Christian Kurtsiefer

Multiphoton entanglement and interferometry

Instytut Fizyki Teoretycznej i Astrofizyki Uniwersytet Gda´nski, 80-952 Gda ´nsk, PolandReceived 14 June 2002, accepted 21 June 2002Published online 30 April 2003Multiphoton entanglement is the basis of many quantum communication schemes, quantum cryptographicprotocols, and fundamental tests of quantum theory. Spontaneous parametric down-conversion is the mosteffective source for polarization entangled photon pairs. Here we show, that a class of entangled 4-photonstates can be directly created by parametric down-conversion. These states exhibit perfect quantum correla-tions and a high robustness of entanglement against photon loss. Therefore these states are well suited fornew types of quantum communication.

Single photon emission from SiV centres in diamond produced by ion implantation

We report the observation of single photon emission from single SiV (silicon-vacancy) centres in diamond produced by ion implantation. The high photostability and the narrow emission bandwidth of about 5 nm at room temperature make SiV centres interesting as a single photon source in practical quantum cryptography. We discuss problems that arise from the nonradiaditve transitions which lower the brightness of the source.

High-efficiency entangled photon pair collection in type-II parametric fluorescence

We report on a method for optimizing the collection of entangled photon pairs in type-II parametric fluorescence. With this technique, we detected 360000 polarization-entangled photon pairs per second in the near-IR region in single-mode optical fibers. The entanglement of the photon pairs was verified by measuring polarization correlations in different bases of at least 96%.

Experimental detection of multipartite entanglement using witness operators

We present the experimental detection of genuine multipartite entanglement using entanglement witness operators. To this aim we introduce a canonical way of constructing and decomposing witness operators so that they can be directly implemented with present technology. We apply this method to three- and four-qubit entangled states of polarized photons, giving experimental evidence that the considered states contain true multipartite entanglement.

Decoherence-free quantum information processing with four-photon entangled states

Decoherence-free states protect quantum information from collective noise, the predominant cause of decoherence in current implementations of quantum communication and computation. Here we demonstrate that spontaneous parametric down conversion can be used to generate four-photon states which enable the encoding of one qubit in a decoherence-free subspace. The immunity against noise is verified by quantum state tomography of the encoded qubit. We show that particular states of the encoded qubit can be distinguished by local measurements on the four photons only.

Experimental Observation of Four-Photon Entanglement from Parametric Down-Conversion

We observe polarization entanglement between four photons produced from a single down-conversion source. The nonclassical correlations between the measurement results violate a generalized Bell inequality for four qubits. The characteristic properties and its easy generation with high interferometric contrast make the observed four-photon state well suited for implementing advanced quantum communication schemes such as multiparty quantum key distribution, secret sharing, and telecloning.

Experimental Single Qubit Quantum Secret Sharing

We present a simple and practical protocol for the solution of a secure multiparty communication task, the secret sharing, and its proof-of-principle experimental realization. In this protocol, a secret is split among several parties in a way that its reconstruction requires the collaboration of the participating parties. In our scheme the parties solve the problem by sequential transformations on a single qubit. In contrast with recently proposed schemes involving multiparticle Greenberger-Horne-Zeilinger states, the approach demonstrated here is much easier to realize and scalable in practical applications.

Secure communication with single-photon two-qubit states

A ball type valve for high pressure lines is designed so that the spherical valve member is offset from the centerline of the valve stem in such a manner that the spherical valve member in the open position has a cylindrical fluid passage in parallel alignment with the flow axis of the inlet and outlet passages, but in the closed position is transversely disposed across the valve chamber so that the cylindrical flow passage is perpendicular to the flow axis of the inlet and outlet passages. Due to the eccentric placement of the valve stem onto the spherical valve member, and the eccentric placement of the bore of the bonnet in which the valve stem rotates relative to the centerline of the bonnet, and the eccentric placement of the bonnet opening relative to the centerline of the inlet and outlet passages, the spherical valve member rotates in an arc from open position to closed position so as to approach the valve seat tangentially with a wiping motion and at an angle of approach so that the spherical valve member is self-locking. Further, the design of the valve is such that in open position, the surface of the spherical valve member engages the wall of the valve chamber to prevent further rotation. This eliminates the need for an external stop and prevents rotation of the spherical valve member beyond the full open position.

High speed optical quantum random number generation

We present a fully integrated, ready-for-use quantum random number generator (QRNG) whose stochastic model is based on the randomness of detecting single photons in attenuated light. We show that often annoying deadtime effects associated with photomultiplier tubes (PMT) can be utilized to avoid postprocessing for bias or correlations. The random numbers directly delivered to a PC, generated at a rate of up to 50 Mbit/s, clearly pass all tests relevant for (physical) random number generators.

Experimental Demonstration of a Quantum Protocol for Byzantine Agreement and Liar Detection

We introduce a new quantum protocol for solving detectable Byzantine agreement (also called detectable broadcast) between three parties, and also for solving the detectable liar detection problem. The protocol is suggested by the properties of a four-qubit entangled state, and the classical part of the protocol is simpler than that of previous proposals. In addition, we present an experimental implementation of the protocol using four-photon entanglement.