arXiv subjects

Masa-Katsu Fujimoto

Publications and source records attributed to Masa-Katsu Fujimoto.

18 recordsLinked to original sources

arXiv2021

A back-linked Fabry-Perot interferometer for space-borne gravitational wave observations

Direct observations of gravitational waves at frequencies below 10 Hz will play crucial roles for fully exploiting the potential of gravitational wave astronomy. One approach to pursue this direction is the utilization of laser interferometers equipped with the Fabry-Perot optical cavities in space. However, a number of challenges lie in this path practically. In particular, the implementation of precision control for the cavity lengths and the suppression of laser phase noises may prevent a practical detector design. To circumvent such difficulties, we propose a new interferometer topology, named the back-linked Fabry-Perot interferometer, where the precision length controls are not required and an offline subtraction scheme for laser phase noises is readily applicable. This article presents the principle idea and the associated sensitivity analyses. Despite additional noises, a strain sensitivity of $7\times 10^{-23}$ Hz$^{-1/2}$ may be attainable in the deci-Hertz band. Several technological developments and studies must be carried out to pave the way forward for the implementation.

astro-ph.IM↗

arXiv2020

Current status of space gravitational wave antenna DECIGO and B-DECIGO

Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) is the future Japanese space mission with a frequency band of 0.1 Hz to 10 Hz. DECIGO aims at the detection of primordial gravitational waves, which could be produced during the inflationary period right after the birth of the universe. There are many other scientific objectives of DECIGO, including the direct measurement of the acceleration of the expansion of the universe, and reliable and accurate predictions of the timing and locations of neutron star/black hole binary coalescences. DECIGO consists of four clusters of observatories placed in the heliocentric orbit. Each cluster consists of three spacecraft, which form three Fabry-Perot Michelson interferometers with an arm length of 1,000 km. Three clusters of DECIGO will be placed far from each other, and the fourth cluster will be placed in the same position as one of the three clusters to obtain the correlation signals for the detection of the primordial gravitational waves. We plan to launch B-DECIGO, which is a scientific pathfinder of DECIGO, before DECIGO in the 2030s to demonstrate the technologies required for DECIGO, as well as to obtain fruitful scientific results to further expand the multi-messenger astronomy.

gr-qc↗

arXiv2018

Double balanced homodyne detection

In the context of the readout scheme for gravitational-wave detectors, the "double balanced homodyne detection" proposed in [K.~Nakamura and M.-K.~Fujimoto, arXiv:1709.01697.] is discussed in detail. This double balanced homodyne detection enables us to measure the expectation values of the photon creation and annihilation operators. Although it has been said that the operator $\hat{b}_θ:=\cosθ\hat{b}_{1}+\sinθ\hat{b}_{2}$ can be measured through the homodyne detection in literature, we first show that the expectation value of the operator $\hat{b}_θ$ cannot be measured as the linear combination of the upper- and lower-sidebands from the output of the balanced homodyne detection. Here, the operators $\hat{b}_{1}$ and $\hat{b}_{2}$ are the amplitude and phase quadrature in the two-photon formulation, respectively. On the other hand, it is shown that the above double balanced homodyne detection enables us to measure the expectation value of the operator $\hat{b}_θ$ if we can appropriately prepare the complex amplitude of the coherent state from the local oscillator. It is also shown that the interferometer set up of the eight-port homodyne detection realizes our idea of the double balanced homodyne detection. We also evaluate the noise-spectral density of the gravitational-wave detectors when our double balanced homodyne detection is applied as their readout scheme. Some requirements for the coherent state from the local oscillator to realize the double balanced homodyne detection are also discussed.

quant-ph↗

arXiv2018

Extension of the input-output relation for a Michelson interferometer to arbitrary coherent-state light sources: --- Gravitational-wave detector and weak-value amplification ---

