Master Thesis Defence by Mikkel Thorbjørn Mikkelsen

Multiphoton and heralded entanglement sources for applications in quantum cryptography

Qubit entanglement is a key enabler of secure communication protocols of tomorrow. The robust nature of photons against environmental influences makes these viable encoding candidates for quantum information processing over long distances. Heralded qubit entanglement enables cryptography based on device- independent quantum key distribution(DIQKD), which promises unconditional security, relying on only the validity of quantum physics. The work on this thesis demonstrates heralded entanglement generation using highly indistinguishable pure single-photons. The single-photon source is based on an InAs/GaAs quantum dot coupled to an ’on-chip’ planar nanophotonic waveguide circuit. Upon pulsed excitation of the quantum dot, it emits a temporal chain of single pho- tons which is demultiplexed (temporal-to-spatial modes) to create a four-photon source. A single-photon rate of 2.3Mhz is observed to result in a demultiplexed four-fold coincidence rate of 3.4Hz, with a lower bound indistinguishability of VHOM = (84.± 1.7)% for superimposed output modes. The four-photon source has been employed in a quantum gate for heralded entanglement generation. We here observe a violation of Bell’s inequality (more specifically the Clauser-Horne- Shimony-Holt inequality) within 3σ, i.e. = 2.24±0.08. The quantum correlation fringe visibility achieved of V= (81.± 0.8)% predicts an even greater violation of = 2.30 ± 0.02. Further, the security of E91 based DIQKD protocols are depen- dent on detection loophole free Bell inequality violation. This can be achieved by heralding the presence of an Einstein-Podolsky-Rosen pair with means of the ability to distinguish between Bell states. We here propose a design for a thin-film metasurface implementation of partial Bell state measurement to realise a low-loss device. The estimated output fidelity reaches = 94.1% proving a viable platform for this purpose.

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