Single-photon indistinguishability of nanowire quantum dots for entanglement swapping

Abstract

Entanglement swapping is a crucial technique for realizing quantum networks and the quantum internet. Photons are promising carriers for quantum information processing, and semiconductor quantum dots can act as sources of both single and entangled photons. An ideal quantum dot based entangled photon source should provide on-demand emission, high brightness, suppressed multiphoton emission, high indistinguishability, and high en- tanglement fidelity. In this thesis, nanowire quantum dots (NWQDs) are used to generate entangled photon pairs. Using a Hanbury Brown–Twiss (HBT) measurement, we demon- strate single-photon emission by observing strong suppression of multiphoton events. The NWQDs achieve a brightness of around 10 k counts per second, which is advantageous for quantum communications which often operates over large optical losses of 30dB or higher. The generated entangled states exhibit entanglement fidelities above 95%, indicating that the entanglement produced by the NWQDs is reliable. The main focus of this work is the characterization of the indistinguishability of photons emitted by a NWQD. Indistinguishability is assessed using Hong Ou Mandel (HOM) inter- ference, where the HOM visibility is used as a proxy for photon indistinguishability. The relevant optical transitions correspond to exciton, biexciton, and trion states (positive and negative), which are treated as analogous to atomic emission lines. Previous measurements on the exciton transition reported a HOM visibility of about 40%(on a 0−100% scale), mo- tivating investigation into noise mechanisms, particularly charge noise, that may degrade indistinguishability in the semiconductor environment. For this purpose, the NWQD is placed in a quadrupole gate structure that enables application of a lateral electric field. HOM measurements are performed under different gate voltages to study the effect of the electric field to evaluate whether electrical control of the charged environment improves HOM visibility. Additionally, a wavelength shift of approximately 0.1 nm is observed for both exciton and biexciton transitions when changing from 0 V to 300 V. Indistinguisha- bility is calculated using two complementary methods which are named the area method and the fitting method. While a small systematic change in HOM visibility is observed across the investigated voltage range, any apparent trends are not fully conclusive, as they may be influenced by the fitting procedures employed to extract the HOM visibility.