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 $$f=4.2~\hbox {GHz}$$ and an amplitude around $$10^{-20}$$ .

Japan

Sugumi Kanno

Jiro Soda

Akira Taniguchi

Search for high-frequency gravitational waves with Rydberg atoms

Chromonatural inflation is a model where non-Abelian gauge fields are sustained by the coupling of the axion with the gauge field through the Chern-Simons term, while minimal warm inflation is a model where the axion produces a thermal bath of non-Abelian gauge particles through the Chern-Simons term. Since both axion inflation models are based on the same action, a natural question is if they are compatible or not. We study axion inflation with the Chern-Simons term and find that chromonatural inflation can accommodate radiation with a temperature much larger than the Hubble parameter during inflation, which is a characteristic feature of warm inflation. Thus, we conclude that chromonatural warm inflation exists, which must have phenomenologically interesting consequences.

Ann Mukuno

Chromonatural warm inflation

We study a scattering problem of gravitational waves (GWs) by an axion domain wall in Chern-Simons (CS) gravity. We find that a 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 axions.

Circularly polarized gravitational waves in Chern-Simons gravity originating from an axion domain wall

Detecting gravitational waves with frequencies higher than 10 kHz requires new strategies. In previous papers, we proposed magnon gravitational wave detectors and gave the first limit on gigahertz gravitational waves by reinterpreting the existing data from axion dark matter experiments. In this paper, we show that the sensitivity can be improved by constructing the detector specific to gravitational waves. In particular, we employ an infinite sum of terms in the expansion of Fermi normal coordinates to probe gravitational waves with a wavelength comparable to the detector size. As a consequence, we obtain sensitivity of around  $$h_c \sim 10^{-20}$$  <math> <mrow> <msub> <mi>h</mi> <mi>c</mi> </msub> <mo>∼</mo> <msup> <mn>10</mn> <mrow> <mo>-</mo> <mn>20</mn> </mrow> </msup> </mrow> </math>  .

Asuka Ito

Exploring high-frequency gravitational waves with magnons

It is known that magnetic fields exist near black holes and photons can go around black holes due to strong gravity. Utilizing these facts, we can probe hypothetical pseudoscalar particles, so-called axions. In fact, photons can be converted into axions when they propagate in a magnetic field. The conversion of such photons into axions leads to a dimming of the photon ring around the black hole shadow. We show that photon ring dimming can occur efficiently for supermassive black holes. Remarkably, it turns out that the maximal dimming rate of the photon ring is 25%. In the case of <math><mrow><mi>M</mi><msup><mrow><mn>87</mn></mrow><mrow><mo>*</mo></mrow></msup></mrow></math>, dimming of 10% will be observed in the x-ray and gamma-ray bands if the angular resolution of <math><msup><mn>10</mn><mrow><mo>−</mo><mn>5</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>arcsec</mi></math> is achieved. The frequency band and the magnitude of the dimming depend on the axion-photon coupling and axion mass. Hence, the distorted spectrum of the photon ring provides a novel tool for detecting axions.

Kimihiro Nomura

Kaishu Saito

Observing axions through photon ring dimming of black holes

Anisotropic inflation is a model that succeeded in explaining statistical anisotropy. Warm inflation is a model that succeeded in providing a mechanism of reheating during inflation. We study anisotropic warm inflation focusing on the cosmic no-hair conjecture. In the anisotropic warm inflation, the condition for making anisotropy survive is clarified. By assuming a constant value for the dissipation ratio, we find exact solutions of power-law anisotropic warm inflation, and investigate the phase space structure of general solutions. It turns out that whether the anisotropy during inflation survives or not depends on the competition of the potential that drives anisotropic inflation against dissipation of an inflaton field. Anisotropic warm inflation will be realized if the decaying process is not efficient.

