Letters in 2014

An optical-frequency-comb laser manipulating a dipole response can imprint the comb on an excited transition with a high photon energy. The concept can be implemented using existing X-ray technology.

The long-standing question of information velocity in slow- and fast-light media is investigated by measuring the propagation time of random and correlated noise. The mutual information shared between two modes of an entangled state of light was found to advance when one mode propagates through the fast-light medium.

A new ‘photon–photon collider’, which may enable elusive Breit–Wheeler pair production in an optics laboratory setting, is predicted. Using this concept, it is potentially possible to produce 10⁵ Breit–Wheeler electron–positron pairs by firing a gamma-ray beam into a high-temperature radiation field of a laser-heated hohlraum cavity.

Little attention has been devoted to development and characterization of below-threshold harmonic sources, which are critical for extending time-resolved photoemission spectroscopy to megahertz repetition rates and for developing high-average-power vacuum-ultraviolet sources. Now, a new regime of below-threshold harmonic generation accompanied by the bright, coherent emission of vacuum-ultraviolet lines is reported.

An optical parametric oscillator in the telecom wavelength range is realized in a diamond system consisting of a ring resonator coupled to a diamond waveguide. Threshold powers as low as 20 mW are measured and up to 20 new wavelengths are generated from a single-frequency pump laser.

With the help of two photonic controlled-NOT gates, a three-logical-qubit concatenated Greenberger–Horne–Zeilinger (C-GHZ) state encoded by a six-photon graph state is experimentally created. Observation of the dynamics of distillability evolving under a collective noisy environment revealed that the C-GHZ state is more robust than the conventional GHZ state.

The first X-ray-pump–X-ray-probe measurement of the nonlinear response of a plasma amplifier perturbed by a ultrashort soft-X-ray pulse is demonstrated. Two time-delayed 18.9-nm-wavelength pulses were incident on a plasma, and the gain depletion induced by saturated amplification of the pump was measured with a femtosecond resolution.

Kerr frequency combs are well suited for high-capacity data transmission with phase-sensitive modulation formats. This work demonstrates error-free transmission with data rates of up to 1.44 Tbit s⁻¹, spectral efficiencies of up to 6 bit s⁻¹ Hz⁻¹ and transmission distances of up to 300 km.

Violation of the classical bound of the three-particle Mermin inequality by nine standard deviations is experimentally demonstrated by closing both the locality and freedom-of-choice loopholes; only the fair-sampling assumption is required. To achieve this, a light source for producing entangled multiphoton states and measurement technologies for precise timing and efficient detection were developed.

A simple experiment enables simultaneous long-range spatial structuring of a cold atomic cloud and an optical pump field, with an adjustable length scale.

Plasma channels induced in air by femtosecond-laser filamentation are useful for many applications, including attosecond physics and spectroscopy and remote sensing. By appropriately employing a surrounding auxiliary dressing beam to continuously supply energy to the filament, the natural range of the plasma column has been extended by at least one order of magnitude.

An optical memory is demonstrated in a kagome photonic crystal fibre whose 26-μm-diameter hollow core is loaded with cesium atoms. Gigahertz-bandwidth light is stored using a far-detuned Raman interaction. It has a memory efficiency is 27 ± 1% and a signal-to-noise ratio of 2.6:1 — the highest at the single-photon level of any memory at room temperature.

By exploiting a self-bending point spread function based on Airy beams, a three-dimensional super-resolution fluorescence imaging is realized. A three-dimensional localization precision in the range 10–15 nm was obtained at an imaging depth of 3 µm from ∼2,000 photons per localization.

Researchers demonstrate unequivocal quantum interference between plasmons in a Hong–Ou–Mandel experiment. The results may be important for quantum information applications of plasmonics.

Polarization-entangled photon pairs are generated from an In(Ga)As quantum dot by setting the pump intensity such that the inversion of the quantum dot from the ground to the biexcitonic state is the most probable transition. On-demand generation is demonstrated with an ultrahigh purity, a high entanglement fidelity and high two-photon-interference non-post-selective visibilities.

The first observation of a third-order process induced by an X-ray beam from a free-electron laser is realized in germanium using a 5.6-keV X-ray beam. Two-photon absorption is confirmed, suggesting that X-ray analogues of other third-order nonlinear processes may be available for exploitation in X-ray experiments.

A photodiode-based logic device employing scalable heterojunctions of carbon nanotubes and silicon whose output currents can be manipulated by both optical and electrical inputs is developed. Bidirectional phototransistors and novel clock-triggerable logic elements, such as a mixed optoelectronic AND gate, a 2-Bit optoelectronic ADDER/OR gate and a 4-Bit optoelectronic D/A converter, are also demonstrated.

A phase modulator that is only 29 µm long and operates at 65 GHz is demonstrated using plasmonics and the Pockels effect in a nonlinear polymer. The device operates across a 120-nm-wide wavelength range centred on 1,550 nm and at temperatures up to 85 °C.

A quantum memory for orbital angular momentum qubits is demonstrated in the single-photon regime. It is based on cold cesium atoms and the dynamic electromagnetically induced transparency protocol. Retrieved states were analysed by quantum tomography, and fidelities after readout of over 92% were obtained, confirming the quantum functionality of the storage process.

An optical-frequency comb-based scheme is demonstrated that transfers a 4.5 × 10⁻¹⁶ fractional frequency stability from a 1,062-nm-wavelength laser to a 1,542-nm-wavelength laser. Transfer is also reported down to 4 × 10⁻¹⁸ at 1 s, which is one order of magnitude below that of previously reported work with comparable systems.