We demonstrate a collectively-encoded qubit based on a single Rydberg excitation-a Rydberg polariton-stored in an ensemble of N entangled atoms. Qubit rotations are performed by applying microwave fields that drive excitations between Rydberg states. Coherent read-out is preformed by mapping the polariton into a single photon. Ramsey interferometry is used to probe the coherence of the qubit, and test the robustness to external perturbations. We show that the Ramsey fringe visibility is independent of atom loss, and that dephasing due to electric field noise scales as the forth power of field amplitude. These results show that robust quantum information processing can be achieved via collective encoding using Rydberg polaritons, and hence this system could provide attractive alternative coding strategies for quantum computation and networking.

The combination of electromagnetically-induced transparency (EIT) and Rydberg excitations in atomic media is a compelling and versatile platform for both applications in quantum information, and fundamental studies of nonlinear quantum optics and non-local quantum dynamics. In this paper, we study the dynamics of a Rydberg-EIT system in a medium that allows for more than one Rydberg excitation in the propagation direction. We study the cross-over between coherent collective emission ('flash') of two-level atoms to a Rydberg dressed regime. The complex dynamics are studied using both intensity and time correlation measurements. We show that while steady-state EIT gives a second order correlation g (2) = 0.79 ± 0.04, the Rydberg-dressed flash exhibits anti-bunching down to 0.2 ± 0.04.

We present a simple permanent magnet set-up that can be used to measure the Faraday effect in gases, liquids and solids. By fitting the transmission curve as a function of polarizer angle (Malus' law) we average over fluctuations in the laser intensity and can extract phase shifts as small as ± 50 µrads. We have focused on measuring the Faraday effect in olive oil and find a Verdet coefficient of V = 192 ± 1 deg T −1 m −1 at approximately 20 • C for a wavelength of 659.2 nm. We show that the Verdet coefficient can be fit with a Drude-like dispersion law A/(λ 2 − λ 2 0 ) with coefficients A = 7.9 ± 0.2 × 10 7 deg T −1 m −1 nm 2 and λ 0 = 142 ± 13 nm.

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Active frequency stabilization of a laser to an atomic or molecular resonance underpins many modern-day AMO physics experiments. With a flat background and high signal-to-noise ratio, modulation transfer spectroscopy (MTS) offers an accurate and stable method for laser locking. Despite its benefits, however, the four-wave mixing process that is inherent to the MTS technique entails that the strongest modulation transfer signals are only observed for closed transitions, excluding MTS from numerous applications. Here, we report for the first time the observation of a magnetically tunable MTS error signal. Using a simple two-magnet arrangement, we show that the error signal for the 87 Rb F = 2 → F = 3 cooling transition can be Zeemanshifted over a range of >15 GHz to any arbitrary point on the rubidium D 2 spectrum. Modulation transfer signals for locking to the 87 Rb F = 1 → F = 2 repumping transition as well as 1 GHz red-detuned to the cooling transition are presented to demonstrate the versatility of this technique, which can readily be extended to the locking of Raman and lattice lasers.