arXiv subjects

Christoph Simon

Publications and source records attributed to Christoph Simon.

At least 163 records · Page 9Linked to original sources

arXiv2007

Entangling Independent Photons by Time Measurement

A quantum system composed of two or more subsystems can be in an entangled state, i.e. a state in which the properties of the global system are well defined but the properties of each subsystem are not. Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in information processing and quantum communication. Remarkably, entanglement can be "swapped": if one prepares two independent entangled pairs A1-A2 and B1-B2, a joint measurement on A1 and B1 (called a "Bell-State Measurement", BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted or shared any common past[1,2]. Experiments using twin photons produced by spontaneous parametric down-conversion (SPDC) have already demonstrated entanglement swapping[3-6], but here we present its first realization using continuous wave (CW) sources, as originally proposed[2]. The challenge was to achieve sufficiently sharp synchronization of the photons in the BSM. Using narrow-band filters, the coherence time of the photons that undergo the BSM is significantly increased, exceeding the temporal resolution of the detectors. Hence pulsed sources can be replaced by CW sources, which do not require any synchronization[6,7], allowing for the first time the use of completely autonomous sources. Our experiment exploits recent progress in the time precision of photon detectors, in the efficiency of photon pair production by SPDC with waveguides in nonlinear crystals[8], and in the stability of narrow-band filters. This approach is independent of the form of entanglement; we employed time-bin entangled photons[9] at telecom wavelengths. Our setup is robust against thermal or mechanical fluctuations in optical fibres thanks to cm-long coherence lengths.

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arXiv2007

Quantum Repeaters with Photon Pair Sources and Multi-Mode Memories

We propose a quantum repeater protocol which builds on the well-known DLCZ protocol [L.M. Duan, M.D. Lukin, J.I. Cirac, and P. Zoller, Nature 414, 413 (2001)], but which uses photon pair sources in combination with memories that allow to store a large number of temporal modes. We suggest to realize such multi-mode memories based on the principle of photon echo, using solids doped with rare-earth ions. The use of multi-mode memories promises a speedup in entanglement generation by several orders of magnitude and a significant reduction in stability requirements compared to the DLCZ protocol.

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arXiv2007

Detection loophole in asymmetric Bell experiments

The problem of closing the detection loophole with asymmetric systems, such as entangled atom-photon pairs, is addressed. We show that, for the Bell inequality I_3322, a minimal detection efficiency of 43% can be tolerated for one of the particles, if the other one is always detected. We also study the influence of noise and discuss the prospects of experimental implementation.

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arXiv2007

Analysis of a quantum memory for photons based on controlled reversible inhomogeneous broadening

We present a detailed analysis of a quantum memory for photons based on controlled and reversible inhomogeneous broadening (CRIB). The explicit solution of the equations of motion is obtained in the weak excitation regime, making it possible to gain insight into the dependence of the memory efficiency on the optical depth, and on the width and shape of the atomic spectral distributions. We also study a simplified memory protocol which does not require any optical control fields.

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arXiv2006

Quantum Communication with Quantum Dot Spins

Single electron spins in quantum dots are attractive for quantum communication because of their expected long coherence times. We propose a method to create entanglement between two remote spins based on the coincident detection of two photons emitted by the dots. Local nodes of several qubits can be realized using the dipole-dipole interaction between trions in neighboring dots and spectral addressing, allowing the realization of quantum repeater protocols. We have performed a detailed feasibility study of our proposal based on tight-binding calculations of quantum dot properties.

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arXiv2005

Multi-Partite Entanglement Inequalities via Spin Vector Geometry

We introduce inequalities for multi-partite entanglement, derived from the geometry of spin vectors. The criteria are constructed iteratively from cross and dot products between the spins of individual subsystems, each of which may have arbitrary dimension. For qubit ensembles the maximum violation for our inequalities is larger than that for the Mermin-Klyshko Bell inequalities, and the maximally violating states are different from Greenberger-Horne-Zeilinger states. Our inequalities are violated by certain bound entangled states for which no Bell-type violation has yet been found.

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arXiv2005

Precision of single-qubit gates based on Raman transitions

We analyze the achievable precision for single-qubit gates that are based on Raman transitions between two near-degenerate ground states via a virtually excited state. In particular, we study the errors due to non-perfect adiabaticity and due to spontaneous emission from the excited state. For the case of non-adabaticity, we calculate the error as a function of the dimensionless parameter $χ=Δτ$, where $Δ$ is the detuning of the Raman beams and $τ$ is the gate time. For the case of spontaneous emission, we give an analytical argument that the gate errors are approximately equal to $Λγ/Δ$, where $Λ$ is the rotation angle of the one-qubit gate and $γ$ is the spontaneous decay rate, and we show numerically that this estimate holds to good approximation.

