Dik Bouwmeester

Experimental quantum teleportation

Quantum teleportation — the transmission and reconstruction over arbitrary distances of the state of a quantum system — is demonstrated experimentally. During teleportation, an initial photon which carries the polarization that is to be transferred and one of a pair of entangled photons are subjected to a measurement such that the second photon of the entangled pair acquires the polarization of the initial photon. This latter photon can be arbitrarily far away from the initial one. Quantum teleportation will be a critical ingredient for quantum computation networks.

Experimental test of quantum nonlocality in three-photon Greenberger–Horne–Zeilinger entanglement

Bells theorem states that certain statistical correlations predicted by quantum physics for measurements on two-particle systems cannot be understood within a realistic picture based on local properties of each individual particle—even if the two particles are separated by large distances. Einstein, Podolsky and Rosen first recognized the fundamental significance of these quantum correlations (termed ‘entanglement’ by Schrödinger) and the two-particle quantum predictions have found ever-increasing experimental support. A more striking conflict between quantum mechanical and local realistic predictions (for perfect correlations) has been discovered; but experimental verification has been difficult, as it requires entanglement between at least three particles. Here we report experimental confirmation of this conflict, using our recently developed method to observe three-photon entanglement, or ‘Greenberger–Horne–Zeilinger’ (GHZ) states. The results of three specific experiments, involving measurements of polarization correlations between three photons, lead to predictions for a fourth experiment; quantum physical predictions are mutually contradictory with expectations based on local realism. We find the results of the fourth experiment to be in agreement with the quantum prediction and in striking conflict with local realism.

Observation of Three-Photon Greenberger-Horne-Zeilinger Entanglement

We present the experimental observation of polarization entanglement for three spatially separated photons. Such states of more than two entangled particles, known as GHZ states, play a crucial role in fundamental tests of quantum mechanics versus local realism and in many quantum information and quantum computation schemes. Our experimental arrangement is such that we start with two pairs of entangled photons and register one photon in a way that any information as to which pair it belongs to is erased. The registered events at the detectors for the remaining three photons then exhibit the desired GHZ correlations.

Towards quantum superpositions of a mirror.

We propose an experiment for creating quantum superposition states involving of the order of 10(14) atoms via the interaction of a single photon with a tiny mirror. This mirror, mounted on a high-quality mechanical oscillator, is part of a high-finesse optical cavity which forms one arm of a Michelson interferometer. By observing the interference of the photon only, one can study the creation and decoherence of superpositions involving the mirror. A detailed analysis of the requirements shows that the experiment is within reach using a combination of state-of-the-art technologies.

Experimental quantum cloning of single photons

Although perfect copying of unknown quantum systems is forbidden by the laws of quantum mechanics, approximate cloning is possible. A natural way of realizing quantum cloning of photons is by stimulated emission. In this context, the fundamental quantum limit to the quality of the clones is imposed by the unavoidable presence of spontaneous emission. In our experiment, a single input photon stimulates the emission of additional photons from a source on the basis of parametric down-conversion. This leads to the production of quantum clones with near-optimal fidelity. We also demonstrate universality of the copying procedure by showing that the same fidelity is achieved for arbitrary input states.

Stimulated emission of polarization-entangled photons

Entangled photon pairs—discrete light quanta that exhibit non-classical correlations—play a crucial role in quantum information science (for example, in demonstrations of quantum non-locality, quantum teleportation and quantum cryptography). At the macroscopic optical-field level non-classical correlations can also be important, as in the case of squeezed light, entangled light beams and teleportation of continuous quantum variables. Here we use stimulated parametric down-conversion to study entangled states of light that bridge the gap between discrete and macroscopic optical quantum correlations. We demonstrate experimentally the onset of laser-like action for entangled photons, through the creation and amplification of the spin-1/2 and spin-1 singlet states consisting of two and four photons, respectively. This entanglement structure holds great promise in quantum information science where there is a strong demand for entangled states of increasing complexity.

Experimental Violation of a Spin-1 Bell Inequality Using Maximally Entangled Four-Photon States

We demonstrate the experimental violation of a spin-1 Bell inequality. The spin-1 inequality is based on the Clauser, Horne, Shimony, and Holt formalism. For entangled spin-1 particles, the maximum quantum-mechanical prediction is 2.55 as opposed to a maximum of 2, predicted using local hidden variables. We obtained an experimental value of 2.27+/-0.02 using the four-photon state generated by pulsed, type-II, stimulated parametric down-conversion. This is a violation of the spin-1 Bell inequality by more than 13 standard deviations.

Multiphoton entanglement concentration and quantum cryptography.

Multiphoton states from parametric down-conversion can be entangled both in polarization and photon number. Maximal high-dimensional entanglement can be concentrated postselectively from these states via photon counting. This makes them natural candidates for quantum key distribution, where the presence of more than one photon per detection interval has up to now been considered undesirable. We propose a simple multiphoton cryptography protocol for the case of low losses.

High-fidelity teleportation of independent qubits

Abstract Quantum teleportation is one of the essential primitives of quantum communication. We suggest that any quantum teleportation scheme can be characterized by its efficiency, i.e. how often it succeeds to teleport, its fidelity, i.e. how well the input state is reproduced at the output, and by its insensitivity to cross talk, i.e. how well it rejects an input state that is not intended to teleport. We discuss these criteria for the two teleportation experiments of independent qubits which have been performed thus far. In the first experiment (1997, Nature, 390, 575) where the qubit states were various different polarization states of photons, the fidelity of teleportation was as high as 0.80 ± 0.05 thus clearly surpassing the limit of 2/3 which can, in principle, be obtained by a direct measurement on the qubit and classical communication. This high fidelity is confirmed in our second experiment (1998, Phys. Rev. Lett., 80, 3891), demonstrating entanglement swapping, that is, realizing the teleportation of a qubit which itself is still entangled to another one. This experiment is the only one to date that demonstrates the teleportation of a genuine unknown quantum state.

Reply: A posteriori teleportation

Bouwmeester et al. reply — Braunstein and Kimble observe correctly that, in the Innsbruck experiment, one does not always observe a teleported photon conditioned on a coincidence recording at the Bell-state analyser. In their opinion, this affects the fidelity of the experiment, but we believe, in contrast, that it has no significance, and that when a teleported photon appears, it has all the properties required by the teleportation protocol. These properties can never be achieved by “abandoning teleportation altogether and transmitting randomly selected polarization states” as Braunstein and Kimble suggest. The fact that there will be events where no teleported photons are created merely affects the efficiency of the experiment. This suggests that the measure of fidelity used by Braunstein and Kimble is unsuitable for our experiment.