arXiv subjects

Sreenath K. Manikandan

Publications and source records attributed to Sreenath K. Manikandan.

At least 19 recordsLinked to original sources

arXiv2026

The Transient Counting Statistics of Autonomous Quantum Clocks

At the heart of every atomic clock is a quantum coherent emitter, the laser, that could in principle be treated autonomously. While their frequency or phase stability is typically studied in the semiclassical regime of a large number of emitted quanta, recent works on autonomous quantum clocks suggest that the few-quanta regime can offer statistical information in the temporal domain. This provides complementary means to probe the fundamental limits to precise timekeeping in the quantum regime. Motivated by these works, we consider simple models for two- and three-level quantum emitters subject to an athermal fueling through dephasing, which generically results in coherences in the steady state. A large deviation principle with finite-time contributions is used to identify the transient counting statistics and their dependence on the initial conditions. We conclude by discussing the implications for precise timekeeping and quantum sensing using autonomous quantum emitters as clocks in their transient regime.

quant-ph↗

arXiv2026

Correlated Quantum Sensing at the Seemingly Classical Limit

It is a difficult task to detect the indivisible quanta of weakly interacting radiation fields, and even more challenging to probe their quantum statistics. Nevertheless, if barely functional high-quality resonant detectors are feasible for weakly interacting radiation fields, they do come with certain statistical advantages to probe quantum effects at the seemingly classical limit of a large number of quanta of the incoming radiation field. We present correlated counting, homodyne, and heterodyne detection strategies using high-quality resonant quantum harmonic detectors operating at this limit, initialized in bolometry-inspired zero-mean preparations such as thermal states. We compare the bolometric regime of good resonant harmonic detectors in quantum optics to the bolometric regime of barely functional resonant mass quadrupole oscillators as detectors for quantum gravity. Simple statistical tests are proposed using symmetric correlators for two and three such barely functional resonant mass detectors that could reveal the complementary quantum noise characteristics of gravitons in tabletop experiments.

quant-ph↗

arXiv2026

Squeezed Quasinormal Modes from Nonlinear Gravitational Effects

We estimate the degree of squeezing possible in gravitational waves due to nonlinear gravitational effects in the weakly perturbative regime. Using the predicted amplitude ratios for higher harmonic generation in the ringdown phase of a black hole merger event, we estimate the relevant degree of squeezing produced by a Schwarzschild singularity to be of the order of one percent.

gr-qc↗

arXiv2026

Quantum Sensing with Joint Emitter-Fluorescence Measurements

We present an analytically tractable model of a driven quantum harmonic emitter, such as an oscillating charged dipole, emitting radiation via resonance fluorescence. With this model we are able to characterize the quantum mechanical correlations that are built up at early times between the drive, the resonant emitter, and its fluorescence. We describe detection strategies that can reveal these quantum signatures in experiments by performing joint measurements on the quantum emitter and its fluorescence field. In particular, we show that simultaneous quantum measurements of a driven quantum emitter and its fluorescence can be used to probe the quantum noise of the driving field, relative to the maximally classical coherent state of the driving field, in short-time experiments. We conclude by discussing the applications to quantum sensing in quantum optical, quantum acoustic, and quantum gravitational scenarios of interest.

quant-ph↗

arXiv2026

Stimulated absorption of single gravitons: First light on quantum gravity

In a recent work we showed that the detection of the exchange of a single graviton between a massive quantum resonator and a gravitational wave can be achieved. Key to this ability are the experimental progress in preparing and measuring massive resonators in the quantum regime, and the correlation with independent LIGO detections of gravitational waves that induce stimulated absorption. But do stimulated single-graviton processes imply the quantization of gravity? Here we analyze this question and make a historic analogy to the early days of quantum theory. We discuss in what ways such experiments can indeed probe key features of the quantized interaction between gravity and matter, and outline five experimental tests. This capability would open the first window into experimental exploration of quantum gravity.

gr-qc↗

arXiv2026

Revealing the fuel of a quantum continuous measurement-based refrigerator

While quantum measurements have been shown to constitute a resource for operating quantum thermal machines, the nature of the energy exchanges involved in the interaction between system and measurement apparatus is still under debate. In this work, we show that a microscopic model of the apparatus is necessary to unambiguously determine whether quantum measurements provide energy in the form of heat or work. We illustrate this result by considering a measurement-based refrigerator, made of a double quantum dot embedded in a two-terminal device, with the charge of one of the dots being continuously monitored. Tuning the parameters of the measurement device interpolates between a heat- and a work-fueled regimes with very different thermodynamic efficiency. Notably, we demonstrate a trade-off between a maximal thermodynamic efficiency when the measurement-based refrigerator is fueled by heat and a maximal measurement efficiency quantified by the signal-to-noise ratio in the work-fueled regime. Our analysis offers a new perspective on the nature of the energy exchanges occurring during a quantum measurement, paving the way for energy optimization in quantum protocols and quantum machines.

