PUBLICATIONS
Kimihiro Nomura, Akira Taniguchi, Kazushige Ueda
"Violation of the Leggett-Garg inequality in photon-graviton conversion''
Published in: Phys. Rev. D 114, 023530 (2026)
▼Abstract▼
PUBLICATIONS
Kimihiro Nomura, Akira Taniguchi, Kazushige Ueda
"Violation of the Leggett-Garg inequality in photon-graviton conversion''
Published in: Phys. Rev. D 114, 023530 (2026)
▼Abstract▼
The Leggett-Garg inequality (LGI) is a temporal analogue of Bell’s inequality and provides a quantitative test of the nonclassicality of a system through its violation. We analytically investigate the violation of the LGI in the context of photon-graviton conversion in a magnetic field background, motivated by its potential applications to testing the nonclassicality of gravity. When gravitational perturbations are quantized as gravitons, the conversion of an initial single photon state gives rise to a superposition of photon and graviton states. We show that the temporal correlations obtained from successive projective measurements on the photon-graviton system violate the LGI. Observation of such a violation would provide a novel avenue for probing the quantum nature of gravity.
Sugumi Kanno, Jiro Soda, Akira Taniguchi
"Binary gravitational waves as probes of quantum graviton states''
Published in: Phys. Rev. D 113, 123542 (2026)
▼Abstract▼
It is well known that the most reliable way to reveal the quantum nature of light is through photon number statistics, since photons exhibiting sub-Poissonian statistics unambiguously demonstrate their quantum behavior. In this paper, we show that gravitons emitted by binary systems can, in principle, exhibit analogous sub-Poissonian statistics. The key idea is that the vacuum state of gravitons may not be the standard Minkowski vacuum but rather a nonclassical state imprinted with the physics of the early Universe, such as inflation. Accordingly, gravitational waves from binary systems provide a means to probe the graviton states generated in the early Universe. As a concrete example, we show that squeezed graviton states originating from inflation can, in principle, imprint nonclassical graviton number statistics on gravitational waves from binary systems. In particular, we identify the frequency range in which the resulting coherent-squeezed graviton state can exhibit sub-Poissonian statistics. A realistic assessment of observational feasibility is left for future work.
Sugumi Kanno, Jiro Soda, Akira Taniguchi
"Coherent State Description of Gravitational Waves from Binary Black Holes''
Published in: Phys. Rev. Lett. 136, 061404 (2026)
▼Abstract▼
Quantum mechanics is the fundamental framework of nature, and gravitational waves from binary black holes during the inspiral phase should likewise be analyzed quantum mechanically. It is commonly assumed that their classical description corresponds to a coherent state, so any deviation would signal genuinely quantum nature of gravity. We show that the coherent-state description reproduces classical gravitational waves at leading order, while next-order effects generate squeezed states of gravitons. For GW150914, we estimate the squeezing parameter to be ∼ 10^(-4). We find that gravitational waves from binary black holes are well described by a coherent state.
Sugumi Kanno, Jiro Soda, Akira Taniguchi
"Parity violation in photon quasinormal modes of black holes''
Published in: Phys. Rev. D 112, 6, 063525 (2025)
▼Abstract▼
Given that black holes are ubiquitous in the universe and axion-like scalar fields are potential candidates for dark energy and/or dark matter, it is natural to consider cosmological black holes endowed with axion. We investigate the photon quasinormal modes of a Schwarzschild black hole with axion where the electromagnetic field is coupled to the axion field via a Chern-Simons interaction. We derive the master equations for the electromagnetic field as a set of coupled equations for parity-even and parity-odd modes and numerically compute quasinormal modes by using Leaver’s continued fraction method. We find parity violation in the polarization of photons within the quasinormal mode spectrum. This parity violation in electromagnetic signals could serve as a new probe to explore the nature of the dark sector.
Sugumi Kanno, Jiro Soda, Akira Taniguchi
"Search for high-frequency gravitational waves with Rydberg atoms"
Published in: Eur. Phys. J. C 85, 31 (2025)
▼Abstract▼
We propose high-frequency gravitational wave (GW) detectors with Rydberg atoms. Rydberg atoms are ultra-sensitive detectors of electric fields. By setting up a constant magnetic field, a weak electric field is generated upon the arrival of GWs. The weak electric field signal is then detected by an electromagnetically induced transparency (EIT) in the system of the Rydberg atoms. Recently, the minimum detectable electric field with the Rydberg atoms is further improved by employing superheterodyne detection method. Hence, even the weak signal generated by GWs turns out to be detectable. We calculate the amplitude of Rabi frequency of the Rydberg atoms induced by the GWs and show that the sensitivity of the Rydberg atoms becomes maximum when the size of the Rydberg atoms is close to the wavelength of GWs. We evaluate the minimum detectable amplitude of GWs with Rubidium Rydberg atoms and find that the detector can probe GWs with a frequency f = 4.2 GHz and an amplitude around 10^(−20).
Sugumi Kanno, Jiro Soda, Akira Taniguchi
"Circularly polarized gravitational waves in Chern-Simons gravity originating from an axion domain wall"
Published in: Phys. Rev. D 108, 083525 (2023)
▼Abstract▼
We study a scattering problem of gravitational waves (GWs) by an axion domain wall in Chern-Simons (CS) gravity. We find that circular polarization of GWs is produced after passing through the domain wall. It turns out that the circular polarization is sizable if the frequency of the GW is comparable to a critical value determined by the characteristic CS length scale and the energy scale of the axion domain wall. Thus, observations of the circular polarization could give a stringent constraint on the characteristic CS length scale or could be a new avenue to search for axion.