arXiv subjects

Igor Pikovski

Publications and source records attributed to Igor Pikovski.

At least 19 recordsLinked to original sources

arXiv2026

Quantum State Characterization of Gravitational Waves via Graviton Counting Statistics

Although gravitational waves are now routinely observed, the detection of individual gravitons has long been regarded as impossible. Recent work, however, has demonstrated that single-graviton detection can be achieved and may be feasible in the near future. Here we show that beyond mere particle detection, these detectors provide access to the quantum state and particle statistics of gravitational waves. We show that graviton detection probabilities enable the discrimination between squeezed, coherent, and thermal radiation. We further demonstrate that the full quantum statistics contained in the second-order correlation function of the passing wave can be directly measured at the detector, independent of the weak gravitational interaction strength. Building on recent quantum-optical techniques, this capability opens the way to full quantum state tomography of Gaussian states. Our results demonstrate that single-graviton detection is not only of foundational significance but also of practical value, allowing for the characterization of quantum statistics and the states of the gravitational radiation field, which remain currently unknown.

quant-ph↗

arXiv2025

Quantum signatures of proper time in optical ion clocks

Optical clocks based on atoms and ions probe relativistic effects with unprecedented sensitivity by resolving time dilation due to atom motion or different positions in the gravitational potential through frequency shifts. However, all measurements of time dilation so far can be explained effectively as the result of dynamics with respect to a classical proper time parameter. Here we show that atomic clocks can probe effects where a classical description of the proper time dynamics is insufficient. We apply a Hamiltonian formalism to derive time dilation effects in harmonically trapped clock atoms and show how second-order Doppler shifts (SODS) due to the vacuum energy (vSODS), squeezing (sqSODS) and quantum corrections to the dynamics (qSODS) arise. We also demonstrate that the entanglement between motion and clock evolution can become observable in state-of-the-art clocks when the motion of the atoms is strongly squeezed, realizing proper time interferometry. Our results show that experiments with trapped ion clocks are within reach to probe relativistic evolution of clocks for which a quantum description of proper time becomes necessary.

quant-ph↗

arXiv2025

A One-sided Witness for the Quantumness of Gravitational Dynamics

Quantum information concepts and quantum technologies have opened the prospect to probe quantum gravity in table-top experiments. Many proposals rely on witnessing entanglement generation as a means to probe whether gravity is a quantum channel. Here we formulate a different and conclusive indirect test of the quantum nature of the gravitational interaction. Our witness is based on the concept of verifiable quantum memory in the dynamics of a quantum system. This allows us to assess the quantumness of an interaction between two systems by local measurements on one subsystem only. Our approach enables the first one-sided verification of the quantum nature of gravity, and provides a quantum signature of the interaction that is not fully covered by existing proposals. Our results open novel ways to witnessing the quantum nature of gravity in table-top experiments and clarify how {decisive tests can be designed even with measurements on only the probe system

quant-ph↗

arXiv2025

Testing quantum theory on curved space-time with quantum networks

Quantum technologies present new opportunities for fundamental tests of nature. One potential application is to probe the interplay between quantum physics and general relativity - a field of physics with no empirical evidence yet. Here we show that quantum networks open a new window to test this interface. We demonstrate how photon mediated entanglement between atomic or atom-like systems can be used to probe time-dilation induced entanglement and interference modulation. Key are non-local measurements between clocks in a gravitational field, which can be achieved either through direct photon interference or by using auxiliary entanglement. The resulting observable depends on the interference between different proper times, and can only be explained if both quantum theory and general relativity are taken into account. The proposed protocol enables clock interferometry on km-scale separations and beyond. Our work thus shows a realistic experimental route for a first test of quantum theory on curved space-time, opening up new scientific opportunities for quantum networks.

quant-ph↗

arXiv2025

Probing classical and quantum violations of the equivalence of active and passive gravitational mass

The equivalence of active and passive (EAP) gravitational mass is one of the most fundamental principles of gravity. But in contrast to the usual equivalence of inertial and (passive) gravitational mass, the EAP has not received much attention. Here we revisit this principle and show how it can be used to probe quantum gravity in laboratory-based experiments. We first examine how the dynamics under EAP violations affects classical systems and show that new laboratory tests can be performed, to improve over the current experimental bounds and to test new manifestations of EAP violations. We then extend the analysis to the quantum domain, where quantized energy contributes to mass and the EAP principle can thus shed light on how quantum source masses would gravitate. We show that experiments with cold polar molecules, and future experiments with nuclear atomic clocks, can test the quantum EAP in a regime where quantum gravity phenomenology could become relevant. Our results open new opportunities for fundamental tests of gravity in high-precision laboratory experiments that can shed light on foundational principles of gravity and its interface with quantum theory.

