Narrowband Single Photons for Light-Matter Interfaces by Markus Rambach

Author:Markus Rambach
Language: eng
Format: epub, pdf
ISBN: 9783319971544
Publisher: Springer International Publishing

Another advantage of the HWP is that it does not increase the linewidth of the single photons, because the HWP itself is (almost) lossless. The additional losses, due to extra reflections, transversed surfaces and absorption, reduce the finesse to , half of its original value without any compensation. However, as the FSR is halved concurrently, we preserve the original linewidth of kHz.

A drawback of the method is that we lose around half of the brightness (coincidences) to the HH and VV photon pairs that are not separated at the polarising beam splitter and therefore not counted as coincidences. It also influences the characterisation measurements, e.g. , as the probability of accidental coincidences increases. A solution for this problem could be switching the polarisation of one of the photons after the cavity, conditioned on the arrival of the first one. In multimode operation, where the source emits many narrowband modes at different frequencies, the arrival times of the photons are well-defined and a fast Pockels cell could flip the polarisation at times when HH or VV pairs are expected. Additionally, implementing this method before filtering allows to resolve the issue for the single-mode operation as well. A more technical problem is that theoretically the HWP should not interact with the pump frequency, but in practise it causes significant losses in certain circumstances, further discussed in Sect. 3.4.2. The effects may be reduced by adjusting the incoming polarisation and careful aligning the HWP itself, but cannot be balanced out completely.

The last method to compensate for birefringence effects is simple alignment of the ppKTP and its temperature. High control over all angular and translational degrees of freedom paired with high precision temperature control allows to tune small clusters, containing a few modes, on resonance, while other modes get suppressed. This method is combining birefringence compensation with an already frequency pre-filtered output. Here, we will focus on the compensation aspects of the method, while the filtering is discussed in the Sect. 3.2.2.

In order to successfully implement this method in a triply resonant cavity, the available degrees of freedom have to be sufficient to overlap the resonances of all three fields involved, while offering enough birefringence to keep the number of modes per cluster small. The technique has been utilised for vastly different wavelengths of signal and idler [21, 28], small cavities of length similar to the crystal length [22, 29, 30] or a combination of both [31]. These implementations lead to a large tunability of the difference in FSR by small temperature adjustments within the phase-matching range. Realising the clustering effect in our setup is fairly challenging as we generate frequency degenerate photon pairs and our cavity is roughly 50 times larger than the crystal. A way to introduce enough birefringence and at the same time keep the physical cavity length at its current value is shown in Fig. 3.6. Signal and idler photons travel collinearly through the major part of the cavity, but are eventually separated in a beam displacer. One of the

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