Synthesis of the Einstein-Podolsky-Rosen entanglement in a sequence of two single-mode squeezers

Synthesis of the Einstein-Podolsky-Rosen entangled state --- the primary entangled resource in continuous-variable quantum-optical information processing --- is a technological challenge of great importance. Here we propose and implement a new scheme of generating this state. Two nonlinear optical crystals, positioned back-to-back in the waist of a pump beam, function as single-pass degenerate optical parametric amplifiers and produce single-mode squeezed vacuum states in orthogonal polarization modes, but in the same spatiotemporal mode. A subsequent pair of waveplates acts as a beam splitter, entangling the two polarization modes to generate the Einstein-Podolsky-Rosen state. This technique takes advantage of the strong nonlinearity associated with type-I phase-matching configuration while at the same time eliminating the need for actively stabilizing the optical phase between the two squeezers, which typically arises if these squeezers are spatially separated. We demonstrate our method in an experiment, preparing a 1.4 dB two-mode squeezed state and characterizing it via two-mode homodyne tomography.Comment: 4 pages, 3 figure

Undoing the effect of loss on quantum entanglement

Entanglement distillation is a process via which the strength and purity of quantum entanglement can be increased probabilistically. It is a key step in many quantum communication and computation protocols. In particular, entanglement distillation is a necessary component of the quantum repeater, a device which counters the degradation of entanglement that inevitably occurs due to losses in a communication line. Here we report an experiment on distilling the Einstein-Podolsky-Rosen (EPR) state of light, the workhorse of continuous-variable entanglement, using the technique of noiseless amplification. In contrast to previous implementations, the entanglement enhancement factor achievable by our technique is not fundamentally limited and permits recovering an EPR state with a macroscopic level of entanglement no matter how low the initial entanglement or how high the loss may be. In particular, we recover the original level of entanglement after one of the EPR modes has passed through a channel with a loss factor of 20. The level of entanglement in our distilled state is higher than that achievable by direct transmission of any state through a similar loss channel. This is a key bench-marking step towards the realization of a practical continuous-variable quantum repeater and other CV quantum protocols.Comment: 8 pages, 5 figure

Dead time duration and active reset influence on the afterpulse probability of InGaAs/InP SPAD based SPDs

We perform the detailed study of the afterpulse probability's dependence in the InGaAs/InP sine-gated SPAD on the dead time and the used approach for its implementation. We have found that the comparator's simple latching can significantly reduce afterpulses' probability, even without using a dead time pulse that lowers the diode bias voltage. We have found that with a low probability of afterpulse ( 10 mus), it is sufficient to use a circuit with latching of the comparator, which will significantly simplify the development of an SPD device for applications in which such parameters are acceptable. We also proposed a precise method for measuring and the afterpulse and presented a model describing the recurrent nature of this effect. We have shown that it should not use a simple model to describe the afterpulse probability due to rough underlying physical processes. A second-order model is preferable

Semi-Empirical Satellite-to-Ground Quantum Key Distribution Model for Realistic Receivers

Satellite-based link analysis is valuable for efficient and secure quantum communication, despite seasonal limits and restrictions on transmission times. A semi-empirical quantum key distribution model for satellite-based systems was proposed that simplifies simulations of communication links. Unlike other theoretical models, our approach was based on the experimentally-determined atmospheric extinction coefficient typical for mid-latitude ground stations. The parameter was measured for both clear and foggy conditions, and it was validated using published experimental data from the Micius satellite. Using this model, we simulated secure QKD between the Micius satellite and ground stations with 300 mm and 600 mm aperture telescopes

Gas-phase equilibrium molecular structures and ab initio thermochemistry of anthracene and rubrene

Otlyotov A, Kurochkin IY, Minenkov Y, et al. Gas-phase equilibrium molecular structures and ab initio thermochemistry of anthracene and rubrene. Physical Chemistry Chemical Physics. 2022.Semi-experimental gas-phase structures of anthracene and rubrene (5,6,11,12-tetraphenyltetracene) were determined by means of gas electron diffraction (GED). The use of the flexible restraints in the refinement of the GED data successfully resolves non-equivalent C–C bond lengths. The tetracene core of an isolated rubrene molecule was found to exhibit twist distortion of about 18°; this is less than DFT calculations predict (30–40°). The modified Feller-Peterson-Dixon method in conjunction with high-level DLPNO-CCSD(T) calculations was employed to resolve the discrepancy between the available experimental gas-phase enthalpies of formation for rubrene. The theoretical value of ΔfHmo(g, 298 K) = 759.4 ± 5.9 kJ mol−1 meets its recent experimental counterpart (765.6 ± 8.4 kJ mol−1) and is in strong disagreement with the previous estimation (882 kJ mol−1)