Entangled Across Kilometers: A New Test for Gravity’s Quantum Reach

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Author: Denis Avetisyan

Researchers have built a 50-kilometer fiber optic interferometer sensitive enough to detect the subtle effects of gravity on quantum interference.

A 50-km fiber interferometer achieves sufficient phase sensitivity to resolve gravitational redshifts and explore the interplay between general relativity and quantum phenomena.

Despite the foundational roles of quantum mechanics and general relativity, experimentally bridging these frameworks remains a significant challenge. Here, we report the realization of a 50-km fiber interferometer-detailed in ’50-km fiber interferometer for testing gravitational signatures in quantum interference’-capable of detecting minute phase shifts indicative of gravitational effects at the single-photon level. This instrument achieves a phase sensitivity sufficient to resolve laboratory-scale gravitational redshifts, demonstrating a clear modulated gravity-induced signal. Could this pave the way for novel quantum sensing techniques and more rigorous tests of fundamental physics at the intersection of these two pillars of modern science?

The Subtle Dance of Light and Gravity

The subtle interplay between gravity and light, particularly the phenomenon of gravitational redshift, presents a persistent challenge to modern physics. This redshift – the stretching of light’s wavelength as it escapes a gravitational field – is predicted by Einstein’s theory of general relativity, but its precise measurement demands extraordinary sensitivity. Existing classical methods often lack the necessary precision to discern these minute shifts, especially when dealing with weak gravitational fields or distant light sources. The difficulty arises from the fact that gravitational redshift manifests as incredibly small phase changes in the light’s wave, easily masked by background noise and instrumental limitations. Consequently, advancements in measurement techniques are crucial not only to confirm the predictions of general relativity with increasing accuracy, but also to potentially uncover deviations that might hint at new physics beyond the standard model. The pursuit of more precise measurements continues to drive innovation in fields like atomic clocks and interferometry, aiming to capture these elusive signals from the universe.

Traditional techniques for measuring gravitational redshift encounter fundamental limitations due to the incredibly small phase shifts induced by gravity on light. These shifts, representing alterations in the wave’s crests and troughs, are often buried within the noise inherent in conventional instruments. Detecting such subtle changes requires an unprecedented level of precision, pushing the boundaries of what is achievable with classical methods. Consequently, investigations into areas like tests of general relativity, the search for dark matter through gravitational lensing, and precise timekeeping – all reliant on accurately measuring these phase shifts – are hampered by this sensitivity barrier. The faintness of the signal necessitates improvements in measurement technology to unlock further insights into the universe and the fundamental laws governing it.

Exploiting the principles of quantum interference presents a promising route to dramatically improve the measurement of subtle gravitational effects, such as redshift. This approach hinges on the wave-like nature of photons, allowing for constructive and destructive interference that can amplify exceedingly small phase shifts induced by gravity. However, realizing this potential demands an unprecedented level of control over individual photons; each photon must be isolated, its quantum state precisely known, and its path meticulously managed. Maintaining this delicate control is exceptionally challenging, as even minor disturbances from the environment can disrupt the quantum coherence necessary for observable interference patterns. Researchers are actively developing advanced techniques – including novel photonic circuits and ultra-stable laser systems – to tame these challenges and unlock the full sensitivity offered by quantum metrology in the pursuit of testing fundamental physics and refining $GR$ predictions.

A Tabletop Universe: Constructing a Fiber Interferometer

A 50-kilometer Mach-Zehnder fiber interferometer was constructed as a compact, tabletop instrument for detecting gravitational effects. This length is achieved by winding polarization-maintaining fiber, allowing for substantial light travel distance within a small physical space. The interferometer operates by splitting a coherent light source into two paths; the interference pattern created upon recombination is sensitive to differences in path length, including those induced by gravitational phenomena. A 50-km path length is necessary to accumulate a measurable phase shift, on the order of $\lambda$, enabling the detection of minute changes in spacetime geometry.

The interferometer’s design incorporates Polarization Maintaining Fiber (PMF) to address the sensitivity of photon quantum states to environmental factors. Standard optical fiber induces depolarization due to bending and stress, altering the photons’ polarization and introducing noise into the gravitational measurements. PMF, however, constrains photons to a specific polarization axis through internal stress, actively suppressing polarization drift. This ensures the preservation of the photons’ initial quantum state – specifically, their polarization – as they traverse the 50-km fiber length, maintaining the coherence necessary for detecting minute phase shifts induced by gravitational effects. The use of PMF is critical for achieving the required signal-to-noise ratio in a tabletop gravitational wave detector.

