Recent developments in the theory of measurement-based quantum computing reduce the problem of building a quantum computer to that of achieving high quality rotation and measurement of single qubits. The first generation of such machines may well therefore consist of individual modules each containing a single quantum system that embodies the qubit. The first demonstrations of entanglement of electronic qubits by measurement have been performed recently in ion traps. The leading contenders for physical qubits in the solid state are the negatively charged nitrogen-vacancy defect in diamond and the Stranski Krastanow quantum dot, each of which offers long electronic spin dephasing times and convenient spin-sensitive optical transitions. In this article we will compare the strengths and weaknesses of these two systems and discuss some of the challenges to be met in constructing a measurement based quantum computer in the solid state.
Recent developments in the theory of measurement-based quantum computing reduce the problem of building a quantum computer to that of achieving high quality rotation and measurement of single qubits. The first generation of such machines may well therefore consist of individual modules each containing a single quantum system that embodies the qubit. The first demonstrations of entanglement of electronic qubits by measurement have been performed recently in ion traps. The leading contenders for physical qubits in the solid state are the negatively charged nitrogen-vacancy defect in diamond and the Stranski Krastanow quantum dot, each of which offers long electronic spin dephasing times and convenient spin-sensitive optical transitions. In this article we will compare the strengths and weaknesses of these two systems and discuss some of the challenges to be met in constructing a measurement based quantum computer in the solid state.
Abstract This article aims to review the developments, both theoretical and experimental, that have in the past decade laid the ground for a new approach to solid state quantum computing. Measurement‐based quantum computing (MBQC) requires neither direct interaction between qubits nor even what would be considered controlled generation of entanglement. Rather it can be achieved using entanglement that is generated probabilistically by the collapse of quantum states upon measurement. Single electronic spins in solids make suitable qubits for such an approach, offering long coherence times and well defined routes to optical measurement. We will review the theoretical basis of MBQC and experimental data for two frontrunner candidate qubits – nitrogen‐vacancy (NV) centres in diamond and semiconductor quantum dots – and discuss the prospects and challenges that lie ahead in realising MBQC in the solid state.
This work investigates the applicability of the nitrogen-vacancy (NV) defect center in diamond for quantum computation and metrology purposes. The NV center is a molecule like defect in the diamond lattice which is optically active and possesses an electron spin in its ground state. The spin can be used as a quantum bit and as a field sensor. The NV center has three major properties enabling individual control of single defects. Its optical properties depend on the spin state, the spin can be polarized optically and the spin state has an exceptionally long lifetime. Single NV centers can be controlled by optical and microwave radiation. Generally, the research on individually controllable quantum systems is not only motivated by quantum computation and communication. With the ongoing miniaturization nowadays electronic and electro-optic devices will soon reach the limit of single quantum building blocks. While in these applications the full complexity of single quantum systems might not be needed, small sensors based on single atoms or molecules might very well take advantage of the susceptibility of quantum coherences to environmental conditions. Major challenges for all approaches towards single quantum system control are the individual addressing, the measurement of the quantum state and the protection of quantum coherence from environmental noise. Today, there is a variety of systems (including the NV center) that allow control on the single quantum level. These are for example single photons, single ions and atoms in traps, single spins in semiconductors or single molecules in solid state systems. Rather mesoscopic examples are superconducting devices, nanomechanical oscillators or surface plasmon polaritons. Regarding the NV center it has been shown that the magnetic moment of its electron spin allows individual sensing and coupling to other surrounding electron and nuclear spins. Especially single nuclear spins were shown to be exceptional quantum memories. In this work small quantum registers of up to 5 spins have been used to test major requirements for a feasible quantum processor. Therefore, mainly artificially created high purity diamond samples with controlled spin and impurity concentration were used. The NV centers were either grown in or created via ion implantation techniques in a controlled fashion. As a first requirement for quantum computation entanglement among up to three spins has been generated and was verified by quantum state tomography. This proves the access of the full Hilbert space. Especially, entanglement among nuclear spins was very long-living. In further experiments this processor was used for first quantum algorithms like super-dense coding and the Deutsch algorithm. In addition, this work demonstrates new methods to initialize nuclear spin qubits into a desired quantum state. This is particularly important as improper initialization inhibits quantum computation. One of these methods is related to dynamic nuclear spin polarization and the other to measurement based state preparation. Nuclear spins in the diamond lattice originate mainly from the C-13 isotope. By artificially altering its content the influence on the NV center spin was investigated. The coherence times of the NV center were increasing inversely with the nuclear spin density. Even increasing nuclear spin density to offer more nuclear spin qubits to an operational quantum register was possible. As an alternative route of scaling up the quantum register pairs of closely spaced NV centers were investigated. For the first time two qubit gates among two NV centers mediated by magnetic dipolar coupling have been realized. Furthermore the sensing ability was demonstrated by measuring the relative distance of the two centers (10 nm) with sub-nm precision. Eventually, this work contains new insight into the electronically excited state of the NV center which allowed a more advanced control of the electron and nuclear spins. A comparably high magnetic field was used to decouple electron and nuclear spins. Finally, this allowed quantum nondemolition measurements on single nuclear spins which manifests in the observation of quantum jumps. The new insight was used to drastically improve the performance of quantum state readout. Summarizing, it was shown that single NV centers and proximal nuclear spins form versatile small quantum registers which can be interconnected with neighboring such NV quantum registers. Although the number of qubits used in this work is still small the experimental results propose a way of how to proceed when building a quantum computer. Regarding quantum metrology, however, the use of such a small register which can sense fields and store and process the results would already outperform a single NV electron spin by orders of magnitude.
