arXiv subjects

Vivishek Sudhir

Publications and source records attributed to Vivishek Sudhir.

At least 19 recordsLinked to original sources

arXiv2026

Observing and evading quantum back-action on a kilogram-scale oscillator

Continuous quantum displacement measurements are fundamentally limited by a trade-off between readout imprecision and measurement back-action, constrained by the Heisenberg uncertainty principle. In the Laser Interferometric Gravitational-Wave Observatory (LIGO), these two quantum noise components dominate much of the observation band, making it an excellent testbed. We induce a sub-Hz-linewidth optomechanical mode by trapping the differential motion of the 40-kg mirrors in a band where radiation-pressure back-action dominates the motion. Engineering the quantum state entering the dark port creates correlations between imprecision and back-action that partially cancel their contributions, reducing observed motion near resonance by ~47%. A framework resolving the imprecision, back-action, and correlation terms identifies the origin of this suppression. These results demonstrate quantum back-action evasion and quantum reservoir engineering in a macroscopic optomechanical system.

quant-ph↗

arXiv2026

A Quantum-Enhanced Feedback Oscillator

Feedback oscillators, such as lasers and masers, serve as time references in modern computing, communication, and measurement. Quantum fluctuations ultimately limit their phase stability and ability to keep time precisely; in the absence of quantum engineering, their phase stability is bounded by a standard quantum limit (SQL). Techniques to improve the frequency stability of feedback oscillators beyond the SQL have been theorized, but have not yet been demonstrated. We demonstrate an opto-electronic oscillator (OEO), a type of feedback oscillator, with phase stability approaching the SQL. We then engineer the OEO's quantum state to improve its phase stability, thereby demonstrating the essential principle quantum-enhancement of feedback oscillators. Similar techniques may be employed to evade the SQL in other feedback oscillators such as masers and lasers.

quant-ph↗

arXiv2026

Continuum-field quantum optics of frequency comb metrology

Frequency combs enable precision measurements across timekeeping, spectroscopy, ranging and astronomy, and are now extending to integrated and field-deployable platforms. Realizing their full performance demands a comprehensive account of the quantum noise that arises when broadband optical fields are converted into finite-bandwidth electrical signals. Here we present a rigorous first-principles quantum-mechanical framework for optical frequency-comb metrology based on continuous-mode field quantization and a comb-line-resolved description of quantum fluctuations. The theory describes how quantum fluctuations of the comb field are transduced into electrical measurement noise. Formulated at the level of the comb field, the framework unifies the standard quantum limits of optical frequency division (OFD) and dual-comb spectroscopy (DCS) within a single treatment, and provides a general recipe for other comb-based measurements. On this footing, we identify practical, resource-efficient routes to quantum enhancement through engineered comb states, laying a foundation for the design of next-generation frequency combs operating at and beyond standard quantum limits.

quant-ph↗

arXiv2026

Completely-positive non-signalling non-Markovian dynamics

We define non-Markovian quantum dynamics as evolution in which the current state depends on all past states, and completely characterize its structure under the assumptions of complete positivity and non-signalling. The resulting continuous-time dynamics is an integro-differential equation that augments the Gorini-Kossakowski-Sudarshan-Lindblad equation with a memory integral, and is capable of describing the quantum state of systems exposed to noise with any integrable power spectral density with no further approximations. We then establish a formalism to evaluate multi-time correlations of measurement outcomes in this general setting, obviating the need for a regression theorem. As an application, we derive the emission spectrum of a driven two-level system coupled to a non-Markovian bath: the familiar Mollow triplet acquires a frequency-dependent linewidth that encodes the memory of the bath. Our work provides a rigorous yet transparent description of the quantum state of non-Markovian systems, opening the door for state estimation and state-based quantum control beyond the Markovian regime.

