Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G3: Coupled Superconducting Qubits |
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Sponsoring Units: DCMP Chair: Steven Girvin, Yale University Room: Baltimore Convention Center Ballroom I |
Tuesday, March 14, 2006 8:00AM - 8:36AM |
G3.00001: Coupled superconducting qubits: A theoretical overview. Invited Speaker: In recent years we have witnessed great progress in quantum state engineering with superconducting qubits. Most experiments have been done with single qubits whereas several groups have reported multi-qubit manipulations. Having controllable coupling between the qubits is absolutely necessary for efficient quantum state engineering. I will overview various coupling schemes suggested in the literature. Among them are capacitive, inductive, nanomechanical, and cavity couplings. Special attention will be paid to the questions of accuracy to which the coupling can be switched off and of speed at which it can be manipulated. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 9:12AM |
G3.00002: High-Fidelity Measurements of Josephson Phase Qubits Invited Speaker: The Josephson junction is an ideal solid-state system for building electrical ``atoms'' that can function as quantum bits for a quantum computer. Recent advances in the materials and design of phase qubits have dramatically improved their coherence, with energy relaxation times as long as 500 ns and, in a separate device, measurement fidelities as high as 90{\%}. Combined with advances in microwave electronics, full characterization of single and coupled qubit gates are now possible using quantum tomographic techniques. I will report on several ongoing experiments in our group: measurement of the evolution of a quantum state from incomplete measurement, violation of Bell's inequality in a coupled qubit, and the implementation and characterization of a CNOT gate. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:48AM |
G3.00003: Qubits in Cavities for Quantum Optics and Computing Invited Speaker: I will describe a platform for quantum computation and quantum optics using superconducting circuits as both the linear and non-linear elements. One dimensional transmission line cavities realize well-defined microwave linear photon modes, while Cooper-pair boxes act as artificial atoms for use as qubits and strongly non-linear elements [1]. We have observed the coherent interaction between a qubit and a single photon, a feat previously only achievable in atomic systems [2]. Using such a coupling a high quality factor cavity can act as an entanglement bus where real or virtual photons mediate interactions between distant qubits [3]. The qubit-photon coupling is exploited to realize a quantum non-demolition measurement of the qubit state [4] and should also allow for direct measurement of the photon number state of the cavity [5]. Using this measurement technique we have shown $>$90{\%} visibility and long coherence times (T$_{1}\sim $7us T$_{2}$* $>$ 500ns) [6]. Off-resonant microwave irradiation is used to adjust qubit-cavity and qubit-qubit detunings for decoherence free control of interaction strengths. Pulsed two photon sideband transitions are shown to mediate off-resonant cavity-qubit interactions, as required to implement non-local two qubit gates. Such techniques could also be used to generate non-classical states of light. We will also discuss initial measurements on two qubits. \newline \newline [1] A. Wallraff et al. Nature (London) 431, 162 (2004) \newline [2] A. Blais et al. Phys. Rev. A 69, 062320 (2004) \newline [3] A. Blais et al. Phys. Rev. A submitted \newline [4] D. I. Schuster et al. Phys. Rev. Lett. 94, 123602 (2005) \newline [5] J. Gambetta et al. Phys. Rev. A submitted \newline [6] A. Wallraff et al. Phys. Rev. Lett. 95 060501 (2005) [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:24AM |
G3.00004: Spectroscopy of two coupled superconducting flux qubits Invited Speaker: We present measurements on a system of two flux or persistent current qubits, which are coupled by a shared Josephson junction. Spectroscopy measurements reveal the four energy levels in great detail, also showing transitions involving two photons. Remarkably good agreement was found when fitting to a simple model Hamiltonian of two magnetically coupled spins. From the fit the qubit persistent currents and energy gaps as well as the qubit-qubit coupling strength and asymmetry were derived. Coherent Rabi oscillations were induced between energy states of the coupled system, from which relaxation and dephasing times could be extracted. Conditional spectroscopy was performed by manipulating one qubit coherently and measuring the excitation spectrum of the second. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 11:00AM |
G3.00005: Non-local Andreev Reflection as a Source of Entangled Electrons Invited Speaker: Cooper pairs in conventional superconductors form a robust reservoir of spin-entangled singlet ``particles''. This has led to many theoretical proposals for the realization of ``entangler'' devices --i.e. devices capable of sourcing currents carried by entangled pairs into nanoelectronic circuits- that are of fundamental interest in the field of quantum information. These proposals rely on the possibility to ``split'' Cooper pairs by injecting the two constituent electrons into two spatially separated normal metal leads attached to a superconductors. It has been shown theoretically that such a Cooper-pair ``splitting'' process is equivalent to the process of non-local Andreev reflection, in which an electron (hole) injected from a metallic electrode connected to a superconductor is transmitted into a hole (electron) into a second, spatially separated electrode. In this talk I will discuss recent experiments[1] performed in Delft that demonstrate the occurrence of non-local Andreev reflection as a quantum mechanically phase coherent process. [1] S. Russo \textit{et al.}, Phys. Rev. Lett. \textbf{95}, 027002 (2005) [Preview Abstract] |
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