Future gravitational-wave detectors will use frequency-dependent squeezed vacuum states to obtain broadband reduction of quantum noise. Quantum noise is one of the major limitations to the sensitivity of these detectors. Advanced LIGO+, Advanced Virgo+, and KAGRA plan to generate frequency-dependent squeezed states by coupling a frequency-independent squeezed light state with a filter cavity. An alternative technique is under consideration, based on conditional squeezing with quantum entanglement: Einstein–Podolsky–Rosen (EPR) squeezing. In the EPR scheme, two vacuum entangled states, the signal field at ω 0 and the idler field at ω 0 + Δ , must be spatially separated with an optical resonator and sent to two separate homodyne detectors. In this framework, we have designed and tested a solid Fabry–Perot etalon, to be used in an EPR table-top experiment prototype, thermally controlled without the use of a control probe optical beam. This device can also be used in optical experiments where the use of a bright beam to control an optical resonator is not possible, or where a simpler optical device is preferred.
Future gravitational-wave detectors will use frequency-dependent squeezed vacuum states to obtain broadband reduction of quantum noise. Quantum noise is one of the major limitations to the sensitivity of these detectors. Advanced LIGO+, Advanced Virgo+, and KAGRA plan to generate frequency-dependent squeezed states by coupling a frequency-independent squeezed light state with a filter cavity. An alternative technique is under consideration, based on conditional squeezing with quantum entanglement: Einstein–Podolsky–Rosen (EPR) squeezing. In the EPR scheme, two vacuum entangled states, the signal field at ω 0 and the idler field at ω 0 + Δ , must be spatially separated with an optical resonator and sent to two separate homodyne detectors. In this framework, we have designed and tested a solid Fabry–Perot etalon, to be used in an EPR table-top experiment prototype, thermally controlled without the use of a control probe optical beam. This device can also be used in optical experiments where the use of a bright beam to control an optical resonator is not possible, or where a simpler optical device is preferred.
Frequency-dependent squeezing (FDS) represents a well established way to address quantum noise (QN) in Gravitational-Wave (GW) Earth-based detectors, such as LIGO, Virgo, and KAGRA. This technique is realized with the use of an external detuned optical resonator, the filter cavity. The experiment we present here is a table-top prototype that will probe a cheaper, more compact and more flexible strategy for broadband QN reduction (Ma et al., 2017). This scheme is based on two-mode Einstein–Podolsky–Rosen (EPR) entangled squeezed light, and it works without any filter cavity. The EPR-entangled beams will propagate in a small-scale suspended interferometer with high-finesse arm-cavities. This experiment aims at validating the EPR conditional squeezing at audio frequencies, suited for GW detection, implementing also innovative optical techniques.
Abstract The sensitivity of gravitational wave interferometric detectors is ultimately limited by the quantum noise, which arises from the quantum nature of light and it is driven by vacuum fluctuations of the optical field entering from the dark port of the interferometer. One way to improve the sensitivity of gravitational wave interferometers is to inject squeezed vacuum into the dark port. This has been already demonstrated for the main gravitational wave detectors (GEO, Advanced LIGO and Advanced VIRGO). We are studying tricks to produce a “frequency- dependent squeezing”: a standard method is to filter the squeezed optical field with one or more optical cavities (300 m long cavities). An alternative method using a pair of squeezed EPR (Einstein-Podolsky-Rosen) entangled beams to produce frequency-dependent squeezing by a non-degenerate OPO (Optical Parametric Oscillator) will be discussed in this paper. This method promises to achieve a frequency-dependent optimization of the injected squeezed light fields without the need for an external filter cavity.
The astrophysical reach of current and future ground-based gravitational-wave detectors is mostly limited by quantum noise, induced by vacuum fluctuations entering the detector output port. The replacement of this ordinary vacuum field with a squeezed vacuum field has proven to be an effective strategy to mitigate such quantum noise and it is currently used in advanced detectors. However, current squeezing cannot improve the noise across the whole spectrum because of the Heisenberg uncertainty principle: when shot noise at high frequencies is reduced, radiation pressure at low frequencies is increased. A broadband quantum noise reduction is possible by using a more complex squeezing source, obtained by reflecting the squeezed vacuum off a Fabry-Perot cavity, known as filter cavity. Here we report the first demonstration of a frequency-dependent squeezed vacuum source able to reduce quantum noise of advanced gravitational-wave detectors in their whole observation bandwidth. The experiment uses a suspended 300-m-long filter cavity, similar to the one planned for KAGRA, Advanced Virgo, and Advanced LIGO, and capable of inducing a rotation of the squeezing ellipse below 100 Hz.