quant-ph↗

arXiv2026

Noise-Resilient Quantum Metrology

Quantum metrology seeks to leverage the richness of quantum systems for making better measurements than are possible using only classical resources in order to gain a ``quantum advantage''. Quantum metrology schemes must also be resilient against noise to be useful in practice. Simultaneously achieving quantum advantage and noise resilience requires an end-to-end analysis of quantum measurement schemes to assess their theoretical sensitivity, feasibility, and noise robustness. We demonstrate this approach through the development of a novel optical interferometer based on squeezed vacuum light. We propose a scheme that relies on a nonlinear phase estimation procedure, which allows us to shift the frequency of noise away from the signal band, resulting in a high degree of noise resilience. This enables us to achieve sensitivity with Heisenberg scaling in the lossless limit and sensitivity below the standard quantum limit (SQL) in practice. It also enables the first experimental demonstration of quantum-optimal Bayesian signal estimation in a balanced interferometer. We expect this end-to-end design approach to enable the development of a variety of useful quantum measurement protocols going forward.

quant-ph↗

arXiv2026

Forecasting in the presence of scale-free noise

The extraction of signals from noise is a common problem in all areas of science and engineering. A particularly useful version is that of forecasting: determining a causal filter that estimates a future value of a hidden process from past observations. Current techniques for deriving the filter require that the noise be well described by rational power spectra. However, scale-free noises, whose spectra scale as a non-integer power of frequency, are ubiquitous in practice. We establish a method, together with performance guarantees, that solves the forecasting problem in the presence of scale-free noise. Via the duality between estimation and control, our technique can be used to design control for distributed systems. These results will have wide-ranging applications in neuroscience, finance, fluid dynamics, and quantum measurements.

math.OC↗

arXiv2025

Wave-particle duality in the measurement of gravitational radiation

In a consistent description of the quantum measurement process, whether the wave or particle-like aspect of a system is revealed depends on the details of the measurement chain, and cannot be interpreted as an objective fact about the system independent of the measurement. We show precisely how this comes to be in the measurement of gravitational radiation. Whether a wave or particle-like aspect is revealed is a property of the detector employed at the end of the quantum measurement chain, rather than of the meter, such as a gravitational-wave (GW) antenna or resonant bar, used to couple the radiation to the detector. A linear detector yields no signal for radiation in a Fock state and a signal proportional to the amplitude in a coherent state -- supporting a wave-like interpretation. By contrast, the signal from a detector coupled to the meter's energy is non-zero only when the incident radiation contains at least a single graviton. Thus, conceptually simple modifications of contemporary GW antennae can reveal wave-particle duality in the measurement of gravitational radiation.

quant-ph↗

arXiv2025

Thermality and athermality in the Unruh effect

In idealized treatments of the Unruh effect, a two-level atom is accelerated in a prescribed classical trajectory through the vacuum of a quantum field -- the Unruh bath -- which causes the atom's internal state to thermalize to a temperature proportional to the acceleration. This happens via emission and absorption of quanta by the atom, leading to a detailed balance between fluctuations in the Unruh bath and associated dissipation of the atom's internal energy. In any physical manifestation of the Unruh effect, the center-of-mass (CoM) degree of freedom of the atom is dynamical, and is therefore coupled to the Unruh bath via momentum recoil during the absorption/emission process. We study this scenario and show how the fluctuation-dissipation theorem fails for the CoM degree of freedom, while still being upheld for the internal energy. That is, the CoM of an accelerated atom is not in thermal equilibrium with the Unruh bath, while its internal level can be.

gr-qc↗

arXiv2025

Scheme for continuous force detection with a single electron at the level of $10^{-27}\ \mathrm{N}$

The detection of weak forces is a central problem in physics and engineering, ranging in importance from fundamental pursuits such as precision tests of gravity, gravitational-wave detection, and searches for dark matter, to applications such as force microscopy. These pursuits require a low-mass mechanical force transducer with a high quality factor, whose motion can be measured in a quantum-noise-limited manner. Here we study the ultimate example of such a transducer: a single trapped electron. We propose and analyze in detail a new scheme for high-sensitivity continuous force detection using a single trapped electron whose motion is coupled to a microwave cavity field via image currents induced in an antenna. We derive the fundamental and technical limits to the sensitivity of this scheme and show that despite the disparity in size between that of a single electron and the wavelength of the microwave field, it is possible to continuously monitor the charge's zero-point motion and use it as a force detector with a sensitivity as low as $6\times 10^{-27}\ \mathrm{N} / \sqrt{\mathrm{Hz}}$ in the gigahertz regime. This sensitivity improves on the state-of-the-art by four orders of magnitude and thus paves the way to novel precision experiments.

