Fault tolerant quantum information processing requires specific nonlinear interactions acting within the Hilbert space of the physical system that implements a logical qubit. The requiredorder of nonlinearity is often not directly available in the natural interactions of the system. Here, we experimentally demonstrate a route to obtain higher-order nonlinearities by combining more easily available lower-order nonlinear processes, using a generalization of the Raman transitions. In particular, we demonstrate a Raman-assisted transformation of four photons of a high-Q superconducting cavity into two excitations of a superconducting transmon mode and vice versa. The resulting six-quanta process is obtained by cascading two fourth-order nonlinear processes through a virtual state. This process is a key step towards hardware efficient quantum error correction using Schrödinger cat-states.
In quantum mechanics, continuously measuring an observable steers the system into one eigenstate of that observable. This property has interesting and useful consequences when the observableis a joint property of two remotely separated qubits. In particular, if the measurement of the two-qubit joint observable is performed in a way that is blind to single-qubit information, quantum back-action generates correlation of the discord type even if the measurement is weak and inefficient. We demonstrate the ability to generate these quantum correlations in a circuit-QED setup by performing a weak joint readout of two remote, non-interacting, superconducting transmon qubits using the two non-degenerate modes of a Josephson Parametric Converter (JPC). Single-qubit information is erased from the output in the limit of large gain and with properly tailored cavity drive pulses. Our results of the measurement of discord are in quantitative agreement with theoretical predictions, and demonstrate the utility of the JPC as a which-qubit information eraser.
Quantum jumps of a qubit are usually observed between its energy eigenstates, also known as its longitudinal pseudo-spin component. Is it possible, instead, to observe quantum jumpsbetween the transverse superpositions of these eigenstates? We answer positively by presenting the first continuous quantum nondemolition measurement of the transverse component of an individual qubit. In a circuit QED system irradiated by two pump tones, we engineer an effective Hamiltonian whose eigenstates are the transverse qubit states, and a dispersive measurement of the corresponding operator. Such transverse component measurements are a useful tool in the driven-dissipative operation engineering toolbox, which is central to quantum simulation and quantum error correction.
Experimental quantum information processing with superconducting circuits is rapidly advancing, driven by innovation in two classes of devices, one involving planar micro-fabricated(2D) resonators, and the other involving machined three-dimensional (3D) cavities. We demonstrate that circuit quantum electrodynamics (cQED), which is based on the interaction of low-loss resonators and qubits, can be implemented in a multilayer superconducting structure, which combines 2D and 3D advantages, hence its nickname „2.5.“ We employ standard micro-fabrication techniques to pattern each layer, and rely on a vacuum gap between the layers to store the electromagnetic energy. Planar superconducting qubits are lithographically defined as an aperture in a conducting boundary of multilayer resonators, rather than as a separate metallic structure on an insulating substrate. In order to demonstrate the potential of these design principles, we implemented an integrated, two-cavity-modes, one-transmon-qubit system for cQED experiments. The measured coherence times and coupling energies suggest that the 2.5D platform would be a promising base for integrated quantum information processing.