Deny R. Hamel

Strong Loophole-Free Test of Local Realism

We performed an loophole-free test of Bells inequalities. The probability that local realism is compatible with our results is less than 5.9×10^-9.

Experimental investigation of the uncertainty principle in the presence of quantum memory and its application to witnessing entanglement

The uncertainty principle tells us that two associated properties of a particle cannot be simultaneously known with infinite precision. However, if the particle is entangled with a quantum memory, the uncertainty of a measurement is reduced. This concept is now observed experimentally.

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.

Three-photon energy–time entanglement

Many-particle entangled states and entanglement between continuous properties are valuable resources for quantum information, but are notoriously difficult to generate. An experiment now entangles the energy and emission times of three photons, creating generalized Einstein–Podolsky–Rosen correlations.

Direct generation of three-photon polarization entanglement

A three-photon entangled Greenberger–Horne–Zeilinger state is directly produced by cascading two entangled down-conversion processes. Experimentally, 11.1 triplets per minute are detected on average. The three-photon entangled state is used for state tomography and as a test of local realism by violating the Mermin and Svetlichny inequalities.

Cluster-state quantum computing enhanced by high-fidelity generalized measurements.

We introduce and implement a technique to extend the quantum computational power of cluster states by replacing some projective measurements with generalized quantum measurements (POVMs). As an experimental demonstration we fully realize an arbitrary three-qubit cluster computation by implementing a tunable linear-optical POVM, as well as fast active feedforward, on a two-qubit photonic cluster state. Over 206 different computations, the average output fidelity is 0.9832+/-0.0002; furthermore the error contribution from our POVM device and feedforward is only of O(10(-3)), less than some recent thresholds for fault-tolerant cluster computing.

Observation of genuine three-photon interference

Three photons can display qualitatively new interference phenomena such as genuine three-photon interference. Here we show how to isolate three-photon interference with more than 90 % visibility, completely suppressing two-photon and single-photon interference.

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

Summary form only given. The essence of wave-particle duality is that particles passing through slits form a pattern that can only be explained by resorting to wave mechanics. Here we present results that provide insight into the buildup of the multiple-slit interference pattern. The perfect time resolved measurements would ideally involve a single-particle source and an array of single-particle noiseless detectors featuring perfect quantum efficiency and time resolution. In previous experiments for electrons [1], photons [2] and molecules [3], detection of single particles was limited by minimum acquisition times, thus denying access to precise timing information. We make a significant step forward, approaching the perfect measurement very closely and showing real-time interference pattern buildup of single-photon detections as a result of passing through a system of multiple slits. These measurements accessed precise timing information, allowing us to analyse the interference patterns formation process and effects of entanglement.

Single-photon test of hyper-complex quantum theories using a metamaterial

In standard quantum mechanics, complex numbers are used to describe the wavefunction. Although this has so far proven sufficient to predict experimental results, there is no theoretical reason to choose them over real numbers or generalizations of complex numbers, that is, hyper-complex numbers. Experiments performed to date have proven that real numbers are insufficient, but the need for hyper-complex numbers remains an open question. Here we experimentally probe hyper-complex quantum theories, studying one of their deviations from complex quantum theory: the non-commutativity of phases. We do so by passing single photons through a Sagnac interferometer containing both a metamaterial with a negative refractive index, and a positive phase shifter. To accomplish this we engineered a fishnet metamaterial to have a negative refractive index at 780 nm. We show that the metamaterial phase commutes with other phases with high precision, allowing us to place limits on a particular prediction of hyper-complex quantum theories.

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 photons 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.