Project A12

Quantum Correlations in Cavity-Waveguide Networks

PI: Dr. M. Hartmann

Solid state photonic devices offer a high degree of scalability and integrability. Cavities can have very low photon losses and couple very strongly to optical emitters. In recent years great advances have been achieved in efficiently coupling two or more cavities either directly or via a waveguide. This development gives rise to networks of coupled cavities and waveguides that offer great possibilities for implementing functional devices and generating and manipulating quantum correlations. Building on these possibilities, the project A12 "Quantum Correlations in Cavity-Waveguide Networks" will develop the theory for designing functional quantum devices and generating quantum correlations in solid state based cavity-waveguide networks. It will thereby focus on implementations in photonic band gap structures that interact with quantum dots and superconducting transmission lines that couple to superconducting qubits. These structures which are experimentally explored within the collaborative research center 631 in projects A3, A8 and B3 are ideally suited for pursuing the approaches we investigate.

In our research, we will explore paradigm examples of small networks, where two or three cavities are either directly coupled or linked via waveguides. For these structures we will design approaches to the optimal transfer of quantum states from one network node to another one, devise schemes for generating entangled output fields at two or more network ports and develop a strategy for implementing a quantum gate for photons. These approaches will be complemented by a theoretical analysis of the stationary state regimes and phases of larger networks of coupled cavities in a driven dissipative scenario and a development of tools for studying open quantum many-body systems with strong correlations.

Besides intensive collaborations with the experimental research groups engaged in projects A3, A8 and B3 we work together with theory projects A5 and A10 on topics that involve superconducting qubits and their interactions with circuit cavities, with project B7 on the development of tools to study open quantum many-body systems with strong correlations that will be applied in the analysis of steady states in driven dissipative cavity networks and with project A9 on the design of optimal pulses for state transfer and driving multi-photon transitions.

For more information, see


figure 1: Possible setups for Y-shaped junctions in photonic band gap structures. The areas of light emitter coupling are depicted as a red disks and the input and output fields as red arrows in the photonic crystal plane. Left: A single cavity that couples to one input and two output waveguides. The dot is tuneable via an applied DC-voltage. Right: Three cavities coupled to one input and two output waveguides. Here, either only the central cavity or all three cavities couple to quantum dots. DC-electrodes are omitted in this sketch.


figure 2: Envisioned implementation of two superconducting resonators that interact via two coupled qubits. This device will allow for strong cross phase modulation of individual photons and can be employed in order to realize a quantum gate for microwave photons.