Single electrons localized in a semiconductor can nowadays be addressed using nano-optical techniques. Dilute impurity centers represent an example where one or two localized electrons can be addressed selectively. I will discuss theoretically optical techniques to control the spin of one and two electrons at the quantum level in these systems. In the case of two spin, this entails the ability of generating and controlling entanglement.
Single electrons localized in a semiconductor can nowadays be addressed using nano-optical techniques. Dilute impurity centers represent an example where one or two localized electrons can be addressed selectively. I will discuss theoretically optical techniques to control the spin of one and two electrons at the quantum level in these systems. In the case of two spin, this entails the ability of generating and controlling entanglement.
Implementations of quantum information processing systems based on optically controlled electron spins in semiconductor quantum dots are particulary appealing due to several features. These features include inherent ultrafast gate operation times, reasonably long decoherence times, small optical control power and a natural ability to link to optical fiber communication networks. We will discuss the current state of the art in the experimental implementations of the main elements of semiconductor spin qubits: qubit initialization, single-qubit gates, two-qubit gates, entanglement distribution, projective measurement, quantum memory and indistinguishable single-photon generation.
Spins localized in semiconductor nanostructures have been intensively investigated for quantum spintronics. These include the spin of single electron localized by quantum dots or impurities, and spins of the lattice nuclei. These localized spins can be exploited as carriers of quantum information, while in some circumstances they also play the role of deleterious noise sources for other quantum objects through their couplings. Quantum control of the spins in semiconductor nanostructures is therefore of central interest for quantum applications. In this thesis, we address several problems related to the quantum control of electron or hole spin and nuclear spins in semiconductor quantum dots and impurity centers. The first problem studied is the control of nuclear spin bath for a hole spin qubit in III-V semiconductor quantum dot. In quantum dots formed on III-V compounds, the direct band gap of the host material allows ultrafast optical addressability of a single electron or hole spin qubit. However, nonzero nuclear spins of group III and group V elements result in a large statistical fluctuation in the Zeeman splitting of the spin qubit which then dephases in nanosecond time scale. We present a novel feedback scheme to suppress the statistical fluctuation of the nuclear spin field for enhancing the coherence time of the hole spin qubit. We also find positive feedback control which can amplify the magnitude of the nuclear field, so that a bimodal distribution can develop, realizing a quantum environment that can not be described by a single temperature. The second problem addressed here is the control of donor spin qubits in silicon architecture which have ultra-long quantum coherence time. We developed the quantum control scheme to realize the quantum metrology of magnetic field gradient, based on the celebrated Kane’s architecture for quantum computation. The scheme can also be generalized to calibrate the locations of the donors. In the third part of the thesis, we investigate a novel type of quantum dot formed in a new class of two-dimensional semiconductors, monolayer transition metal dichalcogenides (TMDs), which exhibit interesting spin and pseudospin physics. This novel quantum dot system may offer new opportunity for quantum spintronics in the ultimate 2D limit, and we investigate here the valley pseudospin as a possible quantum bit carrier. A main finding is that, contrary to the intuition, the lateral confinement by the quantum dot potential does not lead to noticeable valley hybridization, and therefore the valley pseudospin in monolayer TMDs QD can well inherit the valley physics such as the valley optical selection rules from the 2D bulk which implies a variety of quantum control possibilities.
