This thesis investigates the use of single Cooper-pair transistor (SCPT) for fast and sensitive detection of quasiparticle dynamics. This investigation is motivated by the possibility of quantum information processing using superconducting nanoscale circuits, such as the SCPT and the Cooper-pair-box (CPB). In the SCPT coherent charge transport can be temporarily halted due to quasiparticle tunnelling, known as quasiparticle poisoning. Quasiparticle poisoning can be reduced by the use of engineered island and lead gap energies. The thesis begins by reporting measurements of the superconducting gap in aluminium - aluminium-oxide - aluminium tunnel junctions, as a function of film thickness. We have observed an increase in the superconducting energy gap of aluminium with decreasing film thickness. This method is used to engineer the island and gap energies in a SCPT and consequently we observe reduced poisoning and a modification of the thresholds for finite bias transport processes. Radio-frequency reflectometry is used to perform high-bandwidth measurements of quasiparticle tunnelling in a gap engineered SCPT. A model for the radio-frequency (rf) operation of the SCPT is presented and shows close agreement with experiment. Thermal activation of the quasiparticle dynamics is investigated, and consequently, we are able to determine energetics of the poisoning and unpoisoning processes. This enables an effective quasiparticle temperature to be determined, allowing the poisoning to be parametrised. An investigation of the use of normal metal quasiparticle traps for suppression of quasiparticle poisoning in SCPT devices is performed. To date, there has been little quantitative information about the behaviour of quasiparticle traps even though they have been used extensively. The work presented serves to clarify the nature of quasiparticle trap performance. Finally the single-quasiparticle sensitivity of the SCPT is employed to directly probe a few quasiparticle gas in a small superconducting volume. The quasiparticle population is monitored both in the steady-state and under non-equilibrium conditions of injection. In the non-equilibrium regime the quasiparticle recombination time is accessed from the response of the SCPT to pulsed injection. Agreement to previous experimental studies of recombination times in aluminium is found.

This thesis investigates the use of single Cooper-pair transistor (SCPT) for fast and sensitive detection of quasiparticle dynamics. This investigation is motivated by the possibility of quantum information processing using superconducting nanoscale circuits, such as the SCPT and the Cooper-pair-box (CPB). In the SCPT coherent charge transport can be temporarily halted due to quasiparticle tunnelling, known as quasiparticle poisoning. Quasiparticle poisoning can be reduced by the use of engineered island and lead gap energies. The thesis begins by reporting measurements of the superconducting gap in aluminium - aluminium-oxide - aluminium tunnel junctions, as a function of film thickness. We have observed an increase in the superconducting energy gap of aluminium with decreasing film thickness. This method is used to engineer the island and gap energies in a SCPT and consequently we observe reduced poisoning and a modification of the thresholds for finite bias transport processes. Radio-frequency reflectometry is used to perform high-bandwidth measurements of quasiparticle tunnelling in a gap engineered SCPT. A model for the radio-frequency (rf) operation of the SCPT is presented and shows close agreement with experiment. Thermal activation of the quasiparticle dynamics is investigated, and consequently, we are able to determine energetics of the poisoning and unpoisoning processes. This enables an effective quasiparticle temperature to be determined, allowing the poisoning to be parametrised. An investigation of the use of normal metal quasiparticle traps for suppression of quasiparticle poisoning in SCPT devices is performed. To date, there has been little quantitative information about the behaviour of quasiparticle traps even though they have been used extensively. The work presented serves to clarify the nature of quasiparticle trap performance. Finally the single-quasiparticle sensitivity of the SCPT is employed to directly probe a few quasiparticle gas in a small superconducting volume. The quasiparticle population is monitored both in the steady-state and under non-equilibrium conditions of injection. In the non-equilibrium regime the quasiparticle recombination time is accessed from the response of the SCPT to pulsed injection. Agreement to previous experimental studies of recombination times in aluminium is found.

