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Progress towards a spin photon interface

Felipe Ortiz Huerta

Published Jan 1, 2016• 0 citations
OpenAlex

Abstract

The demanding requirements for a controllable large scale quantum system made of only one type of physical qubit, such as photons, can lead to complex quantum computational architectures which are challenging to realise in practise. This has driven interest towards hybrid quantum systems which can combine the relative strengths of different physical qubits to vastly simplify architecture. Here we study the key building block for a hybrid spin-photon system: the spin-photon interface (SPI). The negatively charge nitrogen-vacancy centre (NV- of research attention in recent years due to the ability to optically polarise and measure its ground state electron spin. Its long electronic spin coherence times and optical readout, at room temperature, makes the NVcentre a strong candidate for demonstrating a flexible SPI. Furthermore, coherent electronic interaction with nearby long-lived 13C nuclear spins provide a powerful platform for performing multi-qubit manipulations whilst offering extremely long coherence and storage times. However, engineering strong optical interaction with NVspins has been limited by the inability to either fabricate high quality cavities on demand, or to efficiently couple NVcentres to those cavities. Here we will describe progress towards realising a useful SPI. We detail a novel approach to constructing microcavities which will allow high-fidelity fabrication and efficient coupling of NVcentres in nanodiamond through a two-photon lithography fabrication process. We will present our lithographically defined microcavities and other developments we have made towards a SPI, such as Solid Immersion Lenses (SIL’s) and Distributed Bragg Reflectors (DBR’s). Additionally, we outline progress towards coherent electronic and nuclear spin control, a key requirement of a scalable SPI. The ability to combine the high fidelity fabrication of optical cavities with coherent spin manipulation will allow the creation of a flexible SPI that can form a core component of proposed quantum technologies.

