CPT Violation Experiment (CPT-I and CPT-II)

CPT-I 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 presumably 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.

This effective magnetic field is expected to point a particular direction in space-time that is constant on the scale of our solar system.  Our earthbound experiment searches for a sidereal variation distinct from any diurnal variation as the Earth rotates on its axis.

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 linearly polarized probe beam orthogonal to the pump beam detects this precession through optical rotation. We have demonstrated the operation of this magnetometer at sensitivities surpassing even low temperature SQUIDs by building a SERF magnetometer. The ³He buffer gas 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:

The field cancellation effect endures slow fluctuations of the external magnetic field. The ³He spins adiabatically track a slowly fluctuating 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.

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.

The co-magnetometer will not, however, shield the effect of a CPT-violating field. CPT violating fields presumably do not interact in the same way with different spin species. 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.

Experiments

At present, we are running our second generation apparatus, CPT-II.  The first generation experiment (CPT-I) provided 110 days of good data over a 15 month period.  This experiment produced limits on CPT and Lorentz Violation comparable to existing limits, but did not beat them.  The entire experiment was limited by systematic noise [for details, see T. W. Kornack. "A test of CPT and Lorentz Symmetry Using a K-³He Co-magnetometer." Dissertation, Princeton University (2005).].

CPT-II CPT-I

CPT-II integrates several new features, many to combat long term drift. 

• Short optical path to reduce mechanical drift.
• Bell jar evacuated to ~1 Torr encloses experiment for thermal stability and reduction in noise due to air convection.
• Three layer µ-metal magnetic shields and inner ferrite (non-conductive) magnetic shields reduce magnetic field by 1.2 x 10⁸.
• Rotary platform allows lockin modulation of the proposed CPT and Lorentz violating field by revering the experiment every 20-50 seconds.
• 4.4 kW servo motor enables 180° rotation of entire experiment in 3 s!
• Large Helmholtz coils to reduce Earth's magnetic field around the experiment.
• Rotary vacuum seal and AC slip ring provide vacuum and power to the rotary table.
• Twisted pair of resistive wire heats cell via an AC current.

The co-magnetometer is also a sensitive gyroscope; a rotation appears as an anomalous field.  The effect of Earth's rotation is huge and produces a large systematic which must be accounted for in reversals of the experiment as the sensitive y-direction changes with respect to Earth's rotation axis.  We search for a 0.1-0.01 fT sidereal variation on top of the  ~300 fT gyroscope signal.

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, S. J. Smullin, S.-K. Lee, and M. V. Romalis. "A low-noise ferrite magnetic shield." Appl. Phys. Lett. 90, 223501 (2007).
• 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).

Generation I Pictures (CPT-I)

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.

Generation II Pictures (CPT-II)

Inner ferrite magnetic shield.	Compact AC electrical oven.
Experiment schematic