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Register a new userSynchronized resistance to inhomogeneous magnetic field-induced dephasing of an image stored in a cold atomic ensemble
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Long-lived storage of arbitrary transverse multimodes is important for establishing a high-channel-capacity quantum network. Most of the pioneering works focused on atomic diffusion as the dominant impact on the retrieved pattern in an atom-based memory. In this work, we demonstrate that the unsynchronized Larmor precession of atoms in the inhomogeneous magnetic field dominates the distortion of the pattern stored in a cold-atom-based memory. We find that this distortion effect can be eliminated by applying a strong uniform polarization magnetic field. By preparing atoms in magnetically insensitive states, the destructive interference between different spin-wave components is diminished, and the stored localized patterns are synchronized further in a single spin-wave component; then, an obvious enhancement in preserving patterns for a long time is obtained. The reported results are very promising for studying transverse multimode decoherence in storage and high-dimensional quantum networks in the future.
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Quantum light-matter interfaces, based upon ensembles of cold atoms or other quantum emitters, are a vital platform for diverse quantum technologies and the exploration of fundamental quantum phenomena. Most of our understanding and modeling of such systems are based upon macroscopic theories, wherein the atoms are treated as a smooth, quantum polarizable medium. Although it is known that such approaches ignore a number of microscopic details, such as the granularity of atoms, dipole-dipole interactions and multiple scattering of light, the consequences of such effects in practical settings are usually mixed with background macroscopic effects and difficult to quantify. In this work we demonstrate a time-domain method to measure microscopically-driven optical effects in a background-free fashion, by transiently suppressing the macroscopic dynamics. With the method, we reveal a microscopic dipolar dephasing mechanism that generally limits the lifetime of the optical spin-wave order in a random gas. Theoretically, we show the dephasing effect emerges from the strong resonant dipole interaction between close-by atomic pairs.
We study electromagnetically induced transparency (EIT) of a weakly interacting cold Rydberg gas. We show that the onset of interactions is manifest as a depopulation of the Rydberg state and numerically model this effect by adding a density-dependent non-linear term to the optical Bloch equations. In the limit of a weak probe where the depopulation effect is negligible, we observe no evidence of interaction induced decoherence and obtain a narrow Rydberg dark resonance with a linewidth of <600 kHz, limited by the Rabi frequency of the coupling beam
We report significant improvements in the retrieval efficiency of a single excitation stored in an atomic ensemble and in the subsequent generation of strongly correlated pairs of photons. A 50% probability to transform the stored excitation into one photon in a well-defined spatio-temporal mode at the output of the ensemble is demonstrated. These improvements are illustrated by the generation of high-quality heralded single photons with a suppression of the two-photon component below 1% of the value for a coherent state. A broad characterization of our system is performed for different parameters in order to provide input for the future design of realistic quantum networks.
Quantum memories are an integral component of quantum repeaters - devices that will allow the extension of quantum key distribution to communication ranges beyond that permissible by passive transmission. A quantum memory for this application needs to be highly efficient and have coherence times approaching a millisecond. Here we report on work towards this goal, with the development of a $^{87}$Rb magneto-optical trap with a peak optical depth of 1000 for the D2 $F=2 rightarrow F=3$ transition using spatial and temporal dark spots. With this purpose-built cold atomic ensemble to implement the gradient echo memory (GEM) scheme. Our data shows a memory efficiency of $80pm 2$% and coherence times up to 195 $mu$s, which is a factor of four greater than previous GEM experiments implemented in warm vapour cells.
We study experimentally and theoretically a dense ensemble of negatively charged nitrogen-vacancy centers in diamond coupled to a high $Q$ superconducting coplanar waveguide cavity mode at low temperature. The nitrogen-vacancy centers are modeled as effective spin one defects with inhomogeneous frequency distribution. For a large enough ensemble the effective magnetic coupling of the collective spin dominates the mode losses and inhomogeneous broadening of the ensemble and the system exhibits well resolved normal mode splitting in probe transmission spectra. We use several theoretical approaches to model the probe spectra and the number and frequency distribution of the spins. This analysis reveals an only slowly temperature dependent q-Gaussian energy distribution of the defects with a yet unexplained decrease of effectively coupled spins at very low temperatures below $unit{100}{millikelvin}$. Based on the system parameters we predict the possibility to implement an extremely stable maser by adding an external pump to the system.
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