arXiv subjects

Marko Toroš

Publications and source records attributed to Marko Toroš.

At least 37 records · Page 2Linked to original sources

arXiv2023

Gravitational Optomechanics: Photon-Matter Entanglement via Graviton Exchange

The deflection of light in the gravitational field of the Sun is one of the most fundamental consequences for general relativity as well as one of its classical tests first performed by Eddington a century ago. However, despite its center stage role in modern physics, no experiment has tested it in an ostensibly quantum regime where both matter and light exhibit non-classical features. This paper shows that the interaction which gives rise to the light-bending also induces photon-matter entanglement as long as gravity and matter are treated at par with quantum mechanics. The quantum light-bending interaction within the framework of perturbative quantum gravity highlights this point by showing that the entangled states can be generated already with coherent states of light and matter exploiting the non-linear coupling induced by graviton exchange. Furthermore, the quantum light-bending interaction is capable of discerning between the spin-2 and spin-0 gravitons thus also providing a test for alternative theories of gravity at short distances and at the quantum level. We will conclude by estimating the order of magnitude of the entanglement generated by employing the linear entropy. In particular, we find that a half-ring cavity of radius $0.25$ m placed around a $10$ kg mechanical oscillator operating at $150$ Hz, could be used to generate linear entropy of order unity using a petawatt laser source at optical wavelengths. While the proposed scheme is beyond the current experimental realities it nonetheless initiates the discussion about testing the spin of the gravitational interaction at the quantum level.

gr-qc↗

arXiv2023

Decoherence of a matter-wave interferometer due to dipole-dipole interactions

Matter-wave interferometry with nanoparticles will enable the development of quantum sensors capable of probing ultraweak fields with unprecedented applications for fundamental physics. The high sensitivity of such devices however makes them susceptible to a number of noise and decoherence sources and as such can only operate when sufficient isolation from the environment is achieved. It is thus imperative to model and characterize the interaction of nanoparticles with the environment and to estimate its deleterious effects. The aim of this paper will be to study the decoherence of the matter-wave interferometer due to dipole-dipole interactions which is one of the unavoidable channels for decoherence even for a neutral micro-crystal. We will start the analysis from QED and show that it reduces to the scattering model characterized by the differential cross-section. We will then obtain simple expressions for the decoherence rate in the short and long wavelength limits that can be readily applied to estimate the available coherence time. We will conclude by applying the obtained formulae to estimate the dipole-dipole decoherence rate for the Quantum Gravity-induced Entanglement of Masses (QGEM) protocol and discuss if the effects should be mitigated.

quant-ph↗

arXiv2023

Quantum Gravitational Sensor for Space Debris

Matter-wave interferometers have fundamental applications for gravity experiments such as testing the equivalence principle and the quantum nature of gravity. In addition, matter-wave interferometers can be used as quantum sensors to measure the local gravitational acceleration caused by external massive moving objects, thus lending itself for technological applications. In this paper, we will establish a three dimensional model to describe the gravity gradient signal from an external moving object, and theoretically investigate the achievable sensitivities using the matter-wave interferometer based on the Stern-Gerlach set-up. As an application we will consider the Mesoscopic Interference for Metric and Curvature (MIMAC) and Gravitational wave detection scheme [New J. Phys. 22, 083012 (2020)] and quantify its sensitivity to gravity gradients using frequency-space analysis. We will consider objects near Earth-based experiments and space debris in proximity of satellites and estimate the minimum detectable mass of the object as a function of their distance, velocity, and orientation.

gr-qc↗

arXiv2023

Entanglement Witness for the Weak Equivalence Principle

The Einstein equivalence principle is based on the equality of gravitational mass and inertial mass, which has led to the universality of a free-fall concept. The principle has been extremely well tested so far and has been tested with a great precision. However, all these tests and the corresponding arguments are based on a classical setup where the notion of position and velocity of the mass is associated with a classical value as opposed to the quantum entities. Here, we will provide a simple protocol based on creating large spatial superposition states in a laboratory to test the fully quantum regime of the equivalence principle where both matter and gravity are treated at par as a quantum entity. We will argue that such a quantum protocol is unique with regard to testing especially the generalization of the weak equivalence principle via witnessing quantum entanglement.

