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arXiv:2406.03304

Variational readout through quantum teleportation

Yohei Nishino

Published Jun 5, 2024• 0 citations
arXivHTMLPDF

Abstract

Sensitivity of gravitational-wave detectors (GWDs) is constrained at low frequencies by quantum radiation-pressure noise, a manifestation of the measurement's back action. One strategy to mitigate this back action involves employing variational readout, which entails cross-correlating the measurement shot noise with radiation-pressure noise. Prior research has demonstrated that variational readout in GWDs can be accomplished through the use of a filter cavity. However, current gravitational-wave detectors necessitate filter cavity lengths on the order of $\sim 100$ meters, with future detectors anticipated to reach lengths of a few kilometers. This paper introduces a novel approach to variational readout utilizing principle of quantum teleportation, which eliminates the need for a filter cavity. *This document will not be published any journals, since it turned out to be not very useful for the real detectors. The main reason is that one does not need the EPR entanglement to obtain the same result, which does not even shows true variational readout. In other words, both schemes do not exceed the EPR squeezing. Nevertheless this document will be uploaded to stimulate discussions how to realize "true" back action evasion, such as variational readout or speed meter, via quantum entanglement.

DOI
10.48550/arxiv.2406.03304

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Sensitivity of gravitational-wave detectors (GWDs) is constrained at low frequencies by quantum radiation-pressure noise, a manifestation of the measurement's back action. One strategy to mitigate this back action involves employing variational readout, which entails cross-correlating the measurement shot noise with radiation-pressure noise. Prior research has demonstrated that variational readout in GWDs can be accomplished through the use of a filter cavity. However, current gravitational-wave detectors necessitate filter cavity lengths on the order of ∼100\sim 100∼100 meters, with future detectors anticipated to reach lengths of a few kilometers. This paper introduces a novel approach to variational readout utilizing principle of quantum teleportation, which eliminates the need for a filter cavity. *This document will not be published any journals, since it turned out to be not very useful for the real detectors. The main reason is that one does not need the EPR entanglement to obtain the same result, which does not even shows true variational readout. In other words, both schemes do not exceed the EPR squeezing. Nevertheless this document will be uploaded to stimulate discussions how to realize "true" back action evasion, such as variational readout or speed meter, via quantum entanglement.
We propose a new implementation of a quantum speed meter QND measurement scheme. It employs two independent optical readouts of the interferometer test masses with different values of the bandwidths and of the optical circulating power, whose outputs have to be combined by an additional beamsplitter. Signals at the two outputs of the beamsplitter are proportional to the position and the velocity of the test masses, respectively. The influence of the position meter-like back action force associated with the position signal can be cancelled using the EPR approach by measuring the amplitude quadrature of the beamsplitter common output. • This is an article for the Special Issue in memory of V.B. Braginsky. • We propose a novel and arguably the simplest scheme of a speed meter interferometer. • Speed meters curb quantum noise at low frequencies, boosting the SNR for GW sources. • Combination of readouts of two detectors with distinct bandwidths yields speed signal. • No significant changes of the current GWD infrastructure is necessary for our scheme.
The intracavity topologies of laser gravitational-wave detectors proposed several years ago are the promising way to obtain sensitivity of these devices significantly better than the Standard Quantum Limit (SQL). In essence, the intracavity detector is a two-stage device where the end mirrors displacement created by the gravitational wave is transferred to the displacement of an additional local mirror by means of the optical rigidity. The local mirror positions have to be monitored by an additional local meter. It is evident that the local meter precision defines the sensitivity of the detector. To overcome the SQL, the quantum variational measurement can be used in the local meter. In this method a frequency-dependent correlation between the meter backaction noise and measurement noise is introduced, which allows us to eliminate the backaction noise component from the meter output signal. This correlation is created by means of an additional filter cavity. In this article the sensitivity limitations of this scheme imposed by the optical losses both in the local meter itself and in the filter cavity are estimated. It is shown that the main sensitivity limitation stems from the filter cavity losses. In order to overcome it, it is necessary to increase the filter cavity length. In a preliminary prototype experiment, an approximate 10 m long filter cavity can be used to obtain sensitivity approximately 2--3 times better than the SQL. For future Quantum Non-Demolition (QND) gravitational-wave detectors with sensitivity about 10 times better than the SQL, the filter cavity length should be within kilometer range.
