Marco Barbieri

Boson Sampling on a Photonic Chip

Computing Power of Quantum Mechanics There is much interest in developing quantum computers in order to perform certain tasks much faster than, or that are intractable for, a classical computer. A general quantum computer, however, requires the fabrication and operation a number of quantum logic devices (see the Perspective by Franson). Broome et al. (p. 794, published online 20 December) and Spring et al. (p. 798, published online 20 December) describe experiments in which single photons and quantum interference were used to perform a calculation (the permanent of a matrix) that is very difficult on a classical computer. Similar to random walks, quantum walks on a graph describe the movement of a walker on a set of predetermined paths; instead of flipping a coin to decide which way to go at each point, a quantum walker can take several paths at once. Childs et al. (p. 791) propose an architecture for a quantum computer, based on quantum walks of multiple interacting walkers. The system is capable of performing any quantum operation using a subset of its nodes, with the size of the subset scaling favorably with the complexity of the operation. Optical circuits are used to demonstrate a quantum-enhanced calculation. [Also see Perspective by Franson] 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.

Multiphoton quantum interference in a multiport integrated photonic device

Increasing the complexity of quantum photonic devices is essential for many optical information processing applications to reach a regime beyond what can be classically simulated, and integrated photonics has emerged as a leading platform for achieving this. Here we demonstrate three-photon quantum operation of an integrated device containing three coupled interferometers, eight spatial modes and many classical and nonclassical interferences. This represents a critical advance over previous complexities and the first on-chip nonclassical interference with more than two photonic inputs. We introduce a new scheme to verify quantum behaviour, using classically characterised device elements and hierarchies of photon correlation functions. We accurately predict the devices quantum behaviour and show operation inconsistent with both classical and bi-separable quantum models. Such methods for verifying multiphoton quantum behaviour are vital for achieving increased circuit complexity. Our experiment paves the way for the next generation of integrated photonic quantum simulation and computing devices.

On-chip low loss heralded source of pure single photons.

A key obstacle to the experimental realization of many photonic quantum-enhanced technologies is the lack of low-loss sources of single photons in pure quantum states. We demonstrate a promising solution: generation of heralded single photons in a silica photonic chip by spontaneous four-wave mixing. A heralding efficiency of 40%, corresponding to a preparation efficiency of 80% accounting for detector performance, is achieved due to efficient coupling of the low-loss source to optical fibers. A single photon purity of 0.86 is measured from the source number statistics without narrow spectral filtering, and confirmed by direct measurement of the joint spectral intensity. We calculate that similar high-heralded-purity output can be obtained from visible to telecom spectral regions using this approach. On-chip silica sources can have immediate application in a wide range of single-photon quantum optics applications which employ silica photonics.

Implementation of a Nondeterministic Optical Noiseless Amplifier

Quantum Physics imposes that any phase independent amplification introduces excess noise. Nevertheless this limitation could be ignored with conditioning. We report fully characterization with homodyne tomography of a non-deterministic noiseless amplification of a coherent state.

Quantum teleportation on a photonic chip

Quantum teleportation is a fundamental concept in quantum physics that now finds important applications at the heart of quantum technology, including quantum relays, quantum repeaters and linear optics quantum computing. Photonic implementations have largely focused on achieving long-distance teleportation for decoherence-free quantum communication. Teleportation also plays a vital role in photonic quantum computing for which large linear optical networks will probably require an integrated architecture. Here, we report a fully integrated implementation of quantum teleportation in which all key parts of the circuit - entangled state preparation, Bell-state analysis and tomographic state measurement - are performed on a reconfigurable photonic chip. We also show that a novel element-wise characterization method is critical to the mitigation of component errors, a key technique that will become increasingly important as integrated circuits reach the higher complexities necessary for quantum enhanced operation.