Maintaining the stability of the 50-km fiber interferometer necessitated the implementation of dedicated Vibration Isolation and Thermal Control systems. External vibrations, including seismic noise and mechanical disturbances, introduce phase noise that limits measurement sensitivity; a six-axis vibration isolation table was employed to dampen these effects. Similarly, temperature fluctuations alter the refractive index of the fiber, inducing phase shifts; the interferometer was housed within a thermally regulated enclosure maintained to within $0.1^\circ$C. These systems collectively reduced environmental noise contributions, enabling the detection of subtle phase shifts indicative of gravitational effects.

Whispers of Light: Generating and Detecting Single Photons

Correlated photon pairs were generated using the process of Spontaneous Parametric Down-Conversion (SPDC) within a periodically poled lithium niobate (PPLN) crystal. SPDC involves the nonlinear interaction of a high-energy pump photon with the PPLN crystal, resulting in the creation of two lower-energy photons, known as the signal and idler. The PPLN crystal’s periodic poling allows for quasi-phase matching, maximizing the efficiency of the down-conversion process and ensuring energy and momentum conservation. By carefully controlling the pump wavelength and crystal temperature, the wavelengths of the generated photon pairs can be tuned, and the correlation between the two photons – specifically, their entanglement – is maintained, crucial for quantum applications. The efficiency of this frequency conversion is a key factor in the overall performance of the single-photon source.

The Hong-Ou-Mandel (HOM) effect, observed as a dip in the second-order correlation function $g^{(2)}(\tau)$ at $\tau = 0$, provides definitive evidence of the indistinguishability of the generated photon pairs. This dip arises because indistinguishable photons will either both be reflected or both be transmitted at a beam splitter, interfering destructively and reducing the probability of detecting a photon in coincidence. The depth of the dip, ideally approaching zero, directly correlates with the degree of indistinguishability, which is a crucial requirement for performing quantum interference experiments such as two-photon interference and quantum information processing. Any deviation from a complete dip indicates factors reducing indistinguishability, including spectral or spatial differences between the photons.

Superconducting Nanowire Single-Photon Detectors (SNSPDs) were utilized for single-photon counting due to their demonstrated performance characteristics. These detectors operate on the principle of detecting a single photon-induced resistive transition in a superconducting nanowire, typically composed of niobium nitride. SNSPDs offer high detection efficiency, exceeding 80% at wavelengths relevant to the experiment, and exhibit timing resolution on the order of tens of picoseconds, enabling precise temporal correlation measurements. The detectors were operated at 4K to maintain superconductivity and minimize dark count rates, with careful shielding implemented to reduce background noise and ensure accurate single-photon registration.

Precise phase stabilization was implemented utilizing a Balanced Homodyne Detection scheme, which requires a strong, coherent local oscillator. This local oscillator was provided by a Continuous Wave Laser operating at a wavelength compatible with the generated photon pairs. Balanced Homodyne Detection measures the phase difference between the signal photon and the local oscillator, enabling sensitive phase control. Phase locking techniques were then employed to actively maintain a stable relative phase between these two beams, minimizing phase drift and ensuring consistent interference effects. This stabilization is critical for maintaining the coherence of quantum states and accurate quantum measurements, as even small phase fluctuations can degrade the visibility of interference patterns.

A Universe Unveiled: Towards Enhanced Sensitivity and Future Horizons

A phase sensitivity of $4.42 \times 10^{-6}$ rad RMS has been achieved through this quantum-enhanced measurement technique, marking a significant step towards practical applications of quantum sensing. This level of precision confirms the fundamental viability of utilizing quantum effects to improve the detection of subtle signals, specifically in scenarios where classical methods are limited. The demonstrated sensitivity represents a substantial advancement, opening avenues for exploring previously inaccessible phenomena and refining existing measurement technologies. By harnessing the principles of quantum mechanics, this approach establishes a pathway for building increasingly sensitive instruments capable of resolving extremely weak interactions and gravitational effects. It suggests that the universe doesn’t simply reveal its secrets; it responds to how we choose to listen.

