The Dual Noble Gas Maser

We developed a dual (or two-species) noble gas maser [1-3], the first device to sustain simultaneous active maser oscillations on distinct transitions in two co-located atomic species: the noble gases ³He and ¹²⁹Xe. The dual noble gas maser (DNGM) allows sensitive differential measurements of the ³He and ¹²⁹Xe nuclear spin 1/2 Zeeman transition frequencies, and thus may be useful for symmetry tests and precision measurements such as a search for a permanent electric dipole moment (EDM) of the ¹²⁹Xe atom. The advantage of differential measurements is that they are insensitive to common-mode systematic effects such as uniform magnetic field variations [4]. In the DNGM, one noble gas species serves as a precision magnetometer to stabilize the system's static magnetic field, while the other species is used as a sensitive probe for new physics such as an EDM. The DNGM has an additional important feature: active maser oscillation permits long coherent measurements of the noble gas Zeeman frequencies (on timescales of a few hours). A coherent frequency measurement can achieve greater precision than the incoherent average of a set of shorter measurements made during an equivalent period of time [1].

The DNGM contains dense, co-located ensembles of ³He and ¹²⁹Xe atoms performing active maser oscillations on their nuclear spin 1/2 Zeeman transitions at ~ 4.9 kHzきろへるつ for ³He and ~ 1.8 kHzきろへるつ for ¹²⁹Xe in a static magnetic field of ~ 1.5 gauss. The maser population inversions for the ³He and ¹²⁹Xe ensembles are created by spin exchange collisions between the noble gas atoms and optically-pumped Rb vapor [4,5]. The glass DNGM cell has two chambers, one acting as the spin exchange "pump bulb" and the other serving as the "maser bulb". (A schematic diagram of the DNGM is given in Fig. 1.) This two chamber configuration permits the combination of physical conditions necessary for a high flux of spin-polarized noble gas atoms into the maser bulb, while also maintaining ³He and ¹²⁹Xe maser oscillations with good frequency stability.

Fig. 1. Schematic diagram of the dual noble gas maser.

Figure 2 shows measurements of the frequency stability currently provided by the DNGM when using a two-chamber maser cell without electric field plates installed. These measurements demonstrate a sensitivity to changes in the ¹²⁹Xe Zeeman frequency of approximately 40 nHz in 6,000 s of data acquisition (i.e., the free-running ³He maser stability shown in Fig. 2 divided by ~ 2.7, the ratio of ³He and ¹²⁹Xe magnetic moments). Fig. 2 also shows that the DNGM's frequency stability has been significantly improved over the last two years through system re-design and engineering: e.g., better temperature control, mechanical stability, and electronic shielding; also active regulation of the Rb magnetization in the pump bulb. Shown on the right ordinate axis of Fig. 2 is the one standard deviation statistical sensitivity to a ¹²⁹Xe EDM that would result from: (i) the free-running ³He maser frequency stability given on Fig. 2's left ordinate axis; and (ii) the application of ± 5 kV/cm electric fields across the maser bulb, alternating the field direction every t seconds.

Fig. 2. Comparison of frequency stabilities (Allan deviations) for the current [3] and 1996 versions [2] of the DNGM. The free-running ³He Allan deviation of the upgraded current system decreases to ~ 100 nHz for measurement intervals of 6,000 s. The right ordinate axis shows the EDM sensitivity that would result from this frequency stability in the presence of ± 5 kV/cm electric fields.

Recently, we began using an EDM cell: i.e., a two-chamber maser cell with electric field plates installed (see Fig. 3). These measurements indicate a modest degradation of short-term maser frequency stability with application of ± 3.5 kV/cm electric fields, due to excessive audio noise near the maser frequency (~ 5 kHzきろへるつ) generated by a sub-standard high voltage source. This problem should be corrected with replacement of the high-voltage source.

Fig. 3. Recent measurements of free-running ³He maser frequency stability using a prototype EDM cell. Data is shown with and without application of a 3.5 kV/cm electric field across the maser bulb. The "field-on"; maser frequency stability is degraded by audio noise generated by a sub-standard high voltage source.

Note: Our DNGM investigations have been performed in collaboration with Prof. Timothy Chupp and his group at the University of Michigan.

References:

[1] T.E. Chupp, R.J. Hoare, R.L. Walsworth, and Bo Wu, Phys. Rev. Lett. 72, 2363 (1994).
[2] R.E. Stoner, M.A. Rosenberry, J.T. Wright, T.E. Chupp, E.R. Oteiza, and R.L. Walsworth, Phys. Rev. Lett. 77, 3971 (1996).
[3] D. Bear, T.E. Chupp, K. Cooper, S. DeDeo, M.A. Rosenberry, R.E. Stoner, and R.L. Walsworth, Phys. Rev. A 57, 5006 (1998).
[4] T.E. Chupp, E.R. Oteiza, J.M. Richardson, and T.R. White, Phys. Rev. A 38, 3998 (1988).
[5] G.D. Cates, R.J. Fitzgerald, A.S. Barton, P. Bogorad, M. Gatzke, N.R. Newbury, and B. Saam, Phys. Rev. A 45, 4631 (1992).