Quantum Universe
Cluster of ExcellenceQuantum Universe
Quantum Expander for Gravitational-Wave Observatories
10 December 2019
Ultra-stable laser light that was stored in optical resonators of up to 4 km length enabled the first observations of gravitational waves from inspirals of binary black holes and neutron stars. Due to the rather low bandwidth of the optical resonator system, however, the scientifically highly interesting post-merger signals at frequencies above a few hundred hertz could not be resolved. Such information would give access to the physics of neutron stars, allowing to study the ultra-dense quantum matter and possibly to find the missing link between gravity and quantum physics.
Recently, Quantum Universe scientists Mikhail Korobko and Roman Schnabel from the University of Hamburg and Dr. Yiqiu Ma and Prof. Yanbei Chen from the California Institute of Technology proposed a novel all-optical approach to expanding the detection bandwidth of gravitational-wave observatories towards kilohertz frequencies. Their research results were recently published in the journal Light: Science and Applications.
What they call 'quantum expander' takes advantage of squeezing the quantum uncertainty of the laser light inside the optical resonator system. While squeezing the quantum uncertainty of the laser light before injection into the resonator system is already routinely used in all gravitational-wave observatories since April 2019, the new add-on will specifically improve the signal-to-noise-ratio at kilohertz frequencies, in fact, without deteriorating today's high performance at lower frequencies.
The scientists propose placing a nonlinear crystal inside the so-called signal-recycling cavity, which is a subsystem in every gravitational-wave observatory today and pump this crystal with green laser light having half the wavelength of the main laser light used in the observatory. The interaction between the pump and the main light leads to a squeezed uncertainty in the quantum fluctuations of the main laser. When the signal-recycling cavity length is controlled to remain a non-integer multiple of the laser wavelength, especially the high frequency quantum fluctuations of the laser light are squeezed in addition to any squeezing injected from the outside.
The newly invented 'quantum expander' is fully compatible with previously invented quantum-noise-suppression techniques. It is intrinsically stable and doesn't require significant modifications to the general topology of the observatories. What it does require is a further improved quality of optical components for further reduction in loss of photons. The 'quantum expander' may find applications beyond gravitational-wave detection in the areas of quantum metrology and quantum optomechanics.
Concept of the Quantum Expander
The figure illustrates the concept of the quantum expander for gravitational-wave detectors.
a) The gravitational wave from a cosmic merger event displaces the mirrors of the detector, and the signal is read out on the photodiode (PD). A nonlinear χ(2) crystal inside the detector cavity squeezes quantum uncertainties in the light and enhances the sensitivity of the detector.
b) The quantum expander relies on the parametric process in the nonlinear crystal, and interaction between resonances of coupled optical cavities in the detector.
c) The nonlinear crystal squeezes quantum uncertainty below the vacuum level, reducing quantum noise in the readout. The coupled cavity structure allows to reach maximal squeezing at high frequencies, not disturbing the low-frequency sensitivity.
d) The gravitational-wave strain amplitude from the neutron star merger (light blue trace) is embedded in quantum noise. Due to the optical cavities the signal is lost at high frequency, masking the post-merger oscillations of the newly formed object (yellow trace). The quantum expander allows to resolve the signal at high frequency, and read out the important information about the properties of quantum matter in a formed object (blue trace). The detection bandwidth of the GW detector is thus expanded.
