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Quantum Sensing

Using the quantized states of matter it is possible to build exquisite metrology devices, such as frequency standards and inertial sensors, with new characteristics and improved performance levels.  By monitoring the output of inertial sensors one can in principle calculate the current position of a moving system without input from external measurements.  The accuracy of this calculation of course depends on the noise in the inertial measurement.  Using conventional technology, the performance of inertial navigation systems has in practice reached a plateau.  Matterwave interferometry is widely hypothesized to hold the next advance for inertial measurements due to the potential for outstanding performance in a small package.  Demonstrations measure both rotations and accelerations with outstanding fidelity, stability, and intrinsic accuracy.  This also has applications in seismic sensing where a six-axis matterwave sensor can potentially discriminate between and characterize natural and manmade sources (such as underground facilities and explosions) in new ways as well as independently infer wave speed and direction to reduce event location uncertainty.  We are exploring new techniques to advance the performance of light-pulse atom interferometers, and determine their utility in seismology and inertial guidance applications.  In what follows, we develop a simplified model to explain the measurement dynamics in a light-pulse atom interferometer.  This model neglects physical contributions due to path integrals and wavepacket overlap1 which are small in ordinary measurements but can become important in situations with significant gradients in the gravity field.

In light-pulse matterwave interferometers, the measurement can be approximated as laser ranging of atoms in free fall (see Figure 1).

Use Atom to Stroboscopically measure postion (optical phase) at three equal-spaced points in time

This gives the measurement high sensitivity, intrinsic accuracy, and long-term stability because of the wavelength stability of the laser (exceeding 1 ppb).  The measurement mechanics involve imprinting the optical phase of the laser onto the quantum mechanical phase of the atom’s internal electronic state2.  This results from a resonant interaction of the electronic state with the optical field through stimulated Raman transitions.  This causes the atom’s internal state to oscillate sinusoidally between the hyperfine ground states (see Figure 2). 

Figure 2. Matterwave optics
Figure 2. Matterwave optics

Simultaneously, conservation of momentum demands that the field impart a two-photon recoil momentum kick to the population transferred to the excited state.  Since the atom is very light (10-25 kg) the change in velocity is substantial (~cm/s) and over the duration of the measurement causes the two states to separate far outside their coherence length.  By varying the pulse duration of this coherent process, one can use the velocity recoil to create a matterwave beam splitter (π/2-pulse) and a mirror (π-pulse) from which to construct a π/2-π-π/2 Mach-Zehnder atom interferometer (see Figure 3). 

Figure 3. Matterwave phase shift

Figure 3. Matterwave phase shift

Following the interferometer, the probability for an atom to be in atomic state.......

1K. Bongs, R. Launay, and M. A. Kasevich. High-order inertial phase shifts for time-domain atom interferometers. Applied Physics B-Lasers and Optics, 84, 599, 2006.

2Berman, P. R., [ed.]. Atom Interferometry. San Diego, Academic Press, 1997.


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