Quantum Dot Spin Dynamics and Quantum Randomness for Secure Optical Communication

This thesis investigates two complementary quantum technologies for secure optical communication: spin dynamics in telecom C-band quantum dots and a high-bandwidth source-device-independent quantum random number generator. Together, they address two central requirements of quantum networks: long-lived spin memories for quantum repeaters and secure, high-rate entropy generation for quantum key distribution.

The first part studies the spin dynamics of InAs/InGaAs/InP quantum dots grown by strain-free droplet epitaxy using time-resolved Faraday ellipticity. Compared to Stranski-Krastanov grown telecom dots, droplet epitaxy shows improved spin properties, including an electron spin relaxation time of 3.0 us and reduced in-plane electron g-factor anisotropy, indicating enhanced structural symmetry. Polarization recovery and Hanle measurements reveal nuclear spin lifetimes comparable to electron spin lifetimes, attributed to quadrupolar interactions arising from Ga-In interdiffusion. These results demonstrate the advantages of droplet epitaxy over strain-based growth and indicate further potential through optimized material compositions.

The second part investigates one-, two-, and four-layer InAs/InAlGaAs Stranski-Krastanov quantum dots to assess layer-dependent carrier type and spin coherence. Increasing layer number induces a transition from electron to hole resident carriers due to interlayer tunnelling into lower-energy dots, the emergence of spin-mode locking for four or more layers, and the appearance of a non-oscillating spin component likely associated with reduced heavy-hole/light-hole mixing. Although droplet epitaxy samples exhibit superior spin parameters overall, no spin-mode locking is observed, potentially due to shorter transverse coherence or electron-dominated charging.

The final part presents a source-device-independent quantum random number generator based on heterodyne detection of vacuum fluctuations. Using a 90 degree optical hybrid, balanced photodetectors, a dual-channel 12-bit ADC operating at 3.2 GS/s per channel, and FPGA-implemented Toeplitz hashing with PCIe transfer, a total extracted bandwidth of 33.91 Gbit/s is achieved. The system assumes no trust in the optical source and performs all extraction in hardware, maintaining security even under adversarial state preparation.