September 2022

Experimental demonstration of input-output indefiniteness in a single quantum device

At the fundamental level, the dynamics of quantum fields is invariant under the combination of time reversal, charge conjugation, and parity inversion. This symmetry implies that a broad class of effective quantum evolutions are bidirectional, meaning that the exchange of their inputs and outputs gives rise to valid quantum evolutions. Recently, it has been observed that quantum theory is theoretically compatible with a family of operations in which the roles of the inputs and outputs is indefinite. However, such operations have not been demonstrated in the laboratory so far. Here we experimentally demonstrate input-output indefiniteness in a photonic setup, demonstrating an advantage in a quantum game and showing incompatibility with a definite input-output direction by more than 69 standard deviations. Our results establish input-output indefiniteness as a new resource for quantum information protocols, and enable the table-top simulation of hypothetical scenarios where the arrow of time could be in a quantum superposition.

Witnessing superpositions of causal orders by weak measurements at a given time

The questions we raise in this letter are as follows: What is the most general representation of a quantum state at a single time? Can we adapt the current representations to the scenarios in which the order of quantum operations are coherently or incoherently superposed? If so, what is the relation between the state at a given time and the uncertainty in the order of events before and after it? By establishing the relationship of two-state vector formalism with pseudo-density operators, we introduce the notion of single-time pseudo-state which can be constructed by either ideal or weak measurements. We show that the eigenspectrum in the latter case enables us to discriminate between the coherent and incoherent superpositions of causal orders in which the pre- and post-selection measurements are interchanged with a non-zero probability. Finally, we discuss some of the possible experimental realizations in existing photonic setups.

Witnessing superpositions of causal orders by weak measurements at a given time

The questions we raise in this letter are as follows: What is the most general representation of a quantum state at a single time? Can we adapt the current representations to the scenarios in which the order of quantum operations are coherently or incoherently superposed? If so, what is the relation between the state at a given time and the uncertainty in the order of events before and after it? By establishing the relationship of two-state vector formalism with pseudo-density operators, we introduce the notion of single-time pseudo-state which can be constructed by either ideal or weak measurements. We show that the eigenspectrum in the latter case enables us to discriminate between the coherent and incoherent superpositions of causal orders in which the pre- and post-selection measurements are interchanged with a non-zero probability. Finally, we discuss some of the possible experimental realizations in existing photonic setups.

Addressable quantum gates

We extend the circuit model of quantum computation so that the wiring between gates is soft-coded within registers inside the gates. The addresses in these registers can be manipulated and put into superpositions. This aims at capturing indefinite causal orders and making their geometrical layout explicit: we express the quantum switch and the polarizing beam-splitter within the model. In this context, our main contribution is a full characterization of the anonymity constraints. Indeed, the names used as addresses should not matter beyond the wiring they describe, i.e. quantum evolutions should commute with “renamings”. We show that these quantum evolutions can still act non-trivially upon the names. We specify the structure of “nameblind” matrices.

Addressable quantum gates

We extend the circuit model of quantum computation so that the wiring between gates is soft-coded within registers inside the gates. The addresses in these registers can be manipulated and put into superpositions. This aims at capturing indefinite causal orders and making their geometrical layout explicit: we express the quantum switch and the polarizing beam-splitter within the model. In this context, our main contribution is a full characterization of the anonymity constraints. Indeed, the names used as addresses should not matter beyond the wiring they describe, i.e. quantum evolutions should commute with “renamings”. We show that these quantum evolutions can still act non-trivially upon the names. We specify the structure of “nameblind” matrices.

Poster for Vlatko Vedral's QISS Virtual Seminar on 17 November at 4pm CET: Interference in QFT: detecting ghosts with phases

Vlatko Vedral
University of Oxford Interference in quantum field theory: detecting ghosts with phases

I intend to discuss the implications of the principle of locality for interference in quantum field theory. As an example, I will consider the interaction of two charges via a mediating quantum field and the resulting interference pattern, in the Lorenz gauge. Using the Heisenberg picture, I will claim that detecting relative phases or entanglement between …

Vlatko Vedral
University of Oxford Interference in quantum field theory: detecting ghosts with phases
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poster for the virtual seminar of Martin Plenio, 27 October at 5pm CEST testing quantum aspects of gravity at low energies

Martin Plenio
Ulm UniversityTesting of Quantum Aspects of Gravity at Low Energies: Discussion of Experiment Aspects and Possible Loopholes

Testing the quantum character of weak forces such as gravity requires both exquisite experimental control of technology and careful design by theory of the experiment set-up. At the same time, the interpretation of experiments that probe the interface between quantum physics and gravity also mandates that we examine carefully all the assumptions, however innocuous, that …

Martin Plenio
Ulm UniversityTesting of Quantum Aspects of Gravity at Low Energies: Discussion of Experiment Aspects and Possible Loopholes
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Inference of gravitational field superposition from quantum measurements

Experiments are beginning to probe the interaction of quantum particles with gravitational fields beyond the uniform-field regime. In non-relativistic quantum mechanics, the gravitational field in such experiments can be written as a superposition state. We empirically demonstrate that alternative theories of gravity can avoid gravitational superposition states only by decoupling the gravitational field energy from the quantum particle’s time evolution. Furthermore, such theories must specify a preferred quantum reference frame in which the equations of motion are valid. To the extent that these properties are theoretically implausible, recent experiments provide indirect evidence that gravity has quantum features. Proposed experiments with superposed gravitational sources would provide even stronger evidence that gravity is nonclassical.