Nanophotonic quantum phase switch with a single atom (1404.5615v1)

Abstract: In analogy to transistors in classical electronic circuits, a quantum optical switch is an important element of quantum circuits and quantum networks. Operated at the fundamental limit where a single quantum of light or matter controls another field or material system, it may enable fascinating applications such as long-distance quantum communication, distributed quantum information processing and metrology, and the exploration of novel quantum states of matter. Here, by strongly coupling a photon to a single atom trapped in the near field of a nanoscale photonic crystal cavity, we realize a system where a single atom switches the phase of a photon, and a single photon modifies the atom's phase. We experimentally demonstrate an atom-induced optical phase shift that is nonlinear at the two-photon level, a photon number router that separates individual photons and photon pairs into different output modes, and a single-photon switch where a single "gate" photon controls the propagation of a subsequent probe field. These techniques pave the way towards integrated quantum nanophotonic networks involving multiple atomic nodes connected by guided light.

• The paper demonstrates a single-atom phase switch that uses cavity QED to achieve a near-π optical phase shift at the two-photon level.
• The experiment employs a rubidium atom trapped 200 nm from a photonic crystal cavity to modulate photon routing via coherent control.
• The results reveal scalable potential for quantum photonic circuits, enabling efficient quantum information processing and enhanced photon-atom interactions.

Nanophotonic Quantum Phase Switch with a Single Atom

The paper "Nanophotonic Quantum Phase Switch with a Single Atom" explores a pivotal development in the area of quantum optics and nanophotonics. It investigates the potential use of a single atom as a quantum phase switch within a photonic crystal cavity. This paper holds significant implications for the advancement of quantum circuits and networks, paralleling the role of transistors in classical electronic circuits.

Experimental Setup and Methodology

At the core of this research is the interaction facilitated by cavity quantum electrodynamics (cavity QED), wherein a photon is strongly coupled with a single atom trapped in the near field of a photonic crystal (PC) cavity. The experimental configuration involves trapping a single rubidium ($^{87}$Rb) atom approximately 200 nm from the surface of the cavity. This is achieved by forming an optical lattice through the interference of an optical tweezer and its reflection from the PC cavity. The cavity is one-sided with optical signals coupling in and out through a single port, enabling robust control over photon-atom interactions.

The paper demonstrates several noteworthy phenomena: an atom-induced optical phase shift nonlinear at the two-photon level, an efficient photon number router, and a single-photon switch where a single gate photon modulates the propagation of subsequent photons. These results are reflective of an achieved single-atom cooperativity ($\eta$) of 7.7, evidenced by significant modifications in the atomic excited-state lifetime in the cavity.

Results and Observations

A significant milestone achieved is the manipulation of photons by a single atom to switch phases, which is quantified by the phase shift of reflected photons. Experimentally, the reflection amplitude changes depending on whether the cavity is coupled with an atom, illustrating a phase shift of approximately $\pi$. This phase switching functionality is further demonstrated through coherent manipulation involving atomic superposition states, where the state of a rubidium atom governs the routing of photons into distinct output modes.

The system's saturation properties and non-linearity effects are investigated, with anti-bunching and bunching behaviors in photon streams highlighting the device's potential as a photon router. The research outlines that even in regimes approaching atomic saturation, the device demonstrates coherent phase switching, reinforcing its feasibility for quantum information applications.

Implications and Future Prospects

The successful implementation of a quantum optical switch using a single atom in a nanophotonic setup opens up practical pathways for developing integrated quantum photonic circuits. The findings suggest potential for broad applications such as quantum communication over extensive ranges, distributed quantum processing, and novel quantum state explorations.

Practically, the scalable nature of nanophotonic systems, combined with the robustness offered by atomic coherence times, provides a compelling avenue toward creating complex quantum networks with multiple atomic nodes. The prospect of high-fidelity atom-photon entanglement generation and non-demolition optical measurements further accentuates the potential for advancements in quantum optics and information technology.

Conclusion

The research presented establishes a foundational experiment in nanoscale quantum phase manipulation with implications for the future development of quantum technologies. The integration of photonic and atomic systems at such scales represents a significant step toward realizing more advanced quantum computing and communication systems. The continued development in this field promises even greater breakthroughs, paving the way for more resilient and scalable quantum technologies.