We report experiments on superconducting flux qubits in a circuit quantum electrodynamics (cQED) setup. Two qubits, independently biased and controlled, are coupled to a coplanar waveguide resonator. Dispersive qubit state readout reaches a maximum contrast of 72 %. We find intrinsic energy relaxation times at the symmetry point of 7 µs and 20 µs and levels of flux noise of 2.6 µΦ0/ √ Hz and 2.7 µΦ0/ √ Hz at 1 Hz for the two qubits. We discuss the origin of decoherence in the measured devices. These results demonstrate the potential of cQED as a platform for fundamental investigations of decoherence and quantum dynamics of flux qubits.PACS numbers: 85.25. Cp, 42.50.Dv , 03.67.Lx, 74.78.Na Superconducting qubits are one of the main candidates for the implementation of quantum information processing [1] and a rich testbed for research in quantum optics, quantum measurement, and decoherence [2]. Among various types of superconducting qubits, flux-type superconducting qubits have unique features. Strong and tunable coupling to microwave fields enables fundamental investigations in quantum optics [3][4][5] and relativistic quantum mechanics [6]. The large magnetic dipole moment is a key ingredient in flux noise measurements [5], sensitive magnetic field measurements [8], microwave-optical interfaces [9], and hybrid systems formed with nanomechanical resonators [10]. Finally, flux qubits have a large degree of anharmonicity which is an advantage for fast quantum control [11]. Progress on these diverse research avenues has been hampered by relatively low and irreproducible coherence times compared to other types of superconducting qubits.In the last decade, circuit quantum electrodynamics (cQED) [12,13] has become increasingly popular. In cQED, resonators provide a controlled electromagnetic environment protecting qubits from energy relaxation. In addition, resonators are used for qubit state measurement [2] and as quantum buses for qubit-qubit coupling [15]. In this letter, we present an implementation of cQED with flux qubits strongly coupled to a superconducting coplanar waveguide resonator. The qubits and the resonator are made of aluminum. Local biasing and control lines provide a mean to implement fast single qubit gates as well as controlled two-qubit interactions. We measure energy relaxation times around 10 µs, an improvement over previous experiments with flux qubits coupled to coplanar waveguide resonators [16,17], and comparable with the longest measured to date on flux qubits [5,18]. We characterize in detail the decoherence of the flux qubits coupled to the resonator. Based on decoherence measurements, we extract levels of flux noise of 2.6 µΦ 0 / √ Hz and 2.7 µΦ 0 / √ Hz at 1 Hz for the two qubits. We also present a spectroscopic measurement of a resonator-mediated qubit-qubit coupling, which is relevant for implementation of two-qubit gates. These results demonstrate the versatility of cQED with flux qubits, and its potential for further understanding and improvements of decoherence of these qubits.T...
Entangled photon pairs generated within integrated devices must often be spatially separated for their subsequent manipulation in quantum circuits. Separation that is both deterministic and universal can in principle be achieved through anticoalescent two-photon quantum interference. However, such interference-facilitated pair separation (IFPS) has not been extensively studied in the integrated setting, where the strong polarization and wavelength dependencies of integrated couplers -as opposed to bulk-optics beamsplitters -can have important implications for performance beyond the identical-photon regime. This paper provides a detailed review of IFPS and examines how these dependencies impact separation fidelity and interference visibility. Focus is given to IFPS mediated by an integrated directional coupler. The analysis applies equally to both on-chip and in-fiber implementations, and can be expanded to other coupler architectures such as multimode interferometers. When coupler dispersion is present, the separation performance can depend on photon bandwidth, spectral entanglement, and the linearity of the dispersion. Under appropriate conditions, reduction in the separation fidelity due to loss of non-classical interference can be perfectly compensated for by classical wavelength demultiplexing effects. This work informs the design as well as the performance assessment of circuits for achieving universal photon pair separation for states with tunable arbitrary properties.
Integrated optics has brought unprecedented levels of stability and performance to quantum photonic circuits. However, integrated devices are not merely micron-scale equivalents of their bulk-optics counterparts. By exploiting the inherently dispersive characteristics of the integrated setting, such devices can play a remarkably more versatile role in quantum circuit architectures. We show this by examining the implications of linear dispersion in an ordinary directional coupler. Dispersion unlocks several novel capabilities for this device, including in-situ control over photon spectral and polarization entanglement, tunable photon time-ordering, and entanglement-sensitive two-photon coincidence generation. Also revealed is an ability to maintain perfect two-photon anti-coalescence while tuning the interference visibility, which has no equivalent in bulk-optics. The outcome of this work adds to a suite of state engineering and characterization tools that benefit from the advantages of integration. It also paves the way for re-evaluating the possibilities offered by dispersion in other on-chip devices.
We present a modular design for integrated programmable multimode sources of arbitrary Gaussian states of light. The technique is based on current technologies, in particular recent demonstrations of on-chip photon manipulation and the generation of highly squeezed vacuum states in semiconductors. While the design is generic and independent of the choice of integrated platform, we adopt recent experimental results on compound semiconductors as a demonstrative example. Such a device would be valuable as a source for many quantum protocols that range from imaging to communication and information processing.
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