Qiss

We propose a new set of BMS charges at null infinity, characterized by a super-translation flux that contains only the `hard’ term. This is achieved with a specific corner improvement of the symplectic 2-form, and we spell the conditions under which it is unique. The charges are associated to a Wald-Zoupas symplectic potential, and satisfy all standard criteria: they are covariant, provide a center-less realization of the symmetry algebra, have vanishing flux in non-radiative spacetimes, and vanish in Minkowski. We use them to define a certain notion of localized energy density of gravitational waves. They have potential applications to the generalized second law and to soft theorems.

Characterizing the set of distributions that can be realized in the triangle network is a notoriously difficult problem. In this work, we investigate inner approximations of the set of local (classical) distributions of the triangle network. A quantum distribution that appears to be nonlocal is the Elegant Joint Measurement (EJM) [Entropy. 2019; 21(3):325], which motivates us to study distributions having the same symmetries as the EJM. We compare analytical and neural-network-based inner approximations and find a remarkable agreement between the two methods. Using neural network tools, we also conjecture network Bell inequalities that give a trade-off between the levels of correlation and symmetry that a local distribution may feature. Our results considerably strengthen the conjecture that the EJM is nonlocal.

We introduce two quantitative measures of the strength of causal relations. These two measures capture the maximum and minimum changes in a quantum system induced by changes in another system. We show that both measures possess important properties, such as continuity and faithfulness, and can be evaluated through optimization over orthogonal input states. For the maximum causal effect, we provide numerical lower bounds based on a variational algorithm, which can be used to estimate the strength of causal relations without performing a full quantum process tomography. To illustrate the application of our algorithm, we analyze two paradigmatic examples, the first involving a coherent superposition of direct cause and common cause and the second involving communication through a coherent superposition of two completely depolarizing channels.

This is the second in a series of “graphical grokking” papers in which we study how stabiliser codes can be understood using the ZX calculus. In this paper we show that certain complex rules involving ZX diagrams, called spider nest identities, can be captured succinctly using the scalable ZX calculus, and all such identities can be proved inductively from a single new rule using the Clifford ZX calculus. This can be combined with the ZX picture of CSS codes, developed in the first “grokking” paper, to give a simple characterisation of the set of all transversal diagonal gates at the third level of the Clifford hierarchy implementable in an arbitrary CSS code.

We study non-terminating graph rewriting models, whose local rules are applied non-deterministically — and yet enjoy a strong form of determinism, namely space-time determinism. Of course in the case of terminating computation it is well-known that the mess introduced by asynchronous rule applications may not matter to the end result, as confluence conspires to produce a unique normal form. In the context of non-terminating computation however, confluence is a very weak property, and (almost) synchronous rule applications is always preferred e.g. when it comes to simulating dynamical systems. Here we provide sufficient conditions so that asynchronous local rule applications conspire to produce well-determined events in the space-time unfolding of the graph, regardless of their application orders. Our first example is an asynchronous simulation of a dynamical system. Our second example features time dilation, in the spirit of general relativity.

The Eigenstate Thermalization Hypothesis (ETH) has played a key role in recent advances in the high energy and condensed matter communities. It explains how an isolated quantum system in a far-from-equilibrium initial state can evolve to a state that is indistinguishable from thermal equilibrium, with observables relaxing to almost time-independent results that can be described using traditional statistical mechanics ensembles. In this work we probe the limits of ETH, pushing it outside its prototypical applications in several directions. We design a qutrit lattice system with conserved quasilocal charge, in which we verify a form of generalized eigenstate thermalization. We also observe signatures of thermalization in states well outside microcanonical windows of both charge and energy, which we dub `generic ETH.’

As fundamental scaling limits start to stifle the evolution of complementary metal–oxide–semiconductor transistor technology, interest in potential alternative computing platforms grows. One such alternative is wave-based computation. In this work, we propose a general string diagrammatic formalism for wave-based computation with phase encoding applicable to a wide range of emerging architectures and technologies, including quantum-dot cellular automata, single-electron circuits, spin torque majority gates, and DNA computing. We demonstrate its applicability for design, analysis, and simplification of Boolean logic circuits using the example of spin-wave circuits.

The formulation of a measurement theory for relativistic quantum field theory (QFT) has recently been an active area of research. In contrast to the asymptotic measurement framework that was enshrined in QED, the new proposals aim to supply a measurement framework for measurements in local spacetime regions. This paper surveys episodes in the history of quantum theory that contemporary researchers have identified as precursors to their own work and discusses how they laid the groundwork for current approaches to local measurement theory for QFT.

Many experiments in the field of optical levitation with nanoparticles today are limited by the available technologies for particle loading. Here we introduce a new particle loading method that solves the main challenges, namely deterministic positioning of the particles and clean delivery at ultra-high vacuum levels as required for quantum experiments. We demonstrate the efficient loading, positioning, and repositioning of nanoparticles in the range of $100-755,mathrm{nm}$ diameter into different lattice sites of a standing wave optical trap, as well as direct loading of nanoparticles at an unprecedented pressure below $10^{-9},mathrm{mbar}$. Our method relies on the transport of nanoparticles within a hollow-core photonic crystal fiber using an optical conveyor belt, which can be precisely positioned with respect to the target trap. Our work opens the path for increasing nanoparticle numbers in the study of multiparticle dynamics and high turn-around times for exploiting the quantum regime of levitated solids in ultra-high vacuum.

Erik Curiel Universität Bonn