Gravitational waves (GWs) are ripples in spacetime generated by the violent motion of extremely massive objects, such as black holes, and propagate through the Universe as waves. In 2015, GWs produced by the merger of a binary black hole were directly detected for the first time, marking the beginning of GW astronomy. At present, observed GWs are primarily described as classical waves within the framework of general relativity. On the other hand, modern physics assumes that, at a fundamental level, physical phenomena are governed by quantum mechanics. From this perspective, GWs should also ultimately be described within a quantum-mechanical framework. However, because GWs emitted by astrophysical sources are macroscopic phenomena, their quantum properties have not been extensively investigated.

We therefore analyzed GWs emitted by binary black holes from a quantum-mechanical perspective. We showed that the GWs observed in practice are very well described by a coherent state, a quantum state corresponding to a classical wave, while higher-order interactions generate a small squeezed-state component [1].

Furthermore, we demonstrated that, if the quantum properties of GWs generated during inflation in the very early Universe have survived until the present day, this primordial quantum information can be inherited by GWs emitted from binary black holes, giving rise to features in the graviton number statistics that cannot occur in classical theory [2]. Such signatures may, in principle, be tested in the future through measurements of GW intensity correlations. These studies open up a new possibility of using GWs from observed binary black holes to probe both the quantum nature of gravity and the quantum state of the very early Universe.

[1] S. Kanno, J. Soda, A. Taniguchi, "Coherent State Description of Gravitational Waves from Binary Black Holes'', Phys. Rev. Lett. 136, 061404 (2026)

[2] S. Kanno, J. Soda, A. Taniguchi, "Binary gravitational waves as probes of quantum graviton states'', Phys. Rev. D 113, 123542 (2026)

Current GW observations mainly target frequencies from several tens to several thousand hertz. In contrast, high-frequency GWs in the MHz–GHz range are difficult to detect with current GW interferometers and remain largely unexplored. However, this unexplored frequency range may contain GWs generated by inflation, phase transitions in the early Universe, or primordial black holes. Primordial black holes are thought to form from highly dense regions in the early Universe, unlike ordinary black holes formed by stellar collapse, and they are also considered possible candidates for dark matter. Confirming their existence would provide an important clue to the physics of the early Universe.

We therefore proposed a new high-frequency GW detector using Rydberg atoms for quantum sensing. The key idea is not to measure GWs directly, but to convert their effect into an extremely weak electric field generated through the interaction between GWs and a magnetic field, and then detect this electric field with Rydberg atoms. By using electromagnetically induced transparency (EIT) and superheterodyne detection, the small effect of GWs can be measured as a change in the absorption of a probe laser (Fig. 1).

We theoretically evaluated the sensitivity of the Rydberg-atom GW detector and showed that it can probe a high-frequency range of approximately 0.3 ~ 16 GHz [3]. We also found that, in the GHz range, the detector may be sensitive to GWs with amplitudes as small as about 10^{−20}. By applying quantum technologies to GW observations, we aim to open a previously unexplored frequency window and use it to search for primordial black holes, the early Universe, and new fundamental physics.

[3]  S. Kanno, J. Soda, A. Taniguchi, "Search for high-frequency gravitational waves with Rydberg atoms", Eur. Phys. J. C 85, 31 (2025)

Axions may behave as dark matter (an unknown form of matter that accounts for about 27% of the total energy content of the Universe) or dark energy (an unknown form of energy that accounts for about 68% of the total energy content of the Universe), depending on their mass. However, axions have not yet been detected. We study possible signatures left by axions in GWs and in electromagnetic waves around black holes, with the aim of searching for their existence indirectly.

In study [4], we analyzed GWs passing through an axion domain wall, which may have formed in the early Universe. We found that the interaction between axions and gravity causes right- and left-handed GWs to propagate differently, so that the GWs become circularly polarized after passing through the domain wall. This effect can become large at certain frequencies, suggesting that observations of circular polarization may provide a new way to search for axions.

In study [5], we investigated how electromagnetic waves behave around a black hole in the presence of an axion field. Black holes have characteristic oscillation and decay patterns, known as quasinormal modes, that appear after a perturbation. We found that the interaction with the axion field splits the electromagnetic modes into two branches and leads to parity violation.

These studies show that observations of GWs and electromagnetic waves may provide new ways to probe the properties of unknown axions.

[4] S. Kanno, J. Soda, A. Taniguchi, "Circularly polarized gravitational waves in Chern-Simons gravity originating from an axion domain wall", Phys. Rev. D 108, 083525 (2023)

[5] S. Kanno, J. Soda, A. Taniguchi, "Parity violation in photon quasinormal modes of black holes'', Phys. Rev. D 112, 6, 063525 (2025)