Quantum noise limits in white-light-cavity-enhanced gravitational wave detectors

TL;DR

This paper investigates quantum noise limits in white-light-cavity-enhanced gravitational wave detectors, analyzing idealized and explicit negative dispersion media models, and finds that quantum noise constrains the maximum achievable sensitivity-bandwidth enhancement to about 14-18 times.

Contribution

It introduces a detailed quantum noise analysis for WLC-enhanced detectors, providing upper bounds on sensitivity-bandwidth improvements and proposing explicit gain medium systems for practical realization.

Findings

Sensitivity-bandwidth product can be enhanced up to ~18 times.

Quantum noise always remains above the standard quantum limit.

Explicit gain medium system achieves ~17.66 times enhancement.

Abstract

Previously, we had proposed a gravitational wave detector that incorporates the white-light-cavity (WLC) effect using a compound cavity for signal recycling (CC-SR). Here, we first use an idealized model for the negative dispersion medium (NDM) and use the so-called Caves model for a phase-insensitive linear amplifier to account for the quantum noise (QN) contributed by the NDM, in order to determine the upper bound of the enhancement in the sensitivity-bandwidth product. We calculate the quantum noise limited sensitivity curves for the CC-SR design, and find that the broadening of sensitivity predicted by the classical analysis is also present in these curves, but is somewhat reduced. Furthermore, we find that the curves always stay above the standard quantum limit. To circumvent this limitation, we modify the dispersion to compensate the non-linear phase variation produced by the optomechanical resonance effects. We find that the upper bound of the factor by which the sensitivity-bandwidth product is increased, compared to the highest-sensitivity result predicted by Bunanno and Chen [Phys. Rev. D 64, 042006 (2001)], is ~14. We also present a simpler scheme (WLC-SR) where a dispersion medium is inserted into the SR cavity. For this scheme, we found the upper bound of the enhancement factor to be ~18. We then consider an explicit system for realizing the NDM, which makes use of five energy levels in M-configuration to produce gain, accompanied by electromagnetically induced transparency (the GEIT system). For this explicit system, we employ the rigorous approach based on Master Equation to compute the QN contributed by the NDM, thus enabling us to determine the enhancement in the sensitivity-bandwidth product definitively rather than the upper bound thereof. Specifically, we identify a set of parameters for which the sensitivity-bandwidth product is enhanced by a factor of 17.66.