Over the past several decades, quantum information science has emerged to seek answers to the question: can we gain some advantage by storing, transmitting and processing information encoded in systems that exhibit unique quantum properties? Today it is understood that the answer is yes, and many research groups around the world are working towards the highly ambitious technological goal of building a quantum computer, which would dramatically improve computational power for particular tasks. A number of physical systems, spanning much of modern physics, are being developed for quantum computation. However, it remains unclear which technology, if any, will ultimately prove successful. Here we describe the latest developments for each of the leading approaches and explain the major challenges for the future.

The first quantum technology, which harnesses uniquely quantum mechanical effects for its core operation, has arrived in the form of commercially available quantum key distribution systems that achieve enhanced security by encoding information in photons such that information gained by an eavesdropper can be detected. Anticipated future quantum technologies include large-scale secure networks, enhanced measurement and lithography, and quantum information processors, promising exponentially greater computation power for particular tasks. Photonics is destined for a central role in such technologies owing to the need for high-speed transmission and the outstanding low-noise properties of photons. These technologies may use single photons or quantum states of bright laser beams, or both, and will undoubtably apply and drive state-of-the-art developments in photonics

In 2001 all-optical quantum computing became feasible with the discovery that scalable quantum computing is possible using only single photon sources, linear optical elements, and single photon detectors. Although it was in principle scalable, the massive resource overhead made the scheme practically daunting. However, several simplifications were followed by proof-of-principle demonstrations, and recent approaches based on cluster states or error encoding have dramatically reduced this worrying resource overhead, making an all-optical architecture a serious contender for the ultimate goal of a large-scale quantum computer. Key challenges will be the realization of high-efficiency sources of indistinguishable single photons, low-loss, scalable optical circuits, high efficiency single photon detectors, and low-loss interfacing of these components.Over the last few decades quantum information science has emerged to consider what additional power and functionality can be realised in the encoding, transmission and processing of information by specifically harnessing quantum mechanical effects [1]. Anticipated technologies include: quantum key distribution [2], which offers perfectly secure communication; quantum metrology [3], which allows more precise measurements than could ever be achieved without quantum mechanics; and quantum litography [4], which could enable fabrication of devices with features much smaller than the wavelength of light. Perhaps the most startling and powerful future quantum technology is a quantum computer, which promises exponentially faster computation for particular tasks [1,5].The quest to develop a quantum computer will require formidable technical mastery of the fabrication of devices at the nano and possibly atomic scale, and precision control of their quantum mechanical states. The task is also daunting owing to the inherent fragility of quantum states and the fact that quantum entanglement, and its role in a quantum computer, is not yet fully understood. As we engineer devices that exploit quantum mechanical effects, we will gain an unprecedented control over the fundamental workings of nature as well as a deeper understanding of them.The requirements for realizing a quantum computer are confounding: scalable physical qubits-two state quantum systems-that can be well isolated from the environment, but also initialised, measured, and controllably interacted to implement a universal set of quantum logic gates [6]. However, a number of physical implementations are being pursued, including nuclear magnetic resonance, ion, atom, cavity quantum electrodynamics, solid state, and superconducting systems [7]. Over the last few years single particles of light-photons-have emerged as one of several leading approaches [7]. Single Photons as Qubits Single photons are largely free of the noise-or decoherence-that plagues other systems; can be easily manipulated to realize one-qubit logic gates; and enable encoding in any of several degrees of freedom- An arbitrary state can be plotted on the Bloch...

Quantum technologies based on photons are anticipated in the areas of information processing, communication, metrology, and lithography. While there have been impressive proof-of-principle demonstrations in all of these areas, future technologies will likely require an integrated optics architecture for improved performance, miniaturization and scalability. We demonstrated highfidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference with a visibility of 94.8±0.5%; a controlled-NOT gate with logical basis fidelity of 94.3 ± 0.2%; and a path entangled state of two photons with fidelity > 92%.Quantum information science [1] has shown that harnessing quantum mechanical effects can dramatically improve performance for certain tasks in communication, computation and measurement. However, realizing such quantum technologies is an immense challenge, owing to the difficulty in controlling quantum systems and their inherent fragility. Of the various physical systems being pursued, single particles of light-photons-are often the logical choice, and have been widely used in quantum communication [2], quantum metrology [3,4,5], and quantum lithography [6] settings. Low noise (or decoherence) also makes photons attractive quantum bits (or qubits), and they have emerged as a leading approach to quantum information processing [7].In addition to single photon sources [8] and detectors [9], photonic quantum technologies will rely on sophisticated optical circuits involving high-visibility classical and quantum interference. Already a number of photonic quantum circuits have been realized for quantum metrology [3,4,10,11,12,13], lithography [6], quantum logic gates [14,15,16,17,18,19,20], and other entangling circuits [21,22,23,24]. However, these demonstrations have relied on large-scale (bulk) optical elements bolted to large optical tables, thereby making them inherently unscalable and confining them to the research laboratory. In addition, many have required the design of sophisticated interferometers to achieve the sub-wavelength stability required for reliable operation.We demonstrated the fundamental building blocks of photonic quantum circuits using silica waveguides on a silicon chip: high visibility (98.5±0.4%) classical interference; high visibility (94.8±0.5%) two photon quantum interference; high fidelity controlled-NOT (CNOT) entangling logic gates (logical basis fidelity F = 94.3 ± 0.2%); and on-chip quantum coherence confirmed by high fidelity (> 92%) generation of a two-photon path entangled state. The monolithic nature of these devices means that the correct phase can be stably realized in what would otherwise be an unstable interferometer, greatly simplifying the task of implementing sophisticated photonic quantum circuits. We fabricated 100's of devices on a single wafer and find that performance across the devices is robust, repeatable and well understood.A typical photonic quantum circuit takes several optical paths or "modes" (some...

