Quantum interference in a single quantum dot

The ability to process quantum information has the potential to revolutionize the fields of communication and computing. Photons interact minimally with the environment, making them robust carriers of quantum information. However, processing this information remains very challenging. Perhaps the most feasible approach is to transfer the information of a photon to an atomic state. Hence quantum optics, the study of light-matter interaction at the quantum level, occupies a central role in the emerging field of quantum information processing (QIP). Quantum optics with atomic gasses has produced spectacular results, but scaling up to multiple processing elements and incorporation into devices is difficult in such systems. Hence, a new paradigm stimulated by QIP is the application of quantum optics to solid-state media. However, a solid-state environment has many ways of dephasing or destroying the fragile quantum states. Self-assembled quantum dots (QDs), nano-sized islands of one semiconductor surrounded by a sea of another semiconductor with a larger bandgap, offer a route to overcome this obstacle. Their small size leads to strong quantization and also is effective in limiting the detrimental dephasing mechanisms. The strong quantization also allows QDs to emulate artificial atoms in many ways. To date, the most spectacular quantum optics experiments in quantum dots have exploited the atomic-like two-level system, consisting of an empty QD or a QD with an electron-hole pair (called an exciton) created by the absorption of a photon. This near ideal two-level behaviour of a QD has been exploited for high-fidelity single photon sources and basic elements for quantum computation. However, many applications at the forefront of quantum optics require a three level system. This can be achieved by charging the QD with a single carrier. Due to the two eigenstates for spin (up and down), a 4-level system is created, opening up many new possibilities. At zero magnetic field, the spin states are degenerate. Application of a magnetic field lifts the spin state degeneracy due to the Zeeman effect. Another way to go beyond the two-level system is by adding a second exciton to the QD. The expansion to a QD system with more than two levels coupled optically lies at the heart of this proposal. To date, complete coherent optical manipulation of QD states has not been realized. To achieve this, one can use two laser fields, where each field is resonant with a certain optical transition. If the dephasing of the QD levels is sufficiently suppressed (i.e. they are coherent), dramatic quantum interference effects can be manifest. One such phenomenon is electromagnetic induced transparency (EIT), whereby destructive quantum interference causes the absorption to vanish at the optical resonance of an atomic transition. Such an effect can also lead to radically new optical properties which permit the ability for instance to stop light and is important in the fields of non-linear optics and quantum information processing. The strength of the quantum interference in such a system is determined by the coherence of the states. This provides a direct way to optically probe the coherence in a system. Up to now, optical studies on self-assembled QDs have focused primarily on spectroscopy, and only recently has sensitivity to relaxation times (i.e. how long a certain state is populated) been realized at the single dot level. However, a vital necessity for potential applications is direct measurement of the coherence times of spins and excitons in a single QD. The experiments proposed here will achieve this. An additional aim of this proposal is to manipulate the coherence time of excitonic states in a QD, enabling one to tune the strength and nature, either destructive or constructive, of the quantum interference. Realization of the research outlined here will truly usher QDs and their advantageous solid-state environment into the world of quantum optics.