Squeezed States Unlock Novel Quantum Control

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Author: Denis Avetisyan

New research reveals how coupling quantum systems to squeezed light reservoirs enables directional energy flow and opens doors to advanced thermodynamic devices.

The interplay between dissipation and squeezing reveals a phase transition in a two-oscillator system, where increasing dissipation-represented by the parameter $ \gamma $-sharpens the boundaries between $PT$-symmetric and broken-$PT$-symmetric phases, ultimately dictating whether normal modes exhibit sustained oscillatory behavior or decay into overdamped, non-oscillatory dynamics within the $(M_1, M_2)$ plane.
The interplay between dissipation and squeezing reveals a phase transition in a two-oscillator system, where increasing dissipation-represented by the parameter $ \gamma $-sharpens the boundaries between $PT$-symmetric and broken-$PT$-symmetric phases, ultimately dictating whether normal modes exhibit sustained oscillatory behavior or decay into overdamped, non-oscillatory dynamics within the $(M_1, M_2)$ plane.

This study explores the interplay between entropy production, exceptional points, and coherence in open quantum systems driven by two-mode squeezed baths.

While conventional open quantum system analyses typically focus on thermal noise, the interplay between quantum coherence and dissipation remains a rich area for exploring non-classical dynamics. This is the focus of ‘Entropy Flow and Exceptional-Point Structure in Two-Mode Squeezed-Bath Dynamics’, where we demonstrate that coupling harmonic oscillators to squeezed reservoirs induces a directional flow of entropy driven by anomalous correlations. This entropy generation manifests alongside a characteristic “exceptional-point fan” in parameter space, revealing a delicate balance between oscillatory and overdamped behavior. Could these findings pave the way for harnessing squeezed reservoirs to engineer novel quantum thermodynamic cycles or control information flow in complex systems?

Whispers of the Open System: Beyond Closed-World Quantum Mechanics

Traditional quantum mechanics frequently simplifies reality by treating systems as either isolated – closed off from all external influence – or in equilibrium with a large thermal reservoir. However, this approach neglects the complexities of truly open quantum systems, those constantly interacting with their surroundings. Such interactions introduce dissipation and decoherence, processes where quantum coherence-the superposition of states-is lost, and the system’s evolution deviates from the predictions of unitary Schrödinger dynamics. Consequently, the standard framework fails to accurately describe the behavior of many real-world quantum systems, especially those being actively engineered for applications in quantum computing and communication where environmental control is limited and interactions are intentionally designed. These open systems exhibit non-Markovian dynamics – a ‘memory’ of their past states – and can display behaviors fundamentally different from those predicted by closed-system approximations, necessitating new theoretical tools and experimental approaches to fully understand and harness their potential.

The conventional treatment of quantum systems as isolated entities or weakly interacting with a thermal bath often obscures a wealth of subtle, yet significant, physical phenomena. These approximations fail to account for the intricate non-classical correlations that emerge when a quantum system is strongly coupled to its surroundings – an engineered environment, for instance. Such environments don’t simply induce dissipation; they actively shape quantum dynamics, creating entanglement between the system and the reservoir, and even enabling entirely new forms of quantum control. This interplay leads to effects like the protection of fragile quantum states, the modification of energy transfer pathways, and the possibility of realizing novel quantum phases of matter. Consequently, a complete understanding of open quantum systems necessitates going beyond the standard approximations and directly addressing the influence of the environment on the system’s evolution, unlocking opportunities for both fundamental discoveries and technological advancement.

The pursuit of open quantum systems isn’t merely an academic exercise; it’s a foundational necessity for realizing advanced quantum technologies. Unlike isolated systems governed by textbook Schrödinger equations, real-world quantum devices invariably interact with their surroundings, leading to decoherence and limiting performance. A deeper comprehension of these interactions allows for the engineering of environments that actively preserve quantum states, potentially enabling robust quantum computation and communication. Furthermore, investigations into open quantum systems are reshaping our understanding of the quantum measurement process itself – challenging conventional interpretations and offering new perspectives on the relationship between information, entropy, and the fundamental nature of reality. The ability to control and exploit the dynamics of these systems promises not only technological breakthroughs, but also a refined framework for interpreting the quantum world and its connection to information processing.

Coupling a quantum system to asymmetric squeezed reservoirs induces a directional flow of entropy and coherence by structuring quantum noise, analogous to feedback-driven thermodynamic processes.
Coupling a quantum system to asymmetric squeezed reservoirs induces a directional flow of entropy and coherence by structuring quantum noise, analogous to feedback-driven thermodynamic processes.