Abstract The quantum-mechanical effects typical for single atoms or molecules can be reproduced in micrometer-scale electric devices. In these systems the essential component is a small Josephson junction (JJ) consisting of two superconductors separated by a thin insulator. The quantum phenomena can be controlled in real time by external signals and have a great potential for novel applications. However, their fragility on uncontrolled disturbance caused by typical nearby environments is a drawback for quantum information science, but a virtue for detector technology. Motivated by this we have theoretically studied transistor kind of devices based on single-charge tunneling through small JJs. A common factor of the research is the analysis of the interplay between the coherent Cooper-pair (charge carriers in the superconducting state) tunneling and incoherent environmental processes. In the first work we calculate the current due to incoherent Cooper-pair tunneling through a voltage-biased small JJ in series with large JJs and compare the results with recent experiments. We are able to reproduce the main experimental features and interpret these as traces of energy levels and energy bands of the mesoscopic device. In the second work we analyze a similar circuit (asymmetric single-Cooper-pair transistor) but under the assumption that the Cooper-pair tunneling is mainly coherent. This predicts new resonant transport voltages in the circuit due to higher-order processes. However, no clear traces of most of them are seen in the experiments, and similar discrepancy is present also in the case of the symmetric circuit. We continue to study this problem by modeling the interplay between the coherent and incoherent processes more accurately using a density-matrix approach. By this we are able to demonstrate that in typical conditions most of these resonances are indeed washed out by strong decoherence caused by the environment. We also analyze the contribution of three typical weakly interacting dissipative environments: electromagnetic environment, spurious charge fluctuators in the nearby insulating materials, and quasiparticles. In the last work we model the dynamics of a current-biased JJ perturbed by a smaller JJ using a similar density-matrix approach. We demonstrate that the small JJ can be used also as a detector of the energy-band dynamics in a current biased JJ. The method is also used for modeling the charge transport in the Bloch-oscillating transistor.

No description available.

This thesis presents the results of an experimental study on single-charge effects in nanoscale Josephson junctions and Cooper pair transistors (CPTs). In nanoscale Josephson junctions the charging energy EC becomes significant at sub-Kelvin temperatures and single-charge effects, such as the Coulomb blockade of Cooper pair tunneling, influence the transport properties. In order to observe charging effects in a single Josephson junction, the impedance of the electromagnetic environment surrounding the junction has to be larger than the quantum resistance (RQ=h/4e2≈6.45kΩ). In this work the high impedance environment is obtained by biasing the sample under test (single Josephson junction or CPT) with four one-dimensional Josephson junction arrays having SQUID geometry. The advantage of this configuration is the possibility of tuning in situ the effective impedance of the electromagnetic environment. By applying a magnetic field perpendicular to the SQUID loops, the Josephson energy EJ of the SQUIDs is suppressed, resulting in an increase of the measured zero bias resistance of the arrays of several orders of magnitude (104< R0 (Ω) <109). This bias method enables the measurement of the same sample in environments with different impedance. As the impedance of the environment is increased, the current-voltage characteristics (IVCs) of the single Josephson junction and of the CPT show a well defined Coulomb blockade feature with a region of negative differential resistance, signature of the coherent tunneling of single Cooper pairs. The measured IVCs of a single Josephson junction with SQUID geometry in the high impedance environment show a qualitative agreement with the Bloch band theory as the EJ/EC ratio of the junction is tuned with the magnetic field. We also studied a single nontunable Josephson junction with strong coupling (EJ/EC > 1), where the exact dual of the overdamped Josephson effect is realized, resulting in a dual shape of the IVC, where the roles of current and voltage are exchanged. Here, we make for the first time a detailed quantitative comparison with a theory which includes the effect of fluctuations due to the finite temperature of the environment. The measurements on CPTs in the high impedance environment showed that the Coulomb blockade voltage is modulated periodically by the gate-induced charge. The gate-voltage dependence of the CPT changes from e-periodic to 2e-periodic as the impedance of the environment is increased. The high impedance environment reduces quasiparticle tunneling rates, thereby restoring the even parity of the CPT island. This behavior suggests that high impedance leads can be used to effectively suppress quasiparticle poisoning. / QC 20100928

