Frequency-multiplexed on-demand storage in five modes of atomic frequency comb through simultaneous application of control pulses.

In quantum communication with quantum repeaters, multiplexed quantum memory is expected to enhance communication rates. When using an atomic frequency comb (AFC) for on-demand storage, the frequency mode number is often limited by the optical power of the control pulses. Here, using a space-coupled waveguide electro-optic modulator, we increased the output power, allowing us to apply control pulses to multiple modes simultaneously. Further, through enhancement of an experimental setup that increases power density, we increased the number of modes. Consequently, we pioneered, to the best of our knowledge, on-demand storage using five modes of AFC. This technology is a significant achievement toward frequency-multiplexed on-demand quantum memory.

Sagnac interferometer-type nondegenerate polarization entangled two-photon source with a Fresnel rhomb.

Telecommunication wavelength-entangled photon sources (EPS) are indispensable systems for a fiber-based quantum network. We developed a Sagnac-type spontaneous parametric down conversion system adopting a Fresnel rhomb as a wideband and reasonable retarder. This novelty, to the best of our knowledge, enables the generation of a highly nondegenerate two-photon entanglement comprising the telecommunication wavelength (1550 nm) and quantum memory wavelength (606 nm for Pr:YSO) with only one nonlinear crystal. Quantum state tomography was performed to evaluate the degree of entanglement, and the fidelity with a Bell state |Φ+⟩ with a maximum of 94.4% was obtained. Therefore, this paper shows the potential of nondegenerate EPSs that are compatible with both telecommunication wavelength and quantum-memory wavelength to be installed in quantum repeater architecture.

Evaluation of laser frequency offset locking using an electrical delay line.

Frequency offset locking between two Nd:YAG lasers is performed using frequency locking with an electrical delay line. The relative frequency instability of the offset locking is measured to be 3.5×10-12 for an averaging time of 1 s, which is approximately 77 times smaller than that of the free-running case. The frequency instability of the frequency locking is compared to that of the phase locking between the two Nd:YAG lasers. Furthermore, a compact solid-state laser is frequency locked to an optical frequency comb with a frequency instability of 8.2×10-11 for an averaging time of 1 s, which is improved by approximately 20 times, with respect to the free-running case. The offset-locking scheme using a delay line is useful for various applications including a research on quantum optics, interferometric measurements, and experiments involving laser cooling, such as an optical lattice clock.

High-energy side-peak emission of exciton-polariton condensates in high density regime.

In a standard semiconductor laser, electrons and holes recombine via stimulated emission to emit coherent light, in a process that is far from thermal equilibrium. Exciton-polariton condensates-sharing the same basic device structure as a semiconductor laser, consisting of quantum wells coupled to a microcavity-have been investigated primarily at densities far below the Mott density for signatures of Bose-Einstein condensation. At high densities approaching the Mott density, exciton-polariton condensates are generally thought to revert to a standard semiconductor laser, with the loss of strong coupling. Here, we report the observation of a photoluminescence sideband at high densities that cannot be accounted for by conventional semiconductor lasing. This also differs from an upper-polariton peak by the observation of the excitation power dependence in the peak-energy separation. Our interpretation as a persistent coherent electron-hole-photon coupling captures several features of this sideband, although a complete understanding of the experimental data is lacking. A full understanding of the observations should lead to a development in non-equilibrium many-body physics.

Two-photon interference at telecom wavelengths for time-bin-encoded single photons from quantum-dot spin qubits.

Practical quantum communication between remote quantum memories rely on single photons at telecom wavelengths. Although spin-photon entanglement has been demonstrated in atomic and solid-state qubit systems, the produced single photons at short wavelengths and with polarization encoding are not suitable for long-distance communication, because they suffer from high propagation loss and depolarization in optical fibres. Establishing entanglement between remote quantum nodes would further require the photons generated from separate nodes to be indistinguishable. Here, we report the observation of correlations between a quantum-dot spin and a telecom single photon across a 2-km fibre channel based on time-bin encoding and background-free frequency downconversion. The downconverted photon at telecom wavelengths exhibits two-photon interference with another photon from an independent source, achieving a mean wavepacket overlap of greater than 0.89 despite their original wavelength mismatch (900 and 911 nm). The quantum-networking operations that we demonstrate will enable practical communication between solid-state spin qubits across long distances.

BCS wave-function approach to the BEC-BCS crossover of exciton-polariton condensates.

The crossover between low and high density regimes of exciton-polariton condensates is examined using a BCS wave-function approach. Our approach is an extension of the BEC-BCS crossover theory for excitons, but includes a cavity photon field. The approach can describe both the low density limit, where the system can be described as a Bose-Einstein condensate (BEC) of exciton-polaritons, and the high density limit, where the system enters a photon-dominated regime. In contrast to the exciton BEC-BCS crossover where the system approaches an electron-hole plasma, the polariton high density limit has strongly correlated electron-hole pairs. At intermediate densities, there is a regime with BCS-like properties, with a peak at nonzero momentum of the singlet pair function. We calculate the expected photoluminescence and give several experimental signatures of the crossover.