Session D5: Circuit QED: Superconducting Qubits Coupled to Cavities

Superconducting qubits coupled to resonant cavities

Coupling of superconducting qubits to resonant cavities and mechanical oscillators has opened new possibilities for quantum information processing and for the realization of quantum optics in solid-state devices. Together with steady improvements in superconducting qubits [1], this is due to the qubit-resonator coupling which can readily be made very large with respect to all dissipation rates. As a result, these solid-state systems can reach new parameter regimes currently unexplored in atomic based quantum optics [2]. A resonant cavity can also be used as quantum bus allowing entanglement to be generated controllably between qubits coupled to the same cavity, and regardless of the distance separating the qubits [3]. Because of the relatively large size of these cavities, this allows to couple and entangle multiple qubits, opening new avenues for scalable solid-state quantum computation. In this talk, I will review some of the key properties of superconducting qubits and how they can be strongly coupled to various types of resonant cavities. Focusing on superconducting charge qubits coupled to transmission line resonators, I will explain how the quantum state of the qubits can be coherently manipulated and probed by microwave irradiation of the resonator [4]. I will also present some of the recent theoretical proposals for the generation of entanglement in this system [5]. \newline [1] J. Koch, et al. Phys. Rev. A 76, 042319 (2007). \newline [2] D. I. Schuster, et al. Nature 445, 515 (2007). \newline [3] J. Majer, et al. Nature 449, 443 (2007). \newline [4] J. Gambetta, et al. arXiv:0709.4264 (2007). \newline [5] A. Blais, et al. Phys. Rev. A 75, 032329 (2007).   

Coherent manipulation of quantum information using two Josephson phase qubits coupled to a resonant cavity

We have taken the first step towards the implementation of circuit quantum-electro dynamics (QED) quantum information processing with Josephson phase qubits. We have observed for the first time a coherent interaction between two phase qubits and an LC cavity formed by a ~7 mm long coplanar waveguide resonant at ~9 GHz. When either qubit is resonant with the cavity, we observe the vacuum Rabi splitting of the qubit's spectral line. In a time-domain measurement, we observe coherent vacuum Rabi oscillations between either qubit and the oscillator. Using controllable shift pulses, we have shown coherent transfer of a arbitrary quantum state. We first prepare the first qubit in a superposition state, then this state is transferred to the resonant cavity and then after a short time, we transfer this state into the final qubit. These experiments show that quantum information can be coherently stored and transferred between superconducting quantum bits using a resonant cavity. This opens up new possibilities for performing circuit QED and studying quantum information science.   

Circuit QED: Coupling Superconducting Qubits via a Cavity Bus on a Chip

Circuit quantum electrodynamics is a system, which allows us to do new experiments in quantum optics with a superconducting integrated circuit on a chip. In circuit QED, microwave photons are guided and confined by superconducting transmission lines and cavities, and can then be coherently coupled to a transmon qubit. This system leads to much stronger coupling of the ``light'' and ``matter'' than is possible with traditional atomic systems. Making use of that strong coupling it is possible to couple two qubits via the cavity[1]. I will show how one can use the cavity as a coupling bus which provides non-local and non-nearest neighbor coupling. The interaction is mediated by the exchange of virtual rather than real photons, avoiding cavity-induced loss. The same cavity is also used to perform multiplexed control and read-out of the two qubits. The coupling is effectively switchable which allows for time domain transfer of the quantum states between the qubits. [1] Coupling superconducting qubits via a cavity bus, J. Majer, J. M. Chow, J. M. Gambetta, Jens Koch, B. R. Johnson, J. A. Schreier, L. Frunzio, D. I. Schuster, A. A. Houck, A. Wallraff, A. Blais, M. H. Devoret, S. M. Girvin and R. J. Schoelkopf. Nature 449 443 (2007)   

Single artificial-atom maser

Masers and lasers usually involve ensemble of atoms to be excited and stimulated for emission. As those atoms are only weakly coupled to the cavity mode, a large number of atoms and strong pumping are needed for lasing in order to overcome the cavity loss and the relaxation of atoms due to spontaneous emission into other modes. However, when the coupling becomes strong even a single atom is enough for lasing, as have been demonstrated with atoms in microwave/optical cavities. We have realized an analogous single artificial-atom maser in a superconducting circuit [1]. Josephson-junction charge qubit is used as an artificial atom with a large dipole. The qubit is coupled to a superconducting Nb coplanar-waveguide resonator at around 10~GHz and with a quality factor of 7600. The coupling strength between the qubit and the resonator is 80 MHz. Population inversion is generated by current injection: A current is injected through a voltage-biased electrode attached to the charge qubit via a highly resistive tunnel junction. In the so-called Josephson-quasiparticle process, the qubit is pumped incoherently to the upper state and emits photon into the cavity. This work is in collaboration with O. Astafiev, K. Inomata, A.O. Niskanen, T. Yamamoto, Yu.\ A. Pashkin, and J.S. Tsai. This work has been supported by RIKEN Frontier Research System and CREST-JST. \par Reference: [1] O. Astafiev {\it et al.}, Nature {\bf 449}, 588 (2007).   

Process Tomography of Quantum Memory in a Josephson Phase Qubit

Quantum memory that can protect qubit states against decoherence is an important piece of a scalable quantum computing architecture. Coupling a qubit to a high-Q harmonic oscillator memory element is one example, but many other quantum systems could serve this role. We have used an atomic two-level state (TLS) in the amorphous AlO$_x$ tunnel barrier of a Josephson phase qubit as a prototype quantum memory element. The frequency-tunability of the phase qubit allows us to switch on and off the qubit-TLS coupling and thereby transfer arbitrary qubit states into the TLS, store them for some time and recall them later. We performed quantum process tomography to completely characterize the memory operation and the errors that occur during the state transfer and recall. The overall process fidelity is 78\%. The dominant operator-sum errors are dephasing-like ($\sim$12\%) and relaxation-like ($\sim$9\%), consistent with the measured T$_1$ and T$_2$ of the TLS.