La génération d'objets quantiques intriqués est inévitable pour l'exploration de l'information quantique dans les systèmes de la matière condensée. Alors que des expériences de non-localité on été menées, avec succès, à partir de photons intriqués, il n'a pas encore été possible de réaliser leurs analogues électroniques dans les matériaux solides, où les fermions sont les objets quantiques à intriqués. Les paires de Cooper dans les supraconducteurs sont des candidats prometteurs pour réaliser une source d'électrons intriqués. L'intrication non-local est la clé de voûte et elle peut être induite par le processus de réflexion d'Andreev croisée. Ce phénomène sépare une paire de Cooper en deux électrons, dont les spins sont intriqués, dans deux conducteurs séparés spatialement.J'ai étudié un système composé de trois électrodes supraconductrices (Al) connectées par un métal normal (Cu) formant ainsi une tri-jonction Josephson. De nouvelles structures, dans cette nano-structure hybride, apparaissent dans la conductance différentielle lorsque deux terminaux sont polarisées par des potentiels opposés l'un par rapport à l'autre. Ces anomalies correspondent à des phénomène de transport de paires de Cooper corrélées et sont consistantes avec la prédictiondes Quartets formés par la séparation simultanée de deux paires de Cooper d'un réservoir supraconducteur vers les deux autres électrodes.Dans le but d'expérimenter la cohérence quantique de ces structures, j'ai irradié la tri-jonction à l'aide d'une micro-onde de 14 GHz à très basse température. Des résonances Shapiro apparaissent lorsque la fréquence de la micro-onde est équivalente à la fréquence des courants AC Josephson générés en appliquant des tensions à travers la tri-jonction.Mon étude révèle que les anomalies de type Quartet présentent aussi des résonances Shapiro. Ce résultat démontre que le phénomène de Quartet est un mécanisme quantique cohérent confirmant la séparation cohérente de deux pairs de Cooper à longue portée. / A fundamental route for the exploration of solid state based quantum information is the generation of EPR pairs of quantum-entangled objects. Although experimental tests of nonlocality have been successfully conducted with pair of entangled photons, it has not yet been possible to realize an electronic analogue of it in the solid state, where fermions are the natural quantum objects. However, Cooper pairs in superconductors are known as suitable sources of entanglement. Non-local entanglement is the key and Crossed Andreev reflection process can provide it by converting a Cooper pair into two spin-entangled electrons located in separate conductors.I investigated a device where three superconducting (Al) electrodes are connected by a sub-micron normal metal (Cu) composing a Josephson three-junction. In this hybrid nanostructure, new sub-gap features appear in the differential conductance when two terminals are biased at opposite voltage with respect to the third one. These features correspond to correlated motion of Cooper pairs and are consistent with the prediction of Quartets formed by the simultaneous splitting of two Cooper pairs from one of the superconducting reservoirs and the emission of two phase correlated Cooper pairs in the two other electrodes. In order to probe the quantum coherence of such features, I irradiated the device with a microwave at 14 GHz down to very low temperature (100 mK). Well known Shapiros resonances (both integer and half-integer) are observed when the microwave frequency matches the frequency of the AC Josephson currents generated by applying differents voltages across the three-junction.My study reveals that the quartet feature also shows Shapiro-like resonances. This result demonstrates that the quartet feature is a true quantum mechanism and confirms the quartet scenario that implies the coherent splitting of two Cooper pairs.