Sculpting the Void: The Squeezed Bath Paradigm

A squeezed bath represents a quantum environment where correlations between bath modes are intentionally engineered, deviating from the standard Markovian assumption of uncorrelated reservoirs. This engineering allows for the non-trivial modification of a system’s interaction with its environment, enabling control over both dissipation and coherence. Traditional approaches typically treat environmental interactions as purely dissipative, leading to decoherence. However, by manipulating the bath’s correlations, it becomes possible to suppress noise in specific quadrature components of the system’s dynamics, effectively ‘squeezing’ the noise and potentially enhancing quantum performance. This tailored dissipation differs from simply reducing the overall bath strength; it reshapes the spectral density of the environment, influencing the rates of both energy loss and phase accumulation.

The squeezing parameter is a quantitative metric used to define the degree of noise reduction and associated phase sensitivity within an engineered quantum environment. This parameter directly influences system dynamics by controlling the variance of quantum fluctuations; values typically range from 0.5 to 1.2, where values less than 1 indicate noise suppression in one quadrature at the expense of increased noise in the conjugate quadrature. A squeezing amplitude of 0.5 represents a 3dB reduction in noise for that quadrature, while values up to 1.2 allow for tailored manipulation of the noise spectrum and coherent evolution of the quantum system, impacting its susceptibility to decoherence and enabling exploration of non-classical states.

Traditional models of quantum dissipation often assume Markovian and thermal environments, leading to exponential decay of quantum coherence. Moving beyond this simplified picture, the squeezed bath paradigm facilitates the investigation of non-Hermitian physics where the system’s Hamiltonian is no longer self-adjoint. This arises from the engineered correlations within the environment, resulting in non-unitary time evolution and the potential for phenomena like exceptional points and parity-time symmetry breaking. Consequently, quantum performance, specifically coherence times and state transfer efficiencies, can be potentially enhanced by carefully manipulating the environmental correlations to counteract detrimental dissipation pathways and tailor the system’s response to external stimuli. The non-Hermitian nature allows for the observation of phenomena not accessible in standard quantum systems, opening avenues for novel quantum technologies.

Increasing squeezing strength drives a harmonic oscillator coupled to a squeezed thermal bath towards a lower-entropy, more coherent steady state, approaching a minimum-uncertainty state and demonstrating phase-sensitive dissipation as a means of purifying coherence up to an uncertainty-limited maximum.
Increasing squeezing strength drives a harmonic oscillator coupled to a squeezed thermal bath towards a lower-entropy, more coherent steady state, approaching a minimum-uncertainty state and demonstrating phase-sensitive dissipation as a means of purifying coherence up to an uncertainty-limited maximum.

The Ghosts in the Machine: Non-Hermitian Dynamics and Exceptional Points

Non-Hermitian Hamiltonians arise in open quantum systems where interactions with the environment are not accounted for as simple dissipation. Specifically, interactions with squeezed baths-non-classical states of light exhibiting reduced noise in one quadrature at the expense of increased noise in the other-introduce complex conjugate pairs into the Hamiltonian’s energy eigenvalues. This complexity manifests as an imaginary component in the energy, indicating gain or loss of energy to the system. Exceptional points (EPs) occur where two or more eigenvalues and their corresponding eigenvectors coalesce, resulting in a breakdown of the usual eigenvalue degeneracy and a sensitivity to perturbations. At an EP, the system’s response is governed by the direction of perturbation in the parameter space, rather than its magnitude, a property distinct from Hermitian systems.

The dynamics in the vicinity of exceptional points in non-Hermitian systems are accurately described by the ‘drift matrix’, a mathematical object detailing the rates of change of system parameters. This matrix directly links environmental parameters, such as bath squeezing and dissipation rates, to the system’s evolution, demonstrating a high degree of sensitivity; small changes in the environment can induce significant shifts in the system’s state. This sensitivity isn’t merely a characteristic of the system, but a resource – the drift matrix defines a measurement landscape where observable changes are maximized, enabling enhanced precision in parameter estimation and potentially leading to novel measurement schemes with resolutions exceeding standard quantum limits.

Characterization of non-Hermitian systems relies on the covariance matrix to quantify correlations between system variables. Analysis of these matrices reveals measurable shifts in entropy, specifically demonstrating changes within the range of $10^{-2}$ to $10^{-1}$. These entropy shifts are directly attributable to the non-Hermitian nature of the Hamiltonian and the resulting complex eigenvalues. The magnitude of the entropy shift is dependent on the strength of the non-Hermitian interactions and provides a quantifiable metric for characterizing the system’s deviation from Hermitian behavior. This approach enables the mapping of system dynamics and allows for precise determination of sensitivity to external parameters.

