- Quantum thermodynamics
Landauer argued that information is physical because the process of erasing the information stored in a memory device incurs an energy cost in the form of a minimum amount of mechanical work. We have recently found, however, that this energy cost can be reduced to zero by paying a cost in angular momentum or another conserved quantity. Erasing the memory of Maxwell's demon in this way implies that work can be extracted from a single thermal reservoir at a cost of angular momentum and an increase in total entropy. The new erasure mechanism calls for a fundamental restatement of the Second Law of thermodynamics [Proc. R. Soc. A 467, 1770-1778 (2011), eprint arXiv:1004.5330, Entropy 15, 4956-4968 (2013) ]. It also imposes new restrictions for perpetual machines of the second kind. We have examined the nature of the discrete fluctuations in the cost of erasing information using spin angular momentum [Phys. Rev. Lett. 118, 060602 (2017)]. We are currently exploring experimental implementations of the erasure protocol.
Further information: see the Centre for Quantum Dynamics entry on Quantum Thermodynamics.
- The quantum nature of time
Time reversal invariance (T) refers to the symmetry between the past and future. All physical processes obey this invariance. The one exception is the weak force in the decay of K and B mesons. The violation of T symmetry in these systems signifies a fundamental asymmetry between the past and future. I have recently shown that processes which violate T symmetry induce destructive interference between different paths that the universe can take through time. This work resolves the long-standing problem of modeling the dynamics of T violation processes. It shows that T violation has previously unknown, large-scale physical effects and that these effects underlie the origin of the unidirectionality of time [Found. Phys. 41 1569-1596 (2011) DOI, eprint arxiv:0911.4528, Found. Phys. 45, 691-706 (2015) DOI, eprint arXiv:1503.06523].
Current work is exploring the implications for the difference between space and time [Proc. R. Soc. Lond. A 472, 20150670 (2016) DOI, Book chapter DOI]
New Scientist included my quantum theory of time in the article "One time or another: Our best 5 theories of the fourth dimension" by Anil Ananthaswamy, 1 February 2017.
- Particle-wave duality
The nature of physical objects to have both particle and wave properties is one of the foundational elements of quantum theory. Essentially a particle-like state is represented by a narrow wave function which is displaced by spatial translations. In contrast a wave-like state is represented by a spread out wave function which is invariant to spatial translations. The wave-particle dichotomy can therefore be seen as a competition between displacement and invariance of the state with respect to spatial translations. We have generalised this dichotomy to arbitrary quantum systems with finite dimensional Hilbert spaces as follows. We use arbitrary finite symmetry groups to represent transformations of the quantum system. The symmetry (i.e. invariance) or asymmetry (i.e. displacement) of a given state with respect to transformations of the group are identified with the generalised wave and particle nature, respectively. We adopt a measure of wave and particle properties based on the amount of information that can be encoded in the symmetric and asymmetric parts of the state [Proc. R. Soc. A 468, 1065-1084 (2012) DOI, eprint arXiv:1105.0083].
- Quantum reference frames and entanglement
Our description of physical objects is always relative to reference frames of some sort. For example, the position of a car on campus might be "in the third car park from the main entrance of East Car Park". For this description to make sense we need to know where East Car Park is. Presumably its position is known with respect to the campus site etc. Likewise a description of a quantum system, in the form of a quantum state, is relative to a number of implicit references which are represented by other physical systems. Often the reference systems are large and can be treated as classical. For example, if a spin-1/2 particle is described as being in the "spin-up" state it is presumed that the direction of the positive z-axis ("up") is known, perhaps relative to the orientation of a string bob. However when we include the reference systems in the full quantum description we find that matters change. In particular, the clarity of the description of quantum systems depends on the 'size' of the accompanying quantum references. This has an impact when high-fidelity quantum states are needed in areas such as quantum computing. We are exploring the optimum states for quantum references and the effects on quantum entanglement. [See e.g. Phys. Rev. A 77, 032114 (2008), eprint arxiv:quant-ph/0501121; Phys. Rev. A 79, 032109 (2008), eprint arxiv:0807.0064].
- Quantum data security
Data security is a major issue in everyday life, from electronic fund transfers to voting in an election. A revolution has occurred relatively recently in this field with advent of quantum information science. This new branch of research uses the quantum nature of physical systems as a basis for security. A range of applications have been developed such as quantum secret sharing, quantum data hiding, quantum anonymous transfer, quantum oblivious transfer, quantum broadcast communication, quantum identity authentication, quantum finger printing, quantum seals, quantum signatures and quantum exams. We recently introduced a secure protocol for voting (quantum voting) in elections where the privacy of the vote and the anonymity of the voter is protected by quantum physics [Phys. Rev. A 75, 012333 (2007), eprint arxiv:quant-ph/0504161]. A vote is made by performing an operation at one site, a voting booth if you will, but the information of the vote cannot be accessed from that site alone. This effect is spontaneous and due to the entangled nature of the quantum states used. Areas of interest include exploring how quantum physics beats classical systems in data security, examining the robustness of particular applications and examining security-anonymity trade offs.