Observation of quantum state collapse and revival due to the single-photon Kerr effect (1211.2228v1)

Abstract: Photons are ideal carriers for quantum information as they can have a long coherence time and can be transmitted over long distances. These properties are a consequence of their weak interactions within a nearly linear medium. To create and manipulate nonclassical states of light, however, one requires a strong, nonlinear interaction at the single photon level. One approach to generate suitable interactions is to couple photons to atoms, as in the strong coupling regime of cavity QED systems. In these systems, however, one only indirectly controls the quantum state of the light by manipulating the atoms. A direct photon-photon interaction occurs in so-called Kerr media, which typically induce only weak nonlinearity at the cost of significant loss. So far, it has not been possible to reach the single-photon Kerr regime, where the interaction strength between individual photons exceeds the loss rate. Here, using a 3D circuit QED architecture, we engineer an artificial Kerr medium which enters this regime and allows the observation of new quantum effects. We realize a Gedankenexperiment proposed by Yurke and Stoler, in which the collapse and revival of a coherent state can be observed. This time evolution is a consequence of the quantization of the light field in the cavity and the nonlinear interaction between individual photons. During this evolution non-classical superpositions of coherent states, i.e. multi-component Schroedinger cat states, are formed. We visualize this evolution by measuring the Husimi Q-function and confirm the non-classical properties of these transient states by Wigner tomography. The single-photon Kerr effect could be employed in QND measurement of photons, single photon generation, autonomous quantum feedback schemes and quantum logic operations.

• The paper demonstrates quantum state collapse and revival in a superconducting 3D circuit QED system using the single-photon Kerr effect.
• It utilizes high-resolution state tomography with Husimi Q and Wigner functions to confirm the production of multi-component Schrödinger cat states.
• The work paves the way for advances in quantum computing by enabling strong photon-photon interactions and potential quantum logic operations.

Quantum State Collapse and Revival Induced by the Single-Photon Kerr Effect

The paper presented by Kirchmair et al. explores the observation of quantum state collapse and revival enabled by the single-photon Kerr effect. It highlights significant advancements in the manipulation of nonclassical light states using engineered quantum systems, specifically addressing the limitation on direct photon-photon interactions in quantum optics. Employing 3D circuit quantum electrodynamics (QED), the researchers engineered an artificial Kerr medium that demonstrates strong photon interactions exceeding the decay rate, categorized as the single-photon Kerr regime.

Key Findings and Methodologies

The paper's primary achievement was the realization of a theoretical experiment proposed by Yurke and Stoler, observing the collapse and revival of a coherent quantum state within a Kerr medium. The authors utilized a superconducting "vertical" transmon qubit coupled to high-quality-factor 3D waveguide cavities. The engineered Kerr medium within these cavities allowed for a direct observation of coherent state dynamics not previously achievable.

A notable highlight of this work is the creation and coherent control of multi-component Schrödinger cat states, resulting from the nonlinear photon-photon interaction. This was effectively depicted via the Husimi Q-function and confirmed through Wigner tomography, ensuring the non-classical properties of the quantum states. The methodology involved a detailed experimental sequence manipulating the Kerr medium to create coherent state superpositions, followed by high-resolution state tomography to visualize the quantum phenomena.

Theoretical and Practical Implications

The results presented offer significant theoretical implications in quantum optics, particularly concerning the manipulation and measurement of quantum states at the single-photon level. The successful transition to the single-photon Kerr regime, where interaction strengths per photon exceed decay rates, facilitates multiple applications previously thought to be beyond reach. This includes quantum non-demolition measurements, single-photon generation, and potential quantum logic operations, all conducted via direct photon-photon interactions.

From a practical perspective, the engineered Kerr medium could serve as a fundamental building block for future quantum computing and communication systems. The ability to produce and control cat states furthers the potential for enhancement in quantum error correction schemes and continuous variable quantum information protocols, underscoring the vast potential for integrating such systems with superconducting circuits.

Future Directions

Considering the promising outcomes of this research, future developments could explore scaling these interactions for more complex quantum systems and integrating with other quantum technologies. Enhancing the robustness of the Kerr effect and minimizing photon decay rates would be critical for practical applications in quantum networks. These advancements might eventually facilitate more sophisticated quantum logic operations and the development of autonomous quantum feedback mechanisms.

In conclusion, the paper delivers substantial contributions to the field of quantum optics, specifically in the realms of photon interaction and state manipulation. The demonstration of quantum state collapse and revival due to the single-photon Kerr effect not only enriches the theoretical landscape but also sets a solid groundwork for future experimental and technological advancements in quantum information science.