We have constructed a new type of amplifier whose primary purpose is the readout of superconducting quantum bits. It is based on the transition of an RF-driven Josephson junction between two distinct oscillation states near a dynamical bifurcation point. The main advantages of this new amplifier are speed, high-sensitivity, low back-action, and the absence of on-chip dissipation. Pulsed microwave reflection measurements on nanofabricated Al junctions show that actual devices attain the performance predicted by theory.Quantum measurements of solid-state systems, such as the readout of superconducting quantum bits [1,2,3,4,5,6,7], challenge conventional low-noise amplification techniques. Ideally, the amplifier for a quantum measurement should minimally perturb the measured system while maintaining sufficient sensitivity to overcome the noise of subsequent elements in the amplification chain. Additionally, the characteristic drift of materials properties in solid-state systems necessitates a fast acquisition rate to permit measurements in rapid succession. To meet these inherently conflicting requirements, we propose to harness the sensitivity of a dynamical system -a single RF-driven Josephson tunnel junction -tuned near a bifurcation point. The superconducting tunnel junction is the only electronic dipolar circuit element whose nonlinearity remains unchanged at arbitrary low temperatures. As the key component of present superconducting amplifiers [8,9,10], it is known to exhibit a high degree of stability. Moreover, all available degrees of freedom in the dynamical system participate in information transfer and none contribute to unnecessary dissipation resulting in excess noise. The operation of our Josephson bifurcation amplifier is represented schematically in Fig. 1. The central element is a Josephson junction whose critical current I 0 is modulated by the input signal using an application-specific coupling scheme (input port), such as a SQUID loop [4] or a SSET [2]. The junction is driven with an sinusoidal signal i RF sin(ωt) fed from a transmission line through a directional coupler (drive port). In the underdamped regime, when the drive frequency ω is detuned form the natural oscillation frequency ω p and when the drive current IB < i RF < I B ≪ I 0 , the system has two possible oscillation states which differ in amplitude and phase [11,12]. Starting in the lower amplitude state, at the bifurcation point i RF = I B the system becomes infinitely sensitive, in absence of thermal and quantum fluctuations, to variations in I 0 . The energy stored in the oscillation can always be made larger than thermal fluctuations by increasing the scale of I 0 , thus preserving sensitivity at finite temperature. The reflected component of the drive signal, measured through another transmission line connected to the coupler (output port), is a convenient signature of the junction oscillation state which carries with it information about the input signal. This arrangement minimizes the back-action of the amplifier since the...

We have extracted the phase coherence time τ φ of electronic quasiparticles from the low field magnetoresistance of weakly disordered wires made of silver, copper and gold. In samples fabricated using our purest silver and gold sources, τ φ increases as T −2/3 when the temperature T is reduced, as predicted by the theory of electron-electron interactions in diffusive wires. In contrast, samples made of a silver source material of lesser purity or of copper exhibit an apparent saturation of τ φ starting between 0.1 and 1 K down to our base temperature of 40 mK. By implanting manganese impurities in silver wires, we show that even a minute concentration of magnetic impurities having a small Kondo temperature can lead to a quasi saturation of τ φ over a broad temperature range, while the resistance increase expected from the Kondo effect remains hidden by a large background. We also measured the conductance of Aharonov-Bohm rings fabricated using a very pure copper source and found that the amplitude of the h/e conductance oscillations increases strongly with magnetic field. This set of experiments suggests that the frequently observed "saturation" of τ φ in weakly disordered metallic thin films can be attributed to spin-flip scattering from extremely dilute magnetic impurities, at a level undetectable by other means. I. MOTIVATIONSThe time τ φ during which the quantum coherence of an electron is maintained is of fundamental importance in mesoscopic physics. The observability of many phenomena specific to this field relies on a long enough phase coherence time. 1 Amongst these are the weak localization correction to the conductance (WL), the universal conductance fluctuations (UCF), the Aharonov-Bohm (AB) effect, persistent currents in rings, the proximity effect near the interface between a superconductor and a normal metal, and others. Hence it is crucial to find out what mechanisms limit the quantum coherence of electrons.In metallic thin films, at low temperature, electrons experience mostly elastic collisions from sample boundaries, defects of the ion lattice and static impurities in the metal. These collisions do not destroy the quantum coherence of electrons. Instead they can be pictured as resulting from a static potential on which the diffusivelike electronic quantum states are built.What limits the quantum coherence of electrons are inelastic collisions. These are collisions with other electrons through the screened Coulomb interaction, with phonons, and also with extrinsic sources such as magnetic impurities or two level systems in the metal. Whereas above about 1 K electron-phonon interactions are known to be the dominant source of decoherence, 2 electron-electron interactions are expected to be the leading inelastic process at lower temperatures in samples without extrinsic sources of decoherence. 3 The theory of electron-electron interactions in the diffusive regime was worked out in the early 1980's (for a review see 4 ). It predicts a power law divergence of τ φ when the temperature T goes to zero....