An extension of the input-output relation for a conventional Michelson interferometric gravitational-wave detector is carried out to treat an arbitrary coherent state for the injected optical beam. This extension is one of necessary researches toward the clarification of the relation between conventional gravitational-wave detectors and a simple model of a gravitational-wave detector inspired by weak-measurements in [A.~Nishizawa, Phys. Rev. A {\bf 92} (2015), 032123.]. The derived input-output relation describes not only a conventional Michelson-interferometric gravitational-wave detector but also the situation of weak measurements. As a result, we may say that a conventional Michelson gravitational-wave detector already includes the essence of the weak-value amplification as the reduction of the quantum noise from the light source through the measurement at the dark port.

quant-ph↗

arXiv2018

An application of the measurement of expectation values for the photon annihilation and creation operators

Motivated by the readout scheme in interferometric gravitational-wave detectors, we consider the device which measures the expectation value of the photon annihilation and creation operators for output optical field from the main interferometer. As the result, the eight-port homodyne detection is rediscovered as such a device. We evaluate the noise spectral density in this measurement. We also briefly discuss on the application of our results to the readout scheme of gravitational-wave detectors. We call this measurement scheme to measure these expectation values as "double balanced homodyne detection."

quant-ph↗

arXiv2017

Guided Lock of a Suspended Optical Cavity Enhanced by a Higher Order Extrapolation

Lock acquisition of a suspended optical cavity can be a highly stochastic process and is therefore nontrivial. Guided lock is a method to make lock acquisition less stochastic by decelerating the motion of the cavity length based on an extrapolation of the motion from an instantaneous velocity measurement. We propose an improved scheme which is less susceptible to seismic disturbances by incorporating the acceleration as a higher order correction in the extrapolation. We implemented the new scheme in a 300-m suspended Fabry-Perot cavity and improved the success rate of lock acquisition by a factor of 30.

physics.ins-det↗

arXiv2012

Weak value amplification in a shot-noise limited interferometer

We study the weak-value amplification (WVA) in a phase measurement with an optical interferometer in which shot noise limits the sensitivity. We compute the signal and the shot noise including the full-order interaction terms of the WVA, and show that the shot-noise contribution to a phase shift in a pointer variable is always larger than the final variance of the pointer variable. This yields difference in estimating noise level up to a factor of 1.5. To clarify an advantage for practical uses of the WVA, we discuss signal-to-noise ratio and its optimization in the presence of the shot noise.

quant-ph↗

arXiv2012

Reduction of thermal fluctuations in a cryogenic laser interferometric gravitational wave detector

The thermal fluctuation of mirror surfaces is the fundamental limitation for interferometric gravitational wave (GW) detectors. Here, we experimentally demonstrate for the first time a reduction in a mirror's thermal fluctuation in a GW detector with sapphire mirrors from the Cryogenic Laser Interferometer Observatory at 17\,K and 18\,K. The detector sensitivity, which was limited by the mirror's thermal fluctuation at room temperature, was improved in the frequency range of 90\,Hz to 240\,Hz by cooling the mirrors. The improved sensitivity reached a maximum of $2.2 \times 10^{-19}\,\textrm{m}/\sqrt{\textrm{Hz}}$ at 165\,Hz.

gr-qc↗

arXiv2011

All-order evaluation of weak measurements: --- The cases of an operator ${\bf A}$ which satisfies the property ${\bf A}^{2}=1$ ---

Some exact formulae of the expectation values and probability densities in a weak measurement for an operator ${\bf A}$ which satisfies the property ${\bf A}^{2}=1$ are derived. These formulae include all-order effects of the unitary evolution due to the von-Neumann interaction. These are valid not only in the weak measurement regime but also in the strong measurement regime and tell us the connection between these two regime. Using these formulae, arguments of the optimization of the signal amplification and the signal to noise ratio are developed in two typical experimental setups.

quant-ph↗

arXiv2010

Thermal-noise-limited underground interferometer CLIO

We report on the current status of CLIO (Cryogenic Laser Interferometer Observatory), which is a prototype interferometer for LCGT (Large Scale Cryogenic Gravitational-Wave Telescope). LCGT is a Japanese next-generation interferometric gravitational wave detector featuring the use of cryogenic mirrors and a quiet underground site. The main purpose of CLIO is to demonstrate a reduction of the mirror thermal noise by cooling the sapphire mirrors. CLIO is located in an underground site of the Kamioka mine, 1000 m deep from the mountain top, to verify its advantages. After a few years of commissioning work, we have achieved a thermal-noise-limited sensitivity at room temperature. One of the main results of noise hunting was the elimination of thermal noise caused by a conductive coil-holder coupled with a pendulum through magnets.

