Quantum Systems Stabilized by Energy Dissipation

A new method for stabilising complex quantum states, typically vulnerable to degradation through unwanted excitation, has been investigated by Lorenz Wanckel and André Eckardt at the Max Planck Institute for Physics and Astronomy. They employ a dissipative strategy using thermal baths to suppress ‘Floquet heating’ and guide a driven quantum system towards a steady state with a substantial occupation of its ground state. The approach circumvents limitations inherent in traditional Floquet engineering, offering a pathway to prepare and maintain gapped many-body phases, as verified through the Floquet-Born-Markov master equation applied to a strongly driven Bose-Hubbard chain. The research represents a key advance in controlling and harnessing driven quantum systems for potential applications in quantum technologies.

Dissipative coupling extends coherence beyond Floquet heating limitations in a Bose-Hubbard chain

Stabilisation times for effective ground states in a strongly driven Bose-Hubbard chain have increased to exceeding 1000 driving cycles, a significant improvement over the few cycles previously limited by Floquet heating. This threshold is important because previous limitations prevented sustained observation of many-body phenomena requiring long coherence times; the system was overheating too quickly to reveal its underlying physics. Researchers at Technische Universität Berlin have demonstrated a dissipative strategy, coupling a driven quantum system to a thermal bath, to simultaneously suppress unwanted energy gain and guide the system into a stable, non-equilibrium state.

Ground state stability in a driven Bose-Hubbard chain, a model system for studying interactions between atoms, now extends beyond 1000 driving cycles. Previously, these systems suffered from ‘Floquet heating’, where the driving force caused unwanted energy gain, limiting stability to just a few cycles and preventing detailed observation of complex quantum behaviours. The team at Technische Universität Berlin introduced a ‘dissipative strategy’, effectively connecting the quantum system to a ‘thermal bath’ to simultaneously reduce energy absorption and steer the system towards a stable state. This approach allowed for the creation of a non-equilibrium steady state, with a significant proportion of atoms occupying the desired ground state, despite generally non-thermal populations in excited states. Achieving practical applications, however, still requires overcoming challenges in controlling the thermal bath and scaling these systems to larger, more complex configurations.

Dissipative stabilisation of driven quantum systems via engineered thermal environments

The technique proved central to coupling the driven quantum system to a thermal bath; this ‘bath’ acts as a controlled energy sink, absorbing excess energy and preventing the build-up known as Floquet heating, similar to repeatedly pushing a swing slightly off-beat, causing ever-increasing, ultimately unsustainable, motion. This dissipation isn’t merely about damping unwanted energy, however, but actively guides the system towards a stable, non-equilibrium state where the effective ground state is strongly populated. By carefully tuning the properties of this thermal environment, scientists could sculpt the quantum system’s behaviour, stabilising fragile states that would otherwise quickly decay due to interactions and continuous driving. A dissipative strategy was employed to stabilise effective quantum ground states, overcoming limitations of previous Floquet engineering techniques which suffered from unwanted resonant excitation and ‘Floquet heating’. This approach couples the driven quantum system to a thermal bath, carefully controlling its properties to suppress heating and guide the system towards a stable, non-equilibrium state. The team modelled this using the Floquet-Born-Markov master equation, specifically examining a strongly driven Bose-Hubbard chain exhibiting a gapped Mott-insulator ground state; parameters such as the system-bath coupling strength (γ) and a cutoff energy (Ecut) were used to define the thermal environment.

Active steering of quantum systems surpasses simple decay mitigation

Stabilising quantum states against degradation is important for building future technologies reliant on their delicate properties. A method has now been demonstrated for not only suppressing energy build-up, known as Floquet heating, but also actively steering quantum systems towards a stable condition, a significant step beyond mitigating decay. The team acknowledges that while their approach successfully populates the ground state, it doesn’t achieve perfect fidelity, leaving residual, non-thermal excitations present.

Nevertheless, the presence of residual excitations does not invalidate this progress. Demonstrating active steering towards a stable quantum state represents a considerable advancement over passive decay mitigation, even if achieving perfect stabilisation remains a challenge. This technique offers a pathway to prolong coherence times, key for practical quantum devices, even if it doesn’t yet eliminate all sources of error. Controlling these systems, rather than observing their degradation, unlocks new possibilities for manipulating quantum information.

An active technique to stabilise quantum systems has been demonstrated, steering them towards a ground state despite inherent energy build-up. This dissipative strategy utilises a thermal bath to suppress unwanted excitation and promote stability, though residual excitations persist. This research establishes a method for sustaining driven quantum systems by actively managing energy flow, rather than minimising energy gain. By coupling a quantum system to a carefully tuned ‘thermal bath’, scientists have demonstrated a pathway to both suppress unwanted excitation, termed Floquet heating, and guide the system towards a stable, non-equilibrium state. This dissipative strategy allows for the prolonged occupation of an effective ground state, exceeding previous limits imposed by rapid energy accumulation; the Bose-Hubbard chain, a model for interacting atoms, saw stability extended beyond 1000 driving cycles.

By actively controlling energy flow, researchers have demonstrated a method for sustaining driven quantum systems and stabilising them against degradation. This strategy utilises a thermal bath to suppress unwanted excitation, known as Floquet heating, and guide the system towards a stable, non-equilibrium state with a large occupation of the effective ground state. The team showed this approach extended stability beyond 1000 driving cycles in a Bose-Hubbard chain, representing a considerable advancement over simply mitigating decay. While residual excitations remain, this work establishes a pathway for prolonging coherence times in quantum systems.