The team achieved this by isolating the rotor and resolving thermal fluctuations with a damping rate of only 7 nHz, enabling continuous quantum measurement of its motion. This control allows for conditional states with entropic temperatures as low as 140 nK, two orders of magnitude colder than previously achieved in similar experiments, and opens new avenues for testing modifications of quantum mechanics and probing the quantum-classical interface.

Milligram Rotor Isolates Free-Particle Dynamics

This exceptionally low damping rate signifies a substantial reduction in external disturbances affecting the rotor’s motion, allowing for the observation of subtle quantum effects previously masked by noise. Researchers realized this isolation within a suspended rotor operating in a free-particle regime, a configuration that supports advancing quantum sensing and fundamental tests of physics. Continuous quantum measurement of the rotor localizes its motion, generating conditional states with entropic temperatures reaching 140 nK, a value significantly lower than previously attainable in tabletop optomechanical experiments.

This temperature represents a quantitative measure of the control achieved over the free-particle demonstrating a performance improvement of two orders of magnitude over existing methods. The technique relies on tracking the rotor’s yaw motion in phase space, with an inference uncertainty that shrinks to a steady state, revealing the dynamics of the free mass.

This approach differs from previous work that focused on either precision force sensing with free particles or low-entropy states in trapped oscillators. The physics governing a continuously measured free particle is best understood independently from the harmonic oscillator model, according to the study. Researchers employed a novel measurement-based strategy using smoothed states as a reference to clearly characterize the intrinsically squeezed conditional states of the free particle.

The experimental setup features a fused-silica rotor coated with dielectric mirrors, suspended in an ultrahigh-vacuum chamber, and its yaw motion is measured using a ‘double’ optical lever. This lever counter-circulates polarized beams, with displacements observed differentially on a quadrant photodiode, in an operation designated A-B-C+D. Initial alignment of the rotor is achieved using a push beam via radiation pressure, bringing the reflected lever beam into the detection range.

The rotor is then actively trapped at 240 mHz linear detection range under feedback control, again using the push-beam actuation. To investigate measurement-conditioned dynamics, the rotor is released from the induced trap by locking the push-beam power at its pre-release mean value, and the angular evolution is then monitored. “Extended intervals of free expansion help engineer interesting quantum states or enhance sensitivity to physical phenomena, while trapping primarily serves to initialize experiments,” the paper explains, highlighting the interplay between free expansion and controlled trapping.

This advancement opens possibilities for exploring fundamental physics at the intersection of quantum mechanics and gravity, potentially revealing new insights into the nature of spacetime and the limits of quantum theory. The work demonstrates a significant step towards manipulating macroscopic objects with quantum precision.

Continuous Quantum Measurement Localizes Rotor Motion

The ability to directly observe thermal fluctuations at a damping rate of 7 nHz enabled unprecedented control over the rotor’s motion, a feat previously limited by environmental noise and dissipation. This exceptionally low damping rate demonstrates a new level of environmental isolation, important for maintaining the quantum properties of a relatively massive object in free space. Researchers Austria achieved this isolation, allowing for the resolution of minute movements that would otherwise be obscured by external disturbances.

The team established a framework for understanding these conditional states, linking continuous quantum measurement to temperature through the von Neumann entropy, a measure of quantum purity that allows for unambiguous comparison with oscillating systems. This connection facilitates a rigorous understanding of the rotor’s quantum behavior. This level of control supports fundamental science, including setting tighter constraints on modifications to quantum mechanics and benchmarking entanglement-to-decoherence rate ratios.

The researchers demonstrated these utilities by achieving ratios nearly two orders of magnitude beyond the current state of the art for tests probing the quantum-classical interface, specifically in gravity. The rotor’s energy damping rate of only 7 nHz was directly resolved through the combination of environmental isolation and a low-noise measurement architecture. The anticipated damping rate at the reference angular frequency was estimated from the behavior of 65 mHz torsional oscillations, ensuring accurate modeling of the system.

The approach differs from traditional methods relying on confinement, instead using continuous quantum measurement to access localized conditional states, as recently proposed for generating nonclassical states of macroscopic objects. This alternative strategy offers advantages for exploring and exploiting these states, potentially surpassing the performance of trapped systems.

As Umang Mishra Austria and colleagues demonstrate, the combination of exceptional isolation and low-noise measurement architecture is key to taming a macroscopic free mass through continuous quantum measurement. The work establishes a pathway for future investigations into the fundamental limits of quantum mechanics and the exploration of quantum phenomena in increasingly massive systems.

Entropic Temperature Defines Control at 140 nK

This achievement isn’t simply about reaching a lower temperature; it provides a new quantitative measure of control over the free-particle dynamics, allowing researchers to define the system’s state through entropy rather than traditional confinement methods. The ability to assign an entropic temperature, calculated using the canonical definition relating changes in entropy to energy, allows for an unambiguous comparison between free particles and harmonic oscillators, systems previously assessed using different metrics.

This equation predicts a fundamental limit to temperature reduction, suggesting a minimum achievable temperature of 15 nK given the rotor’s level of environmental isolation, a constraint stemming from the inherent limitations of position measurements. The work demonstrates that even with exceptional isolation, reaching absolute zero temperature remains fundamentally non-measurable for free particles or oscillators alike. Establishing this level of control required overcoming significant technical hurdles, particularly in mitigating noise and achieving the necessary environmental isolation.

