We have designed and operated a superconducting tunnel junction circuit that behaves as a twolevel atom: the "quantronium". An arbitrary evolution of its quantum state can be programmed with a series of microwave pulses, and a projective measurement of the state can be performed by a pulsed readout sub-circuit. The measured quality factor of quantum coherence Qϕ ≃ 25000 is sufficiently high that a solid-state quantum processor based on this type of circuit can be envisioned.Can we build machines that actively exploit the fundamental properties of quantum mechanics, such as the superposition principle or the existence of entangled states? Applications such as the transistor or the laser, often quoted as developments based on quantum mechanics, do not actually answer this question. Quantum mechanics enters into these devices only at the level of material properties but their state variables such as voltages and currents remain classical. Proposals for true quantum machines emerged in the last decades of the 20th century and are now being actively explored: quantum computers [1], quantum cryptography communication systems [2] and detectors operating below the standard quantum limit [3]. The major difficulty facing the engineer of a quantum machine is decoherence [4]. If a degree of freedom needs to be manipulated externally, as in the writing of information, its quantum coherence usually becomes very fragile. Although schemes that actively fight decoherence have recently been proposed [5,6], they need very coherent quantum systems to start with. The quality of coherence for a two-level system can be quantitatively described by the quality factor of quantum coherence Q ϕ = πν 01 T ϕ where ν 01 is its transition frequency and T ϕ is the coherence time of a superposition of the states. It is generally accepted that for active decoherence compensation mechanisms, Q ϕ 's larger than 10 4 ν 01 t op are necessary, t op being the duration of an elementary operation [7].Among all the practical realizations of quantum machines, those involving integrated electrical circuits are particularly attractive. However, unlike the electric dipoles of isolated atoms or ions, the state variables of a circuit like voltages and currents usually undergo rapid quantum decoherence because they are strongly coupled to an environment with a large number of uncontrolled * To whom correspondence should be addressed; E-mail: vion@drecam.saclay.cea.fr † Member of CNRS. ‡ Present address: Applied Physics, Yale University, New Haven, CT 6520, USA degrees of freedom [8]. Nevertheless, superconducting tunnel junction circuits [9,10,11,12,13] have displayed Q ϕ 's up to several hundred [14] and temporal coherent evolution of the quantum state has been observed on the nanosecond time scale [10,15] in the case of the single Cooper pair box [16]. We report here a new circuit built around the Cooper pair box with Q ϕ in excess of 10 4 , whose main feature is the separation of the write and readout ports [17,18]. This circuit, which behaves as a tunable ar...

Spintronics aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them 1-3 . This approach is illustrated by the operation of the most basic spintronic device, the spin valve 4-6 , which can be formed if two ferromagnetic electrodes are separated by a thin tunnelling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction 7,8 . The relative difference in resistance, the so-called magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin-or energy-dependent 3 , the opposite can occur and the magnetoresistance is negative 9 . The next step is to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate 10,11 . In practice though, implementing such a device has proved difficult. Here, we report on a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.Early work on spin transport in multiwall carbon nanotubes (MWNTs) with Co contacts showed that spins could propagate coherently over distances as long as 250 nm (ref. 12). The tunnel magnetoresistance (TMR) = (R AP − R P )/R P , defined as the relative difference between the resistances R AP and R P in the antiparallel and parallel magnetization configuration, was found to be positive and amounted to +4% in agreement with Jullière's formula for tunnel junctions 4,13 . A negative TMR of about −30% was reported later for MWNTs contacted with similar Co contacts 14 . In these experiments, the nanotubes did not show quantum dot behaviour. It has been shown, however, that single-wall carbon nanotubes (SWNTs) and MWNTs contacted with nonferromagnetic metals could behave as quantum dots and FabryPérot resonators [15][16][17][18][19] , in which one can tune the position of discrete energy levels with a gate electrode. From this, one can expect to be able to tune the sign and the amplitude of the TMR in nanotubes, in a similar fashion as predicted originally for semiconductor heterostructures 11 .In this letter, we report on TMR measurements of MWNTs and SWNTs that are contacted with ferromagnetic electrodes and capacitively coupled to a back-gate 20 . A typical sample geometry is shown in the inset of Fig. 1. As a result of resonant tunnelling, we observe a striking oscillatory amplitude and sign modulation of the TMR as a function of the gate voltage. We have studied and observed the TMR on nine samples (seven MWNTs and two SWNTs) with various tube lengths L between the ferromagnetic electrodes (see the Methods section). We present here results for one MWNT device and one SWNT device.We first discu...

