Direct Characterization of Ultrafast Energy-Time Entangled Photon Pairs.

Energy-time entangled photons are critical in many quantum optical phenomena and have emerged as important elements in quantum information protocols. Entanglement in this degree of freedom often manifests itself on ultrafast time scales, making it very difficult to detect, whether one employs direct or interferometric techniques, as photon-counting detectors have insufficient time resolution. Here, we implement ultrafast photon counters based on nonlinear interactions and strong femtosecond laser pulses to probe energy-time entanglement in this important regime. Using this technique and single-photon spectrometers, we characterize all the spectral and temporal correlations of two entangled photons with femtosecond resolution. This enables the witnessing of energy-time entanglement using uncertainty relations and the direct observation of nonlocal dispersion cancellation on ultrafast time scales. These techniques are essential to understand and control the energy-time degree of freedom of light for ultrafast quantum optics.

Quantum-coherent mixtures of causal relations.

Understanding the causal influences that hold among parts of a system is critical both to explaining that system's natural behaviour and to controlling it through targeted interventions. In a quantum world, understanding causal relations is equally important, but the set of possibilities is far richer. The two basic ways in which a pair of time-ordered quantum systems may be causally related are by a cause-effect mechanism or by a common-cause acting on both. Here we show a coherent mixture of these two possibilities. We realize this nonclassical causal relation in a quantum optics experiment and derive a set of criteria for witnessing the coherence based on a quantum version of Berkson's effect, whereby two independent causes can become correlated on observation of their common effect. The interplay of causality and quantum theory lies at the heart of challenging foundational puzzles, including Bell's theorem and the search for quantum gravity.

Observation of Genuine Three-Photon Interference.

Multiparticle quantum interference is critical for our understanding and exploitation of quantum information, and for fundamental tests of quantum mechanics. A remarkable example of multi-partite correlations is exhibited by the Greenberger-Horne-Zeilinger (GHZ) state. In a GHZ state, three particles are correlated while no pairwise correlation is found. The manifestation of these strong correlations in an interferometric setting has been studied theoretically since 1990 but no three-photon GHZ interferometer has been realized experimentally. Here we demonstrate three-photon interference that does not originate from two-photon or single photon interference. We observe phase-dependent variation of three-photon coincidences with (92.7±4.6)% visibility in a generalized Franson interferometer using energy-time entangled photon triplets. The demonstration of these strong correlations in an interferometric setting provides new avenues for multiphoton interferometry, fundamental tests of quantum mechanics, and quantum information applications in higher dimensions.

Phonon-Mediated Nonclassical Interference in Diamond.

Quantum interference of single photons is a fundamental aspect of many photonic quantum processing and communication protocols. Interference requires that the multiple pathways through an interferometer be temporally indistinguishable to within the coherence time of the photon. In this Letter, we use a diamond quantum memory to demonstrate interference between quantum pathways, initially temporally separated by many multiples of the optical coherence time. The quantum memory can be viewed as a light-matter beam splitter, mapping a THz-bandwidth single photon to a variable superposition of the output optical mode and stored phononic mode. Because the memory acts both as a beam splitter and as a buffer, the relevant coherence time for interference is not that of the photon, but rather that of the memory. We use this mechanism to demonstrate nonclassical single-photon and two-photon interference between quantum pathways initially separated by several picoseconds, even though the duration of the photons themselves is just ∼250 fs.

An experimental test of noncontextuality without unphysical idealizations.

To make precise the sense in which nature fails to respect classical physics, one requires a formal notion of classicality. Ideally, such a notion should be defined operationally, so that it can be subject to direct experimental test, and it should be applicable in a wide variety of experimental scenarios so that it can cover the breadth of phenomena thought to defy classical understanding. Bell's notion of local causality fulfils the first criterion but not the second. The notion of noncontextuality fulfils the second criterion, but it is a long-standing question whether it can be made to fulfil the first. Previous attempts to test noncontextuality have all assumed idealizations that real experiments cannot achieve, namely noiseless measurements and exact operational equivalences. Here we show how to devise tests that are free of these idealizations. We perform a photonic implementation of one such test, ruling out noncontextual models with high confidence.

Frequency and bandwidth conversion of single photons in a room-temperature diamond quantum memory.

