A nanometre-scale superconducting electrode connected to a reservoir via a Josephson junction constitutes an arti®cial twolevel electronic system: a single-Cooper-pair box. The two levels consist of charge states (differing by 2e, where e is the electronic charge) that are coupled by tunnelling of Cooper pairs through the junction. Although the two-level system is macroscopic, containing a large number of electrons, the two charge states can be coherently superposed 1±4 . The Cooper-pair box has therefore been suggested 5±7 as a candidate for a quantum bit or qubit'Ðthe basic component of a quantum computer. Here we report the observation of quantum oscillations in a singleCooper-pair box. By applying a short voltage pulse via a gate electrode, we can control the coherent quantum state evolution: the pulse modi®es the energies of the two charge states nonadiabatically, bringing them into resonance. The resulting stateÐ a superposition of the two charge statesÐis detected by a tunnelling current through a probe junction. Our results demonstrate electrical coherent control of a qubit in a solid-state electronic device.Rapidly improving nanofabrication technologies have made quantum two-level systems in solid-state devices promising for functional quantum circuit integration. To coherently control an individual two-level system as a unit of such circuits, several systems have been examined, such as electronic 8±10 and spin 11 states in quantum dots, nuclear spins of impurity atoms embedded in a substrate 12 , and magnetic-¯ux states in a superconducting ring 13,14 . However, only optical coherent control has been realized experimentally 10 .A single-Cooper-pair box 1 (Fig. 1) is a unique arti®cial solid-state system in the sense that: (1) although there are a large number of electrons in the metal`box' electrode, under superconductivity they all form Cooper pairs and condense into a single macroscopic ground state, |ni, separated by a superconductivity gap ¢ from the excited states with quasiparticles. (Here |ni denotes the chargenumber state with the excess number of electrons n the box, n.) (2) The only low-energy excitations are the transitions between different |ni states due to Cooper-pair tunnelling through the Josephson junction, if ¢ is larger than the single-electron charging energy of the box E C . (3) E C , if larger than the Josephson energy E J and the thermal energy k B T, suppresses a large¯uctuation of n. Hence, we can consider the system an effective two-level system by taking into account the two lowest-energy states which differ by one Cooper pair. (4) In addition, the relative energy of the two levels can be controlled through the gate voltage. For example, as shown in Fig. 2a, the electrostatic energies, E C n 2 Q t =e 2 , of two such charge states |0i and |2i change as a function of the total gate-induced charge Q t and cross each other at Q t =e 1. (The parabolic background energy is subtracted.) In the presence of Josephson coupling, and with weak enough dissipation 15 , these charge states w...

An atom in open space can be detected by means of resonant absorption and reemission of electromagnetic waves, known as resonance fluorescence, which is a fundamental phenomenon of quantum optics. We report on the observation of scattering of propagating waves by a single artificial atom. The behavior of the artificial atom, a superconducting macroscopic two-level system, is in a quantitative agreement with the predictions of quantum optics for a pointlike scatterer interacting with the electromagnetic field in one-dimensional open space. The strong atom-field interaction as revealed in a high degree of extinction of propagating waves will allow applications of controllable artificial atoms in quantum optics and photonics.A single atom interacting with electromagnetic modes of free space is a fundamental example of an open quantum system (Fig. 1A) [1]. The interaction between the atom (or molecule, quantum dot, et cetera) and a resonant electromagnetic field is particularly important for quantum electronics and quantum information processing. In three-dimensional (3D) space, however, although perfect coupling (with 100% extinction of transmitted power) is theoretically feasible [2], experimentally achieved extinction has not exceeded 12% [3][4][5][6][7] because of spatial mode mismatch between incident and scattered waves. This problem can be avoided by an efficient coupling of the atom to the continuum of electromagnetic modes confined in a 1D transmission line (Fig. 1B) as proposed in [8,9]. Here we demonstrate extinction of 94% on an artificial atom coupled to the open 1D transmission line. The situation with the atom interacting with freely propagating waves is qualitatively different from that of the atom interacting with a single cavity mode; the latter has been used to demonstrate a series of cavity quantum electrodynamics (QED) phenomena [10][11][12][13][14][15][16][17][18]. Moreover in open space, the atom directly reveals such phenomena known from quantum optics as anomalous dispersion and strongly nonlinear behavior in elastic (Rayleigh) scattering near the atomic resonance[1]. Furthermore, spectrum of inelastically scattered radiation is observed and exhibits the resonance fluorescence triplet (the Mollow triplet) [19][20][21][22][23] under a strong drive.Our artificial atom is a macroscopic superconducting loop, interrupted by Josephson junctions (Fig. 1B) [identical to a flux qubit [24]] and threaded by a bias flux Φ b close to a half flux quantum Φ 0 /2, and shares a segment with the transmission line [25], which results in a loop-line mutual inductance M mainly due to kinetic * On leave from Physical-Technical Institute, Tashkent 100012, Uzbekistan † On leave from Lebedev Physical Institute, Moscow 119991, Russia inductance of the shared segment [26]. The two lowest eigenstates of the atom are naturally expressed via superpositions of two states with persistent current, I p , flowing clockwise or counterclockwise. In energy eigenbasis the lowest two levels |g and |e are described by the truncated...

A practical quantum computer, if built, would consist of a set of coupled two-level quantum systems (qubits). Among the variety of qubits implemented, solid-state qubits are of particular interest because of their potential suitability for integrated devices. A variety of qubits based on Josephson junctions have been implemented; these exploit the coherence of Cooper-pair tunnelling in the superconducting state. Despite apparent progress in the implementation of individual solid-state qubits, there have been no experimental reports of multiple qubit gates--a basic requirement for building a real quantum computer. Here we demonstrate a Josephson circuit consisting of two coupled charge qubits. Using a pulse technique, we coherently mix quantum states and observe quantum oscillations, the spectrum of which reflects interaction between the qubits. Our results demonstrate the feasibility of coupling multiple solid-state qubits, and indicate the existence of entangled two-qubit states.