gr-qc↗

arXiv2008

Search for a stochastic background of 100-MHz gravitational waves with laser interferometers

This letter reports the results of a search for a stochastic background of gravitational waves (GW) at 100 MHz by laser interferometry. We have developed a GW detector, which is a pair of 75-cm baseline synchronous recycling (resonant recycling) interferometers. Each interferometer has a strain sensitivity of ~ 10^{-16} Hz^{-1/2} at 100 MHz. By cross-correlating the outputs of the two interferometers within 1000 seconds, we found h_{100}^2 Omega_{gw} < 6 times 10^{25} to be an upper limit on the energy density spectrum of the GW background in a 2-kHz bandwidth around 100 MHz, where a flat spectrum is assumed.

gr-qc↗

arXiv2007

Current status of Japanese detectors

Current status of TAMA and CLIO detectors in Japan is reported in this article. These two interferometric gravitational-wave detectors are being developed for the large cryogenic gravitational wave telescope (LCGT) which is a future plan for detecting gravitational wave signals at least once per year. TAMA300 is being upgraded to improve the sensitivity in low frequency region after the last observation experiment in 2004. To reduce the seismic noises, we are installing new seismic isolation system, which is called TAMA Seismic Attenuation System, for the four test masses. We confirmed stable mass locks of a cavity and improvements of length and angular fluctuations by using two SASs. We are currently optimizing the performance of the third and fourth SASs. We continue TAMA300 operation and R&D studies for LCGT. Next data taking in the summer of 2007 is planned. CLIO is a 100-m baseline length prototype detector for LCGT to investigate interferometer performance in cryogenic condition. The key features of CLIO are that it locates Kamioka underground site for low seismic noise level, and adopts cryogenic Sapphire mirrors for low thermal noise level. The first operation of the cryogenic interferometer was successfully demonstrated in February of 2006. Current sensitivity at room temperature is close to the target sensitivity within a factor of 4. Several observation experiments at room temperature have been done. Once the displacement noise reaches at thermal noise level of room temperature, its improvement by cooling test mass mirrors should be demonstrated.

gr-qc↗

arXiv2004

Observational constraints on cosmology from modified Friedmann equation

Recent measurements of type Ia supernovae as well as other concordant observations suggest that the expansion of our universe is accelerating. A dark energy component has usually been invoked as the most feasible mechanism for the acceleration. However, the effects arising from possible extra dimensions can mimic well the role of a dark energy through a modified Friedmann equation. In this work, we investigate some observational constraints on a scenario in which this modification is given by $H^2 = {8πG \over 3} (ρ+Cρ^n)$. We mainly focus our attention on the constraints from recent measurements of the dimensionless coordinate distances to type Ia supernovae and Fanaroff-Riley type IIb radio galaxies compiled by Daly and Djorgovski (2003) and the X-ray gas mass fractions in clusters of galaxies published by Allen et al. (2002,2003). We obtain the confidence region on the power index $n$ of the modificative term and the density parameter $Ω_m$ of the universe from a combined analysis of these databases. It is found that $n=0.06^{+0.22}_{-0.18}$ and $Ω_m=0.30^{+0.02}_{-0.02}$, at the 95.4% confidence level, which is consistent within the errors with the standard $Λ$CDM model. These parameter ranges give a universe whose expansion swithes from deceleration to acceleration at a redshift between 0.52 to 0.73.

astro-ph↗

arXiv2004

Determining the equation of state of dark energy from angular size of compact radio sources and X-ray gas mass fraction of galaxy clusters

