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Quantum repeaters are foundational to establishing the long distance quantum communication channels necessary to create a global communication network [1]. A quantum memory capable of storing optical quantum states is an integral component of the quantum repeater.

We introduce hypercube states, a new class of quantum states. These feature with sub-Planck scale resolution allowing increased sensitivity to small phase space perturbations. We investigate their sensitivity in a variety of relistic circumstances.).

We implement a general imaging method by measuring the complex degree of coherence using linear optics and photon number resolving detectors. In the absence of collective or entanglement assisted measurements, our method is optimal over a large range of practically relevant values of the complex degree of coherence. We measure the size and position of a small distant source of pseudo-thermal light, and show that our method outperforms the traditional imaging method by an order of magnitude in precision. Finally, we show that a lack of photon number resolution in the detectors has only a modest detrimental effect on measurement precision and simulate imaging using the new and traditional methods with an array of detectors; showing that the new method improves both image clarity and contrast., Comment: The main document contains 5 pages and 5 figures. The supplementary material contains 4 pages and 3 figures

We introduce quantum hypercube states, a class of continuous-variable quantum states that are generated as orthographic projections of hypercubes onto the quadrature phase-space of a bosonic mode. In addition to their interesting geometry, hypercube states display phase-space features much smaller than Planck's constant, and a large volume of Wigner-negativity. We theoretically show that these features make hypercube states sensitive to displacements at extremely small scales in a way that is surprisingly robust to initial thermal occupation and to small separation of the superposed state-components. In a high-temperature proof-of-principle optomechanics experiment we observe, and match to theory, the signature outer-edge vertex structure of hypercube states., Main consists of 5 pages and 5 figures. Supplementary material consists of 5 pages and 6 figures

Cavity-enhanced single photon sources exhibit mode-locked biphoton states with comb-like correlation functions. Our ultrabright source additionally emits either single photon pairs or two-photon NOON states, dividing the output into an even and an odd comb respectively. With even-comb photons we demonstrate revivals of the typical non-classical Hong-Ou-Mandel interference up to the 84th dip, corresponding to a path length difference exceeding 100 m. With odd-comb photons we observe single photon interference fringes modulated over twice the displacement range of the Hong-Ou-Mandel interference., 10 pages, 7 figures

Exploring the quantum behaviour of macroscopic objects provides an intriguing avenue to study the foundations of physics and to develop a suite of quantum-enhanced technologies. One prominent path of study is provided by quantum optomechanics which utilizes the tools of quantum optics to control the motion of macroscopic mechanical resonators. Despite excellent recent progress, the preparation of mechanical quantum superposition states remains outstanding due to weak coupling and thermal decoherence. Here we present a novel optomechanical scheme that significantly relaxes these requirements allowing the preparation of quantum superposition states of motion of a mechanical resonator by exploiting the nonlinearity of multi-photon quantum measurements. Our method is capable of generating non-classical mechanical states without the need for strong single photon coupling, is resilient against optical loss, and offers more favourable scaling against initial mechanical thermal occupation than existing schemes. Moreover, our approach allows the generation of larger superposition states by projecting the optical field onto NOON states. We experimentally demonstrate this multi-photon-counting technique on a mechanical thermal state in the classical limit and observe interference fringes in the mechanical position distribution that show phase superresolution. This opens a feasible route to explore and exploit quantum phenomena at a macroscopic scale., Comment: 16 pages, 4 figures. v1: submitted for review on 28 Jan 2016. v2: significantly revised manuscript. v3: some further revisions and some extra results included. v3: new results added, extra author added, close to published version, supplementary material available with published version

The quest for nonclassicality is the search for the border between classical and quantum physics. Our work aims to show that nonclassical correlations can be observed even in the absence of entanglement and quantum discord.

One of the cornerstones of quantum mechanics is that matter can possess both particle- and wave-like properties. Since the inception of quantum mechanics, such wavelike behaviour has been observed in ever more massive systems — ranging from electrons, neutrons, ultracold atoms, and even large molecules comprising many hundreds of atoms. An exciting route to further extend the exploration of quantum phenomena to a macroscopic regime is through the study of quantum optomechanics, where optical fields are used to manipulate the motion of mechanical resonators using radiation pressure. We introduce a method for preparing non-classical states of motion for an optomechanical system which, using the non-linearity of single-photon measurements, is independent of the initial thermal occupation of the resonator and does not require strong single-photon coupling. We demonstrate this method in a thermal regime and observe interference fringes in the position quadrature distribution of a SiN membrane [11.