By choosing the right system and using the right techniques, it is possible to achieve reliable control of an individual quantum system in a solid. Certain atom-like solid-state systems are especially suited for this goal. The electron spin of the diamond nitrogen-vacancy (NV) impurity center is a leader among such systems and has featured in a great deal of recent experimental work in the context of various quantum technologies. By extending optical control for the NV center we increase the utility of this system, opening it up to fresh applications in quantum optics. \n \nDoing quantum control with a solid-state spin comes with its own challenges. In particular it can be difficult to simultaneously isolate single systems, both for control and from environment-induced decoherence, while also coupling multiple systems together in a controlled way. A goal of the work presented in this dissertation is to develop techniques for answering this problem in the NV center. \n \nOptical control, as opposed to the microwave control usually used for state manipulation in the NV center, would make it easier to address only one spin system at a time. We demonstrate such control using two methods, two-photon optically driven Rabi oscillations and stimulated Raman adiabatic passage. These both have the added advantage that by using Raman-resonant, dipole-detuned optical fields, they protect the spin state from the decoherence normally associated with the optical transitions. Furthermore, we see that this electron spin control is nuclear spin dependent, providing a mechanism for coupling these two spin systems. \n \nWe also investigate a decoherence reduction technique that involves coupling continuous microwave fields to the spin states. The resulting "dressed states" are shielded from spin-bath-induced magnetic field fluctuations. We confirm this using optical coherent population trapping measurements which we have also developed in the NV center. We show that these measurements are sensitive to nuclear spin states as well as to dressed states. \n \nThese results supply the missing piece, optical spin manipulation, to control schemes that are all-optical, and they demonstrate ways to significantly push back the decoherence limit. \n \nThis dissertation includes previously published and unpublished co-authored material.
We review the progress and main challenges in implementing large-scale quantum computing by optical control of electron spins in quantum dots (QDs). Relevant systems include self-assembled QDs of III–V or II–VI compound semiconductors (such as InGaAs and CdSe), monolayer fluctuation QDs in compound semiconductor quantum wells, and impurity centres in solids, such as P-donors in silicon and nitrogen-vacancy centres in diamond. The decoherence of the electron spin qubits is discussed and various schemes for countering the decoherence problem are reviewed. We put forward designs of local nodes consisting of a few qubits which can be individually addressed and controlled. Remotely separated local nodes are connected by photonic structures (microcavities and waveguides) to form a large-scale distributed quantum system or a quantum network. The operation of the quantum network consists of optical control of a single electron spin, coupling of two spins in a local nodes, optically controlled quantum interfacing between stationary spin qubits in QDs and flying photon qubits in waveguides, rapid initialization of spin qubits and qubit-specific single-shot non-demolition quantum measurement. The rapid qubit initialization may be realized by selectively enhancing certain entropy dumping channels via phonon or photon baths. The single-shot quantum measurement may be in situ implemented through the integrated photonic network. The relevance of quantum non-demolition measurement to large-scale quantum computation is discussed. To illustrate the feasibility and demand, the resources are estimated for the benchmark problem of factorizing 15 with Shor's algorithm.
Single spins in semiconductor quantum dots form a promising platform for solid-state quantum information processing. The spin-up and spin-down states of a single electron or hole, trapped inside a quantum dot, can represent a single qubit with a reasonably long decoherence time. The spin qubit can be optically coupled to excited (charged exciton) states that are also trapped in the quantum dot, which provides a mechanism to quickly initialize, manipulate and measure the spin state with optical pulses, and to interface between a stationary matter qubit and a 'flying' photonic qubit for quantum communication and distributed quantum information processing. The interaction of the spin qubit with light may be enhanced by placing the quantum dot inside a monolithic microcavity. An entire system, consisting of a two-dimensional array of quantum dots and a planar microcavity, may plausibly be constructed by modern semiconductor nano-fabrication technology and could offer a path toward chip-sized scalable quantum repeaters and quantum computers. This article reviews the recent experimental developments in optical control of single quantum dot spins for quantum information processing. We highlight demonstrations of a complete set of all-optical single-qubit operations on a single quantum dot spin: initialization, an arbitrary SU(2) gate, and measurement. We review the decoherence and dephasing mechanisms due to hyperfine interaction with the nuclear-spin bath, and show how the single-qubit operations can be combined to perform spin echo sequences that extend the qubit decoherence from a few nanoseconds to several microseconds, more than 5 orders of magnitude longer than the single-qubit gate time. Two-qubit coupling is discussed, both within a single chip by means of exchange coupling of nearby spins and optically induced geometric phases, as well as over longer-distances. Long-distance spin-spin entanglement can be generated if each spin can emit a photon that is entangled with the spin, and these photons are then interfered. We review recent work demonstrating entanglement between a stationary spin qubit and a flying photonic qubit. These experiments utilize the polarization- and frequency-dependent spontaneous emission from the lowest charged exciton state to single spin Zeeman sublevels.