quant-ph↗

arXiv2026

Signatures of coherent initial ensembles on all work moments

Standard treatments of quantum work using projective energy measurements erase initial coherence and alter the dynamics, thereby failing to capture the thermodynamic effects of coherent superpositions of energy eigenstates in an ensemble of initial states. In this article, we use an operational work definition that is non-intrusive, applying it to the case of a driven dissipative qubit, where the qubit's initial preparation comprises coherent superposition states, while the driving is coherence-less. We derive an evolution equation for the moment generating function for this work, faithfully capturing the thermodynamic signature of coherent superpositions in the initial ensemble. We demonstrate that different initial ensembles that correspond to the same density matrix upon ensemble average, while having the same average work, display different work fluctuations. For monotonic driving, we show that fluctuations are maximum for coherence-less initial ensembles. As an application, we consider quantum bit-erasure in finite time and demonstrate significantly different work statistics for erasing a classical bit of information versus a Haar random initial ensemble. Our results indicate that coherence in the initial ensemble can be utilized as a resource for thermodynamic precision without incurring additional dissipative work costs. We also obtain a generalized fluctuation theorem that establishes a new quantum lower bound on the mean dissipated work. This bound, counterintuitively, is also applicable to a "classical" initial ensemble with the same initial density matrix and is connected to quantum absolute irreversibility.

quant-ph↗

arXiv2025

Detector Correlations and Null Tests of the Coherent State Hypothesis

We discuss the statistics of correlations between two resonant detectors. We show that this allows simple null tests of the coherent state hypothesis, free of vacuum (quantum) noise. Complementary aspects of the radiation field, {\it e.g.}, squeezing in number or phase, can be revealed through appropriate detection strategies.

quant-ph↗

arXiv2025

Probing Quantum Structure in Gravitational Radiation

Gravitational radiation from known astrophysical sources is conventionally treated classically. This treatment corresponds, implicitly, to the hypothesis that a particular class of quantum-mechanical states -- the so-called coherent states -- adequately describe the gravitational radiation field. We propose practicable, quantitative tests of that hypothesis using resonant bar detectors monitored in coincidence with LIGO-style interferometers. Our tests readily distinguish fields that contain significant thermal components or squeezing. We identify concrete circumstances in which the classical (i.e., coherent state) hypothesis is likely to fail. Such failures are of fundamental interest, in that addressing them requires us to treat the gravitational field quantum-mechanically, and they open a new window into the dynamics of gravitational wave sources.

gr-qc↗

arXiv2025

Complementary Probes of Gravitational Radiation States

We demonstrate that the statistical fluctuations in resonant radiation detectors operating in homodyne and heterodyne modes offers additional, complementary information to that obtained from their direct operation as click detectors. We use this to refine tests of the coherent state hypothesis of interest in connection with gravitational wave fields.

gr-qc↗

arXiv2025

Quantum Measurement Induced Radiative Processes in Continuously Monitored Optical Environments

We investigate the emission characteristics of a measurement-driven quantum emitter in a continuously monitored optical environment. The quantum emitter is stimulated by observing the Pauli spin along its transition dipole that maximally noncommutes with the Hamiltonian of the emitter. It also exchanges energy resonantly with the optical environment, observable as quantum jumps corresponding to the absorption or emission of a photon and the null events where the quantum emitter did not make a jump. We characterize the finite-time statistics of quantum jumps and estimate their covariance and precision using the large deviation principle. While the statistics of absorption and emission events are generically sub-Poissonian with an improved precision by at most a factor of two compared to Poissonian jumps, our analysis also reveals a spin-measurement-induced transition from super-Poissonian to sub-Poissonian in their sum. We conclude by describing generalized quantum measurement strategies using mode-entangled optical beams to access the predicted counting statistics in experiments, with implications extending to optimal quantum clocks.

quant-ph↗

arXiv2025

Testing the Coherent State Description of Radiation Fields

We propose simple quantitative criteria, based on counting statistics in resonant harmonic detectors, that probe the quantum mechanical character of radiation fields. They provide, in particular, practical means to test the null hypothesis that a given field is ``maximally classical'', i.e., accurately described by a coherent state. We suggest circumstances in which that hypothesis plausibly fails, notably including gravitational radiation involving non-linear or stochastic sourcing.

quant-ph↗

arXiv2024

Detecting single gravitons with quantum sensing

The quantization of gravity is widely believed to result in gravitons -- particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single graviton exchange can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental clue of the quantization of gravity.

