Quantum States Can Remain Stable Despite Energy Loss, Researchers Find

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf, led by Guan-Yu Lai, have explored dissipation-resilient bound states in strongly coupled quantum systems, a development with potential implications for advancements in quantum technologies. Their research introduces an exactly solvable bosonic model to investigate the conditions under which these bound states can exist and validates these findings through a weak-coupling treatment of a supersystem. This supersystem incorporates multiple reaction coordinates, each representing a small energy interval within the reservoir’s spectral function, providing a complementary analytical approach. The analysis demonstrates that the stability of a bound state is critically dependent on its energy being positioned within a band gap of the reservoir, and while interactions inherently limit the bound state’s lifetime, increasing the coupling strength between the system and the reservoir can effectively prolong its coherence.

Indefinite bound state lifetimes achieved via engineered reservoir band gaps

Traditionally, bound state lifetimes in weakly interacting quantum systems have been limited to timescales on the order of fractions of a second. However, this new research indicates that these lifetimes can be extended indefinitely through strong coupling to reservoirs exhibiting band gaps. This represents a significant step towards overcoming a fundamental challenge in quantum information processing: maintaining quantum coherence in open systems. Environmental interactions typically induce rapid decoherence, destroying the delicate quantum states necessary for computation and communication. Engineering the energy structure of the reservoir, specifically, creating band gaps, provides a pathway to circumvent this limitation, as demonstrated by Lai, Queißer, and Schaller at the Helmholtz-Zentrum Dresden-Rossendorf. A band gap represents a range of energies that the reservoir is unable to accept, effectively shielding the bound state from energy exchange and thus preserving its coherence. The bosonic model employed utilises harmonic oscillators to represent both the quantum system and the reservoir, allowing for analytical solutions and a clear understanding of the underlying physics. The strength of the coupling between the system and reservoir is a crucial parameter, influencing both the formation and longevity of the bound state.

Their work, utilising an exactly solvable bosonic model, revealed that stability hinges on the bound state’s energy residing within the reservoir’s band gap, a region where the reservoir cannot accept energy. This is because the absence of available states at that energy prevents the bound state from decaying via energy transfer to the reservoir. Further analysis revealed that even with weak interactions present, bound state lifetime can be prolonged by increasing the strength of the system-reservoir coupling. This suggests that stronger interactions, counterintuitively, can enhance stability by effectively ‘trapping’ the bound state. Investigations into multiple band gaps broadened the understanding of these durable states, showing a more nuanced relationship between reservoir structure and bound state longevity. The presence of multiple, well-defined band gaps can provide multiple ‘safe havens’ for the bound state, further enhancing its resilience. However, the current findings do not yet detail how easily these principles translate to complex, many-body systems, nor address the significant engineering challenges required to create reservoirs with precisely tailored band structures for practical quantum technologies. At the Helmholtz-Zentrum Dresden-Rossendorf, scientists reproduced these findings with a ‘supersystem’ approach, mapping reservoir energy intervals to reaction coordinates; this confirmed the initial observation that stability arises when the bound state’s energy resides within the band gap. This supersystem approach allows for a perturbative treatment of the system-reservoir interaction, providing a complementary perspective to the exactly solvable model. The reaction coordinates effectively represent the degrees of freedom of the reservoir that are relevant to the interaction with the quantum system.

Stabilising quantum information through interactions with energy-gapped materials

Increasingly, researchers are focused on utilising dissipation-resilient bound states as fundamental building blocks for future quantum devices, offering a potential route to overcome the limitations of fragile quantum information. The ability to maintain coherence for extended periods is paramount for performing complex quantum computations and transmitting quantum information over long distances. Understanding these states remains vital, even though the current theoretical framework relies on extending the initial quantum system with representations of the surrounding reservoir’s energy levels, introducing a complexity that may hinder application to more disordered or realistic environments. This extension, while necessary for accurately modelling the system-reservoir interaction, increases the dimensionality of the problem and the computational cost of solving it. These states emerge when quantum systems interact strongly with materials possessing energy gaps, suggesting a pathway towards stable quantum information storage despite the mathematical complexity of modelling these ‘supersystems’. The choice of materials with appropriate band gap structures is therefore crucial for realising these dissipation-resilient bound states. A surprising result is that quantum bound states can remain stable even when strongly interacting with their environment, given the usual tendency of such states to lose coherence. This counterintuitive behaviour is a direct consequence of the engineered reservoir structure, which prevents the usual decay pathways. These ‘dissipation-resilient’ states emerge when a quantum system connects with a ‘reservoir’ that contains band gaps, ranges of energy the reservoir cannot accept. Harmonic oscillators were used to model this interaction, extending it to a ‘supersystem’ incorporating the reservoir’s energy levels; scientists demonstrated that a bound state’s stability depends on its energy residing within one of these band gaps, offering insights into the mechanisms underpinning this durability. The 01 value, representing a specific parameter within the model, highlights the sensitivity of the bound state to the reservoir’s characteristics. Further research will need to explore the robustness of these states to imperfections and variations in the reservoir structure.

The research demonstrated that dissipation-resilient bound states can exist even when quantum systems strongly interact with their environment. This stability arises because the system couples with a ‘reservoir’ material containing band gaps, effectively shielding the quantum state from decay. Scientists modelled this interaction using harmonic oscillators and a ‘supersystem’ approach, finding that the bound state’s energy must lie within a band gap to remain stable. The study highlights the importance of reservoir characteristics, as indicated by the 0.1 parameter, and the authors intend to investigate the impact of imperfections on these states.