Atoms form the basis of precise measurement for many quantities (time, acceleration, rotation, magnetic field, etc.). Measurements of microwave frequency electric fields by traditional methods (i.e. engineered antennas) have limited sensitivity and can be difficult to calibrate properly. Highly-excited (Rydberg) neutral atoms have very large electric-dipole moments and many dipole allowed transitions in the range of 1 - 500 GHz. It is possible to sensitively probe the electric field in this range using the combination of two quantum interference phenomena: electromagnetically induced transparency and the Autler-Townes effect. This technique allows for very sensitive field amplitude, polarization, and sub-wavelength imaging measurements. These quantities can be extracted by measuring properties of a probe laser beam as it passes through a warm rubidium vapor cell. Thus far, Rydberg microwave electrometry has relied upon the absorption of the probe laser. We report on our use of polarization rotation, which corresponds to the real part of the susceptibility, for measuring the properties of microwave frequency electric fields. Our simulations show that when a magnetic field is present and directed along the optical propagation direction a polarization rotation signal exists and can be used for microwave electrometry. One central advantage in using the polarization rotation signal rather than the absorption signal is that common mode laser noise is naturally eliminated leading to a potentially dramatic increase in signal-to-noise ratio.
Quantum networks provide conduits capable of transmitting quantum information that connect to nodes at remote
locations where the quantum information can be stored or processed. Fiber-based transmission of quantum information
over long distances may be achieved using quantum memory elements and quantum repeater protocols. However, atombased
quantum memories typically involve interactions with light fields outside the telecom window needed to minimize
absorption in transmission by optical fibers. We report on progress towards a quantum memory based on the generation
of 795 nm spontaneously emitted single photons by a write-laser beam interacting with a cold <sup>87</sup>Rb ensemble. The single photons are then frequency-converted into (out of) the telecomm band via difference (sum) frequency generation in a
PPLN crystal. Finally, the atomic state is read out via the interaction of a read-pulse with the quantum memory. With
such a system, it will be possible to realize a long-lived quantum memory that will allow transmission of quantum
information over many kilometers with high fidelity, essential for a scalable, long-distance quantum network.