Building a node in a diamond quantum computer

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

If a quantum computer could be built with enough qubits, it would be able to solve problems that are intractable with the classical computers we have now. A leading design for this is to build nodes with five or more interacting qubits, and then link up many of these nodes. Nitrogen vacancy centres (NVC) in diamond at cryogenic temperatures have been used to demonstrate this linking by entangling their electron spins optically. The nuclear spins coupled to NVC can have long coherence times of over 10 seconds.

In this project we will create a single node with five or more qubits using a single NVC and the 13C nuclear spins close to it. We have built most of the equipment and will soon begin testing it at a temperature of 4 K. In addition to being useful as a node in a quantum computer, we will explore how these coupled spins could be used as a sensitive nanoscale magnetometer.

We have also built related experiments including versions that operate only at room temperature, only with a large ensemble of NVC, and only with optically-levitated nanodiamonds. We collaborate with Jason Smith's group in Oxford because they are working on speeding up the optical entanglement of two NVC as part of the Networked Quantum Information Technology (NQIT) program: the UK National Quantum Technology Hub for Quantum Computing. This PhD studentship is fully funded by the EPSRC through NQIT.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1947194 Studentship EP/N509711/1 02/10/2017 29/09/2021 Yashna Lekhai
 
Description Quantum computers offer a viable route to running certain computations much faster than classical computers like those needed to calculate the structure of proteins in the medical field, or those used in simulating particle motion and decay. There are many routes to achieving quantum computers such as Google's superconducting circuit, or ion traps, but they all have a common building block - the qubit. This is a quantum particle that can be deterministically manipulated between two distinct states. For this project we are concerned with solid state spin qubits, electrons in crystals that are bound to the lattice structure and have a property known as spin that can be changed. Specifically, we look at the nitrogen-vacancy (NV) defect in diamond, which can be optically manipulated: a green laser is used to excite electrons between energy levels, whilst microwaves are used to change the spin state of the electron.

Much work has already been completed on the NV defect for use in quantum technologies, specifically its use as a qubit for storing and manipulating data. It is now becoming important to determine ways of fabricating working devices that incorporate and optimise the fundamental physical attributes of this qubit. This project is concerned with building a processor-like device using a technique for generating defects in diamond known as laser-writing; this method deterministically creates NV defects within a crystal that have been shown to have good characteristics such as long state coherence (the spin state given to the system is maintained for long enough to be operated on) and narrow optical linewidths (which is a measure of environment interactions). In addition to this, the laser-writing process can be exploited to create conductive structures that may act as wires imbedded into the diamond plates. These would allow more individual control of single defects, making in effect a processor that could be chained to others to create a network, akin to those currently used in classical computers.

During this project our collaborators in Oxford have succeeded in fabricating complex structures within single samples, including grids of single NV defects surrounded by electrical probes which we are characterising and testing. Combining confocal microscopy with optically detected magnetic resonance allows us to determine parameters of the defects such as their coherence. To test the electrical components requires the development of new and existing techniques such as photolithography. Whilst commonly employed in the silicon and semiconductor industry, it is not optimised for samples made from diamond or of the scale that we use, but will enable us to test the effects of these laser written structures on the NV centres, critical if they are to be used in a functioning device.
Exploitation Route Through this project we are working towards marrying the scientific potential of the NV centre in diamond with the engineering requirements to fabricate useable, scalable devices. The results will define the extent to which laser-writing can integrate various components of a quantum processor node such as the qubits themselves and electrical control structures. The immediate follow-on from our findings will be to fabricate single devices combining the structures, and then utilise more complex properties of the NV defect in conjunction, such as creating 13C nuclei quantum registers or, electron entanglement. Whilst that is still based on an academic level, some of the structures and techniques are applicable to other applications, for example the wires can be optimised for use in particle detectors, whilst the photolithographic patterning of samples connects well to the semiconductor industry.
Sectors Digital/Communication/Information Technologies (including Software)
URL https://link.aps.org/doi/10.1103/PhysRevApplied.12.064005