arXiv subjects

A. Marx

Publications and source records attributed to A. Marx.

51 records · Page 3Linked to original sources

arXiv2009

Fabrication Technology of and Symmetry Breaking in Superconducting Quantum Circuits

Superconducting quantum circuits are promising systems for experiments testing fundamental quantum mechanics on a macroscopic scale and for applications in quantum information processing. We report on the fabrication and characterization of superconducting flux qubits, readout dc SQUIDs, on-chip shunting capacitors, and high-quality coplanar waveguide resonators. Furthermore, we discuss the tunability and fundamental symmetry aspects inherent to all superconducting qubits, which can be regarded as artificial solid-state atoms. Comparing them to their natural counterparts, we discuss first and second-order energy shifts due to static control fields. Additionally, we present an intuitive derivation of the first- and second-order matrix elements for level transitions in the presence of a coherent microwave driving.

cond-mat.supr-con↗

arXiv2008

Two-resonator circuit QED: A superconducting quantum switch

We introduce a systematic formalism for two-resonator circuit QED, where two on-chip microwave resonators are simultaneously coupled to one superconducting qubit. Within this framework, we demonstrate that the qubit can function as a quantum switch between the two resonators, which are assumed to be originally independent. In this three-circuit network, the qubit mediates a geometric second-order circuit interaction between the otherwise decoupled resonators. In the dispersive regime, it also gives rise to a dynamic second-order perturbative interaction. The geometric and dynamic coupling strengths can be tuned to be equal, thus permitting to switch on and off the interaction between the two resonators via a qubit population inversion or a shifting of the qubit operation point. We also show that our quantum switch represents a flexible architecture for the manipulation and generation of nonclassical microwave field states as well as the creation of controlled multipartite entanglement in circuit QED. In addition, we clarify the role played by the geometric interaction, which constitutes a fundamental property characteristic of superconducting quantum circuits without counterpart in quantum-optical systems. We develop a detailed theory of the geometric second-order coupling by means of circuit transformations for superconducting charge and flux qubits. Furthermore, we show the robustness of the quantum switch operation with respect to decoherence mechanisms. Finally, we propose a realistic design for a two-resonator circuit QED setup based on a flux qubit and estimate all the related parameters. In this manner, we show that this setup can be used to implement a superconducting quantum switch with available technology.

cond-mat.mes-hall↗

arXiv2008

Two-photon probe of the Jaynes-Cummings model and symmetry breaking in circuit QED

Superconducting qubits behave as artificial two-level atoms and are used to investigate fundamental quantum phenomena. In this context, the study of multi-photon excitations occupies a central role. Moreover, coupling superconducting qubits to on-chip microwave resonators has given rise to the field of circuit QED. In contrast to quantum-optical cavity QED, circuit QED offers the tunability inherent to solid-state circuits. In this work, we report on the observation of key signatures of a two-photon driven Jaynes-Cummings model, which unveils the upconversion dynamics of a superconducting flux qubit coupled to an on-chip resonator. Our experiment and theoretical analysis show clear evidence for the coexistence of one- and two-photon driven level anticrossings of the qubit-resonator system. This results from the symmetry breaking of the system Hamiltonian, when parity becomes a not well-defined property. Our study provides deep insight into the interplay of multiphoton processes and symmetries in a qubit-resonator system.

cond-mat.supr-con↗

arXiv2007

Orthogonally-Driven Superconducting Qubit in Circuit QED

We consider a superconducting charge qubit coupled to distinct orthogonal electromagnetic field modes belonging to a coplanar wave guide and a microstrip transmission line resonators. This architecture allows the simultaneous implementation of a Jaynes-Cummings and anti-Jaynes-Cummings dynamics, a resonant method for generating mesoscopic qubit-field superpositions and for field-state reconstruction. Furthermore, we utilize this setup to propose a field measurement technique that is, in principle, robust to qubit dephasing and field relaxation due to a fast pre-measurement.

cond-mat.mes-hall↗

arXiv2006

On-Chip Microwave Fock States and Quantum Homodyne Measurements

We propose a method to couple metastable flux-based qubits to superconductive resonators based on a quantum-optical Raman excitation scheme that allows for the deterministic generation of stationary and propagating microwave Fock states and other weak quantum fields. Moreover, we introduce a suitable microwave quantum homodyne technique, with no optical counterpart, that enables the measurement of relevant field observables, even in the presence of noisy amplification devices.

cond-mat.mes-hall↗

arXiv2002

Epitaxial growth and transport properties of Sr$_2$CrWO$_6$ thin films

We report on the preparation and characterization of epitaxial thin films of the double-perovskite Sr$_2$CrWO$_6$ by Pulsed Laser Deposition (PLD). On substrates with low lattice mismatch like SrTiO$_3$, epitaxial Sr$_2$CrWO$_6$ films with high crystalline quality can be grown in a molecular layer-by-layer growth mode. Due to the similar ionic radii of Cr and W, these elements show no sublattice order. Nevertheless, the measured Curie temperature is well above 400 K. Due to the reducing growth atmosphere required for double perovskites, the SrTiO$_3$ substrate surface undergoes an insulator-metal transition impeding the separation of thin film and substrate electric transport properties.

