Network Integration of a Quantum Dot Entangled Photon-pair Source

Change log
Xiang, Ziheng 

Sub-Poissonian entangled photon-pair sources based on semiconductor quantum dots are promising devices for the construction of a secure quantum network, thanks to their natural lack of multi photon-pair generation. In the previous decades, we have witnessed the achievements on a large variety of quantum dot light sources. However, they are still far from being used in an established standard fibre network. On the one hand, most of the sources are still operated in an environment that requires the installation of bulky laser setups, preventing their integration in a network. On the other hand, most of the experiments on entangled qubits generated from quantum dot sources are performed in the laboratory, resulting in a lack of work on the system supporting the remote operation of the device and high-fidelity transmission of entangled qubits over a long-distance in a large geographic scale. In this work, we demonstrate the integration of a quantum dot device in an optical network. To achieve this, both the source and the systems have been developed that favour the remote operation in a non-laboratory environment. For the source, a fully electrically operated telecom entangled on-chip pumping quantum dot device is designed and fabricated. It supports the operation at a low current of 1.6 mA and simultaneously achieves high wavelength tuneability of more than 25 nm by changing the applied bias voltage from 0 to -3.8 V. A in-laboratory entanglement fidelity of 97.3±0.2% is achieved with this on-chip pumping quantum dot light source. Several systems are developed in this work, supporting the integration of this developed source. We have proposed an innovative method for the precise alignment of the detection system to the quantum dot eigenbasis based on analysing time-resolved photon-pair correlations in a set of randomly oriented detection bases. It eliminates the restriction on the placement of the free-space polariser before the light is coupled from the quantum dot emitter into the first single-mode fibre. To support the classical traffic for sending the control signal to required components for source and system operation, we have developed a multiplexing system that allows the transmission of qubits at the telecom O-band with classical traffic at the telecom C-band over the same optical fibre without generating severe background at the quantum channel. To overcome changes of birefringence introduced by the deployed optical fibres, which causes the variation of the polarisation states during qubit transmission, we have developed a polarisation stabilisation system making use of the commercial polarisation controlling components. The system has an extremely low loss and a high duty cycle. The above development has enabled us to achieve a successful deployment and integration of the quantum dot device in the Cambridge Fibre Network. We demonstrate multiplexing of true single entangled photons tuned to 1310 nm from the on-chip pumping device with classical data traffic and achieve entanglement fidelity above 94% for over 40 hours.

Ritchie, David
Quantum Communication, Quantum Dot, Semiconductor Physics
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
Cambridge Trust - CSC Scholarship Toshiba Research Europe Limited