This dissertation introduces some new properties of both superfluid phases of fermions with spin-orbit coupling (SOC) and superradiant phases of photons in an optical cavity. The effects of SOC on the phase transition between normal and superfluid phase are revealed; an unconventional crossover driven by SOC from the Bardeen-Cooper-Schrieffer (BCS) state to the Bose-Einstein condensate (BEC) state is verified in three different systems; and two kinds of excitations, a Goldstone mode and a Higgs mode, are demonstrated to occur in a quantum optical system. We investigate the BCS superfluid state of two-component atomic Fermi gases in the presence of three kinds of SOCs. We find that SOC drives a class of BCS to BEC crossover that is different from the conventional one without SOC. Here, we extend the concepts of the coherence length and Cooper-pair size in the absence of SOC to Fermi systems with SOC. We study the dependence of chemical potential, coherence length, and Cooper-pair size on the SOC strength and the scattering length in three dimensions (3D) (or the twobody binding energy in two dimensions (2D)) for three attractively interacting Fermi gases with 3D Rashba, 3D Weyl, and 2D Rashba SOC respectively. By adding a population imbalance to a Fermi gas with Rashba-type SOC, we also map out the finite-temperature phase diagram. Due to a competition between SOC and population imbalance, the finite-temperature phase diagram reveals a large variety of new features, including the expanding of the superfluid state regime and the shrinking of both the phase separation and the normal regimes. We find that the tricritical point moves toward a regime of low temperature, high magnetic field, and high polarization as the SOC strength increases. Besides Fermi fluids, this dissertation also gives a new angle of view on the superradiant phase in the Dicke model. Here, we demonstrate that Goldstone and Higgs modes can be observed in an optical system with only a few atoms inside a cavity. The model we study is the U(1)/Z2 Dicke model with N qubits (two-level atoms) coupled to a single photon mode.

The work presented in this thesis investigates transitions between quantum states in superconducting double dots (SDDs), a nanoscale device consisting of two aluminium superconducting islands coupled together by a Josephson junction, with each dot connected to a normal state lead. The energy landscape consists of a two level manifold of even charge parity Cooper pair states, and continuous bands corresponding to charge states with single quasiparticles in one or both islands. These devices are fabricated using shadow mask evaporation, and are measured at sub Kelvin temperatures using a dilution refrigerator. We use radio frequency reflectometry to measure quantum capacitance, which is dependent on the quantum state of the device. We measure the quantum capacitance as a function of gate voltage, and observe capacitance maxima corresponding to the Josephson coupling between even parity states. We also perform charge sensing and detect odd parity states. These measurements support the theoretical model of the energy landscape of the SDD. By measuring the quantum capacitance in the time domain, we observe random switching of capacitance between two levels. We determine this to be the stochastic breaking and recombination of single Cooper pairs. By carrying out spectroscopy of the bath responsible for the pair breaking we attribute it to black-body radiation in the cryogenic environment. We also drive the breaking process with a continuous microwave signal, and find that the rate is linearly proportional to incident power. This suggests that a single photon process is responsible, and demonstrates the potential of the SDD as a single photon microwave detector. We investigate this mechanism further, and design an experiment in which the breaking rate is enhanced when the SDD is in the antisymmetric state rather than the symmetric state. We also measure the quantum capacitance of a charge isolated double dot. We observe 2e periodicity, indicating the tunnelling of Cooper pairs and the lack of occupation of quasiparticle states. This work is relevant to the range of experiments investigating the effect of non-equilibrium quasiparticles on the operation of superconducting qubits and other superconducting devices.

