Universal multiport interferometers, which can be programmed to implement any linear transformation between multiple channels, are emerging as a powerful tool for both classical and quantum photonics. These interferometers are typically composed of a regular mesh of beam splitters and phase shifters, allowing for straightforward fabrication using integrated photonic architectures and ready scalability. The current, standard design for universal multiport interferometers is based on work by Reck et al (Phys. Rev. Lett. 73, 58, 1994). We demonstrate a new design for universal multiport interferometers based on an alternative arrangement of beam splitters and phase shifters, which outperforms that by Reck et al. Our design occupies half the physical footprint of the Reck design and is significantly more robust to optical losses.

Although universal quantum computers ideally solve problems such as factoring integers exponentially more efficiently than classical machines, the formidable challenges in building such devices motivate the demonstration of simpler, problem-specific algorithms that still promise a quantum speedup. We constructed a quantum boson-sampling machine (QBSM) to sample the output distribution resulting from the nonclassical interference of photons in an integrated photonic circuit, a problem thought to be exponentially hard to solve classically. Unlike universal quantum computation, boson sampling merely requires indistinguishable photons, linear state evolution, and detectors. We benchmarked our QBSM with three and four photons and analyzed sources of sampling inaccuracy. Scaling up to larger devices could offer the first definitive quantum-enhanced computation.

We present an experimental demonstration of heralded single photons prepared in pure quantum states from a parametric down-conversion source. It is shown that, through controlling the modal structure of the photon pair emission, one can generate pairs in factorable states and thence eliminate the need for spectral filters in multiple-source interference schemes. Indistinguishable heralded photons were generated in two independent spectrally engineered sources and Hong-Ou-Mandel interference observed between them without spectral filters. The measured visibility of 94.4% sets a minimum bound on the mean photon purity.

We study the simultaneous estimation of multiple phases as a discretised model for the imaging of a phase object. We identify quantum probe states that provide an enhancement compared to the best quantum scheme for the estimation of each individual phase separately, as well as improvements over classical strategies. Our strategy provides an advantage in the variance of the estimation over individual quantum estimation schemes that scales as O(d), where d is the number of phases. Finally, we study the attainability of this limit using realistic probes and photon-number-resolving detectors. This is a problem in which an intrinsic advantage is derived from the estimation of multiple parameters simultaneously.Introduction-Recent developments in quantum metrology point to a new frontier of parameter estimation in which exploiting quantum states enables higher precision than can be achieved using only classical resources. Much of the work in this field to date has been directed towards the estimation of a single Hamiltonian parameter. This has been explored both theoretically [1][2][3][4][5][6][7][8][9][10][11][12][13] and experimentally, with the estimation of optical phase shifts by means of interferometry providing the dominant paradigm, in the setting of photonic systems as the leading platform [14][15][16][17][18].One of the most important metrology problems to the wider research community is that of microscopy and imaging. Producing a quantum advantage in imaging would be of significant benefit in fields such as biology, particularly for the imaging of samples that are sensitive to the total illumination. Various approaches to quantum imaging have been proposed, typically exploring methods for increasing the diffraction limited resolution of optical imaging systems [19][20][21][22][23][24][25]. A recent classical investigation of quantum enhanced imaging made use of point estimation theory, quantifying differences between images by means of a single parameter [26]. However, imaging is inherently a multi-parameter estimation problem, and deeper insights can be gained by studying it as such.In this Letter, we consider a discretised model for phase imaging based on this approach. Phase imaging is a cornerstone of optical microscopy, typically realised using the related techniques of phase contrast and differential interference contrast imaging [27], that allows differences in refractive index to be detected in otherwise transparent media. So far, the potential for quantum enhancements to these techniques has yet to be explored. Our approach maps phase imaging onto the problem of multiple simultaneous phase estimation.Our results provide a strategy for the estimation of multiple phases using correlated quantum states, in which the multi-parameter nature of the problem leads to an intrinsic benefit when exploiting quantum resources. A surprising outcome of our analysis is that our quantum strategy provides an O(d) advantage, where d is the number of phases, over the optimal quantum individual estimation scheme of usi...

By using a systematic optimization approach we determine quantum states of light with definite photon number leading to the best possible precision in optical two mode interferometry. Our treatment takes into account the experimentally relevant situation of photon losses. Our results thus reveal the benchmark for precision in optical interferometry. Although this boundary is generally worse than the Heisenberg limit, we show that the obtained precision beats the standard quantum limit thus leading to a significant improvement compared to classical interferometers. We furthermore discuss alternative states and strategies to the optimized states which are easier to generate at the cost of only slightly lower precision.