Abstract: The plasmon resonance of metal nanoparticles can direct light from optical emitters in much the same way that radiofrequency antennas direct the emission from electrical circuits. Recently, rapid progress has been made in the realization of single-element antennas for optical waves. Because most of these devices are designed to optimize the local near-field coupling between the antenna and an emitter, the possibility of modifying the spatial radiation pattern has not yet received as much attention. In the radiofrequency regime, a typical antenna design for high directivity is the Yagi–Uda antenna, which essentially consists of a one-dimensional array of antenna elements driven by a single feed element. By fabricating a corresponding array of nanoparticles, similar radiation patterns can be obtained in the optical regime. Here, we present the experimental demonstration of directional control of radiation from a nano-optical Yagi–Uda antenna composed of appropriately tuned gold nanorods.

Abstract: Quantum cryptography has recently been extended to continuous-variable systems, such as the bosonic modes of the electromagnetic field possessing continuous degrees of freedom. In particular, several cryptographic protocols have been proposed and experimentally implemented using bosonic modes with Gaussian statistics. These protocols have shown the possibility of reaching very high secret key rates, even in the presence of strong losses in the quantum communication channel. Despite this robustness to loss, their security can be affected by more general attacks where extra Gaussian noise is introduced by the eavesdropper. Here, we show a `hardware solution' for enhancing the security thresholds of these protocols. This is possible by extending them to two-way quantum communication where subsequent uses of the quantum channel are suitably combined. In the resulting two-way schemes, one of the honest parties assists the secret encoding of the other, with the chance of a non-trivial superadditive enhancement of the security thresholds. These results should enable the extension of quantum cryptography to more complex quantum communications.

Abstract: Spin and charge terahertz excitations in solids are promising for implementing future technologies such as spintronics and quantum computation, but coherently controlling them has been a significant challenge. Researchers have now manipulated coherent spin waves in an antiferromagnet using the intense magnetic field of ultrashort terahertz pulses.

Abstract: Emission from a resonantly excited quantum emitter is a fascinating research topic within the field of quantum optics and is a useful source for different types of quantum light fields. The resonance spectrum consists of a single spectral line that develops into a triplet above saturation of the quantum emitter. The three closely spaced photon channels from the resonance fluorescence have different photon statistical signatures. We present a detailed photon statistics analysis of the resonance fluorescence emission triplet from a solid-state-based artificial atom, that is, a semiconductor quantum dot. The photon correlation measurements demonstrate both `single' and `cascaded' photon emission from the Mollow triplet sidebands. The bright and narrow sideband emission (5.9 × 106 photons per second into the first lens) can be conveniently frequency-tuned by laser detuning over 15 times its linewidth (Δv ~ 1.0 GHz). These unique properties make the Mollow triplet sideband emission a valuable light source for quantum light spectroscopy and quantum information applications, for example.

Abstract: Extreme ultraviolet attosecond pulses, which emerge from the interaction of atoms with intense laser fields, play a central role in modern ultrafast science and the exploration of electron behaviour. Recent work now shows that catastrophe theory can help optimize the properties of these pulses.

Abstract: At optical frequencies the radiation produced by a source, such as a laser, a black body or a single-photon emitter, is frequently characterized by analysing the temporal correlations of emitted photons using single-photon counters. At microwave frequencies, however, there are no efficient single-photon counters yet. Instead, well-developed linear amplifiers allow for efficient measurement of the amplitude of an electromagnetic field. Here, we demonstrate first- and second-order correlation function measurements of a pulsed microwave-frequency single-photon source integrated on the same chip with a 50/50 beam splitter followed by linear amplifiers and quadrature amplitude detectors. We clearly observe single-photon coherence in first-order and photon antibunching in second-order correlation function measurements of the propagating fields.

Abstract: The advantageous properties of terahertz waves, such as their transmission through objects opaque to visible light, are attracting attention for imaging applications. A promising approach for achieving high spatial resolution is the use of near-field imaging. Although this method has been well established in the visible and microwave regions, it is challenging to perform in the terahertz region. In the terahertz techniques investigated to date, detectors have been located remotely from the probe, which degrades sensitivity, and the influence of far-field waves is unavoidable. Here we present a new integrated detection device for terahertz near-field imaging in which all the necessary detection components — an aperture, a probe and a terahertz detector — are integrated on one semiconductor chip, which is cryogenically cooled. This scheme allows highly sensitive, high-resolution detection of the evanescent field alone and promises new capabilities for high-resolution terahertz imaging.

Abstract: Noise filters based on so-called dynamical decoupling pulse sequences can suppress decoherence in quantum systems. Turning this idea on its head now provides a new technique for studying the noise itself.