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arXiv2004

Realisation of Hardy's Thought Experiment

We present an experimental realisation of Hardy's thought experiment [Phys. Rev. Lett. {\bf 68}, 2981 (1992)], using photons. The experiment consists of a pair of Mach-Zehnder interferometers that interact through photon bunching at a beam splitter. A striking contradiction is created between the predictions of quantum mechanics and local hidden variable based theories. The contradiction relies on non-maximally entangled position states of two particles.

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arXiv2004

Comment on "Separability of quantum states and the violation of Bell-type inequalities"

The statement of E.R. Loubenets, Phys. Rev. A 69, 042102 (2004), that separable states can violate classical probabilistic constraints is based on a misleading definition of classicality, which is much narrower than Bell's concept of local hidden variables. In a Bell type setting the notion of classicality used by Loubenets corresponds to the assumption of perfect correlations if the same observable is measured on both sides. While it is obvious that most separable states do not satisfy this assumption, this does not constitute "non-classical" behaviour in any usual sense of the word.

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arXiv2004

Creating single time-bin entangled photon pairs

When a single emitter is excited by two phase-coherent pulses with a time delay, each of the pulses can lead to the emission of a photon pair, thus creating a ``time-bin entangled'' state. Double pair emission can be avoided by initially preparing the emitter in a metastable state. We show how photons from separate emissions can be made indistinguishable, permitting their use for multi-photon interference. Possible realizations are discussed. The method might also allow the direct creation of n-photon entangled states (n>2).

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arXiv2004

Could Energy Decoherence due to Quantum Gravity be observed?

It has recently been proposed that quantum gravity might lead to the decoherence of superpositions in energy, corresponding to a discretization of time at the Planck scale. At first sight the proposal seems amenable to experimental verification with methods from quantum optics and atomic physics. However, we argue that the predicted decoherence is unobservable in such experiments if it acts globally on the whole experimental setup. This is related to the unobservability of the global phase in interference. We also show how local energy decoherence, which acts separately on system and phase reference, could be detected with remarkable sensitivity and over a wide range of length scales by long-distance Ramsey interferometry with metastable atomic states. The sensitivity of the experiments can be further enhanced using multi-atom entanglement.

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arXiv2004

Robust long-distance entanglement and a loophole-free Bell test with ions and photons

Two trapped ions that are kilometers apart can be entangled by the joint detection of two photons, each coming from one of the ions, in a basis of entangled states. Such a detection is possible with linear optical elements. The use of two-photon interference allows entanglement distribution without interferometric sensitivity to the path length of the photons. The present method of creating entangled ions also opens up the possibility of a loophole-free test of Bell's inequalities.

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arXiv2003

Entanglement Detection Based on Interference and Particle Counting

A sufficient condition for entanglement in two-mode continuous systems is constructed based on interference visibility and the uncertainty of the total particle number. The observables to be measured (particle numbers and particle number variances) are relatively easily accessible experimentally. The method may be used to detect entanglement in light fields or in Bose-Einstein condensates. In contrast to the standard approach based on entanglement witnesses, the condition is expressed in terms of an inequality which is nonlinear in expectation values.

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arXiv2003

Theory of an Entanglement Laser

We consider the creation of polarization entangled light from parametric down- conversion driven by an intense pulsed pump inside a cavity. The multi-photon states produced are close approximations to singlet states of two very large spins. A criterion is derived to quantify the entanglement of such states. We study the dynamics of the system in the presence of losses and other imperfections, concluding that the creation of strongly entangled states with photon numbers up to a million seems achievable.

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arXiv2003

Probabilistic instantaneous quantum computation

The principle of teleportation can be used to perform a quantum computation even before its quantum input is defined. The basic idea is to perform the quantum computation at some earlier time with qubits which are part of an entangled state. At a later time a generalized Bell state measurement is performed jointly on the then defined actual input qubits and the rest of the entangled state. This projects the output state onto the correct one with a certain exponentially small probability. The sufficient conditions are found under which the scheme is of benefit.

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arXiv2002

Towards quantum superpositions of a mirror

We propose a scheme for creating quantum superposition states involving of order $10^{14}$ 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. All experimental requirements appear to be within reach of current technology.

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arXiv2002

Multi-Photon Entanglement Concentration and Quantum Cryptography

Multi-photon states from parametric down-conversion can be entangled both in polarization and photon number. Maximal high-dimensional entanglement can be concentrated 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 as undesirable. We propose a simple multi-photon protocol for the case of low losses and point out the robustness of the entanglement under photon loss.

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arXiv2002

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 based on 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.

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