Ann Nakato

Kazushige Ueda

Anisotropic warm inflation

There exists observational evidence to believe in the existence of primordial magnetic fields generated during inflation. We study primordial gravitational waves (PGWs) during inflation in the presence of magnetic fields sustained by a gauge kinetic coupling. In the model, not only gravitons as excitations of PGWs, but also photons as excitations of electromagnetic fields are highly squeezed. They become entangled with each other through graviton to photon conversion and vice versa. We derive the reduced density matrix for the gravitons and calculate their entanglement entropy. It turns out that the state of the gravitons is not a squeezed state but a mixed state. Although witnessing such an entanglement is not feasible at present, it would be an important experimental challenge.

Impact of quantum entanglement induced by magnetic fields on primordial gravitational waves

It is believed that relic gravitons are squeezed during inflation. If so, quantum noise induced by them can be significantly enhanced in current interferometers. However, decoherence of the gravitons during cosmic history may change the degree of squeezing. As a first step for analyzing the decoherence of the gravitons, we assume the presence of a sizable magnetic field at the beginning of inflation and study conversion processes of the squeezed gravitons into photons during inflation in the case of minimal coupling between gravitons and photons. We solve the dynamical evolution of a coupled system of gravitons and photons perturbatively and obtain squeezing parameters for the system numerically and analytically. It turns out that the gravitons are robust against the decoherence caused by the cosmological magnetic fields. We also find that the conversion rate of gravitons into photons is at a few percent at most.

Conversion of squeezed gravitons into photons during inflation

The axion provides a solution for the strong <math><mi>C</mi><mi>P</mi></math> problem and is one of the leading candidates for dark matter. This paper proposes an axion detection scheme based on quantum nondemolition detection of magnon, i.e., quanta of collective spin excitations in solid, which is expected to be excited by the axion-electron interaction predicted by the Dine-Fischer-Srednicki-Zhitnitsky model. The prototype detector is composed of a ferrimagnetic sphere as an electronic spin target and a superconducting qubit. Both of these are embedded inside a microwave cavity, which leads to a coherent effective interaction between the uniform magnetostatic mode in the ferrimagnetic crystal and the qubit. An upper limit for the coupling constant between an axion and an electron is obtained as <math><msub><mi>g</mi><mrow><mi>a</mi><mi>e</mi><mi>e</mi></mrow></msub><mo>&lt;</mo><mn>2.6</mn><mo>×</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>6</mn></mrow></msup></math> at the 95% confidence level for the axion mass of <math><mrow><mn>33.117</mn><mtext> </mtext><mtext> </mtext><mtext> </mtext><mtext> </mtext><mi>μ</mi><mi>eV</mi><mo>&lt;</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>a</mi></mrow></msub><mo>&lt;</mo><mn>33.130</mn><mtext> </mtext><mtext> </mtext><mi>μ</mi><mi>eV</mi></mrow></math>.

Tomonori Ikeda

Taiwan

Kentaro Miuchi

Hisaya Kurashige

Yutaka Shikano

Axion search with quantum nondemolition detection of magnons

We study inflationary universes with an SU(3) gauge field coupled to an inflaton through a gauge kinetic function. Although the SU(3) gauge field grows at the initial stage of inflation due to the interaction with the inflaton, nonlinear self-couplings in the kinetic term of the gauge field become significant and cause nontrivial dynamics after sufficient growth. We investigate the evolution of the SU(3) gauge field numerically and reveal attractor solutions in the Bianchi type-I spacetime. In general cases where all the components of the SU(3) gauge field have the same magnitude initially, they all tend to decay eventually because of the nonlinear self-couplings. Therefore, the cosmic no-hair conjecture generically holds in a mathematical sense. Practically, however, the anisotropy can be generated transiently in the early Universe. Moreover, we find particular cases for which several components of the SU(3) gauge field survive against the nonlinear self-couplings. It occurs due to flat directions in the potential of a gauge field for Lie groups whose rank is higher than one. Thus, an SU(2) gauge field has a specialty among general non-Abelian gauge fields.