gr-qc↗

arXiv2022

Measurement of single nanoparticle anisotropy by laser induced optical alignment and Rayleigh scattering for determining particle morphology

We demonstrate the measurement of nanoparticle shape by angularly resolved Rayleigh scattering of single optical levitated particles that are oriented in space via the trapping light in vacuum. This technique is applied to a range of particle geometries, from perfect spherical nanodroplets to octahedral nanocrystals. We show that this method can resolve shape differences down to a few nanometers and be applied in both low-damping environments, as demonstrated here, and in traditional overdamped fluids used in optical tweezers.

physics.optics↗

arXiv2022

Generation of entanglement from mechanical rotation

Many phenomena and fundamental predictions, ranging from Hawking radiation to the early evolution of the Universe rely on the interplay between quantum mechanics and gravity or more generally, quantum mechanics in curved spacetimes. However, our understanding is hindered by the lack of experiments that actually allow us to probe quantum mechanics in curved spacetime in a repeatable and accessible way. Here we propose an experimental scheme for a photon that is prepared in a path superposition state across two rotating Sagnac interferometers that have different diameters and thus represent a superposition of two different spacetimes. We predict the generation of genuine entanglement even at low rotation frequencies and show how these effects could be observed even due to the Earth's rotation. These predictions provide an accessible platform in which to study the role of the underlying spacetime in the generation of entanglement.

quant-ph↗

arXiv2022

Gravitational decoherence by the apparatus in the quantum-gravity induced entanglement of masses

One of the outstanding questions in modern physics is how to test whether gravity is classical or quantum in a laboratory. Recently there has been a proposal to test the quantum nature of gravity by creating quantum superpositions of two nearby neutral masses, close enough that the quantum nature of gravity can entangle the two quantum systems, but still sufficiently far away that all other known Standard Model interactions remain negligible. However, the mere process of preparing superposition states of a neutral mass (the light system), requires the vicinity of laboratory apparatus (the heavy system). We will suppose that such a heavy system can be modelled as another quantum system; since gravity is universal, the lighter system can get entangled with the heavier system, providing an inherent source of gravitational decoherence. In this paper, we will consider two light and two heavy quantum oscillators, forming pairs of probe-detector systems, and study under what conditions the entanglement between two light systems evades the decoherence induced by the heavy systems. We conclude by estimating the magnitude of the decoherence in the proposed experiment for testing the quantum nature of gravity.

quant-ph↗

arXiv2022

Simultaneous cooling of all six degrees of freedom of an optically levitated nanoparticle by elliptic coherent scattering

We report on strong cooling and orientational control of all translational and angular degrees of freedom of a nanoparticle levitated in an optical trap in high vacuum. The motional cooling and control of all six degrees of freedom of a nanoparticle levitated by an optical tweezer is accomplished using coherent elliptic scattering within a high finesse optical cavity. Translational temperatures in the 100 $μ$K range were reached while temperatures as low as 5 mK were attained in the librational degrees of freedom. This work represents an important milestone in controlling all observable degrees of freedom of a levitated particle and opens up future applications in quantum science and the study of single isolated nanoparticles.

quant-ph↗

arXiv2022

Mechanism for the quantum natured gravitons to entangle masses

This paper points out the importance of the quantum nature of the gravitational interaction with matter in a linearized theory of quantum gravity induced entanglement of masses (QGEM). We will show how the quantum interaction entangles the steady states of a closed system (eigenstates) of two test masses placed in the harmonic traps, and how such a quantum matter-matter interaction emerges from an underlying quantum gravitational field. We will rely upon quantum perturbation theory highlighting the critical assumptions for generating a quantum matter-matter interaction and showing that a classical gravitational field does not render such an entanglement. We will consider two distinct examples; one where the two harmonic oscillators are static and the other where the harmonic oscillators are non-static. In both the cases it is the quantum nature of the gravitons interacting with the harmonic oscillators that are responsible for creating an entangled state with the ground and the excited states of harmonic oscillators as the Schmidt basis. We will compute the concurrence as a criterion for the above entanglement and highlight the role of the spin-2 nature of the graviton for entangling the two harmonic oscillators.

