Abstract:We introduce a novel time-frequency quantum key distribution (TFQKD) scheme based on photon pairs entangled in these two conjugate degrees of freedom. The scheme uses spectral detection and phase modulation to enable measurements in the temporal basis by means of time-to-frequency conversion. This allows large-alphabet encoding to be implemented with realistic components. A general security analysis for TFQKD with binned measurements reveals a close connection with finite-dimensional QKD protocols and enables analysis of the effects of dark counts on the secure key size. References and links1. JG Rarity, PR Tapster, JG Walker, and S Seward. Experimental demonstration of single photon rangefinding using parametric downconversion.

Interference between independent single photons is perhaps the most fundamental interaction in quantum optics. It has become increasingly important as a tool for optical quantum information science, as one of the rudimentary quantum operations, together with photon detection, for generating entanglement between non-interacting particles. Despite this, demonstrations of large-scale photonic networks involving more than two independent sources of quantum light have been limited due to the difficulty in constructing large arrays of high-quality single photon sources. Here, we solve the key challenge, reporting a novel array of more than eighteen near-identical, low-loss, highpurity, heralded single photon sources achieved using spontaneous four-wave mixing (SFWM) on a silica chip. We verify source quality through a series of heralded Hong-Ou-Mandel experiments, and further report the experimental three-photon extension of the entire Hong-Ou-Mandel interference curves, which map out the interference landscape between three independent single photon sources for the first time.Recently, integration of photon pair sources on-chip has been recognized as one of the most promising approaches to scaling due to their small size, direct compatibility with integrated photonic architectures, reduction in required pump power, and potentially exquisite control of the populated optical modes [10,[21][22][23]. Unfortunately, fabrication imperfections or material limitations frequently spoil this dream. Optical loss is a key parameter for any quantum light source and on-chip sources frequently suffer from large losses due to high scattering and outcoupling mode mismatch [23][24][25]. In addition, the phase-matching conditions for the spontaneous scattering process are highly sensitive to optical dispersion. arXiv:1603.06984v1 [quant-ph]

We experimentally demonstrate the generation of multi-photon Fock states with up to three photons in well-defined spatial-temporal modes synchronized with a classical clock. The states are characterized using quantum optical homodyne tomography to ensure mode selectivity. The three-photon Fock states are probabilistically generated by pulsed spontaneous parametric down conversion at a rate of one per second, enabling complete characterization in 12 hours.

We present a spectrally decorrelated photon pair source bridging the visible and telecom wavelength regions. Tailored design and fabrication of a solid-core photonic crystal fiber (PCF) lead to the emission of signal and idler photons into only a single spectral and spatial mode. Thus no narrowband filtering is necessary and the heralded generation of pure photon number states in ultrafast wave packets at telecom wavelengths becomes possible

High-quality quantum sources are of paramount importance for the implementation of quantum technologies. We present here a heralded single-photon source based on commercial-grade polarization-maintaining optical fiber. The heralded photons exhibit a purity of at least 0.84 and an unprecedented heralding efficiency into a single-mode fiber of 85%. The birefringent phase-matching condition of the underlying four-wave mixing process can be controlled mechanically to optimize the wavelength tuning needed for interfacing multiple sources, as is required for large-scale entanglement generation

Frequency conversion of nonclassical light enables robust encoding of quantum information based upon spectral multiplexing that is particularly well-suited to integrated-optics platforms. Here we present an intrinsically deterministic linear-optics approach to spectral shearing of quantum light pulses and show it preserves the wavepacket coherence and quantum nature of light. The technique is based upon an electro-optic Doppler shift to implement frequency shear of heralded single-photon wave packets by ±200 GHz, which can be scaled to an arbitrary shift. These results demonstrate a reconfigurable method to controlling the spectral-temporal mode structure of quantum light that could achieve unitary operation.The frequency of a single light quantum, or photon, is a key physical property of individual excitations of the quantized electromagnetic field [1], which were introduced to describe the photoelectric effect [2]. Frequency is a mode characteristic, just as polarization, transverse-spatial amplitude, and direction of propagation define the modes of electromagnetic radiation. Thus frequency can be transformed using linear-optical elements in much the same way lenses transform transverse-spatial modes and wave plates manipulate polarization modes. Frequency is not an immutable property of photons-it can be coherently and deterministically modified. For example, retroreflection from a moving mirror results in a frequency shift due to the Doppler effect [3,4]. The various independent degrees of freedom that comprise the modes of light can be used to encode information in the electromagnetic field, namely position-momentum, time-frequency, and polarization. Information-technology applications require precise means for manipulation and measurement of light in the encoding degree of freedom. Many preliminary demonstrations of quantum optical technologies have utilized polarization, path or transverse-spatial mode encoding. These degrees of freedom are limited to relatively few quantum bits that can be practically addressed per photon within an integrated-optics platform, in which high-stability, low-loss multiphoton interference, necessary for optical quantum technologies, can occur. Recently, the time-frequency (TF) mode structure of light has come to the fore in quantum photonics as an ideal means of quantum information encoding for integrated optical quantum technologies [5][6][7][8][9][10][11].Essential to both quantum and classical technologies based upon TF mode encoding is the ability to control the pulsemode structure of light-where the central frequency and arrival time play prominent roles. In the classical domain the primary methods to control an optical pulse are based upon direct modification of the wave packet by amplifying and filtering different frequency and time components [12,13]. This approach to pulse shaping is incompatible with quantum states of light owing to noise and signal degradation arising from amplification and loss, resulting in destruction of the fragile quantum coherences between di...

Low-noise, efficient, phase-sensitive time-domain optical detection is essential for foundational tests of quantum physics based on optical quantum states and the realization of numerous applications ranging from quantum key distribution to coherent classical telecommunications. Stability, bandwidth, efficiency, and signal-to-noise ratio are crucial performance parameters for effective detector operation. Here we present a high-bandwidth, low-noise, ultra-stable time-domain coherent measurement scheme based on balanced homodyne detection ideally suited to characterization of quantum and classical light fields in well-defined ultrashort optical pulse modes.

Tomographic methods are used for the investigation of three-dimensional compressible flow fields by means of interferometric methods. A modified algebraic reconstruction technique algorithm is applied. The algorithm proved to give reliable reconstructions from experimentally measured projection data in the case of an unrestricted angular view. The method was used for the reconstruction of density distributions of weakly perturbed supersonic free jets exiting from a deformed Laval nozzle. Even small perturbations of the jet resulted in significant three-dimensional effects. Reconstruction of a multiple system of jets emanating from a sievelike nozzle showed mutual interactions between the constituent jets. For the investigation of unsteady flows a setup for the recording of holographic interferograms was designed. Here, because of experimental restrictions, only a limited angular range of views was accessible. In the context of this limited-view geometry, reconstructions revealed considerable distortions for objects containing steep gradients.