arXiv subjects

W. Guichard

Publications and source records attributed to W. Guichard.

13 recordsLinked to original sources

arXiv2015

A V-shape superconducting artificial atom based on two inductively coupled transmons

Circuit quantum electrodynamics systems are typically built from resonators and two-level artificial atoms, but the use of multi-level artificial atoms instead can enable promising applications in quantum technology. Here we present an implementation of a Josephson junction circuit dedicated to operate as a V-shape artificial atom. Based on a concept of two internal degrees of freedom, the device consists of two transmon qubits coupled by an inductance. The Josephson nonlinearity introduces a strong diagonal coupling between the two degrees of freedom that finds applications in quantum non-demolition readout schemes, and in the realization of microwave cross-Kerr media based on superconducting circuits.

quant-ph↗

arXiv2014

Bloch band dynamics of a Josephson junction in an inductive environment

We have measured the current-voltage characteristics of a Josephson junction with tunable Josephson energy $E_J$ embedded in an inductive environment provided by a chain of SQUIDs. Such an environment induces localization of the charge on the junction, which results in an enhancement of the zero-bias resistance of the circuit. We understand this result quantitatively in terms of the Bloch band dynamics of the localized charge. This dynamics is governed by diffusion in the lowest Bloch band of the Josephson junction as well as by Landau-Zener transitions out of the lowest band into the higher bands. In addition, the frequencies corresponding to the self-resonant modes of the SQUID array exceed the Josephson energy $E_J$ of the tunable junction, which results in a renormalization of $E_J$, and, as a consequence, of the effective bandwidth of the lowest Bloch band.

cond-mat.mes-hall↗

arXiv2011

Fabrication of stable and reproducible sub-micron tunnel junctions

We have performed a detailed study of the time stability and reproducibility of sub-micron $Al/AlO_{x}/Al$ tunnel junctions, fabricated using standard double angle shadow evaporations. We have found that by aggressively cleaning the substrate before the evaporations, thus preventing any contamination of the junction, we obtained perfectly stable oxide barriers. We also present measurements on large ensembles of junctions which prove the reproducibility of the fabrication process. The measured tunnel resistance variance in large ensembles of identically fabricated junctions is in the range of only a few percents. Finally, we have studied the effect of different thermal treatments on the junction barrier. This is especially important for multiple step fabrication processes which imply annealing the junction.

cond-mat.mes-hall↗

arXiv2011

Experimental demonstration of Aharonov-Casher interference in a Josephson junction circuit

A neutral quantum particle with magnetic moment encircling a static electric charge acquires a quantum mechanical phase (Aharonov-Casher effect). In superconducting electronics the neutral particle becomes a fluxon that moves around superconducting islands connected by Josephson junctions. The full understanding of this effect in systems of many junctions is crucial for the design of novel quantum circuits. Here we present measurements and quantitative analysis of fluxon interference patterns in a six Josephson junction chain. In this multi-junction circuit the fluxon can encircle any combination of charges on five superconducting islands, resulting in a complex pattern. We compare the experimental results with predictions of a simplified model that treats fluxons as independent excitations and with the results of the full diagonalization of the quantum problem. Our results demonstrate the accuracy of the fluxon interference description and the quantum coherence of these arrays.

cond-mat.mes-hall↗

arXiv2010

Quantum dynamics of a dc-SQUID coupled to an asymmetric Cooper pair transistor

We present a theoretical analysis of the quantum dynamics of a superconducting circuit based on a highly asymmetric Cooper pair transistor (ACPT) in parallel to a dc-SQUID. Starting from the full Hamiltonian we show that the circuit can be modeled as a charge qubit (ACPT) coupled to an anharmonic oscillator (dc-SQUID). Depending on the anharmonicity of the SQUID, the Hamiltonian can be reduced either to one that describes two coupled qubits or to the Jaynes-Cummings Hamiltonian. Here the dc-SQUID can be viewed as a tunable micron-size resonator. The coupling term, which is a combination of a capacitive and a Josephson coupling between the two qubits, can be tuned from the very strong- to the zero-coupling regimes. It describes very precisely the tunable coupling strength measured in this circuit and explains the 'quantronium' as well as the adiabatic quantum transfer read-out.

cond-mat.mes-hall↗

arXiv2010

Measurement of the effect of quantum phase-slips in a Josephson Junction chain

We investigate experimentally the physics of quantum phase slips in one-dimensional Josephson Junction chains. These quantum phase-slips are induced by quantum phase fluctuations occurring on single junctions of the chain. In our experiment we can tune the strength of these fluctuations as each chain junction is realized in form of a SQUID leading to tunable Josephson coupling. We determine the ground state of the chain via switching current measurements of the chain shunted by a large Josephson junction. Our results can be well fitted with a tight binding Hamiltonian taking into account quantum phase-slips.

cond-mat.supr-con↗

arXiv2009

Phase-charge duality in Josephson junction circuits: Role of inertia and effect of microwave irradiation

We investigate the physics of coherent quantum phase slips in two distinct circuits containing small Josephson junctions: (i) a single junction embedded in an inductive environment and (ii) a long chain of junctions. Starting from the standard Josephson Hamiltonian, the single junction circuit can be analyzed using quasi-classical methods; we formulate the conditions under which the resulting quasi-charge dynamics is exactly dual to the usual phase dynamics associated with Josephson tunneling. For the chain we use the fact that its collective behavior can be characterized by one variable: the number $m$ of quantum phase slips present on it. We conclude that the dynamics of the conjugate quasi-charge is again exactly dual to the standard phase dynamics of a single Josephson junction. In both cases we elucidate the role of the inductance, essential to obtain exact duality. These conclusions have profound consequences for the behavior of single junctions and chains under microwave irradiation. Since both systems are governed by a model exactly dual to the standard resistively and capacitively shunted junction model, we expect the appearance of current-Shapiro steps. We numerically calculate the corresponding current-voltage characteristics in a wide range of parameters. Our results are of interest in view of a metrological current standard.

