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.

We have observed the coherent exchange of a single energy quantum between a flux qubit and a superconducting LC circuit acting as a quantum harmonic oscillator. The exchange of an energy quantum is known as the vacuum Rabi oscillation: the qubit is oscillating between the excited state and the ground state and the oscillator between the vacuum state and the first excited state. We also show that we can detect the state of the oscillator with the qubit and thereby obtained evidence of level quantization of the LC circuit. Our results support the idea of using oscillators as couplers of solid-state qubits.

In order to gain a better understanding of the origin of decoherence in superconducting flux qubits, we have measured the magnetic field dependence of the characteristic energy relaxation time (T(1)) and echo phase relaxation time (T(2)(echo)) near the optimal operating point of a flux qubit. We have measured T(2)(echo) by means of the phase cycling method. At the optimal point, we found the relation T(2)(echo) approximately 2T(1). This means that the echo decay time is limited by the energy relaxation (T(1) process). Moving away from the optimal point, we observe a linear increase of the phase relaxation rate (1/T(2)(echo)) with the applied external magnetic flux. This behavior can be well explained by the influence of magnetic flux noise with a 1/f spectrum on the qubit.

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We have observed multiphoton transitions between two macroscopic quantum-mechanical superposition states formed by two opposite circulating currents in a superconducting loop with three Josephson junctions. Resonant peaks and dips of up to three-photon transitions were observed in spectroscopic measurements when the system was irradiated with a strong RF-photon field. The widths of the multiphoton absorption dips are shown to scale with the Bessel functions in agreement with theoretical predictions derived from the Bloch equation or from a spin-boson model. PACS numbers: 74.50.+r, 03.67.Lx, 42.50.Hz, 85.25.Dq A macroscopic quantum two-state system (TSS) offers a unique testing ground for exploring the foundations of quantum mechanics [1]. This system can be in a quantum-mechanical superposition of macroscopically distinct states, and its quantum nature can be revealed by measuring the absorption of an integer number of photons from an externally applied photon field [2]. Since a macroscopic quantum system cannot be completely decoupled from its environment, dissipative and decoherence effects are unavoidable [1,2,3]. In addition to the investigation of fundamental physics, the quantum TSS also serves as an elementary carrier of information in a quantum information processor in the form of a quantum bit (qubit) [4]. Artificially designed superconducting circuits with mesoscopic Josephson junctions constitute an important class of macroscopic quantum systems. The charge degree of freedom of Cooper pairs is used to induce coherent quantum oscillations between two charge states of a Cooper pair box [5]. A circuit with a single relatively large Josephson junction, which is current-biased close to its critical current, forms a so-called Josephson phase qubit [6]. Moreover, three Josephson junctions arranged in a superconducting loop threaded by an externally applied magnetic flux constitute a flux qubit [7]. The device could be prepared in a quantum superposition of two states carrying opposite macroscopic persistent currents [8]. Coherent Rabi oscillations have been reported, when the qubit and the readout device are connected to obtain a large signal [9]. Rabi oscillations have also been observed in a system, where the qubit and the readout device are spatially separated [10]. Since these solid-state devices are thought to be scalable up to a large number of qubits, they are of particular interest in the context of solid-state quantum information processing [11].The energy scale of quantum circuits containing Josephson junctions is in the microwave regime. This property was demonstrated in the current-voltage characteristics of a Josephson junction under microwave irradiation displaying the well-known Shapiro steps [12]. They appear at voltages corresponding to integer multiples of the applied microwave energy. With this phenomenon, the superconductor phase difference at the junction, which is a macroscopic degree of freedom, can be treated as a classical degree of freedom moving in the Josephson potential. In co...

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A novel family of azobenzene-based photochromic amorphous molecular materials has been created. They were found to readily form amorphous glasses with well-defined glass-transition temperatures and to exhibit photochromism as amorphous films as well as in solution. It was found that their quantum yields of trans-cis photoisomerization were smaller as amorphous films than in solution and that the backward cis-trans thermal isomerization reactions as amorphous films were either accelerated or retarded relative to those in solution, depending upon their molecular structures. In addition, the rate acceleration for the cis-trans thermal isomerization as amorphous films relative to solution was found to be more prominent as the irradiation time for generating the cis-isomer became shorter.