Light Deflection Experiment Nears Limit of Measurement Precision with Vacuum Nonlinearity

Scientists are striving to verify a fundamental prediction of electrodynamics, that even a vacuum exhibits nonlinear behaviour under intense electromagnetic fields. Ali Aras, Adrien E. Kraych, and Xavier Sarazin, working with colleagues at Université Paris-Saclay, CNRS/IN2P3, and IJCLab, report a new method achieving unprecedented sensitivity in measuring this effect. Their research, conducted within the DeLLight project utilising the LASERIX facility, focuses on interferometrically detecting the deflection of a probe laser pulse caused by a high-intensity pump pulse inducing a vacuum index gradient. Crucially, the team developed a High-Frequency Phase Noise Suppression (HFPNS) technique to mitigate the impact of mechanical vibrations, a major limitation in achieving the required picometer-scale precision. This experimental validation of HFPNS represents a significant advance, paving the way for the direct observation of quantum electrodynamics-induced vacuum refraction.

The core principle involves measuring the deflection of a weak “probe” laser pulse as it traverses the altered vacuum index created by a powerful “pump” laser pulse. HFPNS utilizes a delayed replica of the probe pulse as a reference to actively cancel out noise induced by vibrations, effectively stabilising the interferometric signal.

This work details the experimental validation of HFPNS, demonstrating its ability to suppress phase noise and pave the way for picometer-scale sensitivity, a crucial step towards observing QED-induced vacuum refraction. The successful implementation of HFPNS represents a significant advance in the precision measurement of optical nonlinearity in vacuum.

The DeLLight experiment employs a Sagnac interferometer, where two beams derived from a single probe laser circulate in opposite directions. These beams are then recombined, and any deflection of the probe beam caused by the pump pulse manifests as a shift in the interference pattern. Achieving the necessary precision demands minimising all sources of noise, particularly those arising from mechanical instability.

The HFPNS method addresses this by creating a delayed copy of the probe pulse, allowing for real-time correction of any phase fluctuations affecting the primary probe beam. Experimental results confirm that HFPNS effectively suppresses vibrational noise within the prototype system. This suppression is essential for reaching the quantum noise limit, the fundamental limit imposed by the wave nature of light, and unlocking the potential to observe the subtle effects predicted by QED. The team’s work not only validates the HFPNS technique but also establishes a robust data analysis procedure, bringing the direct observation of vacuum nonlinearity closer to reality and opening new avenues for exploring the fundamental properties of the vacuum itself.

Sagnac interferometry mitigates vibration noise for vacuum refractive index measurement

The DeLLight project employs interferometry to detect subtle changes in the vacuum refractive index induced by intense laser pulses delivered by the LASERIX facility at IJCLab. Specifically, a low-intensity probe pulse traverses the gradient of the vacuum index created by a high-intensity pump pulse, and any resulting deflection is measured via interferometric techniques.

To achieve the necessary precision, a Sagnac interferometer is central to the experimental design, amplifying the deflection signal and enhancing sensitivity to the predicted effect. This configuration allows for measurement of the probe pulse’s position with extreme accuracy, crucial for detecting the minute deflection expected from quantum electrodynamic effects.

However, mechanical vibrations pose a significant challenge to achieving the required spatial resolution. These vibrations induce phase noise within the interferometer, degrading the precision of the deflection measurement. The delayed pulse functions as a reference signal, effectively monitoring and correcting for any phase noise affecting the prompt pulse. By assuming that the interferometric noise is consistent between both pulses, the HFPNS method allows for accurate subtraction of vibrational disturbances.

The experimental setup incorporates precise timing and control of the delayed reference pulse, ensuring its alignment and synchronization with the primary probe beam. This suppression was achieved through the implementation of a delayed reference signal, effectively correcting noise-related signals within the probe beam.

The experimental setup utilised a delayed pulse, separated by several nanoseconds, to concurrently monitor and mitigate off-line interferometric noise, assuming consistent noise profiles for both the prompt and delayed pulses. Analysis of the data confirms complete suppression of vibrational phase noise within the current prototype, paving the way for picometer-scale sensitivity measurements.

The DeLLight experiment employs a Sagnac interferometer to amplify the deflection signal resulting from the interaction between a low-intensity probe pulse and a high-intensity pump pulse. Extinction factors, characterising the residual interference signal, were initially measured and used to calibrate the system.

The focused spot size of both the probe and pump pulses was carefully controlled, with minimum waist at focus values of w0 and W0 respectively, to optimise the interaction region. This interferometric approach is crucial for detecting the extremely small deflection predicted by Quantum Electrodynamics (QED).

Further refinement of the HFPNS method revealed the ability to resolve spatial resolutions approaching the ultimate limit imposed by quantum noise. The off-line data analysis procedure, integrated with the HFPNS technique, allows for precise measurement of the interference signal barycenter. This precise positioning is essential for detecting the subtle refractive effects predicted by QED in the vacuum. The successful implementation of this method represents a key advancement toward observing QED-induced vacuum refraction, a phenomenon previously beyond the reach of experimental capabilities.

The Bigger Picture

Scientists pursuing the elusive signature of vacuum refraction have long been stymied by a deceptively simple problem: measuring incredibly small deflections of light. The DeLLight project, detailed in recent work, represents a significant advance not because it has yet seen this fundamental quantum effect, but because it has demonstrably overcome a major practical hurdle in the search.

For decades, theoretical physics has predicted that intense electromagnetic fields should subtly alter the properties of the vacuum itself, effectively bending light without any material medium. By employing a delayed reference signal, they effectively subtract out the vibrational noise, achieving unprecedented stability. This isn’t merely a technical refinement; it’s a pathway to accessing a regime previously obscured by instrumental limitations.

While other experiments have attempted similar measurements using static magnetic fields or high-repetition pulsed magnets, the DeLLight approach, leveraging ultra-short laser pulses, offers a complementary strategy with distinct advantages. However, the challenge remains formidable. Even with this noise suppression, the predicted effect is extraordinarily weak, and systematic errors could still masquerade as a signal.

Future work will undoubtedly focus on refining the measurement precision and exploring different laser configurations to maximise the interaction with the vacuum. More broadly, this success encourages a re-evaluation of experimental designs across the field of precision optics, suggesting that sophisticated noise cancellation techniques may unlock other subtle phenomena currently beyond our reach. The pursuit of vacuum refraction is, ultimately, a quest to test the very foundations of quantum electrodynamics, and each incremental step towards a definitive measurement brings us closer to a deeper understanding of the universe.