arXiv subjects

André Großardt

Publications and source records attributed to André Großardt.

At least 19 recordsLinked to original sources

arXiv2025

Causal consistency requirements for gravity-induced entanglement in near-relativistic systems with internal energy

We reconsider a thought experiment that employs the entanglement of the gravitational field with position space quantum states as a means for faster-than-light signaling. We present a protocol that includes the excitation to a higher internal energy level to increase sensitivity to gravitational phase shifts. We report that the explanations why previous versions of the thought experiment remain causally consistent are insufficient to avoid any possibility for faster-than-light signals in this case. An alternative resolution to prevent faster-than-light signaling is most reasonably the requirement for a (near) relativistic treatment. One such effect could be a decoherence channel unobserved in a nonrelativistic treatment.

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arXiv2024

State Swapping via Semiclassical Gravity

The prediction of non-local phenomena is a key attribute of quantum mechanics that distinguishes it from classical theories. It was recently suggested that state swapping is one such effect that a fundamentally classical gravitational field cannot give rise to. The reasoning was that the phenomenology of classical fields is restricted to Local Operations and Classical Communication~(LOCC) whereas swapping is a non-LOCC effect, so gravity must not be a classical channel if it can give rise to such a phenomenon. Here, we show that the Hamiltonian of semiclassical gravity results in an evolution of coherent states that is virtually indistinguishable from swapping them, both theoretically and experimentally. Our results indicate that drawing a definitive conclusion about the nature of gravity based solely on its ability to swap a pair of coherent states would be premature. We discuss the implications of these results regarding proposed experimental tests for the quantum nature of gravity and clarify some conceptual issues.

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arXiv2024

Nonlinear-ancilla aided quantum algorithm for nonlinear Schrödinger equations

We present an algorithm that uses a single ancilla qubit that can evolve nonlinearly, and show how to use it to efficiently solve generic nonlinear Schrödinger equations, including nonlocal Hartree equations and the Navier-Stokes equation for an irrotational, non-viscous flow. We propose a realization of such nonlinear qubits via spin-spin coupling of neutral atom qubits to a Bose-Einstein condensate. The results suggest that the use of nonlinear ancillas can provide substantial speedups compared to exclusively linear qubit devices.

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arXiv2023

Self-gravitational dephasing of quasi-classical Stern-Gerlach trajectories

The nonlinear Schrödinger-Newton equation, a prospective semiclassical alternative to a quantized theory of gravity, predicts a gravitational self-force between the two trajectories corresponding to the two z-spin eigenvalues for a particle in a Stern-Gerlach interferometer. To leading order, this force results in a relative phase between the trajectories. For the experimentally relevant case of a spherical particle with localized wave function, we present a re-derivation of that phase which is both rigorous in its approximations and concise, allowing for simple but accurate experimental predictions.

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arXiv2022

Coupling Quantum Matter and Gravity

In this contribution we deal with several issues one encounters when trying to couple quantum matter to classical gravitational fields. We start with a general background discussion and then move on to two more technical sections. In the first technical part we consider the question how the Hamiltonian of a composite two-particle system in an external gravitational field can be computed in a systematic post-Newtonian setting without backreaction. This enables us to reliably estimate the consistency and completeness of less systematic and more intuitive approaches that attempt to solve this problem by adding `relativistic effects' by hand. In the second technical part we consider the question of how quantum matter may act as source for classical gravitational fields via the semiclassical Einstein equations. Statements to the effect that this approach is fundamentally inconsistent are critically reviewed.

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arXiv2022

Is gravitational entanglement evidence for the quantization of spacetime?

Experiments witnessing the entanglement between two particles interacting only via the gravitational field have been proposed as a test whether gravity must be quantized. In the language of quantum information, a non-quantum gravitational force would be modeled by local operations with classical communication (LOCC), which cannot generate entanglement in an initially unentangled state. This idea is criticized as too constraining on possible alternatives to quantum gravity. We present a parametrized model for the gravitational interaction of quantum matter on a classical spacetime, inspired by the de Broglie-Bohm formulation of quantum mechanics, which results in entanglement and thereby provides an explicit counterexample to the claim that only a quantized gravitational field possesses this capability.

