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International Conference on Quantum Information

Tunable set-up for an entire family of four-photon entangled states

Witlef Wieczorek, Ch. Schmid, Nikolai Kiesel, Roland Krischek, Harald Weinfurter

Published Jan 1, 2008• 0 citations
Publisher

Abstract

We report on the experimental observation and analysis of an entire family of four-photon entangled states. We demonstrate how these states can be obtained with a single linear optics set-up and analyze particular entanglement properties.
DOI
10.1364/icqi.2008.qmc3

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•Jan 1, 2008•3 citations
Spontaneous parametric downconversion in combination with linear optics was successfully used to observe a variety of multiphoton entangled states. Yet, experiments performed so far lacked flexibility, as each of the various setups was useful for only a particular multiphoton entangled state. In this paper, we describe how, by using multiphoton interference, one can observe entire families of multiphoton entangled states in the very same linear optical setup. Our method thus goes beyond the commonly used two-photon interference and turns out to be a very useful tool for state observation. We will discuss the interference of four and six photons at different types of beam splitters and show which families of entangled states are observable. The benefits of this approach are demonstrated in a four-photon interference experiment by observing a variety of highly entangled multiphoton states.
Since the awareness of entanglement was raised by Einstein, Podolski, Rosen and Schrödinger in the beginning of the last century, it took almost 55 years until entanglement entered the laboratories as a new resource. Meanwhile, entangled states of various quantum systems have been investigated. Sofar, their biggest variety was observed in photonic qubit systems. Thereby, the setups of today's experiments on multi-photon entanglement can all be structured in the following way: They consist of a photon source, a linear optics network by which the photons are processed and the conditional detection of the photons at the output of the network. In this thesis, two new linear optics networks are introduced and their application for several quantum information tasks is presented. The workhorse of multi-photon quantum information, spontaneous parametric down conversion, is used in different configurations to provide the input states for the networks. The first network is a new design of a controlled phase gate which is particularly interesting for applications in multi-photon experiments as it constitutes an improvement of former realizations with respect to stability and reliability. This is explicitly demonstrated by employing the gate in four-photon experiments. In this context, a teleportation and entanglement swapping protocol is performed in which all four Bell states are distinguished by means of the phase gate. A similar type of measurement applied to the subsystem parts of two copies of a quantum state, allows further the direct estimation of the state's entanglement in terms of its concurrence. Finally, starting from two Bell states, the controlled phase gate is applied for the observation of a four photon cluster state. The analysis of the results focuses on measurement based quantum computation, the main usage of cluster states. The second network, fed with the second order emission of non-collinear type II spontaneous parametric down conversion, constitutes a tunable source of a whole family of states. Up to now the observation of one particular state required one individually tailored setup. With the network introduced here many different states can be obtained within the same arrangement by tuning a single, easily accessible experimental parameter. These states exhibit many useful properties and play a central role in several applications of quantum information. Here, they are used for the solution of a four-player quantum Minority game. It is shown that, by employing four-qubit entanglement, the quantum version of the game clearly outperforms its classical counterpart. Experimental data obtained with both networks are utilized to demonstrate a new method for the experimental discrimination of different multi-partite entangled states. Although theoretical classifications of four-qubit entangled states exist, sofar there was no experimental tool to easily assign an observed state to the one or the other class. The new tool presented here is based on operators which are formed by the correlations between local measurement settings that are typical for the respective quantum state.