We propose a quantum teleportation-based speed meter for interferometric displacement sensing. The primary motivation is to transform a conventional position-sensing interferometer into a quantum non-demolition speed measurement device, without modifying its fundamental optical configuration. Two equivalent implementations are presented: an online approach that uses real-time displacement operation and an offline approach that relies on post-processing. Both implementations reduce quantum radiation pressure noise and surpass the standard quantum limit of measuring displacement, and they can be applied to a wide range of interferometer configurations. We discuss potential applications to gravitational-wave detectors, where our scheme enhances low-frequency sensitivity without requiring modifications to the core optics of a conventional Michelson interferometer (e.g., substrate or coating properties). This approach offers a new path to back-action evasion enabled by quantum entanglement.
A relatively simple method of overcoming the standard quantum limit in the next generation Advanced LIGO gravitational-wave detector is considered. It is based on the quantum variational measurement with a single, short (a few tens of meters) filter cavity. Estimates show that this method allows one to reduce the radiation-pressure noise at low frequencies (<100  Hz<100\text{ }\text{ }\mathrm{Hz}<100  Hz) to the level comparable with or smaller than the low-frequency noises of nonquantum origin (mirrors' suspension noise, mirrors' thermal noise, seismic noise, and gravity gradients fluctuations).
Abstract We propose several quantum noise reduction schemes for gravitational-wave detectors based on quantum teleportation. Gravitational-wave detectors, as highly precise displacement-measurement devices, are ultimately limited by quantum shot noise and radiation pressure noise. The trade-off between these two noise sources imposes the so-called standard quantum limit (SQL), as a natural consequence of Heisenberg’s uncertainty principle. Numerous schemes to enhance the quantum noise- limited sensitivity beyond the SQL have been proposed, some of which require nontrivial modifications to the basic Michelson configuration of the detectors. Recently, it has been found that the principles of quantum teleportation can solve this issue; the detector’s sensitivity can be enhanced beyond the SQL at minimal cost via parallel processing, which is a key concept in quantum information processing. We theoretically demonstrate ways to implement quantum teleportation in (1) frequency-dependent squeezing [1] and (2) quantum speed measurement [2], without requiring additional infrastructure or the manufacture of optics. We conclude that these ideas can be viable candidates for future detectors.
General relativity predicts the existence of Gravitational Waves (GWs), ripples in spacetime that are now observable to highly sensitive interferometers. However, Quantum Noise (QN) poses a significant challenge for Gravitational Wave (GW) interferometers, influencing the entire bandwidth range (10-10,000 Hz) of the current GW detectors. At higher frequencies, Shot Noise (SN) predominates, while at lower frequencies, Radiation Pressure Noise (RPN) becomes the limiting factor, reducing both sensitivity and measurement precision. Squeezing techniques are implemented to address this issue by confining uncertainty within one quadrature of the light field. During O4, LIGO successfully implemented Frequency-Dependent Squeezing (FDS), while Virgo integrated and commissioned the FDS system without actively operating it during the science run. By employing a 300-meterlong detuned Filter Cavity (FC), the squeezing ellipse is rotated, allowing phase squeezing at high frequencies to reduce and amplitude squeezing at low frequencies to mitigate RPN, ultimately refining detection capabilities across the frequency spectrum. This doctoral thesis focuses on a promising alternative technique for future GW detectors, that achieves broadband QN reduction by exploiting Einstein–Podolsky–Rosen (EPR) quantum entanglement. This approach involves injecting two entangled, squeezed beams at different frequencies into the dark port of the interferometer, enabling it to act as both a FC and a GW detector. To experimentally investigate these techniques, the suspended Interferometer for Ponderomotive Squeezing (SIPS) platform adopts a large-scale GW detector layout featuring a Michelson configuration with Fabry-Perot (FP) arm cavities. To guarantee the extreme optical alignment necessary for quantum noise mitigation, a high-precision, real-time local control system was developed for the SIPS main optics. Using a dummy mirror suspended in a double-pendulum configuration with monolithic fibers, a custom real-time signal processing and feedback control architecture was developed. This subsystem utilizes a PXI hardware framework and position-sensitive detectors (Power Spectral Densitys (PSDs)) implemented in LabVIEW to actively damp the mechanical resonances of the suspensions. Extending these core activities toward future infrastructures, this work directly contributed to the realization and commissioning of the new PNRR ETIC-GALILEO quantum optics cleanroom facility in Genoa for the Einstein Telescope (ET). Within this infrastructure, preliminary optical studies were initiated for Einstein Telescope Low Frequency (ET-LF) low-loss Faraday isolators operating at 1550 nm. Concurrently, numerical simulations were performed using Finesse3 to model and investigate candidate excess noise sources currently limiting the sensitivity of Advanced Virgo Plus