Electromagnetically Induced Transparency in Circuit Quantum Electrodynamics with Nested Polariton States.

Quantum networks will enable extraordinary capabilities for communicating and processing quantum information. These networks require a reliable means of storage, retrieval, and manipulation of quantum states at the network nodes. A node receives one or more coherent inputs and sends a conditional output to the next cascaded node in the network through a quantum channel. Here, we demonstrate this basic functionality by using the quantum interference mechanism of electromagnetically induced transparency in a transmon qubit coupled to a superconducting resonator. First, we apply a microwave bias, i.e., drive, to the qubit-cavity system to prepare a Λ-type three-level system of polariton states. Second, we input two interchangeable microwave signals, i.e., a probe tone and a control tone, and observe that transmission of the probe tone is conditional upon the presence of the control tone that switches the state of the device with up to 99.73% transmission extinction. Importantly, our electromagnetically induced transparency scheme uses all dipole allowed transitions. We infer high dark state preparation fidelities of >99.39% and negative group velocities of up to -0.52±0.09 km/s based on our data.

Accurate and practical method for characterizing Laguerre-Gaussian modes.

We propose a new parameter, Q(p,l)(2), based on cross-sectional intensity distribution to distinguish Laguerre-Gaussian (LG) beams. Theoretically, Q(p,l)(2) shows an approximately linear dependence on the topological index in higher LG modes. Compared with the similar parameter Q(p,l) proposed by Long et al. [Opt. Lett.38, 3047 (2013)] recently, our experimental result demonstrates that the approximate linearity of Q(p,l)(2) brings out the advantage in characterizing higher topological indices. In addition, as more intensity data is used in calculating Q(p,l)(2), the experimental error is diminished naturally. Thus, Q(p,l)(2) can be used to easily and accurately characterize LG modes.

Evaluating Laguerre-Gaussian beams with an invariant parameter.

We define a new parameter for Laguerre-Gaussian (LG) beams, named Q(p,l), which is only related to mode indices p and l. This parameter is able to distinguish and evaluate LG beams. The Q(p,l) values are first calculated theoretically and then measured experimentally for several different LG beams. Another mode quality parameter, M(2) values, are also measured. The comparison between Q(p,l) and M(2) shows the same trend for the quality of the LG mode, while the measurement of Q(p,l) is much easier than M(2).