Hybrid quantum technologies seek to combine the advantages of two individual quantum architectures by transferring the information between the two systems. We want to benefit from the high mobility and ease of transmission of photons for quantum communication and exploit the excellent readout and storage capabilities of atomic qubits as a quantum memory, which is essential to build up quantum repeater networks for quantum data processing. Efficient interaction of photons with atoms requires a match of the photons spectral properties to those of the resonances of the atomic species. The atomic transition usually has a much narrower bandwidth than single photons generated by spontaneous parametric down-conversion (SPDC), the current gold standard of producing high-purity heralded single photons at flexible wavelengths. To develop hybrid quantum technologies therefore requires significantly reducing the single photon emission spectra to fulfill these requirements. The way we try to achieve this is by using an optical cavity to enhance the probability of creating the photons in the spectral and spatial resonator mode.

We report 100% duty cycle generation of single photon pairs with a bandwidth of 666 ± 16 kHz at the Rubidium D1 line using cavity-enhanced spontaneous parametric downconversion.

Quantum memories are essential for quantum data processing. Most suggested approaches rely on storage in atomic media, with bandwidths much narrower than those of the single photons used in quantum communication. We present our work on a cavity based system tailored to generate the narrowest photons from parametric down-conversion to date, suitable for storage in gradient echo memories.

Quantum steering allows the verification of shared entanglement even with an untrusted measurement device. We show the first photonic

Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted. A conclusive demonstration of steering through the violation of a steering inequality is of considerable fundamental interest and opens up applications in quantum communication. To date all experimental tests with single photon states have relied on post-selection, allowing untrusted devices to cheat by hiding unfavourable events in losses. Here we close this "detection loophole" by combining a highly efficient source of entangled photon pairs with superconducting transition edge sensors. We achieve an unprecedented ~62% conditional detection efficiency of entangled photons and violate a steering inequality with the minimal number of measurement settings by 48 standard deviations. Our results provide a clear path to practical applications of steering and to a photonic loophole-free Bell test., Preprint of 7 pages, 3 figures; the definitive version is published in Nature Communications, see below. Also, see related experimental work by A. J. Bennet et al., arXiv:1111.0739 and B. Wittmann et al., arXiv:1111.0760

The metastable ^{2}F_{7/2} and ^{2}D_{3/2} states of Yb^{+} are of interest for applications in metrology and quantum information and also act as dark states in laser cooling. These metastable states are commonly repumped to the ground state via the 638.6 nm ^{2}F_{7/2} -- ^{1}D[5/2]_{5/2} and 935.2 nm ^{2}D_{3/2} -- ^{3}D[3/2]_{1/2} transitions. We have performed optogalvanic spectroscopy of these transitions in Yb^{+} ions generated in a discharge. We measure the pressure broadening coefficient for the 638.6 nm transition to be 70 \pm 10 MHz mbar^{-1}. We place an upper bound of 375 MHz/nucleon on the 638.6 nm isotope splitting and show that our observations are consistent with theory for the hyperfine splitting. Our measurements of the 935.2 nm transition extend those made by Sugiyama et al, showing well-resolved isotope and hyperfine splitting. We obtain high signal to noise, sufficient for laser stabilisation applications., 8 pages, 5 figures

Trapped ions are a leading system for realizing quantum information processing (QIP). Most of the technologies required for implementing large-scale trapped-ion QIP have been demonstrated, with one key exception: a massively parallel ion-photon interconnect. Arrays of microfabricated phase Fresnel lenses (PFL) are a promising interconnect solution that is readily integrated with ion trap arrays for large-scale QIP. Here we show the first imaging of trapped ions with a microfabricated in-vacuum PFL, demonstrating performance suitable for scalable QIP. A single ion fluorescence collection efficiency of 4.2 +/- 1.5% was observed, in agreement with the previously measured optical performance of the PFL. The contrast ratio between the ion signal and the background scatter was 23 +/- 4. The depth of focus for the imaging system was 19.4 +/- 2.4 {\mu}m and the field of view was 140 +/- 20 {\mu}m. Our approach also provides an integrated solution for high-efficiency optical coupling in neutral atom and solid state QIP architectures., Comment: 4 pages, 2 figures

International audience; We show that the primary cause of errors in a broad class of optical quantum-logic gates are due to the higherorder photon terms in parametric downconversion sources. A model describing real-life imperfections in these entangling gates is presented and tested in an experiment where we entangle dependent photons from the same downconversion source using a controlled-z gate, and measure the state tomographically. We find good agreement between the modelled and measured results. Our investigations demonstrate that, although small, these noise terms are amplified by the intrinsic non-determinism of the gates. It is worth considering alternative schemes based on weak nonlinearities to see if they are more resilient to this degradation.

