Confining the state of light to a quantum manifold by engineered two-photon loss

Abstract: Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum states, consisting of all coherent superpositions of multiple stable steady states. We have experimentally confined the state of a harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In particular, we have observed a Schrodinger cat state spontaneously squeeze out of vacuum, before decaying into a classical mixture. This was accomplished by designing a superconducting microwave resonator whose coupling to a cold bath is dominated by photon pair exchange. This experiment opens new avenues in the fields of nonlinear quantum optics and quantum information, where systems with multi-dimensional steady state manifolds can be used as error corrected logical qubits.

• The paper demonstrates a method to confine superpositions of coherent states into a quantum manifold via engineered two-photon loss.
• It details an experimental cQED setup with a superconducting microwave resonator and Josephson junction, achieving balanced photon decay and extraction rates.
• The approach offers potential for quantum error correction by harnessing engineered dissipative processes to stabilize logical qubits.

Summary of "Confining the state of light to a quantum manifold by engineered two-photon loss"

This paper investigates the confinement of quantum states in a harmonic oscillator using engineered two-photon loss. The authors present an experimental realization using a superconducting microwave resonator setup, which achieves confinement to a quantum manifold defined by two coherent states of opposite phases. This approach demonstrates an innovative method for stabilizing quantum states by actively utilizing dissipative processes rather than avoiding them.

Core Contributions

The primary contribution of this research is the demonstration of a quantum manifold where the system dynamically stabilizes to a subspace of interest, confining superpositions of coherent states. The authors achieve this in a circuit quantum electrodynamics (cQED) architecture, employing a Josephson junction to induce strong non-linear interactions, critical for establishing the required two-photon exchange mechanisms with an auxiliary environment.

Detailed Technical Achievements:

• Implementation of a superconducting microwave resonator with low single-photon dissipation, allowing the observation of coherent quantum phenomena.
• Engineering of a dynamic process based on a dissipative environment, leading to stabilization of a two-dimensional manifold spanned by coherent states.
• Utilization of Wigner tomography to directly observe dynamic state transformations, starting from a vacuum state evolving into Schrödinger cat states before settling into classical mixtures.

Numerical Results and Observations

One of the striking technical feats is achieving a photon pair extraction rate equal to the single photon decay rate, thus maintaining the coherence required for observing transient cat states. The precision in frequency matching and manipulation of a non-linear oscillator was substantiated by clear experimental observations and corroborative numerical simulations.

Implications and Future Directions

The theoretical implications of this work extend significantly into quantum error correction realms, where such manifolds can potentially host logical qubits protected against certain error types. This experiment lays the groundwork for a paradigm shift in quantum computation strategies, leveraging engineered dissipation for error correction rather than traditional isolation-based techniques.

Future research may explore expanding the dimensionality of stabilizable manifolds or integrating more sophisticated error syndrome detection methods to enhance logical qubit fidelity. Additionally, the balance between single-photon and two-photon dissipation rates can be optimized further to extend coherence times and harness the full potential of dissipation engineering in quantum systems.