Self-Organized Parametric down Conversion in Optical Fibres

K. J. Blow, R. Loudon, Simon J. D. Phoenix

Keith J. Blow, R. Loudon, Simon J. D. Phoenix

Luc Thévenaz A

Jonas Hammer

The spontaneous decay of a pump photon into a tripartite entangled photon state (photon triplets) has been extensively studied in theory. Observation of these photon triplets at optical frequencies is of interest for fundamental science and future applications of quantum optics, such as quantum computing and quantum cryptography. This decay is known as spontaneous third-order parametric down-conversion (TOPDC). Experimentally, TOPDC has not been demonstrated in the optical frequency domain. TOPDC is enabled by the third-order nonlinearity of the material in which the interaction takes place. In optical fibres, the effective nonlinearity of the glass is increased manifold. The reason for this is the long light-matter interaction length and the tight confinement of light in the waveguide. Optical fibres are therefore the ideal platform for the observation of TOPDC, provided that phase matching can be achieved. In this thesis, the use of submicrometre fibre tapers is studied in the context of TOPDC, both in experiment and theory. Comparing different fibre-based TOPDC sources, fibre tapers rank among the highest in terms of expected triplet generation rate. At the same time, the background noise generated in these fibres is very low, which leads to a high expected three-photon coincidence-to-accidental ratio (CAR) on the order of 7000. In the experiments related to TOPDC, the precision of manufacturing fibre tapers is studied. A novel gas-based mechanism for dispersion tuning is demonstrated. Without this mechanism the manufacturing precision of the fibre tapers would not suffice to observe TOPDC in an experiment. The tuning mechanism is demonstrated using third-harmonic generation, which is the inverse process of TOPDC. The resonant third-harmonic radiation generated around 460nm can be tuned at a rate of −0.118nm/bar. This is achieved by introducing pressurised argon in the taper environment. The background noise from fibre tapers is characterised. The effect of the tuning mechanism on the three-photon-state and the signal-to-noise ratio is studied in theory. In another part of the thesis, gas-filled suspended-core fibres (SCFs) are used as a spectrally tunable source of entangled photon pairs (biphotons). Generation of biphotons via spontaneous four-wave mixing (FWM) in fibres offers great advantages over the generation in bulk crystals, which is the commonly employed technique. The most important advantages are the excellent control of the spatial mode structure, the extra degrees of freedom for dispersion design and the possibility to integrate the fibres into existing networks. Yet, fibre biphoton sources lack the convenient spectral tunability of crystal-based sources. Introducing pressurised gas into the hollow channels of the SCF is demonstrated to be a convenient and broadband spectral tuning mechanism. The FWM-sidebands can be continuously tuned at a rate of −0.3 THz/bar, in the quantum and in the seeded scenario. A high two-photon-CAR exceeding 10000 is measured for the biphoton state, and the frequency entangled joint-spectral intensity is measured. Finally, seeded FWM is used to characterise the core size of high-NA submicrometre fibres to a nanometre precision. If ultrafast pulses are used instead of nanosecond pulses, this will present a non-destructive and high precision characterisation technique for such fibres. The results of this work present an important step towards realising TOPDC at optical frequencies in the near future. The experimental realisation of a broadly tunable biphoton source combines the convenient tuning mechanism found in crystal-based sources with the advantages of fibre-based biphoton generation. Such tunable fibre biphoton sources can therefore be competitive with crystal-based systems for quantum optics applications.

Stefania Castelletto, Ivo Pietro Degiovanni, Alan L. Migdall +1 more

Photon-based quantum information schemes have increased the need for light sources that produce individual photons, with many such schemes relying on optical parametric down-conversion (PDC). Practical realizations of this technology require that the PDC light be collected into a single spatial mode defined by an optical fibre. We present two possible models to describe single-mode fibres coupling with PDC light fields in a non-collinear configuration, that lead to two different results. These approaches include factors such as crystal length and walk-off, non-collinear phase-matching and transverse pump field distribution. We propose an experimental test to distinguish between the two. The goal is to help clarify open issues, such as how to extend the theory beyond the simplest experimental arrangements and, more importantly, to suggest ways to improve the collection efficiency.

Bai Yunfei, 刘洪雨 Liu Hongyu, 郜江瑞 Gao Jiangrui +1 more