Narrowband Single Photons for Light-Matter Interfaces

Abstract

Quantum technologies are becoming the driving engine of physical innovations in the 21st century, leading science and humanity onto a path towards a technological revolution. Early experiments have shown the enormous potential of this field, but after more than two decades, quantum technology is still without an outstanding candidate for its underlying architecture. The reason for this are specific inherent weaknesses, found in all quantum platforms available to date. For example, for photons, it is the difficulty to keep them stored locally and implement two-qubit gates due to their low interaction, while systems based on e.g. atoms or ions fall short on mobility and have a high experimental overhead, making them hard to transport. A promising path forward is hybridisation of quantum technologies, seeking to combine individual quantum architectures by transferring the information between two quantum systems of different type, harnessing their strengths, while circumnavigating their weaknesses.We want to benefit from the high mobility and ease of transmission of photons for quantum communication and exploit the excellent readout and storage capabilities of atomic qubits as a quantum memory. To realise this, efficient interaction between the two qubit carriers is necessary.The atomic transition usually has a much narrower bandwidth than single photons generated by spontaneous parametric downconversion (SPDC), the current gold standard of producing high-purity heralded single photons at flexible wavelengths. A high-quality interface of light particles with atoms therefore demands matching the spectral properties between the photons and the resonances of the atomic species. As the manipulation of atomic transitions is limited, the solution is to significantly reduce the single photon emission spectrum to fulfil the requirements. We achieve this is by performing the downconversion process inside an optical cavity in order to enhance the probability of creating the emitted pairs in the spectral and spatial resonator mode.Previous cavity-based SPDC sources have achieved bandwidths comparable to atomic linewidths, however, this is not sufficient to be merged with high-efficiency storage schemes. The atomic memory with the highest demonstrated storage and recall fidelity is the gradient echo memory (GEM) based on rubidium. To achieve these outstanding fidelities, GEM requires photons at sub-natural linewidths, e.g. sub-MHz for rubidium. Additionally, the operation time of most sources is divided into stabilisation and photon production phases, resulting in typical duty cycles < 50%. The so far narrowest photons from SPDC have bandwidths still well above a MHz and only a few sources have demonstrated 100% duty cycle.In this thesis, I realise an efficient light-matter interface to be used with a rubidium-based GEM. My source offers 100% duty cycle generation of sub-MHz single photon pairs at the rubidium D1 line using cavity-enhanced SPDC. I introduce a new technique – the ”flip-trick” – using a half-wave plate inside the cavity to achieve triple resonance of the pump, signal and idler photons. This allows probabilistic creation of single photons at any given time (100% duty cycle) without compromising the achievable linewidth and enables the highest spectral brightness from a SPDC-based source to date. The double-exponential decay of the temporal intensity cross-correlation function exhibits a bandwidth of 429±10 kHz for the single photons, an order of magnitude below the natural linewidth of the target transition and well suited for the implementation with GEM. This is the narrowest bandwidth of single photons from SPDC reported so far.The quantum nature of the source was confirmed by the idler-triggered second-order autocorrelation function at τ = 0 to be g(2)s,s (0) = 0.032 ± 0.003 for a heralding rate of 3.5 kHz, and antibunching below 0.5 was observed up to heralding rates of 70 kHz. The high multi-photon suppression of the source is matched by high indistinguishability of the photons, demonstrated in a Hong-Ou-Mandel (HOM) interference experiment with a visibility V = 96.7 ± 3.4 % of the central dip. In addition, the mode-locked two-photon state of the generated pairs leads to revivals of the HOM dip. We measured these revivals with up to 105 m path difference between signal and idler photons, where V = 38.2 ± 2.4 %, giving independent proof of the exceptional coherence length of our photons.The narrow bandwidth in combination with high brightness, multi-photon suppression and indistinguishability makes our system the perfect source for the future integration with GEM, one of the most promising schemes for quantum memories to date, or hollow-core glass fibres filled with rubidium gas to allow the construction of novel quantum logic gates. Furthermore, the extension of the photon wave packet over more than 100 m can easily cover a whole experimental setup, making the photons an ideal candidate for measurements on quantum foundations, e.g. quantum causality.