quant-ph↗

arXiv2025

Active laser cooling of a centimeter-scale torsional oscillator

Experimental tests of gravity's fundamental nature call for mechanical systems in the quantum regime while being sensitive to gravity. Torsion pendula, historically vital in studies of classical gravity, are ideal for extending gravitational tests into the quantum realm due to their inherently high mechanical quality factor, even when mass-loaded. Here, we demonstrate laser cooling of a centimeter-scale torsional oscillator to a temperature of 10 mK (average occupancy of 6000 phonons) starting from room temperature. This is achieved by optical radiation pressure forces conditioned on a quantum-noise-limited optical measurement of the torsional mode with an imprecision 9.8 dB below its peak zero-point motion. The measurement sensitivity is the result of a novel `mirrored' optical lever that passively rejects extraneous spatial-mode noise by 60 dB. The high mechanical quality ($1.4\times 10^7$) and quantum-noise-limited measurement imprecision demonstrate the necessary ingredients for realizing the quantum ground state of torsional motion -- a pre-requisite for mechanical tests of gravity's alleged quantum nature.

quant-ph↗

arXiv2025

Distinguishable consequence of classical gravity on quantum matter

What if gravity is classical? If true, a consistent co-existence of classical gravity and quantum matter requires that gravity exhibit irreducible fluctuations. These fluctuations can mediate classical correlations, but not quantum entanglement, between the quantized motion of the gravitationally interacting matter. We use a consistent theory of quantum-classical dynamics in the Newtonian limit of gravity to show that experimentally relevant observables can conclusively test the hypothesis that gravity is classical. This can be done for example by letting highly coherent source masses interact with each other gravitationally, and performing precise measurements of the cross-correlation of their motion. Theory predicts a characteristic phase response that distinguishes classical gravity from quantum gravity, and from naive sources of decoherence. Such experiments are imminently viable.

gr-qc↗

arXiv2025

A Generalized Schawlow-Townes Limit

We study a class of a feedback oscillators realized by a phase-insensitive amplifier in positive feedback, where either the amplifier or the feedback element may determine the oscillator's linewidth. The spectral purity of the output of such a device originates from basic demands of quantum mechanics and causality. The resulting expression generalizes the Schawlow-Townes limit, which is itself one component of a standard quantum limit for feedback oscillators. Recently realized bad-cavity oscillators such as super-radiant lasers and solid-state masers can saturate this generalized Schawlow-Townes limit. This limit can be surpassed through appropriate quantum engineering: for example by atomic spin squeezing in a super-radiant laser.

quant-ph↗

arXiv2024

Quantum Linear Time-Translation-Invariant Systems: Conjugate Symplectic Structure, Uncertainty Bounds, and Tomography

Linear time-translation-invariant (LTI) models offer simple, yet powerful, abstractions of complex classical dynamical systems. Quantum versions of such models have so far relied on assumptions of Markovianity or an internal state-space description. We develop a general quantization scheme for multimode classical LTI systems that reveals their fundamental quantum noise, is applicable to non-Markovian scenarios, and does not require knowledge of an internal description. The resulting model is that of an open quantum LTI system whose dilation to a closed system is characterized by elements of the conjugate symplectic group. Using Lie group techniques, we show that such systems can be synthesized using frequency-dependent interferometers and squeezers. We derive tighter Heisenberg uncertainty bounds, which constrain the ultimate performance of any LTI system, and obtain an invariant representation of their output noise covariance matrix that reveals the ubiquity of "complex squeezing" in lossy systems. This frequency-dependent quantum resource can be hidden to homodyne and heterodyne detection and can only be revealed with more general "symplectodyne" detection. These results establish a complete and systematic framework for the analysis, synthesis, and measurement of arbitrary quantum LTI systems.