quant-ph↗

arXiv2025

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↗

arXiv2025

Probing curved spacetime with a distributed atomic processor clock

Quantum dynamics on curved spacetime has never been directly probed beyond the Newtonian limit. Although we can describe such dynamics theoretically, experiments would provide empirical evidence that quantum theory holds even in this extreme limit. The practical challenge is the minute spacetime curvature difference over the length scale of the typical extent of quantum effects. Here we propose a quantum network of alkaline earth(-like) atomic processors for constructing a distributed quantum state that is sensitive to the differential proper time between its constituent atomic processor nodes, implementing a quantum observable that is affected by post-Newtonian curved spacetime. Conceptually, we delocalize one clock between three locations by encoding the presence or absence of a clock into the state of the local atoms. By separating three atomic nodes over $\sim$km-scale elevation differences and distributing one clock between them via a W-state, we demonstrate that the curvature of spacetime is manifest in the interference of the three different proper times that give rise to three distinct beat notes in our non-local observable. We further demonstrate that $N$-atom entanglement within each node enhances the interrogation bandwidth by a factor of $N$. We discuss how our system can probe new facets of fundamental physics, such as the linearity, unitarity and probabilistic nature of quantum theory on curved spacetime. Our protocol combines several recent advances with neutral atom and trapped ions to realize a novel quantum probe of curved spacetime uniquely enabled by quantum networks.

quant-ph↗

arXiv2024

A coupling prescription for post-Newtonian corrections in Quantum Mechanics

The interplay between quantum theory and general relativity remains one of the main challenges of modern physics. A renewed interest in the low-energy limit is driven by the prospect of new experiments that could probe this interface. Here we develop a covariant framework for expressing post-Newtonian corrections to Schrödinger's equation on arbitrary gravitational backgrounds based on a $1/c^2$ expansion of Lorentzian geometry, where $c$ is the speed of light. Our framework provides a generic coupling prescription of quantum systems to gravity that is valid in the intermediate regime between Newtonian gravity and General Relativity, and that retains the focus on geometry. At each order in $1/c^2$ this produces a nonrelativistic geometry to which quantum systems at that order couple. By considering the gauge symmetries of both the nonrelativistic geometries and the $1/c^2$ expansion of the complex Klein--Gordon field, we devise a prescription that allows us to derive the Schrödinger equation and its post-Newtonian corrections on a gravitational background order-by-order in $1/c^2$. We also demonstrate that these results can be obtained from a $1/c^2$ expansion of the complex Klein--Gordon Lagrangian. We illustrate our methods by performing the $1/c^2$ expansion of the Kerr metric up to $\mathcal{O}(c^{-2})$, which leads to a special case of the Hartle--Thorne metric. The associated Schrödinger equation captures novel and potentially measurable effects.

gr-qc↗

arXiv2024

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↗

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↗

arXiv2023

On inference of quantization from gravitationally induced entanglement

Observable signatures of the quantum nature of gravity at low energies have recently emerged as a promising new research field. One prominent avenue is to test for gravitationally induced entanglement between two mesoscopic masses prepared in spatial superposition. Here we analyze such proposals and what one can infer from them about the quantum nature of gravity, as well as the electromagnetic analogues of such tests. We show that it is not possible to draw conclusions about mediators: even within relativistic physics, entanglement generation can equally be described in terms of mediators or in terms of non-local processes -- relativity does not dictate a local channel. Such indirect tests therefore have limited ability to probe the nature of the process establishing the entanglement as their interpretation is inherently ambiguous. We also show that cosmological observations already demonstrate some aspects of quantization that these proposals aim to test. Nevertheless, the proposed experiments would probe how gravity is sourced by spatial superpositions of matter, an untested new regime of quantum physics.

quant-ph↗

arXiv2022

Solving quantum dynamics with a Lie algebra decoupling method

At the heart of quantum technology development is the control of quantum systems at the level of individual quanta. Mathematically, this is realised through the study of Hamiltonians and the use of methods to solve the dynamics of quantum systems in various regimes. Here, we present a pedagogical introduction to solving the dynamics of quantum systems by the use of a Lie algebra decoupling theorem. As background, we include an overview of Lie groups and Lie algebras aimed at a general physicist audience. We then prove the theorem and apply it to three well-known examples of linear and quadratic Hamiltonian that frequently appear in quantum optics and related fields. The result is a set of differential equations that describe the most Gaussian dynamics for all linear and quadratic single-mode Hamiltonian with generic time-dependent interaction terms. We also discuss the use of the decoupling theorem beyond quadratic Hamiltonians and for solving open-system dynamics.

quant-ph↗

arXiv2022

Nonclassicality of axion-like dark matter through gravitational self-interactions

Axion-like particles (ALPs) are promising dark matter candidates. They are typically described by a classical field, motivated by large phase space occupation numbers. Here we show that such a description is accompanied by a quantum effect: squeezing due to gravitational self-interactions. For a typical QCD axion today, the onset of squeezing is reached on $\mathrm{μs}$-scales and grows over millennia. Thus within the usual models based on the classical Schrödinger-Poisson equation, a type of Gross-Pitaevskii equation, any viable ALP is nonclassical. We also show that squeezing may be relevant on the scales of other self-gravitating systems such as galactic haloes, or solitonic cores. Conversely, our results highlight the incompleteness and limitations of the classical single field description of ALPs.