gr-qc↗

arXiv2022

Requirements on Quantum Superpositions of Macro-Objects for Sensing Neutrinos

We examine a macroscopic system in a quantum superposition of two spatially separated localized states as a detector for a stream of weakly interacting relativistic particles. We do this using the explicit example of neutrinos with MeV-scale energy scattering from a solid object via neutral-current neutrino-nucleus scattering. Presuming the (anti-)neutrino source to be a nuclear fission reactor, we utilize the estimated flux and coherent elastic neutrino-nucleus cross section to constrain the spatial separation $Δ$x and describe the temporal evolution of the sensing system. Particularly, we find that a potentially measurable relative phase between quantum superposed components is obtained for a single gram scale mass placed in a superposition of spatial components separated by $10^{-14}$m under sufficient cooling and background suppression.

quant-ph↗

arXiv2022

Infrared scaling for a graviton condensate

The coupling between gravity and matter provides an intriguing length scale in the infrared for theories of gravity within Einstein-Hilbert action and beyond. In particular, we will show that such an infrared length scale is determined by the number of gravitons $N_{g}\gg1$ associated to a given mass in the non-relativistic limit. After tracing out the matter degrees of freedom, the graviton vacuum is found to be in a displaced vacuum with an occupation number of gravitons $N_{g}\gg1$. In the infrared, the length scale appears to be $L=\sqrt{N_{g}}\ell_{p}$, where $L$ is the new infrared length scale, and $\ell_{p}$ is the Planck length. In a specific example, we have found that the infrared length scale is greater than the Schwarzschild radius for a slowly moving in-falling thin shell of matter. We will argue that the appearance of such an infrared length scale in higher curvature theories of gravity, such as in quadratic and cubic curvature theories of gravity, is also expected. Furthermore, we will show that gravity is fundamentally different from the electromagnetic interaction where the number of photons, $N_{p}$, is the fine structure constant after tracing out an electron wave function.

gr-qc↗

arXiv2021

Gravitons in a Box

Gravity and matter are universally coupled, and this unique universality provides us with an intriguing way to quantifying quantum aspects of space-time in terms of the number of gravitons within a given box. In particular, we will provide a limit on the number of gravitons if we trace out the matter degrees of freedom. We will obtain the universal bound on the number of gravitons, which would be given by $N_{g}\approx(m/M_{p})^{2}$. Since the number of gravitons also signify the number of bosonic states they occupy, the number of gravitons will place an indirect constraint on the gravitational entropy of the system. We will show that it saturates Bekenstein bound on the gravitational Area-law of entropy. We will also find that our conclusion is quite robust against the initial state of the matter degrees of freedom. Based on these observations, we will ascertain that the gravitons permeating in the observable Universe always $N_g\gg 1$.

gr-qc↗

arXiv2021

Relative Acceleration Noise Mitigation for Nanocrystal Matter-wave Interferometry: Application to Entangling Masses via Quantum Gravity

Matter wave interferometers with large momentum transfers, irrespective of specific implementations, will face a universal dephasing due to relative accelerations between the interferometric mass and the associated apparatus. Here we propose a solution that works even without actively tracking the relative accelerations: putting both the interfering mass and its associated apparatus in a freely falling capsule, so that the strongest inertial noise components vanish due to the equivalence principle. In this setting, we investigate two of the most important remaining noise sources: (a) the non-inertial jitter of the experimental setup and (b) the gravity-gradient noise. We show that the former can be reduced below desired values by appropriate pressures and temperatures, while the latter can be fully mitigated in a controlled environment. We finally apply the analysis to a recent proposal for testing the quantum nature of gravity [S. Bose et. al. Phys. Rev. Lett 119, 240401 (2017)] through the entanglement of two masses undergoing interferometry. We show that the relevant entanglement witnessing is feasible with achievable levels of relative acceleration noise.