The spectral manipulation of photons is essential for linking components in a quantum network. Large frequency shifts are needed for conversion between optical and telecommunication frequencies, while smaller shifts are useful for frequency-multiplexing quantum systems, in the same way that wavelength division multiplexing is used in classical communications. Here we demonstrate frequency and bandwidth conversion of single photons in a room-temperature diamond quantum memory. Heralded 723.5 nm photons, with 4.1 nm bandwidth, are stored as optical phonons in the diamond via a Raman transition. Upon retrieval from the diamond memory, the spectral shape of the photons is determined by a tunable read pulse through the reverse Raman transition. We report central frequency tunability over 4.2 times the input bandwidth, and bandwidth modulation between 0.5 and 1.9 times the input bandwidth. Our results demonstrate the potential for diamond, and Raman memories in general, as an integrated platform for photon storage and spectral conversion.

Experimental nonlocal and surreal Bohmian trajectories.

Weak measurement allows one to empirically determine a set of average trajectories for an ensemble of quantum particles. However, when two particles are entangled, the trajectories of the first particle can depend nonlocally on the position of the second particle. Moreover, the theory describing these trajectories, called Bohmian mechanics, predicts trajectories that were at first deemed "surreal" when the second particle is used to probe the position of the first particle. We entangle two photons and determine a set of Bohmian trajectories for one of them using weak measurements and postselection. We show that the trajectories seem surreal only if one ignores their manifest nonlocality.

Certifying the Presence of a Photonic Qubit by Splitting It in Two.

We present an implementation of photonic qubit precertification that performs the delicate task of detecting the presence of a flying photon without destroying its qubit state, allowing loss-sensitive quantum cryptography and tests of nonlocality even over long distance. By splitting an incoming single photon in two via parametric down-conversion, we herald the photon's arrival from an independent photon source while preserving its quantum information with up to (92.3±0.6)% fidelity. With reduced detector dark counts, precertification will be immediately useful in quantum communication.

Storage and retrieval of THz-bandwidth single photons using a room-temperature diamond quantum memory.

We report the storage and retrieval of single photons, via a quantum memory, in the optical phonons of a room-temperature bulk diamond. The THz-bandwidth heralded photons are generated by spontaneous parametric down-conversion and mapped to phonons via a Raman transition, stored for a variable delay, and released on demand. The second-order correlation of the memory output is g((2))(0)=0.65±0.07, demonstrating a preservation of nonclassical photon statistics throughout storage and retrieval. The memory is low noise, high speed and broadly tunable; it therefore promises to be a versatile light-matter interface for local quantum processing applications.

Ultrafast time-division demultiplexing of polarization-entangled photons.

Maximizing the information transmission rate through quantum channels is essential for practical implementation of quantum communication. Time-division multiplexing is an approach for which the ultimate rate requires the ability to manipulate and detect single photons on ultrafast time scales while preserving their quantum correlations. Here we demonstrate the demultiplexing of a train of pulsed single photons using time-to-frequency conversion while preserving their polarization entanglement with a partner photon. Our technique converts a pulse train with 2.69 ps spacing to a frequency comb with 307 GHz spacing which may be resolved using diffraction techniques. Our work enables ultrafast multiplexing of quantum information with commercially available single-photon detectors.

Discriminating single-photon states unambiguously in high dimensions.

The ability to uniquely identify a quantum state is integral to quantum science, but for nonorthogonal states, quantum mechanics precludes deterministic, error-free discrimination. However, using the nondeterministic protocol of unambiguous state discrimination enables the error-free differentiation of states, at the cost of a lower frequency of success. We discriminate experimentally between nonorthogonal, high-dimensional states encoded in single photons; our results range from dimension d=2 to d=14. We quantify the performance of our method by comparing the total measured error rate to the theoretical rate predicted by minimum-error state discrimination. For the chosen states, we find a lower error rate by more than 1 standard deviation for dimensions up to d=12. This method will find immediate application in high-dimensional implementations of quantum information protocols, such as quantum cryptography.

Time-resolved double-slit interference pattern measurement with entangled photons.

The double-slit experiment strikingly demonstrates the wave-particle duality of quantum objects. In this famous experiment, particles pass one-by-one through a pair of slits and are detected on a distant screen. A distinct wave-like pattern emerges after many discrete particle impacts as if each particle is passing through both slits and interfering with itself. Here we present a temporally- and spatially-resolved measurement of the double-slit interference pattern using single photons. We send single photons through a birefringent double-slit apparatus and use a linear array of single-photon detectors to observe the developing interference pattern. The analysis of the buildup allows us to compare quantum mechanics and the corpuscular model, which aims to explain the mystery of single-particle interference. Finally, we send one photon from an entangled pair through our double-slit setup and show the dependence of the resulting interference pattern on the twin photon's measured state. Our results provide new insight into the dynamics of the buildup process in the double-slit experiment, and can be used as a valuable resource in quantum information applications.