We demonstrate a hybrid architecture consisting of a quantum dot circuit coupled to a single mode of the electromagnetic field. We use single wall carbon nanotube based circuits inserted in superconducting microwave cavities. By probing the nanotube-dot using a dispersive read-out in the Coulomb blockade and the Kondo regime, we determine an electron-photon coupling strength which should enable circuit QED experiments with more complex quantum dot circuits.PACS numbers: 73.63.Fg An atom coupled to a harmonic oscillator is one of the most illuminating paradigms for quantum measurements and amplification [1]. Recently, the joint development of artificial two-level systems and high finesse microwave resonators in superconducting circuits has brought the realization of this model on-chip [2,3]. This "circuit Quantum Electro-Dynamics" architecture allows, at least in principle, to combine circuits with an arbitrary complexity. In this context, quantum dots can also be used as artificial atoms [4,5]. Importantly, these systems often exhibit many-body features if coupled strongly to Fermi seas, as epitomized by the Kondo effect. Combining such quantum dots with microwave cavities would therefore enable the study of a new type of coupled fermionicphotonic systems.Cavity quantum electrodynamics [6] and its electronic counterpart circuit quantum electrodynamics[1] address the interaction of light and matter in their most simple form i.e. down to a single photon and a single atom (real or artificial). In the field of strongly correlated electronic systems, the Anderson model follows the same purified spirit [7]. It describes a single electronic level with onsite Coulomb repulsion coupled to a Fermi sea. In spite of its apparent simplicity, this model allows to capture non-trivial many body features of electronic transport in nanoscale circuits. It contains a wide spectrum of physical phenomena ranging from resonant tunnelling and Coulomb blockade to the Kondo effect. Thanks to progress in nanofabrication techniques, the Anderson model has been emulated in quantum dots made out of two dimensional electron gas[8], C60 molecules [9] or carbone nanotubes [10]. Here, we mix the two above situations. We couple a quantum dot in the Coulomb blockade or in the Kondo regime to a single mode of the electromagnetic field and take a step further towards circuit QED experiments with quantum dots. * To whom correspondence should be addressed: kontos@lpa.ens. fr FIG. 1: a. Schematics of the quantum dot embedded in the microwave cavity. The transmitted microwave field has different amplitude and phase from the input field as a result of its interaction with the quantum dot inside the cavity. The quantum dot is connected to "wires" and capacitively coupled to a gate electrode in the conventional 3-terminal transport geometry. b. Scanning electron microscope (SEM) picture in false colors of the coplanar waveguide resonator. Both the typical coupling capacitance geometry of one port of the resonator and the 3-terminals geometry are visib...

Electron spins and photons are complementary quantum-mechanical objects that can be used to carry, manipulate and transform quantum information. To combine these resources, it is desirable to achieve the coherent coupling of a single spin to photons stored in a superconducting resonator. Using a circuit design based on a nanoscale spin-valve, we coherently hybridize the individual spin and charge states of a double quantum dot while preserving spin coherence. This scheme allows us to achieve spin-photon coupling up to the MHz range at the single spin level. The cooperativity is found to reach 2.3, and the spin coherence time is about 60ns. We thereby demonstrate a mesoscopic device suitable for nondestructive spin read-out and distant spin coupling.The methods of cavity quantum electrodynamics hold promise for an efficient use of the spin degree of freedom in the context of quantum computation and simulation (1). Realizing a coherent coupling between a single spin and cavity photons could enable quantum nondemolition readout of a single spin, quantum spin manipulation, and facilitate the coupling of distant spins (1,2,3,4). It could also be used in hybrid architectures in which single spins are coupled to superconducting quantum bits (5), or to simulate one-dimensional spin chains (6).The natural coupling of a spin to the magnetic part of the electromagnetic field is weak (7). In order to enhance it, one needs a large spin ensemble, typically of about 10 12 spins (8,9,10,11,12,13), but these ensembles lose the intrinsic non-linearity of a single spin 1/2.Alternatively, several theoretical proposals have been put forward to electrically couple single spins to superconducting resonators in a mesoscopic circuit (14,15,16,17), building on the exquisite accuracy with which superconducting circuits can be used to couple superconducting qubits and photons and manipulate them (18). One such approach is to engineer an artificial spin-photon interaction by using ferromagnetic reservoirs (15).Noteworthy, the spin/photon coupling is also raising experimental efforts in the optical domain (19,20,21,22,23), but the circuit approach presents the significant advantage of scalability.Recent experiments have demonstrated the coupling of double quantum dot charge states to coplanar waveguide resonators, with a coupling strength gcharge ≈ 2  10 -50 MHz (24,25,26,27,28). In Ref,(29), the spin blockade read-out technique in quantum dots (30) was combined with charge sensing with a microwave resonator (31). In contrast to this spinblockade scheme, here we use the ferromagnetic proximity effect in a coherent conductor to engineer a spin-photon coupling. Our scheme relies on the use of a non collinear spin valve geometry, which realizes an artificial spin orbit interaction (15). Specifically, we contact two non collinear ferromagnets on a carbon nanotube double quantum dot.Our device is shown in Fig. 1, A-C. Our resonator is similar to a previous experiment (27) with a coupling scheme adapted from (24). It is a Nb resonator with a qua...

