Experimental characterization of cluster states using fibre sources

Summary form only given. Spontaneous four wave mixing (FWM) in photonic crystal fibre (PCF) is a promising approach to improved sources of photon pairs for quantum information applications. When pumped by a bright pulsed laser, the χ
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nonlinearity of silica can produce correlated photons between a signal and idler wavelength equally spaced above and below the pump in frequency [1]. In PCF, the microstructure surrounding the silica core can be chosen to engineer the dispersion of the fibre, which allows control of the phase-matched signal and idler wavelengths, and even over the degree of spectral correlation between signal and idler - in particular, avoiding all spectral correlation can result in heralded photons in an intrinsically pure quantum state, without the usual need for lossy filtering [2]. Other advantages of PCF include a high brightness with moderate pump power due to the small guided mode area and long interaction length, and good coupling efficiency to single mode fibre for detection because the photons are generated in a similar mode shape.Multi-partite entangled states have many applications across quantum information, but cluster or graph states have recieved particular attention due to the potential for scalable, universal quantum computing based on measurement of a large cluster state [3]. In this paper, we discuss experimental methods for the generation of entanglement between four qubits, using two sources of polarization entangled photon pairs from PCFs setup in Sagnac loops, and quantum interference at a polarizing beamsplitter (Fig. 1a) to perform a fusion operation [4, 5]. We present results for 3 and 4 photon GHZ states with respective fidelities of 75% and 67%, and for a `star-cluster' state equivalent to a rotated GHZ state (Fig. 1b and c). Using this state, we demonstrate logic gates in the measurement based model of quantum computing [6]. We will also discuss progress in using interferometric path degrees of freedom for two of the photons to extend the cluster state with additional qubits (Fig 1d).