On-chip integrated photonic circuits are crucial to further progress towards quantum technologies and in the science of quantum optics. Here we report precise control of single photon states and multi-photon entanglement directly on-chip. We manipulate the state of path-encoded qubits using integrated optical phase control based on resistive elements, observing an interference contrast of 98.2 ± 0.3%. We demonstrate integrated quantum metrology by observing interference fringes with 2-and 4-photon entangled states generated in a waveguide circuit, with respective interference contrasts of 97.2 ± 0.4% and 92 ± 4%, sufficient to beat the standard quantum limit. Finally, we demonstrate a reconfigurable circuit that continuously and accurately tunes the degree of quantum interference, yielding a maximum visibility of 98.2 ± 0.9%. These results open up adaptive and fully reconfigurable photonic quantum circuits not just for single photons, but for all quantum states of light.Controlling quantum systems is not only a fundamental scientific endeavor, but promises profound new technologies 1,2,3 .Quantum photonics already provides enhanced communication security 2,4 ; has demonstrated increased precision by beating the standard quantum limit in metrology 5,6,7,8 and the diffraction limit in lithography 9,10 ; holds great promise for quantum computation 11,12 ; and continues to advance fundamental quantum science. The recent demonstration of on-chip integrated waveguide quantum circuits 13 is a key step towards these new technologies and for further progress in fundamental science applications.Technologies based on harnessing quantum mechanical phenomena require methods to precisely prepare and control the state of quantum systems. Manipulation of a path-encoded qubit-a single photon in an arbitrary superposition of two optical paths, which is the natural encoding for waveguides 13 -requires control of the relative phase φ between the two optical paths and the amplitude in each path.The integrated waveguide device shown schematically in Fig. 1a applies the unitary operation U M Z = U DC e iφσ Z /2 U DC : each 50% splitting ratio (reflectivity η = 0.5) directional coupler implements U DC 14 ; while control over the relative optical phase φ between the two optical paths implements the phase gate e iφσz/2 . A single photon input into mode a is transformed into a superposition across modes c and d :(a single photon input into mode b is transformed into the same superposition but with a relative π phase shift). The relative optical phase is then controlled by the parameter φ, i.e.(2) before the two modes are recombined at the second η = 0.5 coupler. FIG. 1: Manipulating quantum states of light on a chip.a, Schematic of a waveguide circuit with the relative optical phase φ controlled by applying a voltage V across the contact pads p 1 and p 2 (not to scale). b, Illustration of the cross section of one waveguide located beneath a resistive heater. c, The simulated intensity profile of the guided single mode in the silica wavegui...

Entangled states are central to quantum information processing, including quantum teleportation, efficient quantum computation and quantum cryptography. In general, these applications work best with pure, maximally entangled quantum states. However, owing to dissipation and decoherence, practically available states are likely to be non-maximally entangled, partially mixed (that is, not pure), or both. To counter this problem, various schemes of entanglement distillation, state purification and concentration have been proposed. Here we demonstrate experimentally the distillation of maximally entangled states from non-maximally entangled inputs. Using partial polarizers, we perform a filtering process to maximize the entanglement of pure polarization-entangled photon pairs generated by spontaneous parametric down-conversion. We have also applied our methods to initial states that are partially mixed. After filtering, the distilled states demonstrate certain non-local correlations, as evidenced by their violation of a form of Bell's inequality. Because the initial states do not have this property, they can be said to possess 'hidden' non-locality.

A physical random number generator based on the intrinsic randomness of quantum mechanics is described. The random events are realized by the choice of single photons between the two outputs of a beam splitter. We present a simple device, which minimizes the impact of the photon counters' noise, dead-time and after pulses

Quantum entanglement between qudits -the d-dimensional version of qubits -is relevant for advanced quantum information processing and provides deeper insights in the nature of quantum correlations. Encoding qudits in the frequency modes of photon pairs produced by continuous parametric down-conversion enables access to high-dimensional states. By shaping the energy spectrum of entangled photons, we demonstrate the creation, characterization and manipulation of entangled qudits with dimension up to 4. Their respective density matrices are reconstructed by quantum state tomography. For qubits and qutrits we additionally measured the dependency of a d-dimensional Bell parameter for various degrees of entanglement. Our experiment demonstrates the ability to investigate the physics of high-dimensional frequency entangled qudit states which are of great importance for quantum information science.

The performance of three types of InGaAs/InP avalanche photodiodes is investigated for photon counting at 1550 nm in the temperature range of thermoelectric cooling. The best one yields a dark count probability of 2.8 • 10 −5 per gate (2.4 ns) at a detection efficiency of 10% and a temperature of -60 • C. The afterpulse probability and the timing jitter are also studied. The results obtained are compared with those of other papers and applied to the simulation of a quantum key distribution system. An error rate of 10% would be obtained after 54 kilometers.

We apply near-field matter-wave interferometry to determine the absolute scalar polarizability of the fullerenes C60 and C70. A key feature of our experiment is the combination of good transmission and high spatial resolution, gained by wide molecular beams passing through sub-micron gratings. This allows to significantly facilitate the observation of field-dependent beam shifts. We thus measure the polarizability to be α = 88.9 ± 0.9 ± 5.1Å3 for C60 and to α = 108.5 ± 2.0 ± 6.2Å 3 for C70.

Conventional spectroscopy uses classical light to detect matter properties through the variation of its response with frequencies or time delays. Quantum light opens up new avenues for spectroscopy by utilizing parameters of the quantum state of light as novel control knobs and through the variation of photon statistics by coupling to matter. This Roadmap article focuses on using quantum light as a powerful sensing and spectroscopic tool to reveal novel information about complex molecules that is not accessible by classical light. It aims at bridging the quantum optics and spectroscopy communities which normally have opposite goals: manipulating complex light states with simple matter e.g. qubits vs. studying complex molecules with simple classical light, respectively. Articles cover advances in the generation and manipulation of state-of-the-art quantum light sources along with applications to sensing, spectroscopy, imaging and interferometry.

Multisimultaneity is a causal model of relativistic quantum physics which assigns a real time ordering to any set of events, much in the spirit of the pilot-wave picture. Contrary to standard quantum mechanics, it predicts a disappearance of the correlations in a Bell-type experiment when both analyzers are in relative motion such that each one, in its own inertial reference frame, is first to select the output of the photons. We tested this prediction using acousto-optic modulators as moving beam splitters and interferometers separated by 55 m. We did not observe any disappearance of the correlations, in agreement with quantum mechanics.