ORCID Profile
0000-0002-6547-6005
Current Organisations
Nordic Institute for Theoretical Physics
,
University of Queensland
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Quantum Physics | Quantum Optics | Quantum Information, Computation and Communication
Expanding Knowledge in the Physical Sciences | Expanding Knowledge in the Information and Computing Sciences |
Publisher: American Association for the Advancement of Science (AAAS)
Date: 05-08-2016
Abstract: Causation at a distance does not explain quantum correlations.
Publisher: IOP Publishing
Date: 23-02-2017
Publisher: American Physical Society (APS)
Date: 14-11-2019
Publisher: IOP Publishing
Date: 04-11-2009
Publisher: American Physical Society (APS)
Date: 24-06-2010
Publisher: Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften
Date: 17-08-2021
DOI: 10.22331/Q-2021-08-17-525
Abstract: Quantum interference of "clocks", namely of particles with time-evolving internal degrees of freedom (DOFs), is a promising avenue to test genuine general relativistic effects in quantum systems. The clock acquires which path information while experiencing different proper times on traversing the arms of the interferometer, leading to a drop in its path visibility. We consider scenarios where the clock is subject to environmental noise as it transits through the interferometer. In particular, we develop a generalized formulation of interferometric visibility affected by noise on the clock. We find that, for small noise and small proper time difference between the arms, the noise further reduces the visibility, while in more general situations it can either increase or reduce the visibility. As an ex le, we investigate the effect of a thermal environment constituted by a single field mode and show that the visibility drops further as the temperature is increased. Additionally, by considering noise models based on standard quantum channels, we show that interferometric visibility can increase or decrease depending on the type of noise and also the time scale and transition probabilities. The quantification of the effect of noise on the visibility – particularly in the case of a thermal environment paves the way for a better estimate on the expected outcome in an actual experiment.
Publisher: Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften
Date: 26-04-2021
DOI: 10.22331/Q-2021-04-26-440
Abstract: We present a method to detect quantum memory in a non-Markovian process. We call a process Markovian when the environment does not provide a memory that retains correlations across different system-environment interactions. We define two types of non-Markovian processes, depending on the required memory being classical or quantum. We formalise this distinction using the process matrix formalism, through which a process is represented as a multipartite state. Within this formalism, a test for entanglement in a state can be mapped to a test for quantum memory in the corresponding process. This allows us to apply separability criteria and entanglement witnesses to the detection of quantum memory. We demonstrate the method in a simple model where both system and environment are single interacting qubits and map the parameters that lead to quantum memory. As with entanglement witnesses, our method of witnessing quantum memory provides a versatile experimental tool for open quantum systems.
Publisher: Springer Science and Business Media LLC
Date: 02-08-2018
DOI: 10.1038/S41534-018-0086-Y
Abstract: Multipartite quantum correlations are a powerful resource that underpins applications from quantum metrology to quantum computing. While most research has focused on spatial correlations, it is now becoming clear that a sequence of measurements on a single quantum system at different points in time reveals a similarly rich, yet fundamentally different structure of multipartite temporal correlations. Here we experimentally observe genuine multi-time correlations in a sequence of three generalized measurements on a single photon. These correlations, manifest by a simultaneous violation of all pairwise Bell inequalities, cannot be reproduced by any spatial quantum state of equal dimension. Our work lays the foundation for the exploration of temporal correlations arising in quantum networks for quantum information applications.
Publisher: Springer Science and Business Media LLC
Date: 18-10-2011
DOI: 10.1038/NCOMMS1498
Publisher: IOP Publishing
Date: 23-12-2015
Publisher: Springer Science and Business Media LLC
Date: 21-08-2019
DOI: 10.1038/S41467-019-11579-X
Abstract: Time has a fundamentally different character in quantum mechanics and in general relativity. In quantum theory events unfold in a fixed order while in general relativity temporal order is influenced by the distribution of matter. When matter requires a quantum description, temporal order is expected to become non-classical—a scenario beyond the scope of current theories. Here we provide a direct description of such a scenario. We consider a thought experiment with a massive body in a spatial superposition and show how it leads to entanglement of temporal orders between time-like events. This entanglement enables accomplishing a task, violation of a Bell inequality, that is impossible under local classical temporal order it means that temporal order cannot be described by any pre-defined local variables. A classical notion of a causal structure is therefore untenable in any framework compatible with the basic principles of quantum mechanics and classical general relativity.