cond-mat.mes-hall↗

arXiv2008

Quantum dynamics in a camel-back potential of a dc SQUID

We investigate the quantum dynamics of a quadratic-quartic anharmonic oscillator formed by a potential well between two potential barriers. We realize this novel potential shape with a superconducting circuit comprised of a loop interrupted by two Josephson junctions, with near-zero current bias and flux bias near half a flux quantum. We investigate escape out of the central well, which can occur via tunneling through either of the two barriers, and find good agreement with a generalized double-path macroscopic quantum tunneling theory. We also demonstrate that this system exhibits an "optimal line" in current and flux bias space along which the oscillator, which can be operated as a phase qubit, is insensitive to decoherence due to low-frequency current fluctuations.

cond-mat.supr-con↗

arXiv2008

Measurement of the Current-Phase Relation in Josephson Junctions Rhombi Chains

We present low temperature transport measurements in one dimensional Josephson junctions rhombi chains. We have measured the current phase relation of a chain of 8 rhombi. The junctions are either in the classical phase regime with the Josephson energy much larger than the charging energy, $E_{J}\gg E_{C}$, or in the quantum phase regime where $E_{J}/E_{C}\approx 2$. In the strong Josephson coupling regime ($E_{J}\gg E_{C} \gg k_{B}T$) we observe a sawtooth-like supercurrent as a function of the phase difference over the chain. The period of the supercurrent oscillations changes abruptly from one flux quantum $Φ_{0}$ to half the flux quantum $Φ_{0}/2$ as the rhombi are tuned in the vicinity of full frustration. The main observed features can be understood from the complex energy ground state of the chain. For $E_{J}/E_{C}\approx 2$ we do observe a dramatic suppression and rounding of the switching current dependence which we found to be consistent with the model developed by Matveev et al.(Phys. Rev. Lett. {\bf 89}, 096802(2002)) for long Josephson junctions chains.

cond-mat.mes-hall↗

arXiv2007

Strong tunable coupling between a superconducting charge and phase qubit

We have realized a tunable coupling over a large frequency range between an asymmetric Cooper pair transistor (charge qubit) and a dc SQUID (phase qubit). Our circuit enables the independent manipulation of the quantum states of each qubit as well as their entanglement. The measurements of the charge qubit's quantum states is performed by resonant read-out via the measurement of the quantum states of the SQUID. The measured coupling strength is in agreement with an analytic theory including a capacitive and a tunable Josephson coupling between the two qubits.

cond-mat.mes-hall↗

arXiv2006

Phase-Charge Duality of a Josephson junction in a fluctuating electromagnetic environment

We have measured the current-voltage characteristics of a single Josephson junction placed in a high impedance environment. The transfer of Cooper pairs through the junction is governed by overdamped quasicharge dynamics, leading to Coulomb blockade and Bloch oscillations. Exact duality exists to the standard overdamped phase dynamics of a Josephson junction, resulting in a dual shape of the current-voltage characteristic, with current and voltage changing roles. We demonstrate this duality with experiments which allow for a quantitative comparison with a theory that includes the effect of fluctuations due to finite temperature of the electromagnetic environment.

cond-mat.mes-hall↗

arXiv2006

Coulomb blockade of Cooper pair tunneling and parity effects in the Cooper pair transistor

We have measured the Cooper Pair Transistor (CPT) in a tunable electromagnetic environment consisting of four one-dimensional SQUID arrays. The transport properties of the CPT in the high impedance limit, Z_env>>R_Q=6.45~kΩ, are studied for different ratios of the Josephson coupling energy to the charging energy. As the impedance of the environment is increased, the current-voltage characteristic (IVC) of the CPT develops a Coulomb blockade of Cooper pair tunneling and the measured IVCs agree qualitatively with a theory based on quasicharge dynamics for a CPT. Increasing the impedance of the environment induces a transition in the modulation of the IVC with the gate charge from e-periodic to 2e-periodic.

cond-mat.mes-hall↗

arXiv2006

Single-mode heat conduction by photons

Electrical conductance is quantized in units of $σ_{\rm Q}=2e^2/h$ in ballistic one-dimensional conductors. Similarly, thermal conductance at temperature $T$ is expected to be limited by the quantum of thermal conductance of one mode, $G_{\rm Q} = \frac{πk_{\rm B}^2}{6\hbar}T$, when physical dimensions are small in comparison to characteristic wavelength of the carriers. The relation between $σ_{\rm Q}$ and $G_{\rm Q}$ obeys the Wiedemann-Franz law for ballistic electrons (apart from factor 2 in $σ_{\rm Q}$ due to spin degeneracy), but somewhat amazingly the same expression of $G_{\rm Q}$ is expected to hold also for phonons and photons, or any other particles with arbitrary exclusion statistics. The single-mode heat conductance is particularly relevant in nano-structures, e.g., when studying heat conduction by phonons in dielectric materials, or cooling of electrons in metals at very low temperatures. Here we show, based on our experimental results, that at low temperatures heat is transferred by photon radiation, in our case along a superconducting line, when electron-phonon as well as normal electronic heat conduction are frozen out. Thermal conductance is limited by $G_{\rm Q}$, approaching this value towards low temperatures. Our observation has implications on, e.g., performance and design of ultra-sensitive bolometers and electronic micro-refrigerators, whose operation is largely dependent on weak thermal coupling between the device and its environment.

cond-mat.mes-hall↗