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Recently, the results of the first experimental test for entangled photons closing the detection loophole (also referred to as the fair sampling loophole) were published (Vienna, 2013). From the theoretical viewpoint the main distinguishing feature of this long-aspired experiment was that the Eberhard inequality was used. Almost simultaneously another experiment closing this loophole was performed (Urbana-Champaign, 2013) and it was based on the Clauser-Horne inequality (for probabilities). The aim of this note is to analyze the mathematical and experimental equivalence of tests based on the Eberhard inequality and various forms on the Clauser-Horne inequality. The structure of the mathematical equivalence is nontrivial. In particular, it is necessary to distinguish between algebraic and statistical equivalence. Although the tests based on these inequalities are algebraically equivalent, they need not be equivalent statistically, i.e., theoretically the level of statistical significance can drop under transition from one test to another (at least for finite samples). Nevertheless, the data collected in the Vienna-test implies not only a statistically significant violation of the Eberhard inequality, but also of the Clauser-Horne inequality (in the ratio-rate form): for both a violation $>60\sigma.$, Comment: a few misprints were corrected

Indistinguishable quantum states interfere, but the mere possibility of obtaining information that could distinguish between overlapping states inhibits quantum interference. Quantum interference imaging can outperform classical imaging or even have entirely new features. Here, we introduce and experimentally demonstrate a quantum imaging concept that relies on the indistinguishability of the possible sources of a photon that remains undetected. Our experiment uses pair creation in two separate down-conversion crystals. While the photons passing through the object are never detected, we obtain images exclusively with the sister photons that do not interact with the object. Therefore the object to be imaged can be either opaque or invisible to the detected photons. Moreover, our technique allows the probe wavelength to be chosen in a range for which suitable sources and/or detectors are unavailable. Our experiment is a prototype in quantum information where knowledge can be extracted by and about a photon that is never detected., Comment: Comments welcome

We observe strong modal coupling between the TE00 and TM00 modes in Si3N4 ring resonators revealed by avoided crossings of the corresponding resonances. Such couplings result in significant shifts of the resonance frequencies over a wide range around the crossing points. This leads to an effective dispersion that is one order of magnitude larger than the intrinsic dispersion and creates broad windows of anomalous dispersion. We also observe the changes to frequency comb spectra generated in Si3N4 microresonators due polarization mode and higher-order mode crossings and suggest approaches to avoid these effects. Alternatively, such polarization mode-crossings can be used as a novel tool for dispersion engineering in microresonators., Comment: Comments are very welcome (send to corresponding author)

We demonstrate a photonic Ramsey interferometer using four-wave mixing in which the photon energies are analogous to atomic/molecular levels. This technique is highly promising for frequency-based quantum key distribution.

Non-classical states of light, such as entangled photon pairs and number states, are essential for fundamental tests of quantum mechanics and optical quantum technologies. The most widespread technique for creating these quantum resources is the spontaneous parametric down-conversion (SPDC) of laser light into photon pairs. Conservation of energy and momentum in this process, known as phase-matching, gives rise to strong correlations which are used to produce two-photon entanglement in various degrees of freedom. It has been a longstanding goal of the quantum optics community to realise a source that can produce analogous correlations in photon triplets, but of the many approaches considered, none have been technically feasible. In this paper we report the observation of photon triplets generated by cascaded down-conversion. Here each triplet originates from a single pump photon, and therefore quantum correlations will extend over all three photons in a way not achievable with independently created photon pairs. We expect our photon-triplet source to open up new avenues of quantum optics and become an important tool in quantum technologies. Our source will allow experimental interrogation of novel quantum correlations, the post-selection free generation of tripartite entanglement without post- selection and the generation of heralded entangled-photon pairs suitable for linear optical quantum computing. Two of the triplet photons have a wavelength matched for optimal transmission in optical fibres, ideally suited for three-party quantum communication. Furthermore, our results open interesting regimes of non-linear optics, as we observe spontaneous down-conversion pumped by single photons, an interaction also highly relevant to optical quantum computing., 7 pages, 3 figures, 1 table; accepted by Nature