Linear Optical Quantum Computing in a Single Spatial Mode

We present a scheme for linear optical quantum computing using time-bin-encoded qubits in a single spatial mode. We show methods for single-qubit operations and heralded controlled-phase (cphase) gates, providing a sufficient set of operations for universal quantum computing with the Knill-Laflamme-Milburn [Nature (London) 409, 46 (2001)] scheme. Our protocol is suited to currently available photonic devices and ideally allows arbitrary numbers of qubits to be encoded in the same spatial mode, demonstrating the potential for time-frequency modes to dramatically increase the quantum information capacity of fixed spatial resources. As a test of our scheme, we demonstrate the first entirely single spatial mode implementation of a two-qubit quantum gate and show its operation with an average fidelity of 0.84±0.07.

Interfacing GHz-bandwidth heralded single photons with a warm vapour Raman memory

Broadband quantum memories, used as temporal multiplexers, are a key component in photonic quantum information processing, as they make repeat-until-success strategies scalable. We demonstrate a prototype system, operating on-demand, by interfacing a warm vapour, high time-bandwidth-product Raman memory with a travelling wave spontaneous parametric down-conversion source. We store single photons and observe a clear influence of the input photon statistics on the retrieved light, which we find currently to be limited by noise. We develop a theoretical model that identifies four-wave mixing as the sole important noise source and point towards practical solutions for noise-free operation.

Photonic Maxwell's Demon

We report an experimental realization of Maxwells demon in a photonic setup. We show that a measurement at the few-photons level followed by a feed-forward operation allows the extraction of work from intense thermal light into an electric circuit. The interpretation of the experiment stimulates the derivation of an equality relating work extraction to information acquired by measurement. We derive a bound using this relation and show that it is in agreement with the experimental results. Our work puts forward photonic systems as a platform for experiments related to information in thermodynamics.

Direct observation of sub-binomial light

Nonclassical states of light are necessary resources for quantum technologies such as cryptography, computation and the definition of metrological standards. Distinguishing nonclassical states from those that can be described without invoking quantum mechanics is both a necessary practical task and of fundamental interest. In general, signatures of nonclassicality arise from apparently pathological behaviour of the quasi-probability distributions used to describe the states. While tomographic reconstruction of the state is therefore sufficient to show nonclassicality, it is often more convenient to probe photon-counting statistics of on-off detectors directly. This is formalised by the Mandel Q-parameter QM = 〈Δn〉 / 〈n〉 - 1, which is negative for sub-Poissonian statistics arising from nonclassical states. However, the Mandel Q-parameter can fail when using a multiplexed array of on-off detectors to measure states with several photons; splitting in this manner can corrupt its original statistics. A new parameter, QB = N 〈(Δ c) 2 〉 / 〈 c 〉 (N - 〈 c 〉) can be used to witness nonclassicality directly from such click statistics. For QB <; 0, such statistics are thus deemed sub-binomial and therefore arise from a nonclassical state.

Efficient optical pumping and high optical depth in a hollow-core photonic-crystal fibre for a broadband quantum memory

The generation of large multiphoton quantum states—for applications in computing, metrology and simulation—requires a network of high-efficiency quantum memories capable of storing broadband pulses. Integrating these memories into a fibre offers a number of advantages towards realizing this goal: strong light–matter coupling at low powers, simplified alignment and compatibility with existing photonic architectures. Here, we introduce a large-core kagome-structured hollow-core fibre as a suitable platform for an integrated fibre-based quantum memory with a warm atomic vapour. We demonstrate, for the first time, efficient optical pumping in such a system, where 90xa0±xa01% of atoms are prepared in the ground state. We measure high optical depths (3xa0×xa0104) and narrow homogeneous linewidths (6xa0±xa02xa0MHz) that do not exhibit significant transit-time broadening, showing that we can prepare a Λ-level system in a pure state. Our results establish that kagome fibres are suitable for implementing a broadband, room-temperature quantum memory, as well as a range of nonlinear optical effects.