The promise of tremendous computational power, coupled with the development of robust error-correcting schemes, has fuelled extensive efforts to build a quantum computer. The requirements for realizing such a device are confounding: scalable quantum bits (two-level quantum systems, or qubits) that can be well isolated from the environment, but also initialized, measured and made to undergo controllable interactions to implement a universal set of quantum logic gates. The usual set consists of single qubit rotations and a controlled-NOT (CNOT) gate, which flips the state of a target qubit conditional on the control qubit being in the state 1. Here we report an unambiguous experimental demonstration and comprehensive characterization of quantum CNOT operation in an optical system. We produce all four entangled Bell states as a function of only the input qubits' logical values, for a single operating condition of the gate. The gate is probabilistic (the qubits are destroyed upon failure), but with the addition of linear optical quantum non-demolition measurements, it is equivalent to the CNOT gate required for scalable all-optical quantum computation.

Precision measurements are important across all fields of science. In particular, optical phase measurements can be used to measure distance, position, displacement, acceleration, and optical path length. Quantum entanglement enables higher precision than would otherwise be possible. We demonstrated an optical phase measurement with an entangled four-photon interference visibility greater than the threshold to beat the standard quantum limit-the limit attainable without entanglement. These results open the way for new high-precision measurement applications.

We demonstrate complete characterization of a two-qubit entangling process -a linear optics controlled-not gate operating with coincident detection -by quantum process tomography. We use a maximum-likelihood estimation to convert the experimental data into a physical process matrix. The process matrix allows accurate prediction of the operation of the gate for arbitrary input states and calculation of gate performance measures such as the average gate fidelity, average purity and entangling capability of our gate, which are 0.90, 0.83 and 0.73 respectively.PACS numbers: 03.67. Lx, 03.65.Wj, 03.67.Mn, Quantum information science offers the potential for major advances such as quantum computing [1] and quantum communication [2], as well as many other quantum technologies [3]. Two-qubit entangling gates, such as the controlled-not (cnot), are fundamental elements in the archetypal quantum computer [1]. A promising proposal for achieving scalable quantum computing is that of Knill, Laflamme and Milburn (KLM), in which linear optics and a measurement-induced Kerr-like nonlinearity can be used to construct cnot gates [4]. Gates such as these can also be used to prepare the required entangled resource for optical cluster state quantum computation [5]. The nonlinearity upon which the KLM and related [6,7] cnot schemes are built can be used for other important quantum information tasks, such as quantum nondemolition measurements [8,9] and preparation of novel quantum states (for example, [10]). An essential step in realizing such advances is the complete characterization of quantum processes.A complete characterization in a particular input/output state space requires determination of the mapping from one to the other. In discrete-variable quantum information, this map can be represented as a state transfer function, expressed in terms of a process matrix χ. Experimentally, χ is obtained by performing quantum process tomography (QPT) [11,12]. QPT has been performed in a limited number of systems. A one-qubit teleportation circuit [13], and a controlled-NOT process acting on a highly mixed two-qubit state [14] have been investigated in liquid-state NMR. In optical systems, where pure qubit states are readily prepared, one-qubit processes have been investigated by both ancilla-assisted [15,16] and standard [17] QPT. Two-qubit optical QPT has been performed on a beamsplitter acting as a Bellstate filter [18].We fully characterize a two-qubit entangling gatea cnot gate acting on pure input states -by QPT, maximum-likelihood reconstruction, and analysis of the resulting process matrix. The maximum likelihood technique overcomes the problem that the naïve matrix inversion procedure in QPT, when performed on real (i.e., inherently noisy) experimental data, typically leads to an unphysical process matrix. In a previous maximumlikelihood QPT experiment [18], a reduced set of fitting constraints was used. Here we present a fully-constrained fitting technique that can be applied to any physical process. After obtaining our physical...

Integrated quantum optics promises to enhance the scale and functionality of quantum technologies, and has become a leading platform for the development of complex and stable quantum photonic circuits. Here, we report path-entangled photon-pair generation from two distinct waveguide sources, the manipulation of these pairs, and their resulting high-visibility quantum interference, all on a single photonic chip. Degenerate and non-degenerate photon pairs were created via the spontaneous four-wave mixing process in the silicon-on-insulator waveguides of the device. We manipulated these pairs to exhibit on-chip quantum interference with visibility as high as 100.0 ± 0.4%. Additionally, the device can serve as a two-spatial-mode source of photon-pairs: we measured Hong-Ou-Mandel interference, off-chip, with visibility up to 95 ± 4%. Our results herald the next generation of monolithic quantum photonic circuits with integrated sources, and the new levels of complexity they will offer.