The evolution of bosons undergoing arbitrary linear unitary transformations quickly becomes hard to predict using classical computers as we increase the number of particles and modes. Photons propagating in a multiport interferometer naturally solve this so-called boson sampling problem(1), thereby motivating the development of technologies that enable precise control of multiphoton interference in large interferometers(2-4). Here, we use novel three-dimensional manufacturing techniques to achieve simultaneous control of all the parameters describing an arbitrary interferometer. We implement a small instance of the boson sampling problem by studying three-photon interference in a five-mode integrated interferometer, confirming the quantum-mechanical predictions. Scaled-up versions of this set-up are a promising way to demonstrate the computational advantage of quantum systems over classical computers. The possibility of implementing arbitrary linear-optical interferometers may also find applications in high-precision measurements and quantum communication(5)

A boson sampling device is a specialized quantum computer that solves a problem that is strongly believed to be computationally hard for classical computers. Recently, a number of small-scale implementations have been reported, all based on multiphoton interference in multimode interferometers. Akin to several quantum simulation and computation tasks, an open problem in the hard-to-simulate regime is to what extent the correctness of the boson sampling outcomes can be certified. Here, we report new boson sampling experiments on larger photonic chips and analyse the data using a recently proposed scalable statistical test. We show that the test successfully validates small experimental data samples against the hypothesis that they are uniformly distributed. In addition, we show how to discriminate data arising from either indistinguishable or distinguishable photons. Our results pave the way towards larger boson sampling experiments whose functioning, despite being non-trivial to simulate, can be certified against alternative hypotheses

A novel experiment supports quantum computation using photonic circuits to greatly increase quantum device speed.

A macrostate consisting of N approximately 3.5x10{4} photons in a quantum superposition and entangled with a far apart single-photon state (microstate) is generated. Precisely, an entangled photon pair is created by a nonlinear optical process; then one photon of the pair is injected into an optical parametric amplifier operating for any input polarization state, i.e., into a phase-covariant cloning machine. Such transformation establishes a connection between the single photon and the multiparticle fields. We then demonstrate the nonseparability of the bipartite system by adopting a local filtering technique within a positive operator valued measurement.

The main features of quantum mechanics reside in interference deriving from the superposition of different quantum states. While current quantum optical technology enables two-photon interference both in bulk and integrated systems, simultaneous interference of more than two particles, leading to richer quantum phenomena, is still a challenging task. Here we report the experimental observation of three-photon interference in an integrated three-port directional coupler realized by ultrafast laser writing. By exploiting the capability of this technique to produce three-dimensional structures, we realized and tested in the quantum regime a three-port beam splitter, namely a tritter, which allowed us to observe bosonic coalescence of three photons. These results open new important perspectives in many areas of quantum information, such as fundamental tests of quantum mechanics with increasing number of photons, quantum state engineering, quantum sensing and quantum simulation.

Quantum metrology 1 uses entanglement 2-5 and other quantum effects 6 to improve the sensitivity of demanding measurements [7][8][9] .Probing of delicate systems demands high sensitivity from limited probe energy and motivates the field's key benchmark the standard quantum limit 10 . Here we report the first quantum-enhanced measurement of a delicate material system. We non-destructively probe an atomic spin ensemble by near-resonant Faraday rotation, a measurement that is limited by probeinduced scattering in quantum memory and spinsqueezing applications 6,[11][12][13] . We use narrowband, atom-resonant NOON states to beat the standard quantum limit of sensitivity by more than five standard deviations, both on a per-photon and a per-damage basis. This demonstrates quantum enhancement with fully realistic loss and noise, including variable-loss effects [14][15][16] . The experiment points the way to ultragentle probing of single atoms 17 , single molecules 18 , quantum gases 19 and living cells 20 .Linear interferometry with non-entangled states can reach at best the standard quantum limit (SQL) δφ = 1/ √ N , where φ is the interferometric phase to be measured and N is the number of probe particles. When increasing N is not possible, quantum enhancement offers a practical advantage. A key example is interferometric gravitational-wave detection: in current operating conditions, squeezing improves sensitivity whereas increasing photon flux produces deleterious thermal effects 9 . Here we study an analogous number-limited scenario with very broad potential application: the probing of delicate systems, i.e., material systems which suffer significant damage due to the probing process. Examples are found in atomic 17 , molecular 18 , condensed matter 19 and biological 20 science.By the Kramers-Kronig relations, interferometric phase shifts are necessarily accompanied by absorption, implying deposition of energy in the probed medium. Absorption also degrades any quantum advantage, as described by recent theory 16,21,22 . To further complicate matters, in real media the phase shift and absorption may depend on the same unknown quantity. To show advantage in a fully-realistic scenario, we probe a precisely understood material system using a quantum state permitting rigorous sensitivity and damage analysis.Our delicate system is a 85 Rb atomic spin ensemble, similar to ensembles used for optical quantum memories 12 and quantum-enhanced atom interferometry 23 . Non-destructive dispersive measurements on these systems, used for storage and readout of quantum information or to produce spin squeezing, are fundamentally limited by scattering-induced depolarization 6,[11][12][13] . Both the measurement and damage properties of the ensemble can be calculated from first principles, making this an ideal model system.We probe the ensemble with a polarization NOON state, a two-mode entangled state of the form, in which N particles are either all L-or all R-circularly polarized. The use of entangled photons in the single-photon regim...

Quantum metrology is the state-of-the-art measurement technology. It uses quantum resources to enhance the sensitivity of phase estimation over that achievable by classical physics. While single parameter estimation theory has been widely investigated, much less is known about the simultaneous estimation of multiple phases, which finds key applications in imaging and sensing. In this manuscript we provide conditions of useful particle (qudit) entanglement for multiphase estimation and adapt them to multiarm Mach-Zehnder interferometry. We theoretically discuss benchmark multimode Fock states containing useful qudit entanglement and overcoming the sensitivity of separable qudit states in three and four arm Mach-Zehnder-like interferometers - currently within the reach of integrated photonics technology.

Quantum interferometry uses quantum resources to improve phase estimation with respect to classical methods. Here we propose and theoretically investigate a new quantum interferometric scheme based on three-dimensional waveguide devices. These can be implemented by femtosecond laser waveguide writing, recently adopted for quantum applications. In particular, multiarm interferometers include “tritter” and “quarter” as basic elements, corresponding to the generalization of a beam splitter to a 3- and 4-port splitter, respectively. By injecting Fock states in the input ports of such interferometers, fringe patterns characterized by nonclassical visibilities are expected. This enables outperforming the quantum Fisher information obtained with classical fields in phase estimation. We also discuss the possibility of achieving the simultaneous estimation of more than one optical phase. This approach is expected to open new perspectives to quantum enhanced sensing and metrology performed in integrated photonics.