# Research in the Quantum Science Group

Our chief research focus is to create a unified effort leveraging the strengths and techniques of optical, atomic and condensed matter quantum systems towards fundamentally new breakthrough science and applications. This strategy is at the core of our research program, and represents a unique opportunity for, and strength of, the Quantum Science group at Sydney: a highly-integrated effort of leading researchers in both quantum optical/atomic physics and condensed-matter physics, theory and experiment. Below we detail some of the main projects being undertaken in our group.

Our research activities are diverse, spanning theory and experiment, from quantum foundations to precision metrology and mesoscopic physics. This work is supported by our participation in the Centre of Excellence for Engineered Quantum Systems, and various International research programs in Quantum Information Science.

## Semiconductor Spin Qubits

Realising large-scale quantum technologies from single-qubit building-blocks presents formidable scientific challenges. Most pressing is the need to develop means to couple large numbers of qubits and mitigate decoherence, which transforms quantum systems into their cumbersome classical counterparts.

Semiconductor devices are ideally suited to address this first challenge of scalability. With fabrication approaches that parallel the conventional semiconductor industry, modern lithographic techniques are capable of creating large numbers of coupled quantum systems. Decoherence however, represents our most fundamental scientific challenge in the development of quantum technologies based on condensed matter devices.

Our experimental effort aims at demonstrating new quantum control methods to combat decoherence in condensed matter systems. Our initial focus is spin qubits based on gallium arsenide (GaAs) quantum dots. These systems are ideally suited as testbeds for developing new theoretical and experimental approaches that mitigate decoherence arising from the mesoscopic environment. We aim to extend coherence in these systems via the use of quantum feedback, weak measurements, and state-preparation of the mesoscopic environment.

## New Biomedical Technology based on Quantum Systems

The nascent field of quantum nanoscience provides a unique platform for developing new biomedical tools. Nanoparticles, for instance, are well suited to translocation of the circulatory, lymphatic, and nervous systems, acting as vehicles for the targeted delivery of therapeutics and, in biofunctionalised form, as labels of disease.

Detecting and imaging nanoparticles in-vivo with high contrast however, presents formidable challenges that require new and fundamental developments.

In this project we are exploring the use of nanoparticles as fluorescent, field-sensitive biomarkers and simultaneous novel MRI contrast agents. Our approach combines hyperpolarised MRI techniques with single fluorescence sensing to enable the detection and tracking of targeted bioagents both at the intracellular level and scale of macroscopic tissue.

## Quantum Control of Trapped Ions

A primary focus of our research on trapped ions is the development of efficient and robust control techniques for arbitrary quantum systems in the presence of environmental noise. Decoherence - the decay of the "quantumness" of a state - is a major challenge for any quantum system, and requires a dedicated effort to produce error-resistant approaches to quantum control.

Open-loop coherent control protocols provide a means to dynamically suppress random errors in quantum systems, addressing a primary challenge in quantum technology. Our work aims to expand the efficacy and applicability of dynamical decoupling for use in any coherent technology - establishing a fundamental role for these techniques as quantum firmware. We have recently formulated an efficient and user-friendly "filter-design" framework to understanding the performance of various open-loop control protocols. Outstanding challenges include the suppression of universal decoherence, the development of new optimization techniques, and the dynamical protection of nontrivial logic operations.

Our experimental efforts employ trapped atomic ions as a model quantum system, and permit detailed studies of quantum dynamics in noisy environments.

## Quantum-Enabled Sensing

Trapped ions are exquisite sensors of external forces and fields. Experiments have demonstrated that trapped ion crystals are the most sensitive force detectors known, outperforming rival technologies by more than three orders of magnitude. Our work in this field has earned M.J. Biercuk the 2011 NMI Prize for Excellence in Measurement Science.

We are exploiting normal modes of ion motion, spin coherence, and novel quantum control techniques to produce novel force and field sensors with unrivaled performance. Ultimately we hope to produce deployable ion-based sensors leveraging the device fabrication capabilities of the Australian Institute of Nanoscience.

## Quantum Simulation and Large-Scale Entanglement

Our work aims to study the dynamics of large-scale entangled systems and to produce useful, controllable quantum simulators. This work involves detailed theoretical studies and experiments using trapped atomic ions.

Ion crystals in a Penning trap provide a two-dimensional qubit array with regular structure. This system is ideal for the realization of large-scale entanglement via state-selective spin-motional interaction. Our work aims to produce entangled states of more than 100 particles with tunable interactions, for studies of the dynamics of entangled states. The particular states we are aiming to create may prove useful for simple tasks in quantum simulation and the evaluation of the robustness of real quantum simulators to environmental decoherence.

## Quantum Information Theory

Recent years have seen a remarkable synergy between quantum physics and information processing. It has been demonstrated that the rules of quantum physics can protect the distribution of secret cryptographic keys, allowing for unconditionally secure communication between distant parties. Also, there is strong evidence that a quantum computer operating according to quantum physics could change the rules of computer science, solving problems that are intractable on any current computing device. These observations, which promise great future technological advances based on quantum information processing, have gone hand-in-hand with remarkable scientific breakthroughs in our understanding of quantum physics.

What then are the physical limits on transmitting, storing, and processing quantum information? The answer will have implications for both future technologies and fundamental quantum physics and is the topic of the exciting new interdisciplinary field of quantum information theory.

Current areas of research include:

- Resources for measurement-based quantum computing in strongly-correlated quantum many-body systems
- Topological phases for quantum information processing in spin lattices
- Quantum measurement, feedback and control in single-photon optics and spin quantum dots
- Precision measurement at the Heisenberg limit using single-photon optics