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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...

Quantum memories : a review based on the European integrated project "Qubit Applications (QAP)"

We propose a quantum non-demolition method -giant Faraday rotation -to detect a single electron spin in a quantum dot inside a microcavity where negatively-charged exciton strongly couples to the cavity mode. Left-and right-circularly polarized light reflected from the cavity feels different phase shifts due to cavity quantum electrodynamics and the optical spin selection rule. This yields giant and tunable Faraday rotation which can be easily detected experimentally. Based on this spin-detection technique, a scalable scheme to create an arbitrary amount of entanglement between two or more remote spins via a single photon is proposed.PACS numbers: 78.67. Hc, 03.67.Mn, 42.50.Pq, 78.20.Ek Photons and spins hold great potential in quantum information science, especially for quantum communications, quantum information processing and quantum networks [1]. Photons are ideal candidates to transmit quantum information with little decoherence, whereas spins can be used to store and process quantum information due to their long coherence times. Therefore investigations of spin manipulation, spin detection, remote spin entanglement mediated by photons, and quantum state transfer between photons and spins are of great importance [2,3,4,5,6,7].Spin manipulation is well developed using pulsed magnetic resonance techniques, whereas single spin detection remains a challenging task. Electrical detection of single spin has been reported in a gate-defined quantum box [8,9] and in a silicon field-effect transistor [10]. The optically detected magnetic resonance technique (ODMR) proves to be an effective way to detect a single spin either in a single molecule [11,12] or a single N-V center in diamond [13]. However, the ODMR technique is based on the spin dependent fluorescence such that the spin is destroyed after detection. Recently, a non-demolition method to detect a single electron spin has been experimentally reported by Berezovsky et al [14] and Atatüre et al [15]. Both groups detect the tiny Faraday rotation angle induced by a single electron spin in a quantum dot (QD), so the measured signals (even enhanced by a cavity) are rather weak and noisy.It is widely accepted that entanglement is a useful resource in quantum information science. Recently remote entanglement between photons, trapped ions and atom ensembles have been demonstrated [16,17,18], however, all current experimental proposals for entangling two atoms are restricted to one entanglement bit rather than an arbitrary amount of entanglement [19,20]. To our knowledge, entanglement between remote single spins has not yet been achieved due to the lack of realizable proposals [21,22,23].In this Letter, we propose a quantum non-demolition method -giant Faraday rotation -to detect a single electron spin in a single QD inside a microcavity. The different phase shifts for the left and right circularly polarized light reflected from the QD-cavity system yields giant Faraday rotation which can be easily detected experimentally. This giant Faraday rotation induced by a sin...

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.

We report on the experimental implementation of a Bennett-Brassard 1984 (BB84) protocol type quantum key distribution over a 144 km free-space link using weak coherent laser pulses. Optimization of the link transmission was achieved with bidirectional active telescope tracking, and the security was ensured by employing decoy-state analysis. This enabled us to distribute a secure key at a rate of 12.8 bit/s at an attenuation of about 35 dB. Utilizing a simple transmitter setup and an optical ground station capable of tracking a spacecraft in low earth orbit, this outdoor experiment demonstrates the feasibility of global key distribution via satellites.

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