Semiconductor Quantum Dots for Quantum Information Processing : Addressing Key Bottlenecks Related to Emission Rate and Wavelength

Abstract

Photonic quantum information processing represents a promising approach for implementing quantum computing and communication protocols. A solid-state source of non-classical light lies at the heart of such implementations. To this end, III-V semiconductor quantum dots (QDs) have emerged as a leading platform for generating near-ideal photonic qubits in the form of non-classical light states such as single photons and entangled photon pairs. The key advantage of photonic qubits lies in the possibility to transmit them over long distances with minimal decoherence. Bearing this in mind, there has recently been a significant push towards the development of a global-scale quantum internet that promises an exponential speed- up in information processing through distributed quantum computing while guaranteeing unconditional data security via quantum cryptography. To realize this, photonic qubits need to be generated at high repetition rates and transmitted efficiently over long distances. This thesis is oriented towards the development of semiconductor QD sources targeted at addressing these two separate requirements. The first requirement, concerning qubit rate, is addressed by enhancing the emission rate of InAs QDs through the Purcell effect. This is accomplished by coupling single InAs QDs with metal-clad cylindrical GaAs nanocavities to shorten their radiative lifetime by a factor of up to 38. The ultra-small mode volume of 4.5 × 10-4 (<i>λ/n</i>)<SUP>3</SUP> ensures excellent spatial coupling between the QD and cavity. The low quality factor of 62 results in a broadband mode that is 15 nm wide, thus allowing to achieve QD-cavity resonance without employing complicated tuning mechanisms. Ultimately, the results presented here represent a major step towards realizing scalable QD sources generating non-classical light states at GHz-level repetition rates. The second requirement, concerning low-loss transmission, is addressed by developing a new GaSb-based QD source emitting in the 3rd telecom window around 1.5 µm. These QDs, which are grown by in-filling droplet-etched nanoholes, are highly uniform, symmetric, and nominally strain-free. The ensemble emission linewidth of 8 meV attests to the state-of-the-art homogeneity of QD dimensions. The multi-photon emission probability of 16 % confirms the single-photon nature of the QD emission. Overall, these findings highlight the potential for GaSb QDs to generate high-quality photonic qubits in the 3rd telecom window.