The fair-sampling loophole is closed in a Bell inequality violation experiment with entangled photons, making the photon the first physical system for which all the main loopholes have been closed. So-called Bell experiments are used to discriminate between classical ('local realistic') and quantum models of measurable phenomena. In practice, they are subject to various loopholes (arising from non-ideal experimental conditions) that can render the results inconclusive. These authors used a highly efficient source of photon pairs and superconducting transition-edge sensors in a Bell inequality experiment that closes the 'fair-sampling' loophole for entangled photons. The results conflict with local realism, while making the photon the first physical system for which each of the main loopholes has been closed, albeit in different experiments. The violation of a Bell inequality is an experimental observation that forces the abandonment of a local realistic viewpoint—namely, one in which physical properties are (probabilistically) defined before and independently of measurement, and in which no physical influence can propagate faster than the speed of light1,2. All such experimental violations require additional assumptions depending on their specific construction, making them vulnerable to so-called loopholes. Here we use entangled photons to violate a Bell inequality while closing the fair-sampling loophole, that is, without assuming that the sample of measured photons accurately represents the entire ensemble3. To do this, we use the Eberhard form of Bell’s inequality, which is not vulnerable to the fair-sampling assumption and which allows a lower collection efficiency than other forms4. Technical improvements of the photon source5,6 and high-efficiency transition-edge sensors7 were crucial for achieving a sufficiently high collection efficiency. Our experiment makes the photon the first physical system for which each of the main loopholes has been closed, albeit in different experiments.

We present a quantum mechanical analysis of the orbital angular momentum of a class of recently discovered elliptically-symmetric stable light fields --- the so-called Ince-Gauss modes. We study, in a fully quantum formalism, how the orbital angular momentum of these beams varies with their ellipticity and discover several compelling features, including: non-monotonic behavior, stable beams with real continuous (non-integer) orbital angular momenta, and orthogonal modes with the same orbital angular momenta. We explore, and explain in detail, the reasons for this behavior. These features may have application to quantum key distribution, atom trapping, and quantum informatics in general --- as the ellipticity opens up a new way of navigating the photonic Hilbert space., 9 Pages, 4 Figures, Comments Welcome

We report the efficient creation and detection of hybrid entanglement between one photon's polarization and another photon's complex transverse polarization pattern including various polarization singularities. The polarization measurement of the first photon triggers a polarization-sensitive imaging of its partner photon, the vector photon, using a single-photon-sensitive camera. Thereby, the vector photon's complex polarization pattern is reconstructed tomographically dependent on the type of polarization measurement performed on its partner. To evaluate the quality of the observed patterns, we introduce a fidelity-like measure which compares the reconstructed polarization with the theoretically expected one at every transverse spatial position. We find that the measured polarization patterns and the theory overlap by more than 90%. Moreover, we visualize the varying strengths of polarization entanglement for different transverse regions and demonstrate an interesting phenomenon: Each vector photon can be both entangled and not entangled in polarization with its partner photon. We give an intuitive, information-theoretical explanation for our results. Our results pave the way to study quantum properties of the rich field of such complex light modes with the potential to improve various applications in quantum informational tasks.

We demonstrate a compact, robust, and highly efficient source of polarization-entangled photons, based on linear bi-directional down-conversion in a novel ‘folded sandwich’ configuration. Bi-directionally pumping a single periodically poled KTiOPO4 (ppKTP) crystal with a 405-nm laser diode, we generate entangled photon pairs at the non-degenerate wavelengths 784 nm (signal) and 839 nm (idler), and achieve an unprecedented detection rate of 11.8 kcps for 10.4 μW of pump power (1.1 million pairs / mW), in a 2.9-nm bandwidth, while maintaining a very high two-photon entanglement quality, with a Bell-state fidelity of 99.3 ± 0.3%.

We developed a method to create custom tailored photonic entanglement of complex spatial modes. By taking advantage of the high timing and spatial resolution of an intensified CCD camera, we investigate the properties of the entangled photon pair.

Horse-spleen ferritin has recently gained much attention as a nanoscale magnet that demonstrates macroscopic quantum tunneling of its magnetization through the anisotropy barrier. The size of the protein is ideal for the identification and manipulation of single particles using scanned-probe techniques. We demonstrate that accurate positioning of individual, surface-trapped horse-spleen particles can be achieved by mechanical pushing with the tip of an atomic force microscope. This technique offers a route to the incorporation of single ferritin particles as components in future nanoscale devices.

Conductive atomic force microscopy has been used to electrically image quasibreakdown sites in thin, native SiO2 films. Local current–voltage spectroscopy reveals, at individual sites, fluctuations in the breakdown current between well-defined conductivity states. Theoretical modeling has been performed to show that conduction through the film is governed by local trap-assisted tunneling, with typically one or two charge traps contributing to conduction through a quasibreakdown site. Our study provides a semiquantitative analysis to characterize the effective trap states that give rise to local random telegraph signals in the oxide film.

