arXiv subjects

Markus Aspelmeyer

Publications and source records attributed to Markus Aspelmeyer.

90 records · Page 5Linked to original sources

arXiv2009

High-fidelity entanglement swapping with fully independent sources

Entanglement swapping allows to establish entanglement between independent particles that never interacted nor share any common past. This feature makes it an integral constituent of quantum repeaters. Here, we demonstrate entanglement swapping with time-synchronized independent sources with a fidelity high enough to violate a Clauser-Horne-Shimony-Holt inequality by more than four standard deviations. The fact that both entangled pairs are created by fully independent, only electronically connected sources ensures that this technique is suitable for future long-distance quantum communication experiments as well as for novel tests on the foundations of quantum physics.

quant-ph↗

arXiv2008

Monocrystalline AlGaAs heterostructures for high-reflectivity high-Q micromechanical resonators in the MHz regime

We present high-performance megahertz micromechanical oscillators based on freestanding epitaxial AlGaAs distributed Bragg reflectors. Compared with dielectric reflectors, the low mechanical loss of the monocrystalline heterostructure gives rise to significant improvements in the achievable mechanical quality factor Q while simultaneously exhibiting near unity reflectivity. Experimental characterization yields an optical reflectivity exceeding 99.98% and mechanical quality factors up to 20 000 at 4 K. This materials system is not only an interesting candidate for optical coatings with ultralow thermal noise, but also provides a promising path toward quantum optical control of massive micromechanical mirrors.

cond-mat.mes-hall↗

arXiv2008

Space-QUEST: Experiments with quantum entanglement in space

The European Space Agency (ESA) has supported a range of studies in the field of quantum physics and quantum information science in space for several years, and consequently we have submitted the mission proposal Space-QUEST (Quantum Entanglement for Space Experiments) to the European Life and Physical Sciences in Space Program. We propose to perform space-to-ground quantum communication tests from the International Space Station (ISS). We present the proposed experiments in space as well as the design of a space based quantum communication payload.

quant-ph↗

arXiv2008

Heralded generation of multiphoton entanglement

We present a scheme, based only on linear optics and standard photon detection, that allows to generate heralded multiphoton entangled states of arbitrary photon number from spontaneous parametric downconversion (PDC) in the weak interaction regime. The scheme also works in the strong interaction regime, i.e., for the production of large photon numbers, when photon-number resolving detectors with nearly perfect quantum efficiency are used. In addition, the same setup can be used for quantum metrology as multiphoton interferometer with sensitivity at the Heisenberg limit.

quant-ph↗

arXiv2008

Photonic entanglement as a resource in quantum computation and quantum communication

Entanglement is an essential resource in current experimental implementations for quantum information processing. We review a class of experiments exploiting photonic entanglement, ranging from one-way quantum computing over quantum communication complexity to long-distance quantum communication. We then propose a set of feasible experiments that will underline the advantages of photonic entanglement for quantum information processing.

quant-ph↗

arXiv2008

Experimental verification of the feasibility of a quantum channel between Space and Earth

Extending quantum communication to Space environments would enable to perform fundamental experiments on quantum physics as well as applications of quantum information at planetary and interplanetary scales. Here, we report the first experimental implementation of a scheme for single-photon exchange between a satellite and an Earth-based station. We built an experiment that mimics a single photon source on a satellite, exploiting the telescope at the Matera Laser Ranging Observatory of the Italian Space Agency to detect the transmitted photons. Weak laser pulses, emitted by the ground-based station, are directed towards a satellite equipped with cube-corner retroreflectors. These reflect a small portion of the pulse, with an average of less-than-one photon per pulse directed to our receiver, as required for the faint-pulse quantum communication. We were able to detect returns from satellite Ajisai, a low-earth orbit geodetic satellite, whose orbit has a perigee height of 1485 km.

quant-ph↗

arXiv2008

Radiation-pressure self-cooling of a micromirror in a cryogenic environment

We demonstrate radiation-pressure cavity-cooling of a mechanical mode of a micromirror starting from cryogenic temperatures. To achieve that, a high-finesse Fabry-Perot cavity (F\approx 2200) was actively stabilized inside a continuous-flow 4He cryostat. We observed optical cooling of the fundamental mode of a 50mu x 50 mu x 5.4 mu singly-clamped micromirror at ω_m=3.5 MHz from 35 K to approx. 290 mK. This corresponds to a thermal occupation factor of \approx 1x10^4. The cooling performance is only limited by the mechanical quality and by the optical finesse of the system. Heating effects, e.g. due to absorption of photons in the micromirror, could not be observed. These results represent a next step towards cavity-cooling a mechanical oscillator into its quantum ground state.

quant-ph↗

arXiv2007

Experimental test of nonlocal realistic theories without the rotational symmetry assumption

We analyze the class of nonlocal realistic theories that was originally considered by Leggett [Found. Phys. 33, 1469 (2003)] and tested by us in a recent experiment [Nature (London) 446, 871 (2007)]. We derive an incompatibility theorem that works for finite numbers of polarizer settings and that does not require the previously assumed rotational symmetry of the two-particle correlation functions. The experimentally measured case involves seven different measurement settings. Using polarization-entangled photon pairs, we exclude this broader class of nonlocal realistic models by experimentally violating a new Leggett-type inequality by 80 standard deviations.