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
Quantum Noise (QN) is a phenomenon which gives high contribution to the overall noise in the advanced interferometric Gravitational-Wave Detectors (GWDs). In the previous interferometer generation, the most relevant QN component was dominating in the high frequency region (300Hz–10kHz) of the detection band. This component of noise was attenuated by injection of optimal vacuum squeezed state [1]. In fact, Virgo Scientific Collaboration has already implemented Frequency-Independent Squeezing (FIS) injection to the readout port of Advanced Virgo and reported improvement of the sensitivity curve of around 3dB [2]. Currently, the Collaboration focuses on the work on the FrequencyDependent Squeezing (FDS) injection using a Filter Cavity (FC) which will reduce QN either in the high and low frequency range. At the same time, Virgo Squeezing working group is studying a possible alternative way of producing frequency–dependent squeezed states without the use of FC. The mechanism of FDS without filter cavity, called Einstein–Podolsky–Rosen (EPR) squeezing, was theoretically proposed by Ma et al. [3] and subsequently demonstrated feasible by scientific group from ANU and Hamburg using a proof-of-principle setup [4]. At the moment, development facility for the production of squeezed vacuum at EGO/Virgo site is being adapted for the preliminary experiment on the EPR. This task is carried by the INFN group from Naples and Genova in collaboration with APC. The experiment represents a first step toward implementation of the new squeezing generation technique. For the purpose of the project, INFN group from Perugia together with groups from Pisa, Padova and Rome have been developing various types of photo-detectors and software based on Finite-State Machines (FSMs). In this paper, we overview the current state of work on the facility and we summarize in details the tasks related to the development of electronics and software for implementation of EPR squeezer. In particular, we focus on the engineering of analogue electronics for cavity locking and squeezed state measurement. We conclude by explaining the near future steps: FSM application scheme, noise hunting and operator independent lock acquisition. There are three main motivations for this work. In principle the setup must be as accessible as possible to scientists from various fields of expertise. Thus, its handling must not require any complex operation on the feedback controls, like tuning or calibration. This approach is a first step to minimize necessary human interaction with the subsystem and can be obtained by its automation. Accurate automation will also help us in accomplishing the second goal which is the maximization of the subsystem Quality of Service (QoS). Last but not least, we want to reach the low noise characteristics and keep the subsystem compatible with Virgo infrastructure. We describe below the system developed using custom built devices and various devices built for Virgo. From the optical design of the EPR squeezer shown in Figure 1, we inferred the control system design requirements. In total, we need fourteen feedback loops of various kind: two Optical Phase-Locked Loops (OPLLs) to lock lasers in frequency; two locking loops for non-linear crystal cavities and four for Mode-Cleaner (MC) cavities longitudinal lock, implementing Pound–Drever–Hall technique (PDH); one Proportional–Integral–Derivative controller (PID) for Mach–Zehnder interferometer for light intensity stabilization, two PIDs for Coherent Control (CC) loop for squeezing phase stabilization [5] and yet another three for temperature control loops for non-linear crystals and etalon. Several of these devices need to be fed with a reference frequency. For this purpose we use a custom made 14-channel, synchronous Direct Digital Synthesis (DDS) generator. Actuation on the cavity mirrors is done using high-voltage piezoelectric actuators. We use a custom built amplifier to drive them. In the next two paragraphs we describe the hardware and the software structure of the system.
Quantum Noise (QN) is a phenomenon which gives high contribution to the overall noise in the advanced interferometric Gravitational-Wave Detectors (GWDs). In the previous interferometer generation, the most relevant QN component was dominating in the high frequency region (300Hz–10kHz) of the detection band. This component of noise was attenuated by injection of optimal vacuum squeezed state [1]. In fact, Virgo Scientific Collaboration has already implemented Frequency-Independent Squeezing (FIS) injection to the readout port of Advanced Virgo and reported improvement of the sensitivity curve of around 3dB [2]. Currently, the Collaboration focuses on the work on the FrequencyDependent Squeezing (FDS) injection using a Filter Cavity (FC) which will reduce QN either in the high and low frequency range. At the same time, Virgo Squeezing working group is studying a possible alternative way of producing frequency–dependent squeezed states without the use of FC. The mechanism of FDS without filter cavity, called Einstein–Podolsky–Rosen (EPR) squeezing, was theoretically proposed by Ma et al. [3] and subsequently demonstrated feasible by scientific group from ANU and Hamburg using a proof-of-principle setup [4]. At the moment, development facility for the production of squeezed vacuum at EGO/Virgo site is being adapted for the preliminary experiment on the EPR. This task is carried by the INFN group from Naples and Genova in collaboration with APC. The experiment represents a first step toward implementation of the new squeezing generation technique. For the purpose of the project, INFN group from Perugia together with groups from Pisa, Padova and Rome have been developing various types of photo-detectors and software based on Finite-State Machines (FSMs). In this paper, we overview the current state of work on the facility and we summarize in details the tasks related to the development of electronics and software for implementation of EPR squeezer. In particular, we focus on the engineering of analogue electronics for cavity locking and squeezed state measurement. We conclude by explaining the near future steps: FSM application scheme, noise hunting and operator independent lock acquisition. There are three main motivations for this work. In principle the setup must be as accessible as possible to scientists from various fields of expertise. Thus, its handling must not require any complex operation on the feedback controls, like tuning or calibration. This approach is a first step to minimize necessary human interaction with the subsystem and can be obtained by its automation. Accurate automation will also help us in accomplishing the second goal which is the maximization of the subsystem Quality of Service (QoS). Last but not least, we want to reach the low noise characteristics and keep the subsystem compatible with Virgo infrastructure. We describe below the system developed using custom built devices and various devices built for Virgo. From the optical design of the EPR squeezer shown in Figure 1, we inferred the control system design requirements. In total, we need fourteen feedback loops of various kind: two Optical Phase-Locked Loops (OPLLs) to lock lasers in frequency; two locking loops for non-linear crystal cavities and four for Mode-Cleaner (MC) cavities longitudinal lock, implementing Pound–Drever–Hall technique (PDH); one Proportional–Integral–Derivative controller (PID) for Mach–Zehnder interferometer for light intensity stabilization, two PIDs for Coherent Control (CC) loop for squeezing phase stabilization [5] and yet another three for temperature control loops for non-linear crystals and etalon. Several of these devices need to be fed with a reference frequency. For this purpose we use a custom made 14-channel, synchronous Direct Digital Synthesis (DDS) generator. Actuation on the cavity mirrors is done using high-voltage piezoelectric actuators. We use a custom built amplifier to drive them. In the next two paragraphs we describe the hardware and the software structure of the system.