gr-qc↗

arXiv2021

Coherent scattering 2D cooling in levitated cavity optomechanics

The strong light-matter optomechanical coupling offered by Coherent Scattering (CS) set-ups have allowed the experimental realisation of quantum ground state cavity cooling of the axial motion of a levitated nanoparticle [U. Delić et al., Science 367, 892 (2020)]. An appealing milestone is now quantum 2D cooling of the full in-plane motion, in any direction in the transverse plane. By a simple adjustment of the trap polarisation, one obtains two nearly equivalent modes, with similar frequencies $ω_{x}\simω_{y}$ and optomechanical couplings $g_{x}\simeq g_{y}$ -- in this experimental configuration we identify an optimal trap ellipticity, nanosphere size and cavity linewidth which allows for efficient 2D cooling. Moreover, we find that 2D cooling to occupancies $n_{x}+n_{y}\lesssim1$ at moderate vacuum ($10^{-6}$ mbar) is possible in a "Goldilocks" zone bounded by $\sqrt{κΓ/4}\lesssim g_{x},g_{y}\lesssim|ω_{x}-ω_{y}|\lesssimκ$, where one balances the need to suppress dark modes whilst avoiding far-detuning of either mode or low cooperativities, and $κ$ ($Γ$) is the cavity decay rate (motional heating rate). With strong-coupling regimes $g_{x},g_{y}\gtrsimκ$ in view one must consider the genuine three-way hybridisation between $x$, $y$ and the cavity light mode resulting in hybridized bright/dark modes. Finally, we show that bright/dark modes in the levitated set-up have a simple geometrical interpretation, related by rotations in the transverse plane, with implications for directional sensing.

quant-ph↗

arXiv2019

Revealing and concealing entanglement with non-inertial motion

Photon interference and bunching are widely studied quantum effects that have also been proposed for high precision measurements. Here we construct a theoretical description of photon-interferometry on rotating platforms, specifically exploring the relation between non-inertial motion, relativity, and quantum mechanics. On the basis of this, we then propose an experiment where photon entanglement can be revealed or concealed solely by controlling the rotational motion of an interferometer, thus providing a route towards studies at the boundary between quantum mechanics and relativity.

quant-ph↗

arXiv2019

Optimal control for feedback cooling in cavityless levitated optomechanics

We consider feedback cooling in a cavityless levitated optomechanics setup, and we investigate the possibility to improve the feedback implementation. We apply optimal control theory to derive the optimal feedback signal both for quadratic (parametric) and linear (electric) feedback. We numerically compare optimal feedback against the typical feedback implementation used for experiments. In order to do so, we implement a tracking scheme that takes into account the modulation of the laser intensity. We show that such a tracking implementation allows us to increase the feedback strength, leading to faster cooling rates and lower center-of-mass temperatures.

quant-ph↗

arXiv2019

Static force characterization with Fano anti-resonance in levitated optomechanics

We demonstrate a classical analogy to the Fano anti-resonance in levitated optomechanics by applying a DC electric field. Specifically, we experimentally tune the Fano parameter by applying a DC voltage from 0~kV to 10~kV on a nearby charged needle tip. We find consistent results across negative and positive needle voltages, with the Fano line-shape feature able to exist at both higher and lower frequencies than the fundamental oscillator frequency. We can use the Fano parameter to characterize our system to be sensitive to static interactions which are ever-present. Currently, we can distinguish a static Coulomb force of $2.7 \pm 0.5 \times 10^{-15}$~N with the Fano parameter, which is measured with one second of integration time. Furthermore, we are able to extract the charge to mass ratio of the trapped nanoparticle.

quant-ph↗

arXiv2018

Detection of anisotropic particles in levitated optomechanics

We discuss the detection of an anisotropic particle trapped by an elliptically polarized focused Gaussian laser beam. We obtain the full rotational and translational dynamics, as well as, the measured photo-current in a general-dyne detection. As an example, we discuss a toy model of homodyne detection, which captures the main features typically found in experimental setups.

quant-ph↗