Quantum Entanglement is widely regarded as one of the most prominent features of quantum mechanics and quantum information science. Although, photonic entanglement is routinely studied in many experiments nowadays, its signature has been out of the grasp for real-time imaging. Here we show that modern technology, namely triggered intensified charge coupled device (ICCD) cameras are fast and sensitive enough to image in real-time the effect of the measurement of one photon on its entangled partner. To quantitatively verify the non-classicality of the measurements we determine the detected photon number and error margin from the registered intensity image within a certain region. Additionally, the use of the ICCD camera allows us to demonstrate the high flexibility of the setup in creating any desired spatial-mode entanglement, which suggests as well that visual imaging in quantum optics not only provides a better intuitive understanding of entanglement but will improve applications of quantum science., Two supplementary movies available at the data conservancy project

Single photons are an important prerequisite for a broad spectrum of quantum optical applications. We experimentally demonstrate a heralded single-photon source based on spontaneous parametric down-conversion in collinear bulk optics, and fiber-coupled bolometric transition-edge sensors. Without correcting for background, losses, or detection inefficiencies, we measure an overall heralding efficiency of 83 %. By violating a Bell inequality, we confirm the single-photon character and high-quality entanglement of our heralded single photons which, in combination with the high heralding efficiency, are a necessary ingredient for advanced quantum communication protocols such as one-sided device-independent quantum key distribution., 8 pages, 3 figures

Twist and Entangle Entanglement is a key feature in quantum information science and plays an important role in various applications of quantum mechanics. Fickler et al. (p. 640 ) present a method for converting the polarization state of photons into information encoded into spatial modes of a single photon. From this, superposition states and entangled photons with very high orbital angular momentum quantum numbers were generated.

Photons with complex spatial mode structures open up possibilities for new fundamental high-dimensional quantum experiments and for novel quantum information tasks. Here we show entanglement of photons with complex vortex and singularity patterns called Ince-Gauss modes. In these modes, the position and number of singularities vary depending on the mode parameters. We verify two-dimensional and three-dimensional entanglement of Ince-Gauss modes. By measuring one photon and thereby defining its singularity pattern, we nonlocally steer the singularity structure of its entangled partner, while the initial singularity structure of the photons is undefined. In addition we measure an Ince-Gauss specific quantum-correlation function with possible use in future quantum communication protocols. DOI: 10.1103/PhysRevA.87.012326

Integrated photonic circuits offer the possibility for complex quantum optical experiments in higher-dimensional photonic systems. However, the advantages of integration and scalability can only be fully utilized with the availability of a source for higher-dimensional entangled photons. Here, a novel fiber integrated source for path-entangled photons in the telecom band at 1.55\mum using only standard fiber technology is presented. Due to the special design the source shows good scalability towards higher-dimensional entangled photonic states (quNits), while path entanglement offers direct compatibility with on-chip path encoding. We present an experimental realization of a path-entangled two-qubit source. A very high quality of entanglement is verified by various measurements, i.a. a tomographic state reconstruction is performed leading to a background corrected fidelity of (99.45+-0.06)%. Moreover, we describe an easy method for extending our source to arbitrarily high dimensions., 9 pages, 3 figures, to be published in Optics Express Vol. 20, No. 10 (05/07/2012)

Entangled photons play a pivotal role in the distribution of quantum information in quantum networks. However, the frequency bands for optimal transmission and storage of photons are not necessarily the same. Here we experimentally demonstrate the coherent frequency conversion of photons entangled in their polarization, a widely used degree of freedom in photonic quantum information processing. We verify the successful entanglement conversion by violating a Clauser-Horne-Shimony-Holt (CHSH) Bell inequality and fully confirm that our characterised fidelity of entanglement transfer is close to unity using both state and process tomography. Our implementation is robust and flexible, making it a practical building block for future quantum networks., 4 pages, 4 figures

Single photons provide excellent quantum information carriers, but current schemes for preparing, processing and measuring them are inefficient. For example, down-conversion provides heralded, but randomly timed single photons, while linear-optics gates are inherently probabilistic. Here, we introduce a deterministic scheme for photonic quantum information. Our single, versatile process---coherent photon conversion---provides a full suite of photonic quantum processing tools, from creating high-quality heralded single- and multiphoton states free of higher-order imperfections to implementing deterministic multiqubit entanglement gates and high-efficiency detection. It fulfils all requirements for a scalable photonic quantum computing architecture. Using photonic crystal fibres, we experimentally demonstrate a four-colour nonlinear process usable for coherent photon conversion and show that current technology provides a feasible path towards deterministic operation. Our scheme, based on interacting bosonic fields, is not restricted to optical systems, but could also be implemented in optomechanical, electromechanical and superconducting systems which exhibit extremely strong intrinsic nonlinearities., Comment: 12 pages, 9 figures

Entanglement is central to quantum information technologies. In this talk, I will describe recent experimental results in generation of novel entangled photon states using photons. I will show that a single-photon can be split into three using a cascade of spontaneous down-conversion processes. Further I will present a demonstration of bound entanglement, entangled states which cannot be distilled, by applying random polarization rotations on highly entangled pure states.

