Exploring Quantum Mechanics with a Tiny Interferometric Sensor

By Bethan DaviesApr 10 2024Reviewed by Susha Cheriyedath, M.Sc.

In a recent study published in the journal Sensors, a tiny interferometric sensor comprising powerful, mirror-equipped suspended optical cavities was developed to explore macroscopic quantum mechanics at a tabletop scale.

Suspension thermal noise and readout noise currently restrict sensor performance; however, these limitations can be mitigated by reaching quantum radiation pressure noise levels. Advancements in sensor technology not only facilitate the testing of semi-classical and quantum gravity theories but also enable the demonstration of macroscopic entanglement.

Interferometric Devices in Quantum Physics and Gravitational Wave Detection

Interferometric devices excel in the analysis of small signals within quantum physics, owing to their capability to create complex quantum configurations and their inherent high sensitivity. With applications spanning from atom traps to lasers, these devices are particularly valued for their ability to sense minuscule movements with unparalleled precision. This is largely due to their rapid responsiveness to even the slightest alterations in optical components, such as mirrors.

Gravitational wave detection relies heavily on laser interferometry, with setups akin to the advanced LIGO (aLIGO) and advanced Virgo achieving unprecedented precision in displacement sensing.

As gravitational wave detection continues to evolve, it promises to shape the future landscape of particle physics and cosmology. This advancement is anticipated through the deployment of next-generation observatories and the application of interferometry in various research areas, including the detection of dark matter particles, the study of spacetime quantization, and entanglement investigations.

However, the sensitivity of displacement-sensing laser interferometers is inherently limited by the quantum nature of light, specifically due to quantum shot noise and quantum radiation pressure noise. These noises, resulting from fluctuations in the electromagnetic field's amplitude and phase, pose challenges during displacement measurements.

Despite progress in understanding quantum light behavior, experimental validation, especially at the sensitivity required to approach the standard quantum limit (SQL), remains elusive. Achieving this level of sensitivity, crucial for probing quantum gravity properties and investigating new quantum phenomena like macroscopic entanglement, hinges on operating interferometers at the quantum noise level across various frequencies.

Experimental Arrangement

The recent study diligently followed the established practices and insights of the gravitational wave community, motivated by advancements in noise reduction techniques and the development of detectors with sensitivity near-SQL sensitivity.

Consistent with the aLIGO detectors, the system was placed to set the SQL frequency at 100 Hz, and the SQL level was raised to 10 g by decreasing the mass of the cavity mirrors. The system was designed to focus on the internal suspension, optical system, and active isolation system.

Two identical systems were then created using the same optical configuration, and each centered around a distinct suspended Fabry-Perot optical cavity. The only optical components inside the vacuum envelope were the cavity mirrors, which needed to be cooled and isolated. A four-stage pendulum chain with steel wire connections and intermediate aluminum masses was used to achieve horizontal isolation, while triangular blade springs were used for vertical isolation.

Observations

The streamlined design of the interferometric system minimizes the impact of intensity variations on the phase-sensitive beat readout to a second-order effect. This setup means that intensity noise, which usually causes classical radiation pressure fluctuations, is mechanically amplified by the light cavity mirrors. Suppressing this effect led to a significant improvement in the system's performance, resulting in a fivefold enhancement within the 30 to 100 Hz frequency range.

Following the suppression of suspension and seismic thermal noise above the suspension resonances, a broad region dominated by readout noise emerged from 40 Hz onwards. This readout noise was mitigated using an analog whitening filter.

At present, readout noise is primarily limited by the noise from the Analog-to-Digital Converter (ADC). However, there's room for a substantial fivefold reduction in ADC noise, which could greatly enhance system performance until phase lock loop noise becomes the predominant limitation.

However, challenges remain in further reducing readout noise, particularly due to broad peaks between 200 and 400 Hz. These peaks are thought to result from acoustic noise interacting with the vibrations of the cryostat vacuum lid, with ongoing mitigation efforts focusing on environmental noise reduction and acoustic isolation.

Across a wide range of acoustic frequency bands, the system's sensitivity has reached 0.5 fm/√Hz. Notably, at 100 Hz, the device's sensitivity of 2 × 10^-15 m/√Hz exceeded that of table-top configurations, marking a significant achievement compared to current interferometers. 

Interestingly, the device's sensitivity improved with increasing frequency as thermal noise became the dominant factor. However, above 100 Hz, the system's performance was constrained by the high noise profile of the readout electronics.

Conclusions

To conclude, this research successfully demonstrated the development of a table-top interferometric sensor for investigating macroscopic quantum mechanics. The sensor uses the expertise gained within the gravitational-wave community, primarily in reducing seismic and thermal noise and controlling high-finesse interferometers.

This system consists of two chambers with a thickness greater than 10⁵, supported by a four-stage suspension set on a cryostat. Additionally, to reduce seismic noise significantly, active isolation of the cryostat system was performed before the passive isolation of the internal suspension.

The latest version of the sensor signifies a critical advancement in enhancing sensitivity to near the standard quantum limit, distinguished by its unparalleled low-noise performance in cryogenic conditions. Achieving this milestone required significant material innovation to boost its functionality at lower temperatures.

Moreover, achieving the lowest noise mode in cryogenic conditions required significant material adaptations for optimal low-temperature performance. To facilitate this, the switch to materials with superior thermal conductivity—namely high-purity tungsten, aluminum, or copper—was essential, particularly within the suspension system.

Furthermore, the optical system has undergone extensive updates, incorporating silicon resonators and pioneering new optical coatings that are still in the early stages of development. The proposed improvements are projected to bring the system's performance in line with the standard quantum limit within a similar acoustic frequency spectrum.

Journal Reference

Smetana, J.; Yan, T.; Boyer, V.; Martynov, D. A High-Finesse Suspended Interferometric Sensor for Macroscopic Quantum Mechanics with Femtometre Sensitivity. Sensors 2024, 24, 2375. https://doi.org/10.3390/s24072375, https://www.mdpi.com/1424-8220/24/7/2375