Using recent measurements of angular size of high-z milliarcsecond compact radio sources compiled by Gurvits, Kellermann and Frey (1999) and X-ray gas mass fraction of galaxy clusters published by Allen et al. (2002,2003), we explore their bounds on the equation of state, omega_x = p_x/rho_x, of the dark energy, whose existence has been congruously suggested by various cosmological observations. We relaxe the usual constraint omega_x >= -1, and find that combining the two databases yields a nontrivial lower bound on omega_x. Under the assumption of a flat universe, we obtain a bound -2.22 < omega_x < -0.62 at 95.4% confidence level. The 95.4% confidence bound goes to -1 =< omega_x < -0.60 when the constraint omega_x >= -1 is imposed.

astro-ph↗

arXiv2003

Constraints on Cardassian Scenario from the Expansion Turnaround Redshift and the Sunyaev-Zeldovich/X-ray Data

Cosmic acceleration is one of the most remarkable cosmological findings of recent years. Although a dark energy component has usually been invoked as the mechanism for the acceleration, A modification of Friedmann equation from various higher dimensional models provides a feasible alternative. Cardassian expansion is one of these scenarios. In this work, we first consider the constraints on the parameter space from the turnaround redshift, z_{q=0}, at which the universe switches from deceleration to acceleration. We show that, for every Omega_m, there exist a unique n_{peak} (Omega_m), which makes z_{q=0} reach its maximum value, [z_{q=0}]_{max} = exp[1/ (2-3n_{peak})] -1, which is unlinearly inverse to Omega_m. If the acceleration happans earlier than z_{q=0} = 0.6, suggested by Type Ia supernovae measurements, we have Omega_m < 0.328 no matter what the power index is, and moreover, for reasonable matter density, Omega_m ~ 0.3, it is found n ~ (-0.45,0.25). We next test this scenario using the Sunyaev-Zeldovich/X-ray data of a sample of 18 galaxy clusters with 0.14 < z < 0.83 compiled by Reese et al. (2002). The constraints from the current SZ/X-ray data is weak, though a model with lower matter density is prefered. A certain range of the model parameters is also consistent with the data.

astro-ph↗

arXiv2003

Constraints on Cardassian Expansion from Distant type Ia Supernovae

The distant type Ia supernovae data compiled by Perlmutter et al. (1999) are used to analyze the Cardassian expansion scenario, which was recently proposed by Freese and Lewis (2002) as an alternative to a cosmological constant (or more generally a dark energy component) in explaining the currently accelerating universe. We show that the allowed intervals for $n$ and $z_{eq}$, the two parameters of the Cardassian model, will give rise to a universe with a very low matter density, which can hardly be reconciled with the current value derived from the measurements of the cosmic microwave background anisotropy and galaxy clusters (cluster baryon fraction). As a result, this Cardassian expansion proposal does not seem to survive the magnitude-redshift test for the present type Ia supernovae data, unless the universe contains primarily baryonic matter.

astro-ph↗

arXiv2002

Cardassian Expansion: Constraints from Compact Radio Source Angular Size versus Redshift Data

The ``Cardassian Expansion Scenario'' was recently proposed by Freese and Lewis (2002) as an alternative to a cosmological constant in explaining the current accelerating universe. In this paper we investigate observational constraints on this scenario from recent measurements of the angular size of high-$z$ compact radio sources compiled by Gurvits et al. (1999). We show that the allowed intervals for $n$ and $z_{eq}$, the two parameters of the Cardassian model, are heavily dependent on the value of the mean projected linear size $l$. However, the best fit to the current angular size data prefers the conventional flat $Λ$CDM model to this Cardassian expansion proposal, though the latter is cosmologically credible and compatible with the $Θ- z$ diagram for some values of $l$.

astro-ph↗

arXiv2001

Determining the Cosmic Equation of State Using Future Gravitational Wave Detectors

The expected chirp mass distribution of observed events for future gravitational wave detectors is extensively investigated in the presence of an exotic fluid component with an arbitrary equation of state, $-1 \leq ω_x \equiv p_x/ρ_x < 0$, i.e., the so-called dark energy component. The results for a flat model dominated by a dark energy are compared to those for the standard flat model dominated by cold dark matter. It is found that for a flat universe the chirp mass distribution shows a sensitive dependence on $ω_x$, which may provide an independent and robust constraint on the cosmic equation of state.

astro-ph↗