Quantum theory demands that, in contrast to classical physics, not all properties can be simultaneously well defined. The Heisenberg Uncertainty Principle is a manifestation of this fact. Another important corollary arises that there can be no joint probability distribution describing the outcomes of all possible measurements, allowing a quantum system to be classically understood. We provide the first experimental evidence that even for a single three-state system, a qutrit, no such classical model can exist that correctly describes the results of a simple set of pairwise compatible measurements. Not only is a single qutrit the simplest system in which such a contradiction is possible, but, even more importantly, the contradiction cannot result from entanglement, because such a system is indivisible, and it does not even allow the concept of entanglement between subsystems., 11 pages, 4 figures, 2 tables

We have realized a novel method to create entanglement between two photons with a very high difference in their OAM quantum number and momentum respectively. Furthermore we have explored hybrid entanglement of photons in the OAM and polarization degree of freedom.

Tests of the predictions of quantum mechanics for entangled systems have provided increasing evidence against local realistic theories. However, there still remains the crucial challenge of simultaneously closing all major loopholes - the locality, freedom-of-choice, and detection loopholes - in a single experiment. An important sub-class of local realistic theories can be tested with the concept of "steering". The term steering was introduced by Schr\"odinger in 1935 for the fact that entanglement would seem to allow an experimenter to remotely steer the state of a distant system as in the Einstein-Podolsky-Rosen (EPR) argument. Einstein called this "spooky action at a distance". EPR-Steering has recently been rigorously formulated as a quantum information task opening it up to new experimental tests. Here, we present the first loophole-free demonstration of EPR-steering by violating three-setting quadratic steering inequality, tested with polarization entangled photons shared between two distant laboratories. Our experiment demonstrates this effect while simultaneously closing all loopholes: both the locality loophole and a specific form of the freedom-of-choice loophole are closed by having a large separation of the parties and using fast quantum random number generators, and the fair-sampling loophole is closed by having high overall detection efficiency. Thereby, we exclude - for the first time loophole-free - an important class of local realistic theories considered by EPR. As well as its foundational importance, loop-hole-free steering also allows the distribution of quantum entanglement secure from an untrusted party., Comment: 10 pages, 3 figures

A significant limitation of practical quantum key distribution (QKD) setups is currently their limited operational range. It has recently been emphasized (X. Ma, C.-H. F. Fung, and H.-K. Lo., Phys. Rev. A, 76:012307, 2007) that entanglement-based QKD systems can tolerate higher channel losses than systems based on weak coherent laser pulses (WCP), in particular when the source is located symmetrically between the two communicating parties, Alice and Bob. In the work presented here, we experimentally study this important advantage by implementing different entanglement-based QKD setups on a 144~km free-space link between the two Canary Islands of La Palma and Tenerife. We established three different configurations where the entangled photon source was placed at Alice's location, asymmetrically between Alice and Bob and symmetrically in the middle between Alice and Bob, respectively. The resulting quantum channel attenuations of 35~dB, 58~dB and 71~dB, respectively, significantly exceed the limit for WCP systems. This confirms that QKD over distances of 300~km and even more is feasible with entangled state sources placed in the middle between Alice and Bob., 14 pages, 5 figures, 1 table

An essential requirement for any quantum computing architecture is the existence of a universal set of quantum gates. Examples of such sets are the 2-qubit CNOT [1,2] gate in combination with arbitrary 1-qubit gates, and the 3-qubit Toffoli gate [3, 4] together with the 1-qubit Hadamard. A third set, which has not been implemented so far, is Valiant's 2-qubit matchgate G(A,B) which is universal with the SWAP operation [5]. The Valiant gate has the form of equations where A and B are 1-qubit unitaries which act on the respective even and odd parity subspace of the 2-qubit state. One particularly intriguing property of the Valiant gate is that it can be efficiently simulated with classical computers [5] as long as it is restricted to act between nearest neighbors only. Once the interactions are extended to next nearest neighbors, however, the Valiant gate turns our to be universal for quantum computing. In practice, this extension can be achieved with a two-qubit SWAP operation, which is exceedingly simple in linear optics.

