Entangling dopant nuclear spins using double quantum dots

Quantum mechanics has led to a deep and profound understanding of the electronic and optical properties materials, which has underpinned the technological revolution of the past century. Yet, there are key elements of quantum mechanics, specifically ideas such as 'coherent superposition' and 'entanglement', which have still to be harnessed directly in a technological application. With our improving ability to control smaller and smaller devices, with ever greater precision, we begin to enter a regime where such concepts can evolve from abstract 'thought experiments' to phenemona exhibited by real devices. Sufficiently controlled, superposition and entanglement will enable a new set of technologies - termed Quantum Technologies (QTs)- which offer major and fundamental improvements over certain existing technologies. Examples include ultimately secure communication, enhanced sensors, and 'quantum' computers able to solve problems that are simply intractable on any existing computer today.

Silicon devices have demonstrated quantum bit (qubit) characteristics which make them extremely promising for future QTs. As for most potential QT platforms, the next key step is identifying ways to scale up control and interactions between qubits, and, as seen when comparing different QT approaches, there is a compromise between using 'natural' quantum systems such as those based on atoms, and 'artificial' ones such as those based on superconducting circuits or quantum dots.

This project will bring together both such approaches, as is possible within a silicon-based architecture, in order to benefit from their respective advantages. We will use the uniquely long coherence times of donor spins in silicon (which can be as long as hours), with the tunable control of quantum dots in which entangled singlet and triplets are natural basis states. In doing so, we will demonstrate a scalable method to entangle very long-lived quantum bits in silicon, which will enable future applications in metrology and quantum computers.

This project seeks to explore a new idea with the potential to dramatically impact progress towards silicon-based quantum technologies.

Although quantum physics is now over a hundred years old, some of the strangest and most profound outcomes of quantum physics (such as superposition, or entanglement) have yet to be directly exploited in a technology. Such technologies are termed quantum technologies, and we are still learning about the tremendous enhancements they might offer over existing technologies. These include computers ('quantum computers') with the capability of solving problems which are simply intractable on the fastest computers today. The societal impact of such a computational power can not easily be overstated; the full potential is as unforeseeable today as preliminary computers were many decades ago. However, some applications have already been identified, such as quantum simulators, capable of modelling and predicting material, chemical and biochemical properties far more accurately than can be conceived today.
Other identified types of quantum technologies include enhanced sensors and imaging devices, as well as highly secure encrypted data transmission with impacts in individual and industrial data privacy.

In the United State Federal Vision for Quantum Information Science, published in 2009, the following is written about the potential for quantum technologies: "[They] are at an early pre-application stage, but possess a novelty and a richness that suggests the likelihood of even greater unanticipated impact [than the transistor or laser]". It is therefore no surprise that major ICT companies around the world are investing heavily in research into quantum technologies - these companies include Hitachi (our industrial project partner on this proposal) IBM, Hewlett-Packard, Fujitsu, Siemens, Microsoft, Toshiba, Nokia and NTT.

One of the key challenges that must be addressed in advancing progress towards practical quantum technologies is scaling up small demonstrations of coherent quantum control to many-body systems. Meeting this challenge in any material would be a major achievement, but to do so in silicon (as we propose in this project) would be especially exciting, as it the most important material used in today's technologies. This would enable quantum technologies to harness the mature silicon nano-fabrication techniques developed over the past decades, and would permit the integration of quantum and 'classical' forms of information processing on the same chip.

Aside from the main objective of this project to develop quantum technologies, our basic research on dopants in silicon nanodevices may also lead to impact in conventional silicon-based technologies, especially 'beyond CMOS' technologies. The size of transistors in current computers processors are approaching the limit where the device performance is affected by the discrete number of dopant atoms in the channel. A more thorough understanding of how to tune and control the interaction of the dopant atom with charge transport in the nanodevice could impact the design of future transistors.