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The demanding requirements for a controllable large scale quantum system made of only one type of physical qubit, such as photons, can lead to complex quantum computational architectures which are challenging to realise in practise. This has driven interest towards hybrid quantum systems which can combine the relative strengths of different physical qubits to vastly simplify architecture. Here we study the key building block for a hybrid spin-photon system: the spin-photon interface (SPI). The negatively charge nitrogen-vacancy centre (NV- of research attention in recent years due to the ability to optically polarise and measure its ground state electron spin. Its long electronic spin coherence times and optical readout, at room temperature, makes the NVcentre a strong candidate for demonstrating a flexible SPI. Furthermore, coherent electronic interaction with nearby long-lived 13C nuclear spins provide a powerful platform for performing multi-qubit manipulations whilst offering extremely long coherence and storage times. However, engineering strong optical interaction with NVspins has been limited by the inability to either fabricate high quality cavities on demand, or to efficiently couple NVcentres to those cavities. Here we will describe progress towards realising a useful SPI. We detail a novel approach to constructing microcavities which will allow high-fidelity fabrication and efficient coupling of NVcentres in nanodiamond through a two-photon lithography fabrication process. We will present our lithographically defined microcavities and other developments we have made towards a SPI, such as Solid Immersion Lenses (SIL’s) and Distributed Bragg Reflectors (DBR’s). Additionally, we outline progress towards coherent electronic and nuclear spin control, a key requirement of a scalable SPI. The ability to combine the high fidelity fabrication of optical cavities with coherent spin manipulation will allow the creation of a flexible SPI that can form a core component of proposed quantum technologies.
The holy grail for quantum engineers and scientists is to build the quantum internet that spans over the entire globe. This information infrastructure holds the promise for transmitting information securely, scaling up computing power exponentially and setting the standards for precision measurement at the ultimate limit. Solid-state-based spin systems recently emerge as promising building blocks for the quantum internet. Among these candidates, the negatively charged nitrogen vacancy (NV) center in diamond attracted much attention thanks to its optical addressability, long spin coherence times, and well-controlled electronic orbitals and spin states. However, the non-ideal optical properties of NV poses a challenge to its implementation in quantum technologies. This calls for building photonic structures as efficient spin-photon interfaces for realizing strong interactions with photon modes or efficient out-coupling of its fluorescence. Such interfacing structures are also of great importance for other optically active spin-systems newly found. In this dissertation, chirped dielectric cavities are designed for building NV as fast single photon sources via broadband Purcell enhancement, using an inverse simulation approach to maximize the broadband absorption of the atomically thin absorbers. Simulated NV-cavity coupling indicates broadband Purcell factor of ∼> 100. Next, to realize coupled NV-cavity systems over large scale, a self-aligned nano-implantation technique is investigated using a lithographically defined hybrid mask for both precision pattern transfer and nitrogen implantation. Measured results show single-NV per cavity yield of ∼ 26±1% and 5-fold Purcell induced intensity enhancement. Finally, chirped circular gratings are designed for efficient collection from the NV for remote entanglement and precision sensing. Simulated grating structures present near-unity collection efficiencies. These demonstrated techniques and structures are also applicable to other solid-state-based spin systems.
This thesis describes a series of experiments on the control of the optical properties of the nitrogen-vacancy (NV) center in diamond, and on control of the electron and nuclear spin states associated with the NV center. The NV center is a fluorescing atomic defect center in diamond, consisting of a substitutional nitrogen atom adjacent to a vacancy in the diamond carbon lattice. The electron spin of the NV center can be initialized and read out optically, and coherently manipulated with high fidelity even at room temperature. This unique combination of properties has attracted much attention to the NV center in recent years for application in quantum information technologies. The experiments in this work focus on exploiting the intrinsic hybrid nature of the NV center. The NV center forms a hybrid spin register, as its electron spin is always coupled to the nuclear spin of its own nitrogen nucleus, and can furthermore be coupled to additional nuclear carbon-13 spins in the diamond. In addition, the electronic spin state can be mapped onto the state of a photon, in principle allowing long range transmission of quantum information and measurement-based entanglement of distant NV center registers. The long-term goal is to achieve a large scale quantum network where the different constituents each perform the task they are most well suited for: the stable nuclear spins store the quantum information, photons transfer quantum information over long distances, and the fast electron spin interfaces between spin and photon states. For measurement-based entanglement of distant NV centers it is of primary importance to maximize the emission and detection of coherent photons emitted by the NV center. Spontaneous emission can be controlled and enhanced by coupling an emitter to a photonic cavity. Chapters 3, 4, and 5 of this thesis describe experiments aimed at maximizing the coherent photon emission rate of an NV center by coupling the NV center to a photonic crystal cavity. One of the main challenges is to position a single NV center into the nanometer-sized cavity mode. Chapter 3 describes the development of a nanopositioning technique, which allows diamond nanocrystals ~50 nm in size containing single NV centers to be placed at arbitrary locations on a chip, with nanometer precision. By using a sharp etched tungsten tip, individual nanocrystals can be picked up, moved and placed under real-time imaging with an electron microscope. We explicitly demonstrate that the unique optical and spin properties of the NV center are conserved by the nanopositioning process. In chapter 4, we apply the nanopositioning technique to couple single NV centers contained in a diamond nanocrystals to gallium phosphide photonic crystal cavities. These cavities resonate in the visible with high quality factors, and are designed to have the lowest energy mode coincide with the zero-phonon line of the NV center. Efficient coupling is evidenced by a strong enhancement of NV center emission at the cavity wavelength. The high quality factor of a photonic crystal cavity is a result of careful design and fabrication. The introduction of a nanocrystal may therefore be expected to have a detrimental effect on the optical properties of a cavity. In chapter 5 we investigate the effect of a nanocrystal on the optical properties of photonic crystal cavities. Our simulations and measurements show that the effect of a nanocrystal is in fact only minor, a promising result for future work on cavity-QED systems in the solid state and with diamond defect centers in particular. Understanding and mitigating decoherence is a key challenge for quantum science and technology. The main source of decoherence for solid-state spin systems is the uncontrolled spin bath environment. Chapter 6 describes experiments that exploit quantum control of the NV center and recently developed dynamical decoupling techniques to study the properties of the surrounding bath of spins belonging to single substitutional nitrogen atoms. The coherence properties of a single NV center are affected by changes in the state of the bath spins, allowing a single NV center to be used as a probe to detect bath spin resonances and study the quantum dynamics of the spin bath. We use quantum control of the spin bath to extend the dephasing time of the NV center, important for the application of the NV center in DC magnetometry. In chapter 7 we describe the development and implementation of decoherence protected quantum gates for the hybrid spin register formed by the NV center electron spin that is coupled to the nuclear spin of its own nitrogen atom. Electron and nuclear spins evolve and decohere at vastly different rates, making it challenging to use such a hybrid system as a fully functional quantum register. We create a universal set of two-qubit quantum gates by integrating dynamical decoupling techniques into the gate operation. We demonstrate the power of our gate design by implementing for the first time Grover's quantum search algorithm on a solid-state spin register.
Quantum computation and information processing is an exciting multi-disciplinary field, whose objective is to harness the power of superposition and entanglement of quantum states to establish a new regime of computation for Mankind. This work demonstrates our efforts, in both materials and device engineering, towards building an efficient spin-photon interface for a promising solid-state spin qubit, namely the nitrogen-vacancy (NV) centre in diamond. Improvement in optical coherence of NV centre is realised via the novel laser-writing fabrication technique and quantified using high-resolution spectroscopy. A new metric concerning the charge-state stability of the NV centre is introduced to better translate the measured NV quality to entanglement generation efficiency. To address the low branching ratio of NV emission into the zero-phonon line (ZPL) transition, a hybrid open-cavity device incorporating low-dimensional micro-structured NV-containing diamond membrane is fabricated and characterised. In addition to the more commonly quoted emission enhancement factor, the collection efficiency of the cavity device is also quantified via numerical simulations. Combining all observed improvements in key metrics under the framework of double-heralding entanglement schemes, a two order of magnitude increase in entanglement generate rate can be expected. The established simulation methodology is widely applicable in the design and analysis of similar devices with an emitter-in-cavity setup.
A central aim of quantum information processing is the efficient entanglement of multiple stationary quantum memories via photons. Among solid-state systems, the nitrogen-vacancy centre in diamond has emerged as an excellent optically addressable memory with second-scale electron spin coherence times. Recently, quantum entanglement and teleportation have been shown between two nitrogen-vacancy memories, but scaling to larger networks requires more efficient spin-photon interfaces such as optical resonators. Here we report such nitrogen-vacancy-nanocavity systems in the strong Purcell regime with optical quality factors approaching 10,000 and electron spin coherence times exceeding 200 μs using a silicon hard-mask fabrication process. This spin-photon interface is integrated with on-chip microwave striplines for coherent spin control, providing an efficient quantum memory for quantum networks. Nitrogen-vacancy centres in diamond have established themselves as excellent candidates for solid-state quantum memories due to their optical addressability and long coherence times. Here, the authors report on a diamond-nanocavity system with improved spin-photon interface performances.
Optical nanocavities enable a strong interaction between single photons and single emitters. An appealing application is the construction of a quantum interface for photonic and solid state qubits. Since the material of the solid state qubit is often dierent from the nanocavity, there has been considerable interest in combining the two in a hybrid architecture. We describe our recent development of such a hybrid interface based a Gallium Phosphide photonic crystal nanocavity that is scanned and deterministically coupled to single emitters on a surface. The technique is used to couple the cavity to the nitrogen vacancy center in diamond, an emitter system with optically accessible electron spins and the ability to transfer electronic spin states to nuclear spins.