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arXiv2022

Gravitational entanglement and the mass contribution of internal energy in nonrelativistic quantum systems

Recently, interest has increased in the entanglement of remote quantum particles through the Newtonian gravitational interaction, both from a fundamental perspective and as a test case for the quantization of gravity. Likewise, post-Newtonian gravitational effects in composite nonrelativistic quantum systems have been discussed, where the internal energy contributes to the mass, promoting the mass to a Hilbert space operator. Employing a modified version of a previously considered thought experiment, it can be shown that both concepts, when combined, result in inconsistencies, reinforcing the arguments for the necessity of a rigorous derivation of the nonrelativistic limit of gravitating quantum matter from first principles.

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arXiv2022

Three little paradoxes: making sense of semiclassical gravity

I review the arguments most often raised against a fundamental coupling of classical spacetime to quantum matter. I show that an experiment by Page and Geilker does not exclude such a semiclassical theory but mandates an inclusion of an objective mechanism for wave function collapse. In this regard, I present a classification of semiclassical models defined by the way in which the wave function collapse is introduced. Two related types of paradoxes that have been discussed in the context of the necessity to quantize the gravitational field can be shown to not constrain the possibility of a semiclassical coupling. A third paradox, the possibility to signal faster than light via semiclassical gravity, is demonstrably avoided if certain conditions are met by the associated wave function collapse mechanism. In conclusion, all currently discussed models of semiclassical gravity can be made consistent with observation. Their internal theoretical consistency remains an open question.

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arXiv2021

Comment on "Do Gedankenexperiments compel quantization of gravity"

There is no conclusive theoretical argument that would require the quantization of the gravitational field. A paradox presented by Mari et al. [Sci. Rep. 6, 22777 (2016)] and its resolution by Belenchia et al. [Phys. Rev. D 98, 126009 (2018)] did not change anything about this assertion. Despite being interesting in other respects, efforts of a recent work by Ryding, Aurell, and Pikovski [arXiv:2107.07514] to further establish the inconclusiveness of the question of necessity of quantization are futile.

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arXiv2021

Dephasing and inhibition of spin interference from semi-classical self-gravitation

We present a detailed derivation of a model to study effects of self-gravitation from semi-classical gravity, described by the Schrödinger-Newton equation, employing spin superposition states in inhomogeneous magnetic fields, as proposed recently for experiments searching for gravity induced entanglement. Approximations for the experimentally relevant limits are discussed. Results suggest that spin interferometry could provide a more accessible route towards an experimental test of quantum aspects of gravity than both previous proposals to test semi-classical gravity and the observation of gravitational spin entanglement.

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arXiv2020

Acceleration noise constraints on gravity induced entanglement

It has been proposed that quantum features of the gravitational field can be exposed experimentally by employing gravity as a mediator of entanglement. We show that in order to witness this type of entanglement experimentally, strong limits on acceleration noise, which has been neglected in previous work, must be overcome. In the case of two particles of similar mass, Casimir-Polder forces lead to a fundamental limit of tenths of femtometers per second squared per root Hertz. Limits are between three and six orders of magnitude less strict for two particles of unequal mass, depending on collisional decoherence.

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arXiv2017

Gravitational Decoherence

We discuss effects of loss of coherence in low energy quantum systems caused by or related to gravitation, referred to as gravitational decoherence. These effects, resulting from random metric fluctuations, for instance, promise to be accessible by relatively inexpensive table-top experiments, way before the scales where true quantum gravity effects become important. Therefore, they can provide a first experimental view on gravity in the quantum regime. We will survey models of decoherence induced both by classical and quantum gravitational fluctuations; it will be manifest that a clear understanding of gravitational decoherence is still lacking. Next we will review models where quantum theory is modified, under the assumption that gravity causes the collapse of the wave functions, when systems are large enough. These models challenge the quantum-gravity interplay, and can be tested experimentally. In the last part we have a look at the state of the art of experimental research. We will review efforts aiming at more and more accurate measurements of gravity (G and g) and ideas for measuring conventional and unconventional gravity effects on nonrelativistic quantum systems.

