Long-lived non-classical correlations for scalable quantum repeaters at room temperature

Heralded single-photon sources with on-demand readout are promising candidates for quantum repeaters enabling long-distance quantum communication. The need for scalability of such systems requires simple experimental solutions, thus favouring room-temperature systems. For quantum repeater applications, long delays between heralding and single-photon readout are crucial. Until now, this has been prevented in room-temperature atomic systems by fast decoherence due to thermal motion. Here we demonstrate efficient heralding and readout of single collective excitations created in warm caesium vapour. Using the principle of motional averaging we achieve a collective excitation lifetime of $0.27\pm 0.04$ ms, two orders of magnitude larger than previously achieved for single excitations in room-temperature sources. We experimentally verify non-classicality of the light-matter correlations by observing a violation of the Cauchy-Schwarz inequality with $R=1.4\pm 0.1>1$. Through spectral and temporal analysis we identify intrinsic four-wave mixing noise as the main contribution compromising single-photon operation of the source.Comment: 21 pages total, the first 17 pages are the main article and the remaining pages are supplemental materia

Room-temperature single-photon source with near-millisecond built-in memory

Non-classical photon sources are a crucial resource for distributed quantum networks. Photons generated from matter systems with memory capability are particularly promising, as they can be integrated into a network where each source is used on-demand. Among all kinds of solid state and atomic quantum memories, room-temperature atomic vapours are especially attractive due to their robustness and potential scalability. To-date room-temperature photon sources have been limited either in their memory time or the purity of the photonic state. Here we demonstrate a single-photon source based on room-temperature memory. Following heralded loading of the memory, a single photon is retrieved from it after a variable storage time. The single-photon character of the retrieved field is validated by the strong suppression of the two-photon component with antibunching as low as $g^{(2)}_{\text{RR|W=1}} = 0.20 \pm 0.07$. Non-classical correlations between the heralding and the retrieved photons are maintained for up to $\tau_{\text{NC}}^{\mathcal R} = (0.68\pm 0.08)$ ms, more than two orders of magnitude longer than previously demonstrated with other room-temperature systems. Correlations sufficient for violating Bell inequalities exist for up to $\tau_{\text{BI}} = (0.15 \pm 0.03)$ ms.Comment: 7 pages, 4 figures main text plus 6 pages, 5 figures supplemen

Computer simulation of the electro-optical switching process in ferroelectric liquid crystal cells with bookshelf geometry

The electro-optical properties of a ferroelectric liquid crystal in thin surface stabilized cells with bookshelf geometry were studied with respect to the temperature and to the frequency with alternating amplitude and waveform of the electric field applied. Using a previously proposed model of director reorientation, the measurements were simulated, and a consistent description of the electro-optical behaviour of the system achieved. The fit of the simulations to the measurements provided material constants for the compound investigated and these are discussed

Synthesis and characterization of liquid-crystalline side group polymers with benzylideneaniline as mesogenic moiety

Two new thermotropic liquid-crystalline side group polymers were synthesized, characterized and compared with the liquid-crystalline monomeric analogues. Some packing features of these polymeric liquid crystals are discussed. Investigations were carried out by differential calorimetry (D.S.C.), polarization microscopic observation and X-ray diffraction on non-aligned and magnetic field-aligned samples in the wide and small angle region. The synthesized polymers contain as mesogenic moiety a benzylideneaniline group which is attached in the 4 position via a hexamethylene spacer to a poly-methacrylate backbone. The benzylideneaniline group is substituted in 4′ position with an ethoxy or butoxy group (PEt or PBu). The monomeric analogues are denoted MEt and MBu. The two polymers show a phase sequence crystalline-smectic A-nematic-isotropic. The liquid-crystalline temperature range is observed between 90 and 150°C. The monomeric MEt exhibits only a monotropic nematic, MBu an enantiotropic nematic and a smectic A phase

Coupled director and layer reorientation in layer tilted ferroelectric smectic liquid crystal cells

A reversible reorientation of smectic layers in surface stabilized ferroelectric liquid crystal (SSFLC) cells with layer tilted (chevron) geometries was found by electrooptical investigations. The reorientation of the layers superimposes the reorientation of the director along the S*C tilt cone and is suppressed, if the layers are irreversibly fixed in a bookself geometry by high electric fields. Based on a potential density expansion, a dynamical model of a coupled director and layer reorientation is proposed, which leads to excellent agreement with the electrooptical behaviour of the system investigated. The fit of the model to experimental data supplies a simultaneous determination of several material and cell constants. Basic electrooptical parameters, as contrast, switching times and optical overshoot, are reproduced correctly

Director movement and potential function of surface stabilized ferroelectric liquid crystal cells by electrooptical investigations

Electrooptical investigations on a ferroelectric liquid crystal (DOBAMBC) were carried out on a well aligned sample in a 2 μm cell. On the basis of these measurements the complete average director movement during a switching process caused by an electric field can be calculated and will be discussed. The thermodynamic potentials at the SA*-SC* phase transition were determined from the experimental results with a modified Landau theory. They serve as a basis for a discussion of the phase transition and the influence of the surface orientation. Some preliminary results were obtained concerning the electrooptical behaviour of the SH* phase

Detection of low-conductivity objects using eddy current measurements with an optical magnetometer

Detection and imaging of an electrically conductive object at a distance can be achieved by inducing eddy currents in it and measuring the associated magnetic field. We have detected low-conductivity objects with an optical magnetometer based on room-temperature cesium atomic vapor and a noise-canceling differential technique which increased the signal-to-noise ratio (SNR) by more than three orders of magnitude. We detected small containers with a few mL of salt-water with conductivity ranging from 4-24 S/m with a good SNR. This demonstrates that our optical magnetometer should be capable of detecting objects with conductivity 1 and opens up new avenues for using optical magnetometers to image low-conductivity biological tissue including the human heart which would enable non-invasive diagnostics of heart diseases.Comment: Main article with supplemental materia

TGB A* state in a homologous series of diarylethane α-chloroester ferroelectric liquid crystals

A twist grain boundary (TGB A*) or twisted smectic A* state was observed in two compounds of a homologous series of diarylethane α-chloroester ferroelectric liquid crystals. The phases have been characterized by optical polarizing microscopy and differential scanning calorimetry. Textures of the TGB A* state obtained by preparation on a glass slide and in thin homeotropically orienting liquid crystal cells or as free-standing films clearly show the helical structure, whereas preparation in homogeneously orienting LC cells suggests that the helical structure is suppressed by the cell geometry, in a similar way to that observed for S*C phases in the surface stabilized geometry (SSFLCs)

Strong electroacoustic effect in ferroelectric liquid crystal cells

A strong electroacoustic effect of a ferroelectric liquid crystal cell was observed with a 1 MV/m electric square field in the frequency range from 3 kHz to 30 kHz. The sound emitted can be heared with the naked ear and easily detected with a simple microphone. The electroacoustic effect vanishes in the chiral SA-phase

Electrooptical investigation on the three switching states of a chiral smectic side group polymer

Electrooptical properties of a polyacrylate with 1,3-dioxolane-4-carboxylic acid as chiral building block terminally attached to phenylpyridine mesogenic moiety and linked via a C11 spacer were studied. Results showed that the formation of 3 switching states for the polymer occurred upon conformational interactions between the side groups and the main chain