We experimentally demonstrate PT-symmetric optical lattices with periodical gain and loss profiles in a coherently prepared four-level N-type atomic system. By appropriately tuning the pertinent atomic parameters, the onset of PT-symmetry breaking is observed through measuring an abrupt phase-shift jump between adjacent gain and loss waveguides. The experimental realization of such a readily reconfigurable and effectively controllable PT-symmetric waveguide array structure sets a new stage for further exploiting and better understanding the peculiar physical properties of these non-Hermitian systems in atomic settings.

We study the interaction of two photons in a Rydberg atomic ensemble under the condition of electromagnetically induced transparency, combining a semi-classical approach for pulse propagation and a complete quantum treatment for quantum state evolution. We find that the blockade regime is not suitable for implementing photon-photon cross-phase modulation due to pulse absorption and dispersion. However, approximately ideal cross-phase modulation can be realized based on relatively weak interactions, with counter-propagating and transversely separated pulses.Strong nonlinearity at the single-photon level is desirable to the realization of all-optical quantum devices. Ensembles of highly excited Rydberg atoms under electromagnetically induced transparency (EIT) condition combine the advantages of strong atom-field coupling without significant absorption and non-local atomic interaction, and have attracted intensive experimental [1][2][3][4][5][6][7] and theoretical studies [8][9][10][11][12][13][14][15][16][17] recently. The strong correlation directly between single photons inside Rydberg atomic ensemble was observed [7], and the formation of a Wigner crystal of individual photons is also predicted [15]. When such interaction is applied to implement the cross-phase modulation (XPM) between two individual photons with a non-zero relative velocity as in Fig. 1 [18-21], a main difference from a single probe beam propagation in Rydberg EIT medium [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] is that no steady state exists for the pulses, because their interaction varies with the relative distance, pulse velocity that changes pulse sizes, as well as the absorption in medium. The realistic timedependence in the inherent nonlinear dynamics makes a complete solution of the problem rather challenging. With the combination of a semi-classical approach for pulse propagation and a complete quantum approach for pulse quantum state evolution, we find a realistic picture for the dynamical process by showing the concerned figures of merit. We show that our proposed setup outperforms the previously considered Rydberg blockade regime [20] clearly in terms of much lower photon absorption and negligible group velocity dispersion.The detailed two-photon XPM via Rydberg EIT is as follows. One respectively couples the far-away input photons to cold Rydberg atoms under the EIT condition to form the light-matter quasi-particle called darkstate polariton (DSP) [22]. The spatial distribution of the pulses necessitates a quantum many-body description of the process. The prepared DSPs are in the state |1 l

We utilize a homodyne detection technique to achieve a new sensitivity limit for atom-based, absolute radio-frequency electric field sensing of 5 μV cm−1 Hz−1/2. A Mach-Zehnder interferometer is used for the homodyne detection. With the increased sensitivity, we investigate the dominant dephasing mechanisms that affect the performance of the sensor. In particular, we present data on power broadening, collisional broadening and transit time broadening. Our results are compared to density matrix calculations. We show that photon shot noise in the signal readout is currently a limiting factor. We suggest that new approaches with superior readout with respect to photon shot noise are needed to increase the sensitivity further.

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A new approach to detect absolute radio-frequency (RF) electric fields (E-fields) that uses Rydberg atoms at room temperature in vapor cells has recently been demonstrated. The large transition dipole moments between energetically adjacent Rydberg states enable this technique to make traceable E-field measurements with high sensitivity over a large frequency range, from 1 GHz to 1 THz. In this paper, we experimentally investigate how the vapor cell geometry affects the accuracy of the measurements. We find that the effects of the vapor cell on the measured RF E-field can be minimized by making the vapor cell size small compared to the wavelength of the RF E-field.