quant-ph↗

arXiv2007

An experimental test of non-local realism

Most working scientists hold fast to the concept of 'realism' - a viewpoint according to which an external reality exists independent of observation. But quantum physics has shattered some of our cornerstone beliefs. According to Bell's theorem, any theory that is based on the joint assumption of realism and locality (meaning that local events cannot be affected by actions in space-like separated regions) is at variance with certain quantum predictions. Experiments with entangled pairs of particles have amply confirmed these quantum predictions, thus rendering local realistic theories untenable. Maintaining realism as a fundamental concept would therefore necessitate the introduction of 'spooky' actions that defy locality. Here we show by both theory and experiment that a broad and rather reasonable class of such non-local realistic theories is incompatible with experimentally observable quantum correlations. In the experiment, we measure previously untested correlations between two entangled photons, and show that these correlations violate an inequality proposed by Leggett for non-local realistic theories. Our result suggests that giving up the concept of locality is not sufficient to be consistent with quantum experiments, unless certain intuitive features of realism are abandoned.

quant-ph↗

arXiv2006

Experimental interference of independent photons

Interference of photons emerging from independent sources is essential for modern quantum information processing schemes, above all quantum repeaters and linear-optics quantum computers. We report an observation of non-classical interference of two single photons originating from two independent, separated sources, which were actively synchronized with an r.m.s. timing jitter of 260 fs. The resulting (two-photon) interference visibility was 83(+/-)4 %.

quant-ph↗

arXiv2005

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 $2.59\pm 0.08$, which is more than 7 standard deviations larger than the threshold of 2 imposed by local realism.

quant-ph↗

arXiv2005

Nonlocality of cluster states of qubits

We investigate cluster states of qubits with respect to their non-local properties. We demonstrate that a Greenberger-Horne-Zeilinger (GHZ) argument holds for any cluster state: more precisely, it holds for any partial, thence mixed, state of a small number of connected qubits (five, in the case of one-dimensional lattices). In addition, we derive a new Bell inequality that is maximally violated by the 4-qubit cluster state and is not violated by the 4-qubit GHZ state.

quant-ph↗

arXiv2004

Experimental teleportation of quantum entanglement with an optimal linear optics Bell-state analyzer

We demonstrate an experiment on entanglement swapping using an optimal Bell-state measurement capable of identifying two of the four Bell-states for polarization entangled photons, which is the optimum with linear optical elements. The two final photons belong to separately created paris. They are entangled after their original partner photons have been subjected to the Bell state measurement, whose outcome determines the type of entanglement of the final photon pair. The resulting violation of Bell's inequality in both cases confirms the success of the teleportation protocol.

quant-ph↗

arXiv2004

Exploiting the randomness of the measurement basis in quantum cryptography: Secure Quantum Key Growing without Privacy Amplification

We suggest that the randomness of the choices of measurement basis by Alice and Bob provides an additional important resource for quantum cryptography. As a specific application, we present a novel protocol for quantum key distribution (QKD) which enhances the BB84 scheme by encrypting the information sent over the classical channel during key sifting. We show that, in the limit of long keys, this process prevents an eavesdropper from reproducing the sifting process carried out by the legitimate users. The inability of the eavesdropper to sift the information gathered by tapping the quantum channel reduces the amount of information that an eavesdropper can gain on the sifted key. We further show that the protocol proposed is self sustaining, and thus allows the growing of a secret key.

quant-ph↗

arXiv2004

Complementarity and Information in "Delayed-choice for entanglement swapping"

Building on Peres's idea of "Delayed-choice for extanglement swapping" we show that even the degree to which quantum systems were entangled can be defined after they have been registered and may even not exist any more. This does not arise as a paradox if the quantum state is viewed as just a representative of information. Moreover such a view gives a natural quantification of the complementarity between the measure of information about the input state for teleportation and the amount of entanglement of the resulting swapped entangled state.

quant-ph↗

arXiv2003

De Broglie Wavelength of a Nonlocal Four-Photon

Superposition is one of the most distinct features of quantum theory and has been demonstrated in numerous realizations of Young's classical double-slit interference experiment and its analogues. However, quantum entanglement - a significant coherent superposition in multiparticle systems - yields phenomena that are much richer and more interesting than anything that can be seen in a one-particle system. Among them, one important type of multi-particle experiments uses path-entangled number-states, which exhibit pure higher-order interference and allow novel applications in metrology and imaging such as quantum interferometry and spectroscopy with phase sensitivity at the Heisenberg limit or quantum lithography beyond the classical diffraction limit. Up to now, in optical implementations of such schemes lower-order interference effects would always decrease the overall performance at higher particle numbers. They have thus been limited to two photons. We overcome this limitation and demonstrate a linear-optics-based four-photon interferometer. Observation of a four-particle mode-entangled state is confirmed by interference fringes with a periodicity of one quarter of the single-photon wavelength. This scheme can readily be extended to arbitrary photon numbers and thus represents an important step towards realizable applications with entanglement-enhanced performance.

quant-ph↗

arXiv2003

Proof-of-Concept Experiments for Quantum Physics in Space

Quantum physics experiments in space using entangled photons and satellites are within reach of current technology. We propose a series of fundamental quantum physics experiments that make advantageous use of the space infrastructure with specific emphasis on the satellite-based distribution of entangled photon pairs. The experiments are feasible already today and will eventually lead to a Bell-experiment over thousands of kilometers, thus demonstrating quantum correlations over distances which cannot be achieved by purely earth-bound experiments.

quant-ph↗

arXiv2003

Long-Distance Quantum Communication with Entangled Photons using Satellites

The use of satellites to distribute entangled photon pairs (and single photons) provides a unique solution for long-distance quantum communication networks. This overcomes the principle limitations of Earth-bound technology, i.e. the narrow range of some 100 km provided by optical fiber and terrestrial free-space links.

quant-ph↗