arXiv subjects

Germain Tobar

Publications and source records attributed to Germain Tobar.

9 recordsLinked to original sources

arXiv2025

Wave-particle duality in the measurement of gravitational radiation

In a consistent description of the quantum measurement process, whether the wave or particle-like aspect of a system is revealed depends on the details of the measurement chain, and cannot be interpreted as an objective fact about the system independent of the measurement. We show precisely how this comes to be in the measurement of gravitational radiation. Whether a wave or particle-like aspect is revealed is a property of the detector employed at the end of the quantum measurement chain, rather than of the meter, such as a gravitational-wave (GW) antenna or resonant bar, used to couple the radiation to the detector. A linear detector yields no signal for radiation in a Fock state and a signal proportional to the amplitude in a coherent state -- supporting a wave-like interpretation. By contrast, the signal from a detector coupled to the meter's energy is non-zero only when the incident radiation contains at least a single graviton. Thus, conceptually simple modifications of contemporary GW antennae can reveal wave-particle duality in the measurement of gravitational radiation.

quant-ph↗

arXiv2025

Quantum metasurfaces as probes of vacuum particle content

The quantum vacuum of the electromagnetic field is inherently entangled across distinct spatial sub-regions, resulting in entangled particle content across these sub-regions. However accessing this particle content in a controlled laboratory experiment has remained out of experimental reach. Here, we analyse the feasibility of witnessing such vacuum effects with highly non-perturbative boundary condition changes with a quantum metasurface made from a two-dimensional sub-wavelength atomic array. The array response to light is tunable between transmissive and reflective states by a control atom that is excited to a Rydberg state. We find that vacuum photon content from non-perturbative changes of the boundary conditions and therefore distinct spatial sub-regions of the vacuum causes subtle frequency shifts and analyse the accessibility to sub-wavelength atom array platforms. This novel approach to probing vacuum particle content leverages the unique ability to generate coherent dynamics in superpositions of transmissive and reflective states of the array reflectivity, providing a quantum-enhanced platform for observing vacuum particle creation driven by highly non-perturbative changes in the boundary conditions of the electromagnetic vacuum.

quant-ph↗

arXiv2024

The Gravito-Phononic Effect: A Quantum Signature of Linearised Gravity

The photo-electric effect was a historic milestone in the development of quantum theory, revealing the first evidence of discrete energy of the electromagnetic field through hallmark signatures such as the threshold frequency, intensity-independent energy transfer, and the near instantaneous ejection of photo-electrons. Here, we discuss the photo-electric effect through the lens of semi-classical, quantum, and neo-classical models. We provide a pedagogical outline for how two coupled harmonic oscillators, under a beam-splitter interaction, can exhibit hallmark signatures analogous to the photo-electric effect, including resonance conditions and quantised energy absorption. We discuss the implications of this model for recently proposed graviton detection protocols. This further clarifies that a gravitational version of the photo-electric effect, modelled as discrete energy transfer between harmonic oscillators can provide the first evidence of the graviton.

quant-ph↗

arXiv2024

Stimulated absorption of single gravitons: First light on quantum gravity

In a recent work we showed that the detection of the exchange of a single graviton between a massive quantum resonator and a gravitational wave can be achieved. Key to this ability are the experimental progress in preparing and measuring massive resonators in the quantum regime, and the correlation with independent LIGO detections of gravitational waves that induce stimulated absorption. But do stimulated single-graviton processes imply the quantization of gravity? Here we analyze this question and make a historic analogy to the early days of quantum theory. We discuss in what ways such experiments can indeed probe key features of the quantized interaction between gravity and matter, and outline five experimental tests. This capability would open the first window into experimental exploration of quantum gravity.

gr-qc↗

arXiv2024

Detecting kHz gravitons from a neutron star merger with a multi-mode resonant mass detector

We propose a multi-mode bar consisting of mass elements of decreasing size for the implementation of a gravitational version of the photo-electric effect through the stimulated absorption of up to kHz gravitons from a binary neutron star merger and post-merger. We find that the multi-mode detector has normal modes that retain the coupling strength to the gravitational wave of the largest mass-element, while only having an effective mass comparable to the mass of the smallest element. This allows the normal modes to have graviton absorption rates due to the tonne-scale largest mass, while the single graviton absorption process in the normal mode could be resolved through energy measurements of a mass-element in-principle smaller than pico-gram scale. We argue the feasibility of directly counting gravito-phonons in the bar through energy measurements of the end mass. This improves the transduction of the single-graviton signal, enhancing the feasibility of detecting single gravitons.

astro-ph.IM↗

arXiv2023

Detecting single gravitons with quantum sensing

The quantization of gravity is widely believed to result in gravitons -- particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single graviton exchange can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental clue of the quantization of gravity.

quant-ph↗

arXiv2022

Testing spontaneous wavefunction collapse with quantum electromechanics

Theories of spontaneous wavefunction collapse offer an explanation of the possible breakdown of quantum mechanics for macroscopic systems. However, the challenge of resolving predicted collapse signatures above background noise has precluded conclusive tests. Here, we propose to overcome this challenge using a superconducting qubit to precisely readout the collapse-induced heating of a mechanical resonator. We show that the ability to strongly couple the qubit to the resonator can enable both fast measurements and initialization of the qubit close to its ground state. Combined this greatly suppresses the influence of quasiparticle heating of the qubit, which we predict to be the dominant noise source. We find that bulk acoustic wave resonances can amplify the collapse induced heating due to their ultra-low dissipation. Together, this could enable a conclusive test of collapse models.

quant-ph↗

arXiv2022

Mass-energy equivalence in gravitationally bound quantum states of the neutron

Gravitationally bound neutrons have become an important tool in the experimental searches for new physics, such as modifications to Newton's force or candidates for dark matter particles. Here we include the relativistic effects of mass-energy equivalence into the model of gravitationally bound neutrons. Specifically, we investigate a correction in a gravitationally bound neutron's Hamiltonian due to the presence of an external magnetic field. We show that the neutron's additional weight due to mass-energy equivalence will cause a small shift in the neutron's eigenenergies and eigenstates, and examine how this relativistic correction would affect experiments with trapped neutrons. We further consider the ultimate precision in estimating the relativistic correction to the precession frequency and find that, at short times, a joint measurement of both the spin and motional degrees of freedom provides a metrological enhancement as compared to a measurement of the spin alone.

quant-ph↗

arXiv2020

Reversible dynamics with closed time-like curves and freedom of choice

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.

quant-ph↗