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Locality is a central notion in modern physics, but different disciplines understand it in different ways. Quantum field theory focusses on relativistic locality, enforced by microcausality, while quantum information theory focuses on subsystem locality, which regulates how information and causal influences propagate in a system, with no direct reference to spacetime notions. Here we investigate how microcausality and subsystem locality are related. The question is relevant for understanding whether it is possible to formulate quantum field theory in quantum information language, and has bearing on the recent discussions on low-energy tests of quantum gravity. We present a first result in this direction: in the quantum dynamics of a massive scalar quantum field coupled to two localised systems, microcausality implies subsystem locality in a physically relevant approximation.

Efficient characterization of continuous-variable quantum states is important for quantum communication, quantum sensing, quantum simulation and quantum computing. However, conventional quantum state tomography and recently proposed classical shadow tomography require truncation of the Hilbert space or phase space and the resulting sample complexity scales exponentially with the number of modes. In this paper, we propose a quantum-enhanced learning strategy for continuous-variable states overcoming the previous shortcomings. We use this to estimate the point values of a state characteristic function, which is useful for quantum state tomography and inferring physical properties like quantum fidelity, nonclassicality and quantum non-Gaussianity. We show that for any continuous-variable quantum states $rho$ with reflection symmetry – for example Gaussian states with zero mean values, Fock states, Gottesman-Kitaev-Preskill states, Schr”odinger cat states and binomial code states – on practical quantum devices we only need a constant number of copies of state $rho$ to accurately estimate the square of its characteristic function at arbitrary phase-space points. This is achieved by performinig a balanced beam splitter on two copies of $rho$ followed by homodyne measurements. Based on this result, we show that, given nonlocal quantum measurements, for any $k$-mode continuous-variable states $rho$ having reflection symmetry, we only require $O(log M)$ copies of $rho$ to accurately estimate its characteristic function values at any $M$ phase-space points. Furthermore, the number of copies is independent of $k$. This can be compared with restricted conventional approach, where $Omega(M)$ copies are required to estimate the characteristic function values at $M$ arbitrary phase-space points.

Non-abelian symmetries play a central role in many areas in physics, and have been recently argued to result in distinct quantum dynamics and thermalization. Here we unveil the effect that the non-abelian SU(2) symmetry, and the rich Hilbert space structure that it generates for spin 1/2 systems, has on the average entanglement entropy of random pure states and of highly-excited Hamiltonian eigenstates. Focusing on the zero magnetization sector (J_z=0) for different fixed spin J, we show that the entanglement entropy has a leading volume law term whose coefficient s_A depends on the spin density j=2J/L, with s_A(j –> 0)=ln(2) and s_A(j –> 1)=0. We also discuss the behavior of the first subleading corrections.

It is shown that any theory that has certain properties has a measurement problem, in the sense that it makes predictions that are incompatible with measurement outcomes being absolute (that is, unique and non-relational). These properties are Bell Nonlocality, Information Preservation, and Local Dynamics. The result is extended by deriving Local Dynamics from No Superluminal Influences, Separable Dynamics, and Consistent Embeddings. As well as explaining why the existing Wigner’s-friend-inspired no-go theorems hold for quantum theory, these results also shed light on whether a future theory of physics might overcome the measurement problem. In particular, they suggest the possibility of a theory in which absoluteness is maintained, but without rejecting relativity theory (as in Bohm theory) or embracing objective collapses (as in GRW theory).

In physics, it is crucial to identify operational measurement procedures to give physical meaning to abstract quantities. There has been significant effort to define time operationally using quantum systems, but the same has not been achieved for space. Developing an operational procedure to obtain information about the location of a quantum system is particularly important for a theory combining general relativity and quantum theory, which cannot rest on the classical notion of spacetime. Here, we take a first step towards this goal, and introduce a model to describe an extended material quantum system working as a position measurement device. Such a “quantum ruler” is composed of N harmonically interacting dipoles and serves as a (quantum) reference system for the position of another quantum system. We show that we can define a quantum measurement procedure corresponding to the “superposition of positions”, and that by performing this measurement we can distinguish when the quantum system is in a coherent or incoherent superposition in the position basis. The model is fully relational, because the only meaningful variables are the relative positions between the ruler and the system, and the measurement is expressed in terms of an interaction between the measurement device and the measured system.

A pair of interferometers can be coupled by allowing one path from each to overlap such that if the particles meet in this overlap region, they annihilate. It was shown by one of us over thirty years ago that such annihilation-coupled interferometers can exhibit apparently paradoxical behaviour. More recently, Bose et al. and Marletto and Vedral have considered a pair of interferometers that are phase-coupled (where the coupling is through gravitational interaction). In this case one path from each interferometer undergoes a phase-coupling interaction. We show that these phase-coupled interferometers exhibit the same apparent paradox as the annihilation-coupled interferometers, though in a curiously dual manner.

The Wigner’s friend experiment is a thought experiment in which a so-called superobserver (Wigner) observes another observer (the friend) who has performed a quantum measurement on a physical system. In this setup Wigner treats the friend the system and potentially other degrees of freedom involved in the friend’s measurement as one joint quantum system. In general, Wigner’s measurement changes the internal record of the friend’s measurement result such that after the measurement by the superobserver the result stored in the observer’s memory register is no longer the same as the result the friend obtained at her measurement, i.e. before she was measured by Wigner. Here, we show that any awareness by the friend of such a change, which can be modeled by an additional memory register storing the information about the change, conflicts with the no-signaling condition in extended Wigner-friend scenarios.

Light-matter interaction in the ultrastrong coupling regime can be used to generate exotic ground states with two-mode squeezing and may be of use for quantum enhanced sensing. Current demonstrations of ultrastrong coupling have been performed in fundamentally nonlinear systems. We report a cavity optomechanical system that operates in the linear coupling regime, reaching a maximum coupling of $g_x/Omega_x=0.55pm 0.02$. Such a system is inherently unstable, which may in the future enable strong mechanical squeezing.

Most existing proposals to explain the temporal asymmetries we see around us are sited within an approach to physics based on time evolution, and thus they typically put the asymmetry in at the beginning of time in the form of a special initial state. But there may be other possibilities for explaining temporal asymmetries if we don’t presuppose the time evolution paradigm. In this article, we explore one such possibility, based on Kent’s `final-measurement’ interpretation of quantum mechanics. We argue that this approach potentially has the resources to explain the electromagnetic asymmetry, the thermodynamic asymmetry, the coarse-graining asymmetry, the fork asymmetry, the record asymmetry, and the cosmological asymmetry, and that the explanations it offers may potentially be better than explanations appealing to a special initial state. Our hope is that this example will encourage further exploration of novel approaches to temporal asymmetry outside of the time evolution paradigm.

The laws of Physics are time-reversible, making no qualitative distinction between the past and the future — yet we can only go towards the future. This apparent contradiction is known as the `arrow of time problem’. Its resolution states that the future is the direction of increasing entropy. But entropy can only increase towards the future if it was low in the past, and past low entropy is a very strong assumption to make, because low entropy states are rather improbable, non-generic. Recent works, however, suggest we can do away with this so-called `past hypothesis’, in the presence of reversible dynamical laws featuring expansion. We prove that this is the case for a toy model, set in a 1+1 discrete spacetime. It consists in graphs upon which particles circulate and interact according to local reversible rules. Some rules locally shrink or expand the graph. Generic states always expand; entropy always increases — thereby providing a local explanation for the arrow of time.