The achieved sensitivity of $4.42 \times 10^{-6}$ rad RMS enables the detection of exceedingly subtle phenomena, specifically a simulated gravitational phase shift measured at (6.18 ± 0.44) x $10^{-5}$ rad RMS at a frequency of 0.1 Hz. This resolution represents a significant advancement in the field, as such minute phase shifts are characteristic of gravitational waves and other relativistic effects predicted by Einstein’s theory. The ability to resolve these shifts opens avenues for investigating weak gravitational fields and probing the fundamental nature of spacetime, potentially offering new insights into astrophysical events and the universe’s origins. This demonstrated capacity highlights the potential of this quantum-enhanced approach for future gravitational wave detection and precision measurement applications.

Researchers are poised to refine the interferometer’s capabilities through the implementation of Differential Measurement techniques. This approach seeks to amplify sensitivity by deliberately introducing a height differential between the interferometer’s arms. By measuring the difference in phase shift experienced across this height gradient, the system becomes less susceptible to common-mode noise – disturbances affecting both arms equally. This strategy effectively cancels out certain environmental errors and allows for the detection of subtler gravitational effects. Preliminary analysis suggests that optimizing this height difference, coupled with advanced data analysis, could unlock a new order of magnitude in precision, potentially revealing previously undetectable signals from gravitational waves or dark matter interactions.

A complete theoretical framework for interpreting these highly sensitive measurements necessitates the principles of Quantum Field Theory in Curved Spacetime. This approach moves beyond treating gravity as a mere external force and instead considers its geometric effects on the quantum vacuum itself. The curvature of spacetime, as predicted by General Relativity, fundamentally alters the behavior of quantum fields, leading to phenomena like vacuum fluctuations and the creation of entangled particles. Accurately modeling these interactions is crucial for interpreting subtle phase shifts induced by gravitational waves or other spacetime distortions. Furthermore, this theoretical lens allows researchers to disentangle genuine gravitational signals from various sources of noise, ultimately pushing the boundaries of precision measurement and offering a deeper understanding of the universe at its most fundamental level. The successful application of this framework validates the use of quantum technologies in probing the interplay between quantum mechanics and gravity, opening avenues for exploring previously inaccessible regimes of physics. It’s not about controlling the universe, but about attuning to its inherent order.

The research details an instrument capable of discerning minute phase shifts-a feat akin to observing the subtle growth patterns within a complex ecosystem. This work, demonstrating a 50-km fiber interferometer sensitive enough to detect gravitational redshifts, reveals how order emerges from meticulously established local rules – the properties of light within the fiber. Schrödinger famously stated, “The total number of states of a system is finite, but the number of ways of arranging these states is infinite.” This resonates with the experimental setup; while the fundamental laws governing light are finite, the possible interference patterns-and thus the sensitivity to gravitational effects-approach infinite nuance, allowing for unprecedented tests of general relativity and quantum phenomena. The system doesn’t control gravity, but rather influences our ability to observe its effects.

What Lies Ahead?

This demonstration of a 50-km fiber interferometer capable of resolving gravitational redshifts in a controlled environment doesn’t so much answer a question as it re-frames the landscape. The pursuit wasn’t to confirm general relativity – those confirmations accumulate organically – but to establish a platform where quantum phenomena are no longer shielded from relativistic effects. The interferometer itself is less an instrument of control and more a facilitator of emergence; local phase sensitivity, extended over distance, yields a global sensitivity to spacetime curvature.

Limitations, naturally, exist. Maintaining coherence over such distances remains a challenge, a constant negotiation between signal integrity and environmental noise. More intriguing, however, is the question of what constitutes a meaningful test. Simply observing a predicted effect isn’t progress; the true value lies in probing regimes where current theory falters, where the local rules of quantum mechanics and general relativity begin to produce unexpected global patterns.

Future iterations will likely focus on miniaturization, increased bandwidth, and perhaps, most crucially, a shift in focus. The goal isn’t to impose quantum mechanics onto gravity, but to allow their interplay to reveal the inherent self-governance of the universe. Control is illusory; influence, demonstrated through careful observation of emergent phenomena, is the path forward.

Original article: https://arxiv.org/pdf/2511.17022.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

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2025-11-24 10:55