Color, or frequency, is one of the most familiar degrees of freedom (DOFs) of light and has been routinely analyzed in spectroscopy for centuries. However, it has been used relatively little for quantum technologies. Entangled qubits are a key ingredient in many such technologies. For other optical degrees of freedom discretely entangled states have been extensively investigated, e.g. [1–5]. In contrast, discrete frequency entanglement has not yet been unambiguously demonstrated, despite potentially interesting applications such as enhanced clock synchronization beyond the classical limit [6,7] and improved quantum communication in noisy channels [8]. Moreover, quantum information encoded in discrete, tunable frequency bins of photons would be an ideal mediator between stationary qubits with different energy levels. For many of those applications it is essential, that the color-entangled stated can be directly created - in contrast to spectrally post-selecting them from broadband continuous spectral entanglement [9,10]. So far, no experiment was able to achieve this.

We present a two‐qubit quantum algorithm and multiplexed quantum cryptography as examples for quantum information processing using single photons generated in a single InP(GaInP) quantum dot. In the first part, a Deutsch‐Jozsa algorithm is executed with linear optics obtaining a fidelity of up to 79%, and noise‐resistant encoding is implemented by appropriate superpositions of spatial modes. In the second part, multiplexing enhances the bandwidth of quantum key distribution in a BB84 protocol.

We demonstrate a two-qubit Deutsch-Jozsa algorithm with single photons from a single InP quantum dot. The qubits are implemented via the spatial mode and the polarization of a single photon. Our photon source is operated both under continuous and pulsed excitation, the latter allowing deterministic quantum logic by generating photons on demand with a strong suppression of two-photon events. The computation reached a success probability of up to 79%. We also exploit the concept of decoherence-free subspaces that helps to make our experimental setup robust against sources of phase noise.

Publisher Summary This chapter describes single-photon generation with single indium phosphide (InP) quantum dots which emit in the visible spectrum around 690 nm. At this wavelength, highest detection efficiencies with Si-based photo detectors are currently available which makes InP quantum dots preferable for free-space quantum optical applications. The single-photon character of this source enables the performance of fundamental quantum optics experiments, where the wave- and particle-aspect of light can be observed simultaneously. The chapter demonstrates single-photon statistics and cross-correlations of various transitions from multi-excitonic states including biexciton and triexciton decays. Multi-photon generation from single quantum dots may find applications in quantum cryptography devices since a higher rate of photons also enhances the maximum transmission rate of the quantum information. Therefore, a method to perform multiplexing is presented, similar to the classical technique, but on the single-photon level. A typical application— the BB84 quantum key distribution protocol—was performed to demonstrate this method.

In the circuit model, quantum computers rely on the availability of a universal quantum gate set. A particularly intriguing example is a set of two-qubit only gates: matchgates, along with SWAP (the exchange of two qubits). In this paper, we show a simple decomposition of arbitrary matchgates into better known elementary gates, and implement a matchgate in a linear-optics experiment using single photons. The gate performance was fully characterized via quantum process tomography. Moreover, we represent the resulting reconstructed quantum process in a novel way, as a fidelity map in the space of all possible nonlocal two-qubit unitaries. We propose the non-local distance - which is independent of local imperfections like uncorrelated noise or uncompensated local rotations - as a new diagnostic process measure for the non-local properties of the implemented gate., Comment: * published version * extended title reflecting the additions on non-local process analyis in the manuscript

We present a Bell inequality that does not require two-outcome measurements. It is based on an inequality originally derived by Wigner, extending it such that no assumptions other than local-realism, fair-sampling, and freedom-of-choice are necessary.

We demonstrate Hong-Ou-Mandel interference between different transverse spatial modes in a silicon nitride multimode waveguide. We show over 90% visibility between modes, providing a promising route for scalable on-chip quantum information processing.

We demonstrate frequency-degenerate optical parametric oscillation via four-wave mixing using dual pumps in a silicon-nitride microresonator. The system offers potential for realization of coherent optical computing and all-optical quantum random number generation.

A number of key experiments in free-space have shown that polarization-entangled photons are not only of paramount importance to fundamental quantum optics, but can also find real-world applications in space, such as quantum enhanced metrological schemes, quantum communication and quantum cryptography. A crucial requirement for envisaged satellite-link experiments is the development of efficient space-qualified sources of entangled photons.

Indistinguishability of photons that need not be detected results in a novel paradigm of quantum information processing. This confirms that for quantum interference it is important whether two possibilities could in principle be distinguished independent of detection.