Quantum information applications with fibre sources of photon pairs

• Main Author: Bell, Bryn A.
• Published: University of Bristol 2014
• Subjects: 003.54
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.659105

Quantum information science holds immense promise for new and enhanced technologies in communications, computation, and precision measurement, as well as being closely linked to the fundamentals of quantum mechanics. One approach to realising these goals is to encode information onto single photons, the individual quanta of light. In this thesis, I .make use of a source of pairs of photons based on spontaneous four-wave mixing in microstructured fibre, which generates correlated signal and idler photons equally spaced in frequency above and below a bright pump laser powering the process. This source is well suited to demonstrating small-scale quantum information tasks involving a few quantum bits due to its brightness ' (the pump power required to reach a high generation rate is lower than comparable sources using bulk crystals) and its high coupling efficiency into standard optical fibre and photon detectors. Also, the use of a microstructure allows the dispersion properties of the fibre to be precisely engineered, adding control over the photons' wavelengths and spectral properties. Quantum interference is observed between signal photons in a Hong Ou Mandel dip with a visibility of 73%, which is thought to be limited mainly by impurity in the spectral state. Also, polarization entanglement between a signal-idler pair is generated with a fidelity of 87%. The source is then used in an interferometric setting to demonstrate enhanced sensitivity to optical phase changes and path-lengths compared to what can be achieved with classical light, and interference fringes are observed with down to a sixth of the wavelength of the classical pump laser. Using two sources of entangled pairs and a 'fusion' gate, three and four photon entangled GHZ states are produced with fidelities of 75% and 67% respectively. A rotated version of the four photon state is used to demonstrate three logic gates in the one-way model of quantum computing, the Hadamard H, the T phase rotation, and the controlled-NOT. Finally, the entangled state is extended to graph states with more complex structures, using path degrees of freedom to : add extra qubits to photons. A graph-state error correcting code is demonstrated.