Coherent ultrafast measurement of time-bin encoded photons.

Time-bin encoding is a robust form of optical quantum information, especially for transmission in optical fibers. To readout the information, the separation of the time bins must be larger than the detector time resolution, typically on the order of nanoseconds for photon counters. In the present work, we demonstrate a technique using a nonlinear interaction between chirped entangled time-bin photons and shaped laser pulses to perform projective measurements on arbitrary time-bin states with picosecond-scale separations. We demonstrate a tomographically complete set of time-bin qubit projective measurements and show the fidelity of operations is sufficiently high to violate the Clauser-Horne-Shimony-Holt-Bell inequality by more than 6 standard deviations.

Direct generation of photon triplets using cascaded photon-pair sources.

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 spontaneous parametric down-conversion of laser light into photon pairs. Conservation of energy and momentum in this process, known as phase-matching, gives rise to strong correlations that are used to produce two-photon entanglement in various degrees of freedom. It has been a longstanding goal in quantum optics to realize a source that can produce analogous correlations in photon triplets, but of the many approaches considered, none has been technically feasible. Here we report the observation of photon triplets generated by cascaded down-conversion. 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. Our photon-triplet source will allow experimental interrogation of novel quantum correlations, the 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, suitable 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.

Experimental bound entanglement in a four-photon state.

Bound entanglement is central to many exciting theoretical results in quantum information processing, but has thus far not been experimentally realized. In this work, we consider a one-parameter family of four-qubit Smolin states. We experimentally produce these states in the polarization of four optical photons produced from parametric down-conversion. Within a range of the parameter, we show that our states are entangled and undistillable, and thus bound entangled. Using these bound-entangled states we demonstrate entanglement unlocking.

Quantum information: source of triggered entangled photon pairs?

The realization of an entangled photon source will be of great importance in quantum information--for example, for quantum key distribution and quantum computation--and Stevenson et al. have described such a source. However, we show here that first, their source is not entangled; second, they use inappropriate entanglement indicators that rely on assumptions invalidated by their data; and third, their source has insignificant entanglement even after simulating subtraction of the significant quantity of background noise. We therefore find that the standard of proof required for a semiconductor source of triggered entangled photon pairs has not been met by Stevenson et al..

Experimental entangled entanglement.

All previous tests of local realism have studied correlations between single-particle measurements. In the present experiment, we have performed a Bell experiment on three particles in which one of the measurements corresponds to a projection onto a maximally entangled state. We show theoretically and experimentally that correlations between these entangled measurements and single-particle measurements are too strong for any local-realistic theory and are experimentally exploited to violate a Clauser-Horne-Shimony-Holt-Bell inequality by more than 5 standard deviations. We refer to this possibility as "entangled entanglement."

Filtering out photonic Fock states.

Unprecedented optical nonlinearities can be generated probabilistically in simple linear-optical networks conditioned on specific measurement outcomes. We describe a highly controllable quantum filter for photon number states, which takes advantage of such a measurement-induced amplitude nonlinearity. The basis for this filter is multiphoton nonclassical interference which we demonstrate for one- and two-photon states over a wide range of beam splitter reflectivities. Specifically, we show that the transmission probability, conditional on a specific measurement outcome, can be larger for a two-photon state than a one-photon state; this is not possible with linear optics alone.

Experimental violation of a cluster state bell inequality.

Cluster states are a new type of multiqubit entangled states with entanglement properties exceptionally well suited for quantum computation. In the present work, we experimentally demonstrate that correlations in a four-qubit linear cluster state cannot be described by local realism. This exploration is based on a recently derived Bell-type inequality [V. Scarani et al., Phys. Rev. A 71, 042325 (2005)] which is tailored, by using a combination of three- and four-particle correlations, to be maximally violated by cluster states but not violated at all by GHZ states. We observe a cluster-state Bell parameter of 2.59+/-0.08, which is more than 7sigma larger than the threshold of 2 imposed by local realism.

Conditional-phase switch at the single-photon level.

We present an experimental realization of a two-photon conditional phase switch, related to the " c-phis;" gate of quantum computation. This gate relies on quantum interference between photon pairs and generates entanglement between two optical modes through the process of spontaneous parametric down-conversion (SPDC). The interference effect serves to enhance the effective nonlinearity by many orders of magnitude, so it is significant at the quantum (single-photon) level. By adjusting the relative optical phase between the classical pump for SPDC and the pair of input modes, one can impress a large phase shift on one beam which depends on the presence or absence of a single photon in a control mode.