quant-ph↗

arXiv2024

Temperature stabilization of a lab space at $10\,\mathrm{mK}$-level over a day

Temperature fluctuations over long time scales ($\gtrsim 1\,\mathrm{h}$) are an insidious problem for precision measurements. In optical laboratories, the primary effect of temperature fluctuations is drifts in optical circuits over spatial scales of a few meters and temporal scales extending beyond a few minutes. We present a lab-scale environment temperature control system approaching $10\, \mathrm{mK}$-level temperature instability across a lab for integration times above an hour and extending to a few days. This is achieved by passive isolation of the laboratory space from the building walls using a circulating air gap and an active control system feeding back to heating coils at the outlet of the laboratory HVAC unit. The latter achieves 20 dB suppression of temperature fluctuations across the lab, approaching the limit set by statistical coherence of the temperature field.

physics.ins-det↗

arXiv2024

Limits and prospects for long-baseline optical fiber interferometry

Today's most precise optical instruments -- gravitational-wave interferometers and optical atomic clocks -- rely on long storage times for photons to realize their exquisite sensitivity. Optical fiber technology is the most widely deployed platform for realizing long-distance optical propagation. Yet, their application to precision optical measurements is sparse. We review the state-of-the-art in the noise performance of conventional (solid-core) optical fibers from the perspective of precision optical measurements and quantum technology that rely on precise transfer of information over long distances. In doing so, we highlight the limitations of this platform and point to the opportunities that structured fiber technology offers to overcome some of these limitations.

physics.optics↗

arXiv2024

Exceptional-point Sensors Offer No Fundamental Signal-to-Noise Ratio Enhancement

Exceptional-point (EP) sensors are characterized by a square-root resonant frequency bifurcation in response to an external perturbation. This has lead numerous suggestions for using these systems for sensing applications. However, there is an open debate as to whether or not this sensitivity advantage is negated by additional noise in the system. We show that an EP sensor's imprecision in measuring a generalized force is independent of its operating point's proximity to the EP. That is because frequency noises of fundamental origin in the sensor -- due to quantum and thermal fluctuations -- increase in a manner that exactly cancels the benefit of increased resonant frequency sensitivity near the EP. So the benefit of EP sensors is limited to the regime where sensing is limited by technical noises. Finally, we outline an EP sensor with phase-sensitive gain that does have an advantage even if limited by fundamental noises.

quant-ph↗

arXiv2023

Causal State Estimation and the Heisenberg Uncertainty Principle

The observables of a noisy quantum system can be estimated by appropriately filtering the records of their continuous measurement. Such filtering is relevant for state estimation and measurement-based quantum feedback control. It is therefore imperative that the observables estimated through a causal filter satisfy the Heisenberg uncertainty principle. In the Markovian setting, prior work implicitly guarantees this requirement. We show that any causal estimate of linear observables of a linear, but not necessarily Markovian, system will satisfy the uncertainty principle. In particular, this is true irrespective of any feedback control of the system and of where in the feedback loop -- inside or outside -- the measurement record is accessed. Indeed, causal estimators using the in-loop measurement record can be as precise as those using the out-of-loop record. These results clarify the role of causal estimators to a large class of quantum systems, restores the equanimity of in-loop and out-of-loop measurements in their estimation and control, and simplifies future experiments on measurement-based quantum feedback control.

quant-ph↗

arXiv2023

Quantum noise and its evasion in feedback oscillators

We study an abstract model of an oscillator realized by an amplifier embedded in a positive feedback loop. The power and frequency stability of the output of such an oscillator are limited by quantum noise added by two elements in the loop: the amplifier, and the out-coupler. The resulting frequency instability gives the Schawlow-Townes formula. Thus the applicability of the Schawlow-Townes formula is extended to a large class of oscillators, and is shown to be related to the Haus-Caves quantum noise limit for a linear amplifier, while identifying the role of quantum noise added at the out-coupler. By illuminating the precise origin of amplitude and frequency quantum noise in the output of an oscillator, we reveal several techniques to systematically evade them.

quant-ph↗