Publisher: Springer Science and Business Media LLC
Date: 15-06-2015
DOI: 10.1038/NPHYS3366
Publisher: American Physical Society (APS)
Date: 12-04-2021
Publisher: American Physical Society (APS)
Date: 30-09-2016
Publisher: Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften
Date: 14-12-2017
Abstract: The study of correlations with no definite causal order has revealed a rich structure emerging when more than two parties are involved. This motivates the consideration of multipartite "noncausal" correlations that cannot be realised even if noncausal resources are made available to a smaller number of parties. Here we formalise this notion: genuinely N-partite noncausal correlations are those that cannot be produced by grouping N parties into two or more subsets, where a causal order between the subsets exists. We prove that such correlations can be characterised as lying outside a polytope, whose vertices correspond to deterministic strategies and whose facets define what we call "2-causal" inequalities. We show that genuinely multipartite noncausal correlations arise within the process matrix formalism, where quantum mechanics holds locally but no global causal structure is assumed, although for some inequalities no violation was found. We further introduce two refined definitions that allow one to quantify, in different ways, to what extent noncausal correlations correspond to a genuinely multipartite resource.
Publisher: IOP Publishing
Date: 29-10-2019
Abstract: General relativity allows for the existence of closed time-like curves, along which a material object could travel back in time and interact with its past self. This possibility raises the question whether certain initial conditions, or more generally local operations, lead to inconsistencies and should thus be forbidden. Here we consider the most general deterministic dynamics connecting classical degrees of freedom defined on a set of bounded space-time regions, requiring that it is compatible with arbitrary operations performed in the local regions. We find that any such dynamics can be realised through reversible interactions. We further find that consistency with local operations is compatible with non-trivial time travel: three parties can interact in such a way to be all both in the future and in the past of each other, while being free to perform arbitrary local operations.
Publisher: American Physical Society (APS)
Date: 26-08-2021
Publisher: American Physical Society (APS)
Date: 26-07-2018
Publisher: American Physical Society (APS)
Date: 28-04-2021
Publisher: American Physical Society (APS)
Date: 25-09-2014
Publisher: IOP Publishing
Date: 15-08-2022
Abstract: Past studies of the billiard-ball paradox, a problem involving an object that travels back in time along a closed timelike curve (CTC), typically concern themselves with entirely classical histories, whereby any trajectorial effects associated with quantum mechanics cannot manifest. Here we develop a quantum version of the paradox, wherein a (semiclassical) wave packet evolves through a region containing a wormhole time machine. This is accomplished by mapping all relevant paths on to a quantum circuit, in which the distinction of the various paths is facilitated by representing the billiard particle with a clock state. For this model, we find that Deutsch's prescription (D-CTCs) provides self-consistent solutions in the form of a mixed state composed of terms which represent every possible configuration of the particle’s evolution through the circuit. In the equivalent circuit picture (ECP), this reduces to a binomial distribution in the number of loops of time machine. The postselected teleportation prescription (P-CTCs) on the other hand predicts a pure-state solution in which the loop counts have binomial coefficient weights. We then discuss the model in the continuum limit, with a particular focus on the various methods one may employ in order to guarantee convergence in the average number of clock evolutions. Specifically, for D-CTCs, we find that it is necessary to regularise the theory’s parameters, while P-CTCs alternatively require more contrived modification.
Publisher: Springer Science and Business Media LLC
Date: 07-08-2015
DOI: 10.1038/NCOMMS8913
Abstract: Quantum computers achieve a speed-up by placing quantum bits (qubits) in superpositions of different states. However, it has recently been appreciated that quantum mechanics also allows one to ‘superimpose different operations’. Furthermore, it has been shown that using a qubit to coherently control the gate order allows one to accomplish a task—determining if two gates commute or anti-commute—with fewer gate uses than any known quantum algorithm. Here we experimentally demonstrate this advantage, in a photonic context, using a second qubit to control the order in which two gates are applied to a first qubit. We create the required superposition of gate orders by using additional degrees of freedom of the photons encoding our qubits. The new resource we exploit can be interpreted as a superposition of causal orders, and could allow quantum algorithms to be implemented with an efficiency unlikely to be achieved on a fixed-gate-order quantum computer.
