Quantum Systems Reverse Cooling Rules in Unexpected Symmetry Shift

Scientists at Coventry University have investigated a new quantum phenomenon analogous to the classical Mpemba effect, where a hotter system can cool faster than a colder one. Liv Hammer and colleagues demonstrate this effect within open quantum many-body systems, revealing that a more asymmetric quantum state can restore symmetry more rapidly than a less asymmetric one. The findings extend understanding of quantum Mpemba effects beyond closed systems and establish symmetry-breaking phase transitions as a potential means of observing and controlling unusual relaxation behaviours.

Disorder accelerates symmetry restoration via quantum asymmetry in open systems

Asymmetry, defined as a measure of imbalance within an atomic arrangement, now demonstrates the capacity to restore symmetry in open quantum systems up to 15% faster when initiating from a more disordered state compared to a more ordered one. This observation challenges conventional expectations regarding the relationship between order, disorder, and relaxation dynamics. Open systems, characterised by constant energy exchange with their surroundings, exhibit unique behaviours, surpassing previous limitations confined to closed systems where symmetry restoration relied on reaching thermal equilibrium. The concept of open systems is crucial, as most real-world quantum systems are not perfectly isolated; they inevitably interact with an external environment, leading to dissipation and decoherence. The open Dicke model, a system of N two-level atoms interacting with light, served as the platform for demonstrating this quantum Mpemba effect, establishing a new platform for controlling anomalous relaxation. This is akin to observing a complex quantum system ‘bounce’ back to order in an unconventional way, defying intuitive expectations based on classical physics. The Dicke model is particularly relevant due to its widespread use in modelling light-matter interactions, with applications in areas such as cavity quantum electrodynamics and quantum optics.

An imbalance between the rates at which asymmetry increases and decreases within a subsystem of the open Dicke model mirrors observations of unequal heating and cooling rates in classical systems. Specifically, the system’s capacity to increase its asymmetry can differ significantly from its capacity to decrease it, depending on the initial conditions. This asymmetry in the dynamics is not simply a consequence of energy dissipation; it arises from the specific interactions within the system and the way they influence the evolution of the quantum state. Quantifying this asymmetry using the relative entropy of asymmetry, a measure of imbalance within the atomic arrangement based on information theory, reveals non-monotonic behaviour where maximal symmetry breaking precedes restoration. This means that the system first becomes increasingly disordered before unexpectedly transitioning towards a more ordered state. The system’s dynamics, meticulously tracked through numerical simulations, showed that a quantum Mpemba effect emerges as a direct consequence of this non-monotonic evolution, extending the concept to open quantum many-body systems exhibiting symmetry-breaking phase transitions. These phase transitions represent qualitative changes in the system’s behaviour, analogous to the freezing of water or the boiling of a liquid, but occurring at the quantum level.

Disorder accelerating quantum symmetry restoration mirrors the Mpemba effect

Our understanding of how quantum systems settle into order is expanding, revealing surprising behaviours in those that constantly interact with their environment. A quantum effect has been observed where disorder can, counterintuitively, lead to faster symmetry restoration, a phenomenon akin to the classical Mpemba effect but operating on quantum principles. The classical Mpemba effect, famously observed with water, remains a subject of debate, with proposed explanations ranging from dissolved gases to convection currents. The quantum analogue, however, offers a fundamentally different perspective, rooted in the principles of quantum mechanics and the behaviour of many-body systems. However, the analysis remains confined to a specific model, an arrangement of atoms and light, and further research is needed to determine if these findings hold true across a wider range of quantum systems. Investigating different types of quantum systems, such as those based on superconducting circuits or trapped ions, will be crucial for establishing the generality of this effect.

Unlike previous studies of isolated quantum systems, this work extends the quantum Mpemba effect to open quantum systems, those interacting with their surroundings. This is a significant advancement, as it brings the theoretical framework closer to the reality of experimental implementations. The effect occurs alongside symmetry breaking, a change where identical elements become distinct, and revealed an imbalance in how asymmetry increases or decreases within the system; this describes the degree of imbalance within the atomic arrangement. This symmetry breaking is often associated with phase transitions, where the system undergoes a qualitative change in its properties. This is key because it offers a new avenue for potentially controlling relaxation phenomena within complex quantum systems, with implications for future technologies. For example, understanding and manipulating these relaxation processes could be crucial for developing more efficient quantum sensors, quantum memories, and even quantum computers. The ability to accelerate symmetry restoration could potentially be harnessed to speed up certain quantum computations or to enhance the performance of quantum devices. Further investigation into the precise mechanisms underlying this effect could lead to the development of novel control strategies for manipulating quantum systems.

The research highlights the importance of considering non-equilibrium dynamics and the role of asymmetry in understanding the behaviour of complex quantum systems. The observed 15% acceleration in symmetry restoration, while seemingly modest, represents a significant departure from conventional expectations and opens up new possibilities for exploring and controlling quantum phenomena. Future work will focus on exploring the limits of this effect, investigating its dependence on system parameters, and extending the analysis to more complex and realistic quantum systems.

Researchers demonstrated a quantum Mpemba effect in an open quantum many-body system, observing that a more asymmetric state restores symmetry faster than a less asymmetric one. This finding extends the understanding of this effect beyond closed systems and into those interacting with their environment, bringing theory closer to practical experimentation. The study revealed a non-monotonic evolution of asymmetry alongside symmetry breaking, and showed an imbalance in how asymmetry increases or decreases within the system. Authors intend to explore the limits of this effect and investigate its dependence on system parameters.