We study current fluctuations in an interacting three-terminal quantum dot with ferromagnetic leads. For appropriately polarized contacts, the transport through the dot is governed by a novel dynamical spin blockade, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. This leads for instance to positive zero-frequency cross-correlations of the currents in the output leads even in the absence of spin accumulation on the dot. We include the influence of spin-flip scattering and identify favorable conditions for the experimental observation of this effect with respect to polarization of the contacts and tunneling rates.PACS numbers: 72.70.+m,72.25.Rb Quantum fluctuations of current in mesoscopic devices have attracted considerable attention in the last years (for reviews, see Refs. [1,2]). It has been shown that the statistics of non-interacting fermions leads to a suppression of noise below the classical Poisson value [3,4,5] and to negative cross-correlations in multi-terminal structures [6]. This was recently confirmed experimentally in a Hanbury Brown-Twiss setup [7]. The question of the sign of cross-correlations has triggered a lot of activity [8], and different mechanisms to obtain positive crosscorrelations in electronic systems have been proposed. Employing a superconductor as a source, positive crosscorrelations have been predicted for several setups [9]. This is because a superconducting source injects highly correlated electron pairs. Screening currents due to longrange Coulomb interactions lead to positive correlations in the finite-frequency voltage noise measured at two capacitors coupled to a coherent conductor [8,10]. Lastly, positive cross-correlations can occur due to the correlated injection of electrons by a voltage probe [12], or due to correlated excitations in a Luttinger liquid [13].Below we will be interested in noise correlations in a quantum dot. This problem was addressed theoretically in the sequential-tunneling limit [14] and in the cotunneling regime [15]. Noise measurements [16] were in agreement with the Coulomb-blockade picture [14]. Crosscorrelations between particle currents in a paramagnetic multi-terminal quantum dot were studied in Ref. [17], and they were found to be negative. The noise of a twoterminal quantum dot with ferromagnetic contacts was studied in the sequential tunneling limit [18,19], and, interestingly, a super-Poissonian Fano factor was found.In this Letter, we consider an interacting threeterminal quantum dot with ferromagnetic leads. The dot is operated as a beam splitter: one contact acts as source and the other two as drains. Our main finding is that sufficiently polarized contacts can lead to a dynamical spin blockade on the dot, i.e., a spin-dependent bunching of tunneling events not present in the paramagnetic case. A striking consequence of this spin blockade is the FIG. 1: Current-voltage characteristic of a quantum dot connected to three ferromagnetic leads i ∈ {1, 2, 3}, with respective polarizations Pi, throu...

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Boundary conditions in quasiclassical theory of superconductivity are of crucial importance for describing proximity effects in heterostructures between different materials. Although they have been derived for the ballistic case in full generality, corresponding boundary conditions for the diffusive limit, described by Usadel theory, have been lacking for interfaces involving strongly spin-polarized materials, e.g. half-metallic ferromagnets. Given the current intense research in the emerging field of superconducting spintronics, the formulation of appropriate boundary conditions for the Usadel theory of diffusive superconductors in contact with strongly spin-polarized ferromagnets for arbitrary transmission probability and arbitrary spin-dependent interface scattering phases has been a burning open question. Here we close this gap and derive the full boundary conditions for quasiclassical Green functions in the diffusive limit, valid for any value of spin polarization, transmission probability, and spin-mixing angles (spin-dependent scattering phase shifts). Our formulation allows also for complex spin textures across the interface and for channel off-diagonal scattering (a necessary ingredient when the numbers of channels on the two sides of the interface differ). As an example we derive expressions for the proximity effect in diffusive systems involving half-metallic ferromagnets. In a superconductor/half-metal/superconductor Josephson junction we find 0 ϕ -junction behavior under certain interface conditions.

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