Quantum information carriers with higher dimension than the canonical qubit offer significant advantages. However, manipulating such systems is extremely difficult. We show how measurement induced non-linearities can be employed to dramatically extend the range of possible transforms on biphotonic qutrits; the three level quantum systems formed by the polarisation of two photons in the same spatio-temporal mode. We fully characterise the biphoton-photon entanglement that underpins our technique, thereby realising the first instance of qubit-qutrit entanglement. We discuss an extension of our technique to generate qutrit-qutrit entanglement and to manipulate any bosonic encoding of quantum information., Comment: 4 pages, 4 figures

We demonstrate significantly compacted quantum algorithms and demonstrate a Fock-state filter by going outside the qubit corner of Hilbert space. We obtain a complete error budget for an entangling gate when driven with independent photons.

Shor's powerful quantum algorithm for factoring represents a major challenge in quantum computation. Here, we implement a compiled version in a photonic system. For the first time, we demonstrate the core processes, coherent control, and resultant entangled states required in a full-scale implementation. These are necessary steps on the path towards scalable quantum computing. Our results highlight that the algorithm performance is not the same as that of the underlying quantum circuit and stress the importance of developing techniques for characterizing quantum algorithms.

We demonstrate a Fock-state filter which is capable of preferentially blocking single photons over photon pairs. The large conditional nonlinearities are based on higher-order quantum interference, using linear optics, an ancilla photon, and measurement. We demonstrate that the filter acts coherently by using it to convert unentangled photon pairs to a path-entangled state. We quantify the degree of entanglement by transforming the path information to polarisation information, applying quantum state tomography we measure a tangle of T=(20+/-9)%., Comment: 4 pages, 3 figures

We implement an entangling gate for photonic qubits that relies on a single interference condition, offering significantly improved gate fidelity, and describe its use for investigating optical cluster states and performing fully resolving Bell measurements.

We demonstrate a new architecture for an optical entangling gate that is significantly simpler than previous realisations, using partially-polarising beamsplitters so that only a single optical mode-matching condition is required. We demonstrate operation of a controlled-Z gate in both continuous-wave and pulsed regimes of operation, fully characterising it in each case using quantum process tomography. We also demonstrate a fully-resolving, nondeterministic optical Bell-state analyser based on this controlled-Z gate. This new architecture is ideally suited to guided optics implementations of optical gates., 4 pages, 3 figures. v2: additional author, improved data and figures (low res), some other minor changes. Accepted for publication in PRL

Porous polytetrafluoroethylene (PTFE) films were positively or negatively corona charged at room or elevated temperatures. Their charge storage behavior was investigated by means of isothermal surface potential measurements in direct comparison to nominally nonporous samples of the same polymer. It was found that porosity may lead to significantly enhanced surface-charge stability for both polarities. Direct piezoelectricity was studied on quadruple, double, and single layer samples by means of quasi-static measurements. For the determination of indirect piezoelectricity, frequency-dependent acoustical-transducer experiments were carried out. Both applications-relevant measurements yielded piezoelectric d/sub 33/ coefficients of up to approximately 600 pC/N or 600 pm/V. These values are more than one order of magnitude higher than in conventional piezoelectric polymers such as polyvinylidenefluoride (PVDF) and almost comparable to the highest known values of inorganic piezoelectrics. Consequently, the novel piezoelectric porous-fluoropolymer spacecharge electrets exhibit an outstanding potential for various device applications that are very briefly discussed.

Single-film bipolar electrets of porous polytetrafluoroethylene are generated by means of a two-step corona-charging process at elevated temperatures. Quasi-static direct piezoelectric coefficients of up to 0.15 nC/N have been observed on these films. In addition, multiple-layer stacks of porous and non-porous polytetrafluoroethylene films with monopolar charge were also investigated. While the piezoelectric responses of the stacks were not as high as those of the single films, the multiple-film arrangements may have other advantages such as better electrical shielding or tunable mechanical properties and adjustable acoustical impedance. Our new results are discussed in the context of the emerging field of porous polymer electrets with many potential device applications.

We stabilize an ultraviolet diode laser system at 369.5 nm to the optical absorption signal from Yb+ ions in a hollow-cathode discharge lamp. The error signal for stabilization is obtained by Zeeman spectroscopy of the 3 GHz-wide absorption feature. The frequency stability is independently measured by comparison against the fluorescence signal from a laser-cooled crystal of 174Yb+ ions in a linear Paul trap. We measure a frequency fluctuation of 1.7 MHz RMS over 1000 s, and a frequency drift of 20 MHz over 7 days. Our method is suitable for use in quantum information processing experiments with trapped ion crystals., Comment: 4 pages, 3 figures. Submitted to Applied Physics Letters

Porous PTFE films were corona charged with high voltages at room and elevated temperatures and their direct and inverse piezoelectric coefficients were determined. In both cases, piezoelectric d/sub 33/ coefficients of up to 600 pC/N were found. These values are more than one order of magnitude higher than those of conventional piezoelectric polymers such as poly(vinylidene fluoride). They are comparable to the coefficients of inorganic piezoelectrics and of the recently developed porous polypropylene electrets, but have the added advantage of a much better thermal stability.