Electron Spins in Semiconductor Quantum Dots

This thesis describes a series of experiments aimed at understanding and controlling the behavior of the spin degree of freedom of single electrons, confined in semiconductor quantum dots. This research work is motivated by the prospects of using the electron spin as a quantum bit (qubit), the basic building block of a quantum computer. Here, the envisioned basis states (logical 0 and 1) of the qubit are the two possible orientations of the spin in a magnetic field: spin-up (parallel to the field) and spin-down (anti-parallel to the field). In this thesis, a number of important steps towards the use of electron spins as qubits are reported: the isolation of a single electron in a quantum dot, energy spectroscopy of the electron spin states, development of a new technique to probe a nearlyisolated quantum dot, single-shot read-out of the electron spin orientation, and increased understanding of the interaction of the electron spin with its environment. A quantum dot can be thought of as a small box filled with a controllable number of electrons. This box is coupled via tunnel barriers to reservoirs, with which electrons can be exchanged, and is coupled capacitively to one or more gate electrodes that allow the number of electrons on the dot to be varied. Due to the small dot size (typically ? 50 nm), comparable to the Fermi wavelength of the electrons, it exhibits a discrete energy spectrum. The quantum dot devices studied in this work are defined in a two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure, by applying negative voltages to metallic gate electrodes fabricated on top of the heterostructure.Applied Science

Diamond NV centers for quantum computing and quantum networks

The exotic features of quantum mechanics have the potential to revolutionize information technologies. Using superposition and entanglement, a quantum processor could efficiently tackle problems inaccessible to current-day computers. Nonlocal correlations may be exploited for intrinsically secure communication across the globe. Finding and controlling a physical system suitable for fulfi lling these promises is one of the greatest challenges of our time. The nitrogen-vacancy (NV) center in diamond has recently emerged as one of the leading candidates for such quantum information technologies thanks to its combination of atom-like properties and solid-state host environment. We review the remarkable progress made in the past years in controlling electrons, atomic nuclei, and light at the single-quantum level in diamond. We also discuss prospects and challenges for the use of NV centers in future quantum technologies.Quantum NanoscienceApplied Science

Quantum rekenen: Quantumcomputers en qubits

De quantum computer is een computer gebaseerd op quantum bits, kortweg qubits. Dat zijn bits die fysiek gemaakt zijn van quantum systemen, met de speciale eigenschap dat ze in een superpositie tussen twee toestanden kunnen zijn.QN/Quantum NanoscienceApplied Science

Top-down fabrication of plasmonic nanostructures for deterministic coupling to single quantum emitters

Metal nanostructures can be used to harvest and guide the emission of single photon emitters on-chip via surface plasmon polaritons. In order to develop and characterize photonic devices based on emitter-plasmon hybrid structures, a deterministic and scalable fabrication method for such structures is desirable. Here, we demonstrate deterministic and scalable top-down fabrication of metal wires onto preselected nitrogen vacancy centers in nanodiamonds using clean room nano-fabrication methods. We observe a life-time reduction of the emitter emission that is consistent with earlier proof-of-principle experiments that used non-deterministic fabrication methods. This result indicates that top-down fabrication is a promising technique for processing future devices featuring single photon emitters and plasmonic nanostructures.QN/Quantum NanoscienceApplied Science

Quantum internet: A vision for the road ahead

The internet-a vast network that enables simultaneous long-range classical communication-has had a revolutionary impact on our world. The vision of a quantum internet is to fundamentally enhance internet technology by enabling quantum communication between any two points on Earth. Such a quantum internet may operate in parallel to the internet that we have today and connect quantum processors in order to achieve capabilities that are provably impossible by using only classical means. Here, we propose stages of development toward a full-blown quantum internet and highlight experimental and theoretical progress needed to attain them.Accepted Author ManuscriptQuantum Internet DivisionQuTechQuantum Information and SoftwareQID/Elkouss GroupQID/Hanson La

Optimal design of diamond-air microcavities for quantum networks using an analytical approach

Defect centres in diamond are promising building blocks for quantum networks thanks to a long-lived spin state and bright spin-photon interface. However, their low fraction of emission into a desired optical mode limits the entangling success probability. The key to overcoming this is through Purcell enhancement of the emission. Open Fabry-Perot cavities with an embedded diamond membrane allow for such enhancement while retaining good emitter properties. To guide the focus for design improvements it is essential to understand the influence of different types of losses and geometry choices. In particular, in the design of these cavities a high Purcell factor has to be weighed against cavity stability and efficient outcoupling. To be able to make these trade-offs we develop analytic descriptions of such hybrid diamond-and-air cavities as an extension to previous numeric methods. The insights provided by this analysis yield an effective tool to find the optimal design parameters for a diamond-air cavity.QuTechQID/Hanson La

Decay of Rabi Oscillations by Dipolar-Coupled Dynamical Spin Environments

We study the Rabi oscillations decay of a spin decohered by a spin bath whose internal dynamics is caused by dipolar coupling between the bath spins. The form and rate of decay as a function of the intrabath coupling is obtained analytically, and confirmed numerically. The complex form of decay smoothly varies from power law to exponential, and the rate changes nonmonotonically with the intrabath coupling, decreasing for both slow and fast baths. The form and rate of Rabi oscillations decay can be used to experimentally determine the intrabath coupling strength for a broad class of solid-state systems.Kavli Institute of NanoscienceApplied Science

Decoherence dynamics of a single spin versus spin ensemble

We study decoherence of central spins by a spin bath, focusing on the difference between measurement of a single central spin and measurement of a large number of central spins (as found in typical spin-resonance experiments). For a dilute spin bath, the single spin demonstrates Gaussian free-induction decay, in contrast to exponential decay characteristic of spin ensembles. A strong difference between a single spin and a spin ensemble also exists for the Rabi oscillation decay: for a repeated Rabi oscillation experiment, suppression of decoherence happens for a single spin while acceleration takes place for a spin ensemble. The mathematical origin of such behavior is similar to quantum Zeno/anti-Zeno effects.Kavli Institute of NanoscienceApplied Science

Diamond-based quantum technologies

Optically accessible spins associated with defects in diamond provide a versatile platform for quantum science and technology. These spins combine multiple key characteristics, including long quantum coherence times, operation up to room temperature, and the capability to create long-range entanglement links through photons. These unique properties have propelled spins in diamond to the forefront of quantum sensing, quantum computation and simulation, and quantum networks.QN/vanderSarlabQID/Taminiau LabQID/Hanson LabQN/Hanson La