The ability to manipulate individual quantum systems in a precise way has led to a new era of quantum technologies, including quantum computation and quantum simulation. In this thesis we present several new implementations of these quantum technologies. We first aim at addressing current experimental challenges for quantum dot based quantum computing. We propose a systematic way to study the dynamics of nuclear spin, which is responsible for the short electron spin coherence time. Our calculation is based on diffusion model and is consistent with experiments. We also invent a novel protocol to realize high-fidelity ultrafast universal quantum gate in recently-developed quantum dot molecule system. Experimental realization of our protocol requires only a simple time engineering of optical pulses. We then propose a new quantum state transfer scheme for Nitrogen-Vacancy center based quantum computer, which is applicable at room temperature. Our method accomplishes high fidelity robust quantum state transfer through uncontrolled thermal nitrogen spin chain between two remote NV registers. Our next study helps building a hybrid quantum computer by entangling disparate systems using photonic links. The photons emitted from two types of system need to be matched in both frequency and pulse shape. We propose a simple method to match the emitted pulse shape from two qubit systems with different transition linewidths. We then focus on quantum simulation with trapped ions. We show the possibility of observing a novel type of temperature driven structural phase transition in trapped ion chain, which originates from anharmonic interaction between different vibrational modes. Afterward, an experimental protocol to simulate a conceptually new state of matter, called time crystal, is proposed based on ions trapped in a ring trap. Finally, we propose two new applications based on the recently developed trapped ion quantum simulator of spin models: (1) simulation of Haldane-Shastry model, which opens the way of experimental study to a remarkable theoretical model involving spin liquid ground state and fractional excitations (2) observation of prethermalization and dynamical phase transition, which are poorly understood non-equilibrium phenomena in closed quantum many-body system.
The negatively charged tin-vacancy center in diamond (SnV−) is a highly promising candidate for next-generation long-distance quantum networks, owing to its bright emission, robustness against electronic noise, and extended spin coherence times at higher temperatures. We demonstrate the measurement of a single SnV− electronic spin with a single-shot readout fidelity of 87.4%, which can be further enhanced to 98.5% using conditional readout. Our findings show that SnV− centers are compatible with rapid microwave spin control, overcoming the typical trade-off between optical readout and spin control found in group-IV centers in diamond. Additionally, we investigate measurement-induced dephasing through weak quantum measurement, shedding light on the fundamental interplay between measurement and decoherence in quantum mechanics, and utilizing the qubit’s spin coherence as a metrological tool. These advancements represent a significant milestone in SnV− based quantum technologies, providing techniques and insights widely applicable to solid-state quantum emitters.
In this thesis, quantum states of single spin systems in solid state are investigated for potential applications in quantum information processing. The studied system is the nitrogen-vacancy defect in diamond, which can be thought of as a molecule trapped inside the diamond crystal. An issue for high-fidelity state preparation is the intitialization of the system. The photo-induced charge state dynamics of the nitrogen-vacancy defect is determined, and methods for charge state initialization are beeing developed. The electron spin state of single nitrogen-vacancy defects can be read out optically. Here, we focus on nearby nuclear spins coupled to the electron spin. We investigate the quantum evolution of a single nuclear spin, measurement of non-orthogonal quantum states and improved magnetic field sensing. Based on three nuclear spins, a small quantum register is realized. We prepare and measure the GHZ and the W state in this register, and implement an algorithm for quantum error correction.
Quantum computing is an attractive and multidisciplinary field, which became a focus for experimental and theoretical research during the last decade. Among other systems, such as ions in traps and superconducting circuits, solid state based qubits are considered to be promising candidates for use in first experimental tests of quantum hardware. Here we report recent progress in quantum information processing with point defects in diamond. Qubits are defined as single spin states (electron or nuclear). This allows exploration of long coherence times (up to seconds for nuclear spins at cryogenic temperatures). In addition, the optical transition between ground and excited electronic states allows coupling of spin degrees of freedom to the state of the electromagnetic field. Such coupling gives access to spin state read-out via spin-selective scattering of photons. This also allows the use of spin states as robust memory for flying qubits (photons).