The detection and control principles employed build upon previously reported methods, but incorporate key innovations including a reduction in torsional suspension stiffness to minimize thermal noise. A novel “double” optical lever was implemented for detection noise immunity, and a 0. 4 Hz cut-off passively isolated platform was used for precise motion control. an auxiliary suspended-mirror Michelson interferometer was integrated to accurately sense and compensate for platform motion.

The researchers note, highlighting the careful calibration and characterization of the system. The implications of this work extend beyond simply achieving a lower temperature. These improved ratios are essential for tests of gravity at the quantum-classical interface, potentially revealing subtle deviations from established theories.

Rotor’s Phase-Space Uncertainty & Reference Frequency

The steady-state uncertainty ellipse characterizing the rotor’s yaw motion reveals the limits of localization achievable through continuous quantum measurement, its shape dictated by the free mass’s momentum-position dynamics. This shape is a fundamental consequence of the system’s inherent properties, visualized in an inset illustrating how the conditional phase-space area approaches a stable value over time relative to that steady state.

The timescale governing this approach, the inverse of the reference frequency, defines how quickly measurement-conditioned states settle into predictable variances in phase space, an important parameter for manipulating the rotor’s quantum state. Understanding the physics of a continuously measured free particle requires a shift in perspective, moving beyond the traditional framework of harmonic oscillators, according to the research.

The available reference frequency, denoted $Ω_{r e f}$ , is now defined by the time scale, $t_{r e f}$ = 1/Ω_(ref), needed for measurement-conditioned states to reach stable variances in phase space. The well-defined conditional entropy enabled by this setup allows for the assignment of an entropic temperature to the conditional states, using the canonical definition T = (k_B(∂ S)/(∂ E))^(-1), which relates changes in entropy (S) to changes in energy (E), where $k_{B}$ is Boltzmann’s constant.

In one dimension, the researchers derive a relationship, k_BT = 3f(P)/2E, linking the thermal energy (k_BT) to the kinetic energy (E) in the rotor’s reference frame and a function f(P) = 2P/ln(1 + P)/(1-P) approximating to 1 for P < 0. 5.

This finding highlights a fundamental limit: position measurements, whether applied to free particles or oscillators, cannot achieve zero temperature. Radiation pressure, applied via a “push” beam and a feedback loop with the optical lever, manipulates the rotor’s motion for damping, alignment, or trapping. The entire apparatus rests on a passive seismic isolation platform with a resonance frequency of 0. 4 Hz in all directions, further augmented by magnet-coil pairs actuating motion in a feedback loop to dampen higher-order mechanical modes, all housed within a vacuum below mbar.

While technically a 15-second period (65 mHz) torsional pendulum, the rotor operates deeply in the free particle regime given the reference timescale relevant to measurement-based state conditioning. The rotor’s damping follows a structural dissipation model, a multi-time-scale process leading to timescale-dependent friction strength, and ultimately to thermal torque noise with a power spectral density $S_{τ τ}$ = $4 k_{B} T_{0} I γ (ω)$ . However, for quantum control of mechanical motion, the relevant quantity determining decoherence rates is the noise arising from it, not the oscillation decay itself.

Quantum Limits Constrain Modifications of Mechanics

The ability to localize the motion of a 1-milligram rotor through continuous quantum measurement establishes new constraints on proposed modifications to quantum mechanics, according to work Austria. This level of control over a macroscopic free mass is particularly significant for experiments probing the quantum-gravity interface and searching for new forces. This low-noise environment is critical for maintaining quantum coherence and minimizing decoherence, a major obstacle in controlling free-particle dynamics.

The work demonstrates a quantum-gravity figure-of-merit, $η_{Q G}$ , where gravity-mediated entanglement would require $η_{Q G}$ ≥ 1 alongside a sufficiently high achievable single particle purity, providing a benchmark for future experiments. The implications extend to tests of the continuous spontaneous localization (CSL) model, a proposed modification to quantum mechanics that introduces stochastic collapse mechanisms.

The measured angular noise floor achieved in the experiment excludes diffusion rates $λ_{C S L}$ , complementing constraints established by the LISA Pathfinder mission and probing a regime where experimental tests have historically proven difficult. Specifically, the collapse length scale $r_{C S L}$ is conventionally chosen as 100 nm, and the current results refine the boundaries of acceptable parameters within the CSL framework.

The team led by Umang Mishra, Teodor Strömberg, Gabriela Valencia-Guzman, Felipe Gómez-Lozada, Marta Marchese, and Onur Hosten demonstrated that by continuously measuring the rotor’s position, they could effectively localize its motion and achieve the unprecedentedly low entropic temperatures. The rotor’s mass density, thickness and atomic mass unit, $m_{u}$ , all contribute to its behavior, but the ultimate limit to control is set by the quantum realm.

This achievement not only pushes the boundaries of precision measurement but also provides a platform for exploring the interplay between quantum mechanics and gravity with a macroscopic object. The work also has relevance for dark-matter models and the search for new short-range forces, as the highly sensitive measurement setup can be used to detect subtle interactions that might otherwise be masked by noise. The team’s results represent a step towards realizing the promise of macroscopic quantum systems for both sensing and fundamental research.

👉 More information
🗞 Taming a macroscopic free mass through continuous quantum measurement
✍️ Umang Mishra, Teodor Strömberg, Gabriela Valencia-Guzman, Felipe Gómez-Lozada, Marta Marchese and Onur Hosten (Institute of Science and Technology Austria, Klosterneuburg, 3400, Austria)
🧠 ArXiv: https://arxiv.org/html/2611.03159v1

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