Physics is in the midst of the second quantum revolution

It's in this revolution in which we are using the truly weird and wonderful aspects of quantum theory—like Schrödinger’s cat and Heisenberg’s uncertainty principle—to create technological advancements in the 21st century.

At the same time there are still profound problems in physics—in particular how quantum theory can be reconciled with our experience of the macroscopic world and the physical nature of time—which we are trying to solve.

Quantum information theory

Quantum information theory is what lies behind emerging quantum technologies, such as quantum cryptography (with security guaranteed by the laws of physics) and quantum computing (which promises exponentially faster solutions to certain problems).

Our theoretical work at CQD takes place within the Australia-wide Centre for Quantum Computation and Communication Technology, and we work closely with the experimentalists in CQD’s quantum optics information laboratory. Topics of research include new forms of quantum information processing, characterising quantum correlations and modelling quantum hardware.

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Recent key papers

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Theory and control of open quantum systems

An open quantum system is a quantum system that is continuously interacting with its environment. Understanding open quantum systems is vital to quantum technology, in two ways.

First, the coupling means that the quantum system can leave an imprint on the environment. This “imprint” can actually be profound, and technologically important. In particular, the beam of a laser is the imprint of the excitations in the laser cavity on the external electromagnetic field. We are currently investigating ultimate limits to the coherence of that beam as a function of the laser cavity energy.

Second, the coupling leaves an imprint on the quantum system, which can be thought of as noise affecting the system. Achieving the high levels of control needed for quantum technologies requires having intimate knowledge of this noise mechanism. Thus we are working on developing universal system characterization tools, capable of providing precise knowledge about the dynamics of open quantum systems, as well as optimal control tools that exploit this knowledge.

The various projects in this broad topic work are funded by the Australian Research Council (A DECRA Fellowship held by Paz-Silva, a DP held by Wiseman, and the Centre for Quantum Computation and Communication Technology) and by the Australian government via an AUSMURI grant, a multidisciplinary collaboration with machine learning and experimental experts at other leading Australian institutions.

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Quantum measurement and estimation

Unlike classical systems, measurement plays an essential role in quantum mechanics, as measuring a quantum system unavoidably disturbs it. This limits the precision with which one can estimate the parameters influencing a quantum system, and entangled states and adaptive (i.e. actively controlled) measurement schemes are typically necessary to attain these limits. However, measurements also allow systems to be controlled in uniquely quantum ways, enabling, for example, measurement-based quantum computation, and rapid state-preparation using measurement and feedback control. Also, broadening from parameter estimtion to quantum state estimation raises challenges that don't exist in the classical case, in particular with regard to estimating states in the past.

We work across each of these topics, many in collaboration with experimental partners in the Centre for Quantum Computation and Communication Technology and elsewhere.

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Recent key publications

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Foundations and extensions of quantum theory

Quantum mechanics is our best theory of the microscopic world, yielding accurate statistical predictions.  However, significant issues remain to be resolved, including the limits to what can be known about a quantum system, what the theory implies for our understanding of reality, and whether it needs to be extended in some way to reconcile the Schrödinger equation with our perceptions of time and reality.

Topics of research include uncertainty relations and the wave-particle duality, ontological models (general theorems and specific instances e.g. Bohmian mechanics), quantum contextuality, quantum causality, a new quantum theory of time, and many interacting worlds (a new approach to quantum phenomena).

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Recent key papers

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Quantum thermodynamics

Until recently, scientists have believed that erasing information requires energy. Our research shows that this energy cost can be reduced to zero and instead, the cost of erasure can be paid in terms of another conserved quantity, such as spin angular momentum.

The new erasure mechanism calls for a fundamental revision of thermodynamics, including the second law.  It also imposes new restrictions for perpetual machines of the second kind.

We are currently exploring experimental implementations and possible applications.

Team leader

Recent key papers

  • Croucher, T., Bedkihal, S. & Vaccaro, J. A. (2017). Discrete Fluctuations in Memory Erasure without Energy Cost. Phys. Rev. Lett. 118, 060602.
  • Barnett S. Vaccaro J, (2013), Beyond Landauer erasure, Entropy 15, 4956-4968
  • Vaccaro J. Barnett S, (2011), Information erasure without an energy cost, Proceedings of the Royal Society A 467, 1770-1778
  • Vaccaro J. Barnett S, (2009), The Cost of Erasing Information, AIP Conference Proceedings 1110, 37-40

Want to know more?

Contact the Centre for Quantum Dynamics