quant-ph↗

arXiv2017

Reply to Comment on "Quantum mechanics for non-inertial observers"

Our recent paper (arXiv:1701.04298 [quant-ph]) discussed the occurrence of a coupling of centre of mass and internal degrees of freedom for complex quantum systems in non-inertial frames. There, we pointed out that an external force supporting the system against gravity plays a crucial role for the coupling between center of mass position to the internal degrees of freedom. In a comment (arXiv:1702.06670 [quant-ph]) to our paper, Pikovski et al. question our conclusion and present the argument that the lack of the coupling term would be in contradiction with the observation of gravitational time dilation using atomic clocks. Here, we elaborate on our results in reply to their criticism and clarify why our arguments remain valid.

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arXiv2017

Newtonian self-gravity in trapped quantum systems and experimental tests

No experimental evidence exists, to date, whether or not the gravitational field must be quantised. Theoretical arguments in favour of quantisation are inconclusive. The most straightforward alternative to quantum gravity, a coupling between classical gravity and quantum matter according to the semi-classical Einstein equations, yields a nonlinear modification of the Schrödinger equation. Here, effects of this so-called Schrödinger-Newton equation are discussed, which allow for technologically feasible experimental tests.

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arXiv2017

Quantum mechanics for non-inertial observers

A recent analysis by Pikovski et al. [Nat. Phys. 11, 668 (2015)] has triggered interest in the question of how to include relativistic corrections in the quantum dynamics governing many-particle systems in a gravitational field. Here we show how the center-of-mass motion of a quantum system subject to gravity can be derived more rigorously, addressing the ambiguous definition of relativistic center-of-mass coordinates. We further demonstrate that, contrary to the prediction by Pikovski et al., external forces play a crucial role in the relativistic coupling of internal and external degrees of freedom, resulting in a complete cancellation of the alleged coupling in Earth-bound laboratories for systems supported against gravity by an external force. We conclude that the proposed decoherence effect is an effect of relative acceleration between quantum system and measurement device, rather than a universal effect in gravitational fields.

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arXiv2016

Approximations for the free evolution of self-gravitating quantum particles

The evolution of the centre-of-mass wave-function for a mesoscopic particle according to the Schrödinger-Newton equation can be approximated by a harmonic potential, if the wave-function is narrow compared to the size of the particle. It was noticed by Colin et al. [Phys. Rev. A, 93, 062102 (2016)] that, in the regime where self-gravitational effects are weak, intermediate and wider wave-functions may be approximated by a harmonic potential as well, but with a width dependent coupling, leading to a time evolution that is determined only by a differential equation for the width of a Gaussian wave-function as a single parameter. Such an approximation results in considerably less computational effort in order to predict the self-gravitational effects on the wave-function dynamics. Here, we provide an alternative approach to this kind of approximation, including a rigorous derivation of the equations of motion for an initially Gaussian wave packet, under the assumption that its shape is conserved. Our result deviates to some degree from the result by Colin et al., specifically in the limit of wide wave-functions.

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arXiv2015

Optomechanical test of the Schrödinger-Newton equation

The Schrödinger-Newton equation has been proposed as an experimentally testable alternative to quantum gravity, accessible at low energies. It contains self-gravitational terms, which slightly modify the quantum dynamics. Here we show that it distorts the spectrum of a harmonic system. Based on this effect, we propose an optomechanical experiment with a trapped microdisc to test the Schrödinger-Newton equation, and we show that it can be realized with existing technology.

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arXiv2015

Effects of Newtonian gravitational self-interaction in harmonically trapped quantum systems

The Schrödinger-Newton equation has gained attention in the recent past as a nonlinear modification of the Schrödinger equation due to a gravitational self-interaction. Such a modification is expected from a fundamentally semi-classical theory of gravity, and can therefore be considered a test case for the necessity of the quantisation of the gravitational field. Here we provide a thorough study of the effects of the Schrödinger-Newton equation for a micron-sized sphere trapped in a harmonic oscillator potential. We discuss both the effect on the energy eigenstates and the dynamical behaviour of squeezed states, covering the experimentally relevant parameter regimes.

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