Publisher: Springer Science and Business Media LLC
Date: 06-03-2018
DOI: 10.1038/S41534-018-0062-6
Abstract: Finding a causal model for a set of classical variables is now a well-established task—but what about the quantum equivalent? Even the notion of a quantum causal model is controversial. Here, we present a causal discovery algorithm for quantum systems. The input to the algorithm is a process matrix describing correlations between quantum events. Its output consists of different levels of information about the underlying causal model. Our algorithm determines whether the process is causally ordered by grouping the events into causally ordered non-signaling sets. It detects if all relevant common causes are included in the process, which we label Markovian, or alternatively if some causal relations are mediated through some external memory. For a Markovian process, it outputs a causal model, namely the causal relations and the corresponding mechanisms, represented as quantum states and channels. Our algorithm opens the route to more general quantum causal discovery methods.
Publisher: American Physical Society (APS)
Date: 31-08-2018
Publisher: American Physical Society (APS)
Date: 18-12-2014
Publisher: IOP Publishing
Date: 04-05-2018
Publisher: IOP Publishing
Date: 21-09-2020
Abstract: The theory of general relativity predicts the existence of closed time-like curves (CTCs), which theoretically would allow an observer to travel back in time and interact with their past self. This raises the question of whether this could create a grandfather paradox, in which the observer interacts in such a way to prevent their own time travel. Previous research has proposed a framework for deterministic, reversible, dynamics compatible with non-trivial time travel, where observers in distinct regions of spacetime can perform arbitrary local operations with no contradiction arising. However, only scenarios with up to three regions have been fully characterised, revealing only one type of process where the observers can verify to both be in the past and future of each other. Here we extend this characterisation to an arbitrary number of regions and find that there exist several inequivalent processes that can only arise due to non-trivial time travel. This supports the view that complex dynamics is possible in the presence of CTCs, compatible with free choice of local operations and free of inconsistencies.
Publisher: Springer Science and Business Media LLC
Date: 02-10-2012
DOI: 10.1038/NCOMMS2076
Publisher: Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften
Date: 18-05-2018
Abstract: Realist interpretations of quantum mechanics presuppose the existence of elements of reality that are independent of the actions used to reveal them. Such a view is challenged by several no-go theorems that show quantum correlations cannot be explained by non-contextual ontological models, where physical properties are assumed to exist prior to and independently of the act of measurement. However, all such contextuality proofs assume a traditional notion of causal structure, where causal influence flows from past to future according to ordinary dynamical laws. This leaves open the question of whether the apparent contextuality of quantum mechanics is simply the signature of some exotic causal structure, where the future might affect the past or distant systems might get correlated due to non-local constraints. Here we show that quantum predictions require a deeper form of contextuality: even allowing for arbitrary causal structure, no model can explain quantum correlations from non-contextual ontological properties of the world, be they initial states, dynamical laws, or global constraints.
Publisher: IOP Publishing
Date: 18-10-2012
Publisher: World Scientific Pub Co Pte Lt
Date: 12-2018
DOI: 10.1142/S0219749918400105
Abstract: Supervised learning algorithms take as input a set of labeled ex les and return as output a predictive model. Such models are used to estimate labels for future, previously unseen ex les, drawn from the same generating distribution. In this paper, we investigate the possibility of using supervised learning to estimate the dimension of a non-Markovian quantum environment. Our approach uses an ensemble learning method, the Random Forest Regressor, applied to classically simulated datasets. Our results indicate this is a promising line of research.
Publisher: IOP Publishing
Date: 06-2016
Publisher: American Physical Society (APS)
Date: 15-01-2009
Publisher: Springer Science and Business Media LLC
Date: 2016
DOI: 10.1038/NPHYS3650
Publisher: American Physical Society (APS)
Date: 11-2022
Publisher: IOP Publishing
Date: 22-09-2014
Publisher: IOP Publishing
Date: 24-06-2016
Start Date: 2017
End Date: 2019
Funder: Australian Research Council
View Funded ActivityStart Date: 05-2017
End Date: 06-2022
Amount: $360,000.00
Funder: Australian Research Council
View Funded Activity