Researchers have made six lithium atoms effectively transparent to one another, a feat achieved by manipulating ultracold atomic collisions with a technique called Floquet engineering. Alexander Guthmann, Louisa Marie Kienesberger, and colleagues demonstrate the creation of a molecular state above the dissociation threshold by eliminating both elastic and inelastic coupling to the continuum at a critical parameter point. Full coupled-channel calculations identify this state as a Friedrich-Wintgen bound state in the continuum, demonstrating how quantum interference can control “openness” in quantum matter and enable the engineering of non-Hermitian Hamiltonians.
Floquet Engineering Creates Tunable Feshbach Resonances
The creation of a stable quantum state within an otherwise unstable energy range has been demonstrated using six lithium atoms, achieved by precisely manipulating two Feshbach resonances with Floquet engineering. Unlike typical bound states which are energetically isolated, this state exists within the continuum of energies where atoms would normally scatter and dissipate, a phenomenon enabled by destructive interference. Researchers report that this interference effectively localizes the state, preventing its decay and establishing a new method for controlling how quantum systems interact with their environment.
The team used strong magnetic-field modulation to generate additional, tunable Feshbach resonances, effectively tailoring the interactions between the lithium atoms. This approach allows for precise positioning and width adjustment of these resonances, a critical step in achieving the necessary interference conditions. The experimental setup involved loss spectroscopy measurements performed at cloud temperatures below 600 nanokelvin, minimizing broadening effects that could obscure the observation of the bound state.
Interference Stabilizes Bound States in Lithium Collisions
Localized bound states, typically found below the dissociation threshold, were experimentally created above that threshold through the precise manipulation of ultracold lithium collisions. This counterintuitive result stems from destructive interference engineered between two coupled Feshbach resonances, effectively isolating the state from environmental dissipation. The stabilization process relies on coherently coupling the two Feshbach resonances using Floquet engineering, a technique that allows for precise control over the system’s parameters at a critical point.
The researchers report that this interference-driven localization demonstrates a novel method for controlling “openness” in quantum matter, offering a pathway to isolate quantum systems from external influences. This is a departure from traditional bound states, which are inherently isolated by their energy level. Previous observations of bound states in the continuum, such as trapped light within the radiation continuum, required specific material properties or geometries. This work, however, achieves the same effect through purely quantum mechanical control of atomic interactions.
Feshbach Resonances Couple to Form Continuum States
This stabilization process, achieved through Floquet engineering, establishes a superposition decoupled from lossy channels, mirroring mechanisms seen in electromagnetically induced transparency and coherent population trapping. The underlying principle hinges on the coupling of two resonances to a common continuum, providing decay channels that mediate an effective interaction between them. This differs from scenarios involving discrete states coupled through an intermediate state; instead, the observed state arises from the resonances themselves influencing the continuum.
Full coupled-channel calculations confirm the observed state’s characteristics, aligning with theoretical predictions for Friedrich-Wintgen bound states. All scientific data supporting the findings is available via Zenodo, ensuring transparency and reproducibility of the results.
Friedrich-Wintgen BICs: A Theoretical Prediction
While BICs have been recognized as a general interference phenomenon and observed in various systems, a direct quantum-mechanical realization of this specific mechanism remained elusive until now. Earlier studies of autoionization and laser-induced continuum structure revealed narrow spectral features from interference, but lacked the precise control needed to confirm the underlying physics. Theoretical work established the framework for Friedrich-Wintgen BICs, yet identifying the specific control parameters to achieve tunability and observe the avoided crossing between two Feshbach resonances proved challenging.
Laser-based cold atom accelerators offered a potential route for observation, with predictions of a BIC in collisions of 87 rubidium atoms, and coherent optical coupling near a magnetic Feshbach resonance also proposed as a means of engineering the state. However, a direct observation had not been reported until this work, which uses an ultracold atomic gas of 6 lithium.
Quantum Interference Controls Open System Decay
The conventional expectation of environmental interaction driving dissipation does not universally hold; destructive interference can instead localize quantum states within a continuum. This work demonstrates the creation of an interference-stabilized state in ultracold collisions of lithium-6 atoms, a phenomenon where a quantum system resists typical decay pathways. By coherently coupling two tunable Feshbach resonances using Floquet engineering, researchers established a condition where the system’s openness, its susceptibility to environmental interaction, is actively controlled.
This control relies on the destructive interference between direct and continuum-mediated coupling pathways, effectively cancelling the decay of a resonance and forming a bound state in the continuum. Originally theorized by von Neumann and Wigner in 1929, these states represent a departure from the expectation that coupling to external continua always leads to dissipation.
Friedrich and Wintgen later proposed in 1985 that such a state could arise specifically from the interference of two Feshbach resonances, a prediction now experimentally verified. The observed stabilization is not merely a theoretical curiosity. It demonstrates a pathway for manipulating the interaction between a quantum system and its environment. This is particularly relevant as coupling to external continua is often viewed as a limitation in quantum technologies, driving decoherence and irreversibility.
Molecular Potential Defines Bound vs. Scattering States
The shape of a molecular potential dictates whether a quantum system binds or scatters, yet recent work demonstrates a decoupling mechanism altering this fundamental relationship. Traditionally, bound states exist only below a defined dissociation threshold, with energies above that point resulting in unbound, scattering states that can create resonances like those seen in Feshbach spectroscopy. These resonances, important in nuclear and ultracold atom physics, arise from finite-lifetime states mixing localized and scattering channels.
This research introduces a method for creating a bound state above the dissociation threshold, a Friedrich-Wintgen BIC, through the precise control of scattering. The team’s approach relies on inducing an avoided crossing between Feshbach resonances, a phenomenon initially theorized to give rise to these counterintuitive molecular states.
Unlike typical bound states, this BIC cannot be populated from scattering states, effectively eliminating resonant scattering signatures. “Although long-lived due to its decoupling from the continuum, it cannot be populated from scattering states,” explains the paper’s description of the resulting state. Experimental verification involved rf photoassociation spectroscopy, a technique used to probe the kinetic energy above the | a ⟩ -| b ⟩ threshold.
By analyzing the scattering continuum and molecular bound states, researchers confirmed the existence of the BIC at specific Floquet modulation frequencies, 55.95 MHz and 56.74 MHz, demonstrating a parameter point where the system’s openness is minimized. Extended Data shows that above the threshold, the numerically calculated inelastic scattering cross section is significantly reduced, while below the threshold, corresponding molecular bound states are clearly defined.
Non-Hermitian Model Explains BIC Formation
A minimal model captures the mechanism behind the creation of Friedrich-Wintgen bound states in the continuum, revealing how quantum interference dictates the system’s openness. Coefficients quantifying coupling to both entrance and Floquet-dressed continua demonstrate that coherently returned information is essential for interferometric BIC formation, a departure from conventional Markovian bath descriptions. A detailed derivation of the non-Hermitian effective Hamiltonian used in the analysis is available in the Methods section of the published work.
The model uses eigenvalues with real parts defining resonance positions and imaginary parts corresponding to resonance widths. An avoided crossing emerges as the detuning between energy levels changes, a critical feature in establishing the BIC. Full coupled-channel calculations validate the accuracy of the non-Hermitian two-level model, with extracted resonance positions and widths closely aligning with both computational and experimental data. Optimization procedures used to tailor the model to the specific system are detailed in the Methods section.
See today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals.
