Publications (12)140.49 Total impact

Article: Multimode mediated qubitqubit coupling and darkstate symmetries in circuit quantum electrodynamics
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ABSTRACT: Microwave cavities with high quality factors enable coherent coupling of distant quantum systems. Virtual photons lead to a transverse interaction between qubits when they are nonresonant with the cavity but resonant with each other. We experimentally investigate the inverse scaling of the interqubit coupling with the detuning from a cavity mode and its proportionality to the qubitcavity interaction strength. We demonstrate that the enhanced coupling at higher frequencies is mediated by multiple higherharmonic cavity modes. Moreover, we observe dark states of the coupled qubitqubit system and analyze their relation to the symmetry of the applied driving field at different frequencies.  [Show abstract] [Hide abstract]
ABSTRACT: Microwave cavities with high quality factors enable coherent coupling of distant quantum systems. Virtual photons lead to a transverse exchange interaction between qubits, when they are nonresonant with the cavity but resonant with each other. We experimentally probe the inverse scaling of the interqubit coupling with the detuning from a cavity mode and its proportionality to the qubitcavity interaction strength. We demonstrate that the enhanced coupling at higher frequencies is mediated by multiple higherharmonic cavity modes. Moreover, in the case of resonant qubits, the symmetry properties of the system lead to an allowed twophoton transition to the doubly excited qubit state and the formation of a dark state.  [Show abstract] [Hide abstract]
ABSTRACT: The exceptionally strong coupling realizable between superconducting qubits and photons stored in an onchip microwave resonator allows for the detailed study of matterlight interactions in the realm of circuit quantum electrodynamics (QED). Here we investigate the resonant interaction between a single transmontype multilevel artificial atom and weak thermal and coherent fields. We explore up to three photon dressed states of the coupled system in a linear response heterodyne transmission measurement. The results are in good quantitative agreement with a generalized JaynesCummings model. Our data indicates that the role of thermal fields in resonant cavity QED can be studied in detail using superconducting circuits. Comment: ArXiv version of manuscript to be published in the Physica Scripta topical issue on the Nobel Symposium 141: Qubits for Future Quantum Computers(2009), 13 pages, 6 figures, hires version at http://qudev.ethz.ch/content/science/PubsPapers.html  [Show abstract] [Hide abstract]
ABSTRACT: The quantum state of a superconducting qubit nonresonantly coupled to a transmission line resonator can be determined by measuring the quadrature amplitudes of an electromagnetic field transmitted through the resonator. We present experiments in which we analyze in detail the dynamics of the transmitted field as a function of the measurement frequency for both weak continuous and pulsed measurements. We find excellent agreement between our data and calculations based on a set of Blochtype differential equations for the cavity field derived from the dispersive JaynesCummings Hamiltonian including dissipation. We show that the measured system response can be used to construct a measurement operator from which the qubit population can be inferred accurately. Such a measurement operator can be used in tomographic methods to reconstruct single and multiqubit states in ensembleaveraged measurements.  [Show abstract] [Hide abstract]
ABSTRACT: We present an ideal realization of the TavisCummings model in the absence of atom number and coupling fluctuations by embedding a discrete number of fully controllable superconducting qubits at fixed positions into a transmission line resonator. Measuring the vacuum Rabi mode splitting with one, two, and three qubits strongly coupled to the cavity field, we explore both bright and dark dressed collective multiqubit states and observe the discrete square root N scaling of the collective dipole coupling strength. Our experiments demonstrate a novel approach to explore collective states, such as the W state, in a fully globally and locally controllable quantum system. Our scalable approach is interesting for solidstate quantum information processing and for fundamental multiatom quantum optics experiments with fixed atom numbers. 
Article: Measurement of AutlerTownes and Mollow Transitions in a Strongly Driven Superconducting Qubit
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ABSTRACT: We present spectroscopic measurements of the AutlerTownes doublet and the sidebands of the Mollow triplet in a driven superconducting qubit. The ground to first excited state transition of the qubit is strongly pumped while the resulting dressed qubit spectrum is probed with a weak tone. The corresponding transitions are detected using dispersive readout of the qubit coupled off resonantly to a microwave transmission line resonator. The observed frequencies of the AutlerTownes and Mollow spectral lines are in good agreement with a dispersive JaynesCummings model taking into account higher excited qubit states and dispersive level shifts due to offresonant drives.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum state tomography is an important tool in quantum information science for complete characterization of multiqubit states and their correlations. Here we report a method to perform a joint simultaneous readout of two superconducting qubits dispersively coupled to the same mode of a microwave transmission line resonator. The nonlinear dependence of the resonator transmission on the qubit state dependent cavity frequency allows us to extract the full twoqubit correlations without the need for singleshot readout of individual qubits. We employ standard tomographic techniques to reconstruct the density matrix of twoqubit quantum states. 
