respond.

CPT violating field skewering Earth The co-magetometer will not, however, shield the effect of a CPT-violating field. CPT violating fields presumably interact with different spin species differently. Thus, the K component of the magnetometer will be sensitive to a CPT violating field despite the reaction of ³He. We will search for a diurnal signal as the earth rotates through such a CPT violating vector field. Please refer to the following slides and papers for more information.

Relevant Presentations

T. Kornack, M. Romalis "Operation of the K-³He Co-magnetometer." Presented at the Third Meeting on CPT and Lorentz Symmetry 2004.
T. Kornack, T. Jackson, M. Romalis "Progress towards testing CPT and Lorentz symmetry using a self-compensating K-³He co-magnetometer." Presented at DAMOP 2004.
T. Kornack, I. Savukov, M. Romalis "Testing violation of CPT and Lorentz symmetries with a self-compensated ³He-K magnetometer." Presented at DAMOP 2003.
T. W. Kornack, "A test of CPT symmetry using a high sensitivity potassium magnetometer." Thesis proposal presentation. [writeup]
T. Kornack, "Self-compensated ³He-K magnetometer for CPT tests." Presented at DAMOP 2002.
M. Romalis, "Atomic magnetometers: new twists to the old story." Presented at the Pennsylvania State University, State College, PA, March 2001.
M. Romalis, J. Allred, R. Lyman, "K-³He self-compensating co-magnetometer for tests of CPT symmetry." In proceedings of the Conference on CPT Violation, Bloomington, Indiana, August 2001.

Relevant Papers

T. W. Kornack, R. K. Ghosh and M. V. Romalis. "Nuclear Spin Gyroscope Based on an Atomic Co-magnetometer." Phys. Rev. Lett. 95, 230801 (2005).
T. W. Kornack. "A test of CPT and Lorentz Symmetry Using a K-³He Co-magnetometer." Dissertation, Princeton University (2005).
I. K. Kominis, T. W. Kornack, J. C. Allred & M. V. Romalis, "A subfemtotesla multichannel atomic magnetometer." Nature 422, 596 (2003).
T. W. Kornack, M. V. Romalis, "Dynamics of Two Overlapping Spin Ensembles Interacting by Spin Exchange." Phys. Rev. Lett. 89, 253002 (2002).
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

The CPT experiment with the probe beam in the foreground, prominently showing the magnetic sheilds.	The CPT experiment with the probe beam in the foreground, prominently showing the magnetic sheilds.
Charles inspects the magnetic shields with new foam insulation.	Testing the assembled cell oven with its cooling jacket visible.
The cell oven. Hot air flows through the inner and outer walls to uniformly heat the cell.	The K-³He cell ready to be inserted into the oven. The oven heats this cell to 190 C.