[en] A key step in the use of diamond nitrogen vacancy (NV) centers for quantum computational tasks is a single shot quantum non-demolition measurement of the electronic spin state. Here, we propose a high fidelity measurement of the ground state spin of a single NV center, using the effects of cavity quantum electrodynamics. The scheme we propose is based in the one-dimensional atom or Purcell regime, removing the need for high Q cavities that are challenging to fabricate. The ground state spin of the NV center has a splitting of ∼6-10 μeV, which can be resolved in a high-resolution absorption measurement. By incorporating the center in a low-Q and low volume cavity we show that it is possible to perform single shot readout of the ground state spin using a weak laser with an error rate of ∼7x10^-3, when realistic experimental parameters are considered. Since very low levels of light are used to probe the state of the spin we limit the number of florescence cycles, which dramatically reduces the measurement induced decoherence approximating a non-demolition measurement of ground state spin.

[en] The periodic nature of photonic crystals (PCs) (Yablonovitch 1987 Phys. Rev. Lett. 58 2059–62; John 1987 Phys. Rev. Lett. 58 2486–9) has been extensively exploited for the past quarter of a century using photonic bandgap (PBG) effects to manipulate photons in engineered electromagnetic structures. Structures such as photonic crystal nanocavities are widely considered to be key in realizing future nanoscale optoelectronic devices. These cavities are capable of creating resonant modes with high-quality factor (Q) and small mode volume, in other words a large Purcell factor (Purcell 1946 Phys. Rev. 69 681), and have been widely researched in the two-dimensional photonic crystal slab (PCS) defect cavity configuration (Painter et al 1999 J. Opt. Soc. Am. B 16 275–85). Here, we demonstrate for the first time how three confinement mechanisms are thought to coincide to give rise to a high-Q resonance for a slab containing a modified L3 defect where the slab thickness is such that the guiding in the slab is no longer single moded (Tandaechanurat et al 2008 Opt. Express 16 448–55). This is in contrast to the conventional design approach, where the PCS thickness is chosen to be of the order of half a lattice constant to ensure that a PBG exists to confine cavity modes strongly within the slab (Painter et al 1999 J. Opt. Soc. Am. B 16 275–85; Johnson et al 1999 Phys. Rev. B 60 5751–8; Khankhoje et al 2010 Nanotechnology 21 065202). These newly identified high-Q modes can be important in terms of the fabrication of slabs and other devices such as vertical pillars since they allow high-Q factors in thicker and more fabrication tolerant geometries. (paper)

[en] We examine the possibility of secure key exchange between a ground station and a low earth orbit satellite using the technique of quantum cryptography. The study suggests there are no technical obstacles to building a system that could exchange keys at kilobaud rates between a metre diameter telescope on the ground and a satellite with a 10 cm diameter lightweight telescope

[en] We introduce and experimentally demonstrate a method for realising a quantum channel using the measurement-based model. Using a photonic setup and modifying the basis of single-qubit measurements on a four-qubit entangled cluster state, representative channels are realised for the case of a single qubit in the form of amplitude and phase damping channels. The experimental results match the theoretical model well, demonstrating the successful performance of the channels. We also show how other types of quantum channels can be realised using our approach. This work highlights the potential of the measurement-based model for realising quantum channels which may serve as building blocks for simulations of realistic open quantum systems. (paper)

[en] When a one-photon state is mixed with a (separate) weak coherent state at a beamsplitter the probability for seeing one photon in each beamsplitter output approaches zero due to destructive interference. We demonstrate this non-classical interference effect using pulse-gated single photons and weak mode-locked laser pulses

[en] We report on the fusion of photons from two independent photonic crystal fiber sources into polarization entangled states using a fiber-based polarizing beamsplitter. We achieve fidelities of up to F = 0.74 ± 0.01 with respect to the maximally entangled Bell state |φ⁺> using a low pump power of 5.3 mW with a success rate of 3.2 fourfold detections per second. By increasing the pump power we find that success rates of up to 111.6 fourfold detections per second can be achieved, with entanglement still present in the fused state. We characterize the fusion operation by providing a full quantum process reconstruction. Here a model is developed to describe the generation of entanglement, including the main causes of imperfection, and we show that this model fits well with the experimental results. Our work shows how non-ideal settings limit the success of the fusion, providing useful information about the practical requirements for an operation that may be used to build large entangled states in bulk and on-chip quantum photonic waveguides. (paper)

[en] We describe a technique for fabricating micro- and nanostructures incorporating fluorescent defects in diamond with a positional accuracy better than hundreds of nanometers. Using confocal fluorescence microscopy and focused ion beam etching, we initially locate a suitable defect with respect to registration marks on the diamond surface then etch a structure using these coordinates. We demonstrate the technique by etching an 8 μm diameter hemisphere positioned with single negatively charged nitrogen-vacancy defect lies at its origin. Direct comparison of the fluorescence photon count rate before and after fabrication shows an eightfold increase due to the presence of the hemisphere.

[en] We perform high-resolution reflection spectroscopy of a quantum dot resonantly coupled to a pillar microcavity. We show the change in reflectivity as the quantum dot is tuned through the cavity resonance and measure the quantum-dot-induced phase shift using an ultrastable interferometer. The macroscopic phase shift we measure could be extended to the study of charged quantum dot pillar microcavity systems, where it could be exploited to realize a high-efficiency spin photon interface for hybrid quantum information schemes.

[en] We report the characterization of a universal set of logic gates for one-way quantum computing using a four-photon ‘star’ cluster state generated by fusing photons from two independent photonic crystal fibre sources. We obtain a fidelity for the cluster state of 0.66 ± 0.01 with respect to the ideal case. We perform quantum process tomography to completely characterize a controlled-NOT, Hadamard and T gate all on the same compact entangled resource. Together, these operations make up a universal set of gates such that arbitrary quantum logic can be efficiently constructed from combinations of them. We find process fidelities with respect to the ideal cases of 0.64 ± 0.01 for the CNOT, 0.67 ± 0.03 for the Hadamard and 0.76 ± 0.04 for the T gate. The characterization of these gates enables the simulation of larger protocols and algorithms. As a basic example, we simulate a Swap gate consisting of three concatenated CNOT gates. Our work provides some pragmatic insights into the prospects for building up to a fully scalable and fault-tolerant one-way quantum computer with photons in realistic conditions. (paper)

[en] We demonstrate a quantum interference experiment between two photons coming from non-degenerate pairs created by four-wave mixing in two separated micro-structured fibres. When the two heralded photons are made indistinguishable a 95% visibility is demonstrated