Article: Climbing the JaynesCummings Ladder and Observing its Sqrt(n) Nonlinearity in a Cavity QED System
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ABSTRACT: The already very active field of cavity quantum electrodynamics (QED), traditionally studied in atomic systems, has recently gained additional momentum by the advent of experiments with semiconducting and superconducting systems. In these solid state implementations, novel quantum optics experiments are enabled by the possibility to engineer many of the characteristic parameters at will. In cavity QED, the observation of the vacuum Rabi mode splitting is a hallmark experiment aimed at probing the nature of matterlight interaction on the level of a single quantum. However, this effect can, at least in principle, be explained classically as the normal mode splitting of two coupled linear oscillators. It has been suggested that an observation of the scaling of the resonant atomphoton coupling strength in the JaynesCummings energy ladder with the square root of photon number n is sufficient to prove that the system is quantum mechanical in nature. Here we report a direct spectroscopic observation of this characteristic quantum nonlinearity. Measuring the photonic degree of freedom of the coupled system, our measurements provide unambiguous, long sought for spectroscopic evidence for the quantum nature of the resonant atomfield interaction in cavity QED. We explore atomphoton superposition states involving up to two photons, using a spectroscopic pump and probe technique. The experiments have been performed in a circuit QED setup, in which ultra strong coupling is realized by the large dipole coupling strength and the long coherence time of a superconducting qubit embedded in a high quality onchip microwave cavity.  [Show abstract] [Hide abstract]
ABSTRACT: We demonstrate time resolved driving of twophoton blue sideband transitions between superconducting qubits and a transmission line resonator. Using the sidebands, we implement a pulse sequence that first entangles one qubit with the resonator, and subsequently distributes the entanglement between two qubits. We show generation of 75% fidelity Bell states by this method. The full density matrix of the two qubit system is extracted using joint measurement and quantum state tomography, and shows close agreement with numerical simulation. The scheme is potentially extendable to a scalable universal gate for quantum computation. Comment: 4 pages, 5 figures, version with high resolution figures available at http://qudev.ethz.ch/content/science/PubsPapers.html  [Show abstract] [Hide abstract]
ABSTRACT: Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental observation of the Lamb shift in a solidstate system. The strong dispersive coupling of a superconducting electronic circuit acting as a quantum bit (qubit) to the vacuum field in a transmissionline resonator leads to measurable Lamb shifts of up to 1.4% of the qubit transition frequency. The qubit is also observed to couple more strongly to the vacuum field than to a single photon inside the cavity, an effect that is explained by taking into account the limited anharmonicity of the higher excited qubit states.  [Show abstract] [Hide abstract]
ABSTRACT: The field of cavity quantum electrodynamics (QED), traditionally studied in atomic systems, has gained new momentum by recent reports of quantum optical experiments with solidstate semiconducting and superconducting systems. In cavity QED, the observation of the vacuum Rabi mode splitting is used to investigate the nature of matterlight interaction at a quantummechanical level. However, this effect can, at least in principle, be explained classically as the normal mode splitting of two coupled linear oscillators. It has been suggested that an observation of the scaling of the resonant atomphoton coupling strength in the JaynesCummings energy ladder with the square root of photon number n is sufficient to prove that the system is quantum mechanical in nature. Here we report a direct spectroscopic observation of this characteristic quantum nonlinearity. Measuring the photonic degree of freedom of the coupled system, our measurements provide unambiguous spectroscopic evidence for the quantum nature of the resonant atomfield interaction in cavity QED. We explore atomphoton superposition states involving up to two photons, using a spectroscopic pump and probe technique. The experiments have been performed in a circuit QED setup, in which very strong coupling is realized by the large dipole coupling strength and the long coherence time of a superconducting qubit embedded in a highquality onchip microwave cavity. Circuit QED systems also provide a natural quantum interface between flying qubits (photons) and stationary qubits for applications in quantum information processing and communication.  [Show abstract] [Hide abstract]
ABSTRACT: In quantum information science, the phase of a wave function plays an important role in encoding information. Although most experiments in this field rely on dynamic effects to manipulate this information, an alternative approach is to use geometric phase, which has been argued to have potential fault tolerance. We demonstrated the controlled accumulation of a geometric phase, Berry's phase, in a superconducting qubit; we manipulated the qubit geometrically by means of microwave radiation and observed the accumulated phase in an interference experiment. We found excellent agreement with Berry's predictions and also observed a geometrydependent contribution to dephasing.
Publication Stats
813  Citations  
140.49  Total Impact Points  
Top Journals
 Physical Review Letters (3)
 Physical Review A (2)
 Science (2)
 Physical review. B, Condensed matter (1)
 Nature (1)
Institutions

2009

ETH Zurich
 Department of Physics
Zürich, Zurich, Switzerland