Un conducteur est bien caractérisé par sa conductance donnée par la formule de Landauer. Toutefois, le bruit contient davantage d'informations. Il mesure les fluctuations temporelles du courant autour de sa valeur moyenne. De plus, le signe des corrélations croisées est lié à la statistique des porteurs de charge. Cette thèse aborde deux principaux thèmes à savoir le transport dans les liquides de Luttinger et dans les structures hybrides. Dans la première partie, nous commençons par donner une vision détaillée des liquides de Luttinger et des systèmes qu'ils modélisent. Nous parlons également du formalisme de Keldysh permettant de traiter des problèmes hors équilibre. Puis, nous rentrons dans le vif du sujet en étudiant l'effet de la largeur d'un contact ponctuel quantique sur le courant de rétrodiffusion entre les deux états de bords de l'effet Hall quantique. L'augmentation de la largeur du contact ponctuel quantique entraîne une forte diminution du courant de rétrodiffusion. Dans un autre chapitre, nous développons une technique permettant l'utilisation d'un circuit RLC couplé inductivement au circuit mésoscopique pour détecter les corrélations de courant en régime photo-assisté. La mesure de ces corrélations s'effectue à travers la charge aux bornes du condensateur. Dans une deuxième partie, nous consacrons notre étude au transport non-local dans les structures hybrides supraconductrices. L'étude de la réflexion d'Andreev croisée y est détaillée. Finalement, nous étudions une structure en double point quantique reliée à deux électrodes en métal normal et une supraconductrice. Nous mettons en avant la séparation des paires de Cooper en mesurant simultanément les courants de branchement et les corrélations croisées. Nous démontrons que dans le régime antisymétrique, c'est-à-dire lorsque les deux points quantiques ont des niveaux d'énergie opposés par rapport au potentiel chimique du supraconducteur, la réflexion d'Andreev croisée est optimisée. / The conductance is the most natural quantity to characterize a quantum conductor. It is given by the Landauer Formula. However, noise contains more information. It measures the current fluctuations around its average value. Moreover, the sign of the crossed correlations is related to the statistics of carriers. This thesis broaches two main topics which are the transport in the quantum Hall effect and in hybrid circuits.First, we start by introducing the Luttinger liquid and the systems which are modelized by them. Also, we discuss the Keldysh formalism in order to treat nonequilibrium problems. Then, we study the effect of the width of a quantum point contact on the backscattering current between two edge states of the quantum Hall effect. By increasing the width of the quantum point contact, we show that the backscattering current is strongly reduced. In another chapter, we develop a technique to use a RLC circuit inductively coupled to a mesoscopic circuit in order to measure the current correlations in the photo-assisted regime. The measurement of these correlations is performed through the charge on the capacitor plates.Secondly, we present the non-local transport in hybrid structures. The mechanism of Crossed Andreev Reflection is explained. Finally, we study a double quantum dot connected to two normal leads and a superconducting lead. We introduce the separation of the Cooper pair by measuring together the branching currents and the crossed correlations. We demonstrate that in the anti-symmetric regime (the energy level of the two quantum dots have opposite values with respect to the chemical potential of the superconducting lead), crossed Andreev reflection is optimized.

Cette thèse utilise les micro-ondes pour étudier des circuits de boîtes quantiques à base de nanotubes de carbone. Dans une première expérience, l'excitation micro-onde est appliquée directement sur une électrode du circuit pour une boîte quantique dans le régime Kondo. Nous réalisons la première caractérisation fréquence-amplitude de la conductance Kondo à biais nul. Des données préliminaires sont en accord avec la prédiction d'universalité. Nous présentons deux autres expériences, où les boîtes quantiques sont insérées dans des résonateurs micro-ondes. Les photons de la cavité sondent la résistance de relaxation de charge et l'émission de photons dans une boîte quantique couplée à des réservoirs normaux et supraconducteurs, en présence de répulsion coulombienne. Nos observations valident une modélisation en termes de réponse linéaire du circuit. Nous présentons aussi la première implémentation d'une lame séparatrice à paires de Cooper en cavité. Le régime de couplage fort est atteint, une première avec des circuits de boîtes quantiques. Nos résultats renforcent l'idée que l'électrodynamique quantique mésoscopique est une boîte à outils fructueuse, aussi bien dans le contexte du domaine du transport quantique que dans celui de l'information quantique. / This thesis uses microwaves as probe of carbon nanotube quantum dot circuits. In a first experiment, a microwave excitation is directly applied to a circuit electrode for a quantum dot in the Kondo regime. We provide the first frequency-amplitude characterisation of the Kondo zero-bias conductance. Preliminary data are consistent with predicted universal behaviour. We present two other experiments, where quantum dot circuits are embedded in microwave resonators. Cavity photons probe charge relaxation resistance and photon-emission in a quantum dot coupled to normal and superconducting reservoirs in presence of Coulomb repulsion. Our observations validate a modelling in terms of the circuit linear response. We also present the first implementation of a Cooper pair splitter in cavity. The strong coupling regime is achieved, a premiere with quantum dot circuits. Our findings support the idea, that mesoscopic quantum electrodynamics is a fruitful toolbox in the context of both fields of quantum transport and quantum information science.