Towards fibre-integrated optical switching
Date:
This poster received the prize in quantum optics. Link to poster
Thank you for your interest in my poster, this informal summary is intended to be read alongside the poster for further information. A copy of the poster can be found here, and you can find more information about me on this website.
1. Phase modulation of light
The challenge presented here is to induce a phase shift in light without loss and on fast time scales (100MHz or faster). Such a tool would be extremely valuable for implmenting photonic quantum technologies, including state generation, multiplexing and potentially even the implementation of quantum gates. Conventional modulators tend to be either fast but low loss (LiNbO crytals, with 3dB loss) or low loss but slow (acoustic modulators, at a few MHz).
We are aiming to achieve both speed and low loss. Additionally we note that for a phase modulator to be useful, we really want to be reaching around a whole pi phase shift. We won’t quite get there in this demonstration, but we’ll get pretty close.
In this experiment, we have a continuous wave laser as a signal field, counter-propagating through a rubidium 87 vapour cell with strong control pulses. These are near resonance with the S-P and P-D transitions in Rb respectively. With the control off, the signal experiences some absorption through the atoms, and accumulates some phase. When a control pulse is present, the D state becomes strongly coupled to the P state, altering the susceptibility of the Rb and hence a different phase is accumulated, whilst still experiencing low loss.
We observe phase shift of 0.90(5) multiples of pi, with transmission of 74(2)%. Various routes to improvement, including optical pumping of the Rb and enhancement in a cavity or by integration with a waveguide. We have also demonstrated switching of pulsed signals, however that is not detailed here. This shows that progression towards phase modulation of single photons is possible.
2. Enhancement with a cavity
The effect is enhanced inside a cavity, by the product of the finesse of the signal and the finesse of the control. We have begun investigating with a builk cavity (where we can see finesse on the order of 10), however here switching speed becomes limited by the ringdown time of the cavity.
To improve, we aim to use microcavities, such as microbubble resonators filled with Rb. This will allow for much higher finesse and shorter response times. Sufficient enhancement may even pave the way to interaction between a single signal and control photon, which could enable the implmentation of two-photon quantum gates.
3. Integration with optical fibres
We are undertaking work to connectorise hollow core optical fibres (HCF) to single mode fibres (SMF). This requires careful mode matching between the modes in the two fibres, which we achieve by a graded-index (GRIN) fibre lens. Choosing the length of the GRIN fibre such that light exiting is collimated allows for good mode matching into HCF. The HCF is custom made for this purpose, so that its mode closely matches that exiting the SMF.
I am developing novel methods of connectorising HCF to SMF to enable filling of HCF with Rb, and realisation of an in-fibre vapour cell. This would allow enhanced phase modulation by the method described above in a scalable package. (I don’t want to share too many details here due to restrictions with collaborators, but we hope to present something new soon!)
Attribution
This presentation is mainly based on the work I have undertaken with PhD candidate Will Davis at the Univerisy of Bath under supervisor Josh Nunn and second supervisor Pete Mosley. Here I detail our demonstration of fast, low-loss phase modulation of light. I also briefly outline work undertaken by myself and another PhD candidate Tabijah Wasawo on enhancing this interaction in a cavity, and further work that I have towards integration with optical