Project

Quantum Control Technology for Pioneering Gravitational Wave Astronomy

The second-generation gravitational-wave detectors observed more than 100 events including a number of neutron star mergers. A clear footprint of the merger signal will provide information of the remnant and reveal the equation of state of the mysterious object. The merger frequency, estimated to be 3-4kHz, is slightly out of the bandwidth of current detectors. Several techniques have been proposed to improve the sensitivity in this frequency band. A basic idea, first proposed by our research group at Tokyo Tech, is to insert some sort of a quantum filter in the signal recycling cavity. There are several different approaches to make use of this technique. One is to use a parametric amplifier and detune the recycling cavity to form a stiff optical spring that has a resonant frequency at 3-4kHz. Another is to use a delay line in the recycling cavity to form a double pole resonator that has a peak frequency at 3-4kHz (long-SRC effect). A parametric amplifier and delay line can be combined (quantum expander).

In parallel to the development of quantum filters, it is important as well to increase the circulation power. With the current silica glass mirrors, thermal lensing and parametric instability will put a limitation in the laser power. One approach to circumvent the heat problem is to use a cryogenic mirror. The conventional cooling method with heat conduction through the suspension fiber can still afford a decent amount of laser power if we use thicker and shorter suspension fibers, but an alternative way is to use 123K silicon mirror instead of 20K mirrors. At such a intermediate temperature, thermoelastic damping could limit the sensitivity with a mirror made of sapphire or other materials, but the thermal expansion rate of silicon is zero at 123K so the thermoelastic damping is negligible. Compared with the heat conduction at 20K, the radiation at 123K can transport much more heat. The laser wavelength must be revised to be used with silicon mirrors. One decent candidate is 1550nm but it is also proposed to use 2um laser as the scattering loss is lower.

The existance of one high-frequency detector in the global gravitational-wave network would make a big difference. The broadband detectors will trigger the event and localize the source. The high-frequency detector will obtain the information of the merger. People in Australia (OzGrav) plan to propose a new detector built in Australia, called NEMO. KAGRA people plan to propose an upgrade plan, KAGRA+, aiming at the same role in the network. In the next 5 years, under this ASPIRE Japan-Australia bilateral program, people in the two regions work together to develop necessary techniques and to discuss the future plans.

Members

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