astro-ph.CO↗

arXiv2022

Optimal Fidelity Witnesses for Gravitational Entanglement

Optomechanical systems open new possibilities in fundamental research at the interface between quantum information and gravity. Recently, an ambitious experimental proposal was suggested by Bose et al. to measure the entanglement between two optomechanical systems generated by their gravitational interaction. The scheme relies on witnessing entanglement between the two systems. Here we develop a general framework to study the quality of bipartite entanglement witnesses using fidelity witnesses. We then apply this framework to the gravitational entanglement proposal, optimizing for the detection of entanglement. We construct a witness consisting of only 5 non-trivial spin measurements, which we compare with other proposed witnesses. With post-processing our witness can detect entanglement for any choice of phases in the setup, up to a set of measure zero.

quant-ph↗

arXiv2022

Limits on inference of gravitational entanglement

Combining gravity with quantum mechanics remains one of the biggest challenges of physics. In the past years, experiments with opto-mechanical systems have been proposed that may give indirect clues about the quantum nature of gravity. In a recent variation of such tests [D. Carney et al., Phys.Rev.X Quantum 2, 030330 (2021)], the authors propose to gravitationally entangle an atom interferometer with a mesoscopic oscillator. The interaction results in periodic drops and revivals of the interferometeric visibility, which under specific assumptions indicate the gravitational generation of entanglement. Here we study semi-classical models of the atom interferometer that can reproduce the same effect. We show that the core signature -- periodic collapses and revivals of the visibility -- can appear if the atom is subject to a random unitary channel, including the case where the oscillator is fully classical and situations even without explicit modelling of the oscillator. We also show that the non-classicality of the oscillator vanishes unless the system is very close to its ground state, and even when the system is in the ground state, the non-classicality is limited by the coupling strength. Our results thus indicate that deducing entanglement from the proposed experiment is very challenging, since fulfilling and verifying the non-classicality assumptions is a significant challenge on its own right.

quant-ph↗

arXiv2021

Many-body probes for quantum features of spacetime

Many theories of quantum gravity can be understood as imposing a minimum length scale the signatures of which can potentially be seen in precise table top experiments. In this work we inspect the capacity for correlated many body systems to probe non-classicalities of spacetime through modifications of the commutation relations. We find an analytic derivation of the dynamics for a single mode light field interacting with a single mechanical oscillator and with coupled oscillators to first order corrections to the commutation relations. Our solution is valid for any coupling function as we work out the full Magnus expansion. We numerically show that it is possible to have superquadratic scaling of a non-classical phase term, arising from the modification to the commutation relations, with coupled mechanical oscillators.

quant-ph↗

arXiv2021

Do Gedankenexperiments compel quantization of gravity?

Whether gravity is quantized remains an open question. To shed light on this problem, various Gedankenexperiments have been proposed. One popular example is an interference experiment with a massive system that interacts gravitationally with another distant system, where an apparent paradox arises: even for space-like separation the outcome of the interference experiment depends on actions on the distant system, leading to a violation of either complementarity or no-signalling. A recent resolution shows that the paradox is avoided when quantizing gravitational radiation and including quantum fluctuations of the gravitational field. Here we show that the paradox in question can also be resolved without considering gravitational radiation, relying only on the Planck length as a limit on spatial resolution. Therefore, in contrast to conclusions previously drawn, we find that the necessity for a quantum field theory of gravity does not follow from so far considered Gedankenexperiments of this type. In addition, we point out that in the common realization of the setup the effects are governed by the mass octopole rather than the quadrupole. Our results highlight that no Gedankenexperiment to date compels a quantum field theory of gravity, in contrast to the electromagnetic case.

gr-qc↗

arXiv2021

The missing link in gravitational-wave astronomy: A summary of discoveries waiting in the decihertz range

Since 2015 the gravitational-wave observations of LIGO and Virgo have transformed our understanding of compact-object binaries. In the years to come, ground-based gravitational-wave observatories such as LIGO, Virgo, and their successors will increase in sensitivity, discovering thousands of stellar-mass binaries. In the 2030s, the space-based LISA will provide gravitational-wave observations of massive black holes binaries. Between the $\sim 10$-$10^3~\mathrm{Hz}$ band of ground-based observatories and the $\sim10^{-4}$-$10^{-1}~\mathrm{Hz}$ band of LISA lies the uncharted decihertz gravitational-wave band. We propose a Decihertz Observatory to study this frequency range, and to complement observations made by other detectors. Decihertz observatories are well suited to observation of intermediate-mass ($\sim10^2$-$10^4 M_\odot$) black holes; they will be able to detect stellar-mass binaries days to years before they merge, providing early warning of nearby binary neutron star mergers and measurements of the eccentricity of binary black holes, and they will enable new tests of general relativity and the Standard Model of particle physics. Here we summarise how a Decihertz Observatory could provide unique insights into how black holes form and evolve across cosmic time, improve prospects for both multimessenger astronomy and multiband gravitational-wave astronomy, and enable new probes of gravity, particle physics and cosmology.

gr-qc↗