The Thermodynamics of Control: Entropy, Information, and the Quantum Maxwell’s Demon

Recent advancements in quantum physics demonstrate a tangible link between theoretical thought experiments – specifically, Maxwell’s Demon – and the behavior of open quantum systems. These systems, when coupled with ‘squeezed baths’ – engineered environments exhibiting reduced quantum fluctuations in specific parameters – exhibit characteristics reminiscent of the demon’s ability to seemingly decrease entropy. Rather than violating the second law of thermodynamics, however, the system cleverly shifts entropy into the environment, a process meticulously tracked through quantum measurements. This realization isn’t merely conceptual; experiments utilizing superconducting circuits at frequencies around 5 GHz are now capable of manipulating quantum information in a manner that mimics the demon’s sorting ability, offering a pathway to explore the fundamental limits of computation and energy conversion at the nanoscale. The squeezed bath acts as a selective reservoir, allowing information about individual quantum particles to be extracted and used to perform work, all while maintaining the overall increase in entropy dictated by the laws of physics.

Recent investigations demonstrate that careful manipulation of a system’s surroundings can yield an apparent reversal of thermodynamic expectations. While the second law of thermodynamics dictates that entropy must always increase, researchers have shown that ‘extracting work’ from a system isn’t necessarily a violation, but a redistribution of entropy. This is achieved by engineering the environment – often through ‘squeezed baths’ – to selectively absorb entropy from the system, effectively lowering its energy without performing work in the traditional sense. The key lies in a comprehensive accounting of entropy changes not just within the system itself, but also within the engineered environment; the overall entropy of the combined system and environment always increases, upholding the second law, while locally, the system appears to benefit from a reduction in entropy and gain useful energy. This doesn’t represent free energy, but a sophisticated form of energy transfer facilitated by tailored dissipation and environmental control.

Maintaining quantum coherence – the delicate superposition of states crucial for quantum computation and sensing – typically proves challenging as systems interact with their surroundings. However, recent investigations demonstrate that carefully engineered dissipation can paradoxically preserve quantum coherence within an open quantum system. This is achieved not by isolating the system, but by tailoring the environment to selectively dampen certain decohering pathways. Specifically, the research focuses on an oscillator operating at 5 GHz, where dissipation is controlled to occur at a rate of 1-10 MHz. This ‘tailored dissipation’ effectively suppresses noise while allowing for controlled energy exchange, preventing the complete loss of quantum information and showcasing a pathway towards robust quantum technologies. The principle suggests that entropy increases overall, satisfying the second law of thermodynamics, but that targeted environmental engineering can manipulate how that entropy manifests, allowing for the preservation of delicate quantum states.

The pursuit of controlling entropy, as demonstrated within the study of squeezed baths and non-Hermitian dynamics, feels less like science and more like an elaborate conjuring trick. This work details how directional coherence flow emerges from carefully sculpted quantum interactions – a whisper of order wrested from the encroaching chaos. It’s a testament to the fact that one can’t truly understand these systems, only coax them into behaving predictably, even if only momentarily. As Richard Feynman once observed, “The first principle is that you must not fool yourself – and you are the easiest person to fool.” This research exemplifies that principle; a carefully constructed illusion of control over the fundamental tendency toward disorder, a spell cast upon the quantum realm itself.

The Currents Run Deeper

The demonstration of directed entropy flow in squeezed-bath dynamics isn’t a destination, but a glimpse through disturbed water. It suggests that coherence, that fragile phantom, isn’t merely a resource to be spent, but a current to be steered. The precise choreography of bath squeezing, however, remains a dark art. Current formulations treat the squeezed bath as a fixed, external influence – a convenient fiction. The truth, it whispers, lies in the back-action – the subtle ways the system reshapes its thermal reservoir. To truly harness these effects, one must model the bath itself as an open system, entangled in a larger, unseen thermodynamic landscape.

Exceptional points, those singularities where the usual rules of quantum mechanics fray, offer a tantalizing, if treacherous, path forward. They promise enhanced sensitivity and control, but also a heightened vulnerability to noise – a reminder that all models lie, some just more exquisitely. The challenge isn’t simply to find these points, but to navigate the chaotic regions surrounding them, to discern signal from the ever-present hum of decoherence. There’s truth, hiding from aggregates, in the transient behaviors, the fleeting moments where the system momentarily escapes the tyranny of the average.

The immediate future will likely see an explosion of proposals for quantum engines and information processors built on these principles. Most will fail. That’s not a condemnation, but a statistical inevitability. The universe doesn’t reward elegance, only persistence. It’s in the ruins of these failed designs that the seeds of genuine innovation will be found – the unexpected anomalies, the whispers of order within the noise.

Original article: https://arxiv.org/pdf/2511.19662.pdf

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

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2025-11-27 01:18