quant-ph↗

arXiv2023

Autonomous quantum clocks using athermal resources

Here we explore the possibility of precise time-keeping in quantum systems using athermal resources. We show that quantum measurement engineered reservoirs can be used as athermal resources to drive the ticks of a quantum clock. Two and three level quantum systems act as transducers in our model, converting the quantum measurement induced noise to produce a series of ticks. The ticking rate of the clock is maximized when the measured observable maximally non-commutes with the clock's Hamiltonian. We use the large deviation principle to characterize the statistics of observed ticks within a given time-period and show that it can be sub-Poissonian -- quantified by Mandel's Q parameter -- alluding to the quantum nature of the clock. We discuss the accuracy and efficiency of the clock, and extend our framework to include hybrid quantum clocks fueled by both measurements, and thermal resources. We make comparisons to relatable recent proposals for quantum clocks, and discuss alternate device implementations harvesting the quantum measurement engineered non-equilibrium conditions, beyond the clock realization.

quant-ph↗

arXiv2023

Optimal quantum parametric feedback cooling

We propose an optimal protocol using phase-preserving quantum measurements and phase-dependent modulations of the trapping potential at parametric resonance to cool a quantum oscillator to an occupation number of less than one quantum. We derive the optimal phase relationship and duration for the parametric modulations, and compute the lowest-possible occupation number in the steady state. The protocol is robust against moderate amounts of dissipation and phase errors in the feedback loop. Our work has implications for the cooling of levitated mechanical resonators in the quantum regime.

quant-ph↗

arXiv2022

Cyclic Superconducting Quantum Refrigerators Using Guided Fluxon Propagation

We propose cyclic quantum refrigeration in solid-state, employing a gas of magnetic field vortices in a type-II superconductor -- also known as fluxons -- as the cooling agent. Refrigeration cycles are realized by envisioning a racetrack geometry consisting of both adiabatic and isothermal arms, etched into a type-II superconductor. The guided propagation of fluxons in the racetrack is achieved by applying an external electrical current, in a Corbino geometry, through the sample. A gradient of magnetic field is set across the racetrack allowing one to adiabatically cool down and heat up the fluxons, which subsequently exchange heat with the cold, and hot reservoirs, respectively. We characterize the steady state of refrigeration cycles thermodynamically for both $s-$wave and $d-$wave pairing symmetries, and present their figures of merit such as the cooling power delivered, and the coefficient of performance. Our cooling principle can offer significant cooling for on-chip micro-refrigeration purposes, by locally cooling below the base temperatures achievable in a conventional dilution refrigerator. We estimate $10\mathrm{nW}/\mathrm{mm}^2$ of cooling power per unit area under typical operating conditions. Integrating the fluxon fridge to quantum circuits can enhance their coherence time by locally suppressing thermal fluctuations, and improve the efficiency of single photon detectors and charge sensors.

cond-mat.mes-hall↗

arXiv2022

Thermodynamics of Quantum Measurement and the Demon's Arrow of Time

We discuss the thermodynamic aspects of a single qubit based device, powered by weak quantum measurements, and feedback controlled by a quantum Maxwell's demon. We discuss both discrete and time-continuous operation of the measurement based device at finite temperature of the reservoir. In the discrete example where a demon acquires information via discrete weak measurements, we find that the thermodynamic variables including the heat exchanged, extractable work, and the entropy produced are completely determined by an information theoretic measure of the demon's perceived arrow of time. We also discuss a realistic time-continuous operation of the device where the feedback is applied after a sequence of weak measurements. In the time-continuous limit, we derive the exact finite-time statistics of work, heat and entropy changes along individual quantum trajectories of the quantum measurement process, and relate them to the demon's arrow of time.

quant-ph↗

arXiv2022

Efficiently Fuelling a Quantum Engine with Incompatible Measurements

We propose a quantum harmonic oscillator measurement engine fueled by simultaneous quantum measurements of the non-commuting position and momentum quadratures of the quantum oscillator. The engine extracts work by moving the harmonic trap suddenly, conditioned on the measurement outcomes. We present two protocols for work extraction, respectively based on single-shot and time-continuous quantum measurements. In the single-shot limit, the oscillator is measured in a coherent state basis; the measurement adds an average of one quantum of energy to the oscillator, which is then extracted in the feedback step. In the time-continuous limit, continuous weak quantum measurements of both position and momentum of the quantum oscillator result in a coherent state, whose coordinates diffuse in time. We relate the extractable work to the noise added by quadrature measurements, and present exact results for the work distribution at arbitrary finite time. Both protocols can achieve unit work conversion efficiency in principle.

quant-ph↗