During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress. Many foundational experiments have been performed, and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are likely to be insufficient for future large-scale quantum computation. A natural solution to this problem is a dedicated engineered quantum memory based on atomic and molecular systems. The question of whether coherent quantum coupling is possible between such natural systems and a single macroscopic artificial atom has attracted considerable attention since the first demonstration of macroscopic quantum coherence in Josephson junction circuits. Here we report evidence of coherent strong coupling between a single macroscopic superconducting artificial atom (a flux qubit) and an ensemble of electron spins in the form of nitrogen-vacancy colour centres in diamond. Furthermore, we have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3 × 10(7) such colour centres. This provides a foundation for future quantum memories and hybrid devices coupling microwave and optical systems.

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We replace the Josephson junction defining a three-junction flux qubit's properties with a tunable direct current superconducting quantum interference devices (DC-SQUID) in order to tune the qubit gap during the experiment. We observe different gaps as a function of the external magnetic pre-biasing field and the local magnetic field through the DC-SQUID controlled by high-bandwidth on chip control lines. The persistent current and gap behavior correspond to numerical simulation results. We set the sensitivity of the gap on the control lines during the sample design stage. With a tuning range of several GHz on a qubit dynamics timescale, we observe coherent system dynamics at the degeneracy point.PACS numbers: 03.67. Lx,85.25.Hv,85.25.Cp Superconducting flux qubits consisting of three Josephson junctions embedded in a low inductance superconducting loop 1,2 are promising candidates for quantum information processing 3 . With the standard design, the magnetic field inducing an energy difference between the ground state and the first excited state is the only parameter that is adjustable during the experiment. At the degeneracy point, it reaches its minimum value (called the gap ∆), which is related to the quantum mechanical tunnel rate. However, the gap is determined at the time of production by the inherent parameters of the three Josephson junctions. One of the main problems in realizing quantum computation schemes using flux qubits is 1/f flux noise. At the degeneracy point the qubit is decoupled from low frequency flux noise 4,5 , and achieves maximal coherence times of several µs rather than ns at off-degeneracy point, wherefore this is the optimal operation point. Implementing a quantum processor based on flux qubits requires the operation at this optimal point at all times, especially when coupling several qubits. Recently, circuit quantum electrodynamics (QED) (quantum bus, qubus) schemes have been developed for many superconducting qubits 6-10 , however standard flux qubits suffer from the fact that bringing them in resonance with such a quantum bus requires to operate them at the offdegeneracy point.With the flux qubit we present here we aim to overcome these limitations by replacing the smallest junction of the flux qubit with a low inductance DC-SQUID loop 1,2 . Varying the magnetic flux penetrating this loop is equivalent to changing the critical current of the smallest junction, thus making the gap tunable during the experiment. Using this tuning parameter we can control the coupling to a qubus by tuning into or out of the qubus frequency while operating at the optimal point. Moreover, we can implement strong transverse coupling to another qubit or to a resonator 11,12 . a) xbzhu@will.brl.ntt.co.jp b) semba@will.brl.ntt.co.jp Our qubit is shown schematically in Fig. 1. Two identical junctions with Josephson energy E J in series with a symmetric DC-SQUID, in which each junction has a Josephson energy of α 0 E J are enclosed in a superconducting loop. The effective Josephson energy of the DC-SQUID ...

We propose a scheme with dc control of finite bandwidth to implement a two-qubit gate for superconducting flux qubits at the optimal point. We provide a detailed nonperturbative analysis on the dynamic evolution of the qubits interacting with a common quantum bus. An effective qubit-qubit coupling is induced while decoupling the quantum bus with proposed pulse sequences. The two-qubit gate is insensitive to the initial state of the quantum bus and applicable to nonperturbative coupling regime which enables rapid two-qubit operation. This scheme can be scaled up to multiqubit coupling.

We demonstrate experimentally the creation and measurement of an entangled state between a microscopic two level system and a macroscopic superconducting resonator where their indirect interaction is mediated by an artificial atom, a superconducting persistent current qubit (PCQB). We show that the microscopic two level system, formed by a defect in an oxide layer, exhibits an order of magnitude longer dephasing time than the PCQB, while the dephasing time of the entangled states between the microscopic two level system and macroscopic superconducting resonator is significantly longer than the dephasing time in the persistent current qubits. This demonstrates the possibility that a qubit of moderate coherence properties can be used in practice to address low decoherence quantum memories by connecting them to macroscopic circuit QED quantum buses, leading future important implications for quantum information processing tasks.