cond-mat.str-el↗

arXiv2002

Low frequency 1/f noise in doped manganite grain-boundary junctions

We have performed a systematic analysis of the low frequency 1/f-noise in single grain boundary junctions in the colossal magnetoresistance material La_{2/3}Ca_{1/3}MnO_{3-delta}. The grain boundary junctions were formed in epitaxial La_{2/3}Ca_{1/3}MnO_{3-delta} films deposited on SrTiO_3 bicrystal substrates and show a large tunneling magnetoresistance of up to 300% at 4.2 K as well as ideal, rectangular shaped resistance versus applied magnetic field curves. Below the Curie temperature T_C the measured 1/f noise is dominated by the grain boundary. The dependence of the noise on bias current, temperature and applied magnetic field gives clear evidence that the large amount of low frequency noise is caused by localized sites with fluctuating magnetic moments in a heavily disordered grain boundary region. At 4.2 K additional temporally unstable Lorentzian components show up in the noise spectra that are most likely caused by fluctuating clusters of interacting magnetic moments. Noise due to fluctuating domains in the junction electrodes is found to play no significant role.

cond-mat.str-el↗

arXiv2001

Possible pseudogap behavior of electron doped high-temperature superconductors

We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y) and Pr(1.85)Ce(0.15)CuO(4-y) as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature Tc. In contrast, below Tc for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to the superconducting energy gap below Tc. Possible interpretations of this observation are the existence of a normal state pseudogap in the electron doped HTS or the existence of a spatially non-uniform superconducting state.

cond-mat.supr-con↗

arXiv2001

Comparison of Josephson vortex flow transistors with different gate line configurations

We performed numerical simulations and experiments on Josephson vortex flow transistors based on parallel arrays of YBa2Cu3O(7-x) grain boundary junctions with a cross gate-line allowing to operate the same devices in two different modes named Josephson fluxon transistor (JFT) and Josephson fluxon-antifluxon transistor (JFAT). The simulations yield a general expression for the current gain vs. number of junctions and normalized loop inductance and predict higher current gain for the JFAT. The experiments are in good agreement with simulations and show improved coupling between gate line and junctions for the JFAT as compared to the JFT.

cond-mat.supr-con↗

arXiv2000

Transport and Noise Characteristics of Submicron High-Temperature Superconductor Grain-Boundary Junctions

We have investigated the transport and noise properties of submicron YBCO bicrystal grain-boundary junctions prepared using electron beam lithography. The junctions show an increased conductance for low voltages reminiscent of Josephson junctions having a barrier with high transmissivity. The voltage noise spectra are dominated by a few Lorentzian components. At low temperatures clear two-level random telegraph switching (RTS) signals are observable in the voltage vs time traces. We have investigated the temperature and voltage dependence of individual fluctuators both from statistical analysis of voltage vs time traces and from fits to noise spectra. A transition from tunneling to thermally activated behavior of individual fluctuators was clearly observed. The experimental results support the model of charge carrier traps in the barrier region.

cond-mat.supr-con↗

arXiv2000

Voltage and temperature dependence of the grain boundary tunneling magnetoresistance in manganites

We have performed a systematic analysis of the voltage and temperature dependence of the tunneling magnetoresistance (TMR) of grain boundaries (GB) in the manganites. We find a strong decrease of the TMR with increasing voltage and temperature. The decrease of the TMR with increasing voltage scales with an increase of the inelastic tunneling current due to multi-step inelastic tunneling via localized defect states in the tunneling barrier. This behavior can be described within a three-current model for magnetic tunnel junctions that extends the two-current Julliere model by adding an inelastic, spin-independent tunneling contribution. Our analysis gives strong evidence that the observed drastic decrease of the GB-TMR in manganites is caused by an imperfect tunneling barrier.

cond-mat.str-el↗

arXiv1999

Anomalous Low Temperature Behavior of Superconducting Nd(1.85)Ce(0.15)CuO(4-y)

We have measured the temperature dependence of the in-plane London penetration depth lambda(T) and the maximum Josephson current Ic(T) using bicrystal grain boundary Josephson junctions of the electron-doped cuprate superconductor Nd(1.85)Ce(0.15)CuO(4-y). Both quantities reveal an anomalous temperature dependence below about 4 K. In contrast to the usual monotonous decrease (increase) of lambda(T) (Ic(T)) with decreasing temperature, lambda(T) and Ic(T) are found to increase and decrease, respectively, with decreasing temperature below 4 K resulting in a non-monotonous overall temperature dependence. This anomalous behavior was found to be absent in analogous measurements performed on Pr(1.85)Ce(0.15)CuO(4-y). From this we conclude that the anomalous behavior of Nd(1.85)Ce(0.15)CuO(4-y) is caused by the presence of the Nd3+ paramagnetic moments. Correcting the measured lambda(T) dependence of Nd(1.85)Ce(0.15)CuO(4-y) for the temperature dependent susceptibility due to the Nd moments, an exponential dependence is obtained indicating isotropic s-wave pairing. This result is fully consistent with the lambda(T) dependence measured for Pr(1.85)Ce(0.15)CuO(4-y).

cond-mat.supr-con↗

arXiv1998

Andreev Bound States in High Temperature Superconductors

Andreev bound states (ABS) at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of ABS in HTS employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBCO, BSCCO, and LSCO show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS NCCO no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented.

cond-mat.supr-con↗

arXiv1998

Observation of Bound Surface States in Grain Boundary Junctions of High Temperature Superconductors

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and NCCO. In all experiments the tunneling direction was along the CuO planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.

cond-mat.supr-con↗

arXiv1998

Superconducting NdCeCuO Bicrystal Grain Boundary Josephson Junctions

We have studied the electric transport properties of symmetrical [001] tilt NdCeCuO bicrystal grain boundary Josephson junctions (GBJs) fabricated on SrTiO bicrystal substrates with misorientation angles of 24 and 36.8 degree. The superconducting properties of the NdCeCuO-GBJs are similar to those of GBJs fabricated from the hole doped high temperature superconductors (HTS). The critical current density Jc decreases strongly with increasing misorientation angle. The products of the critical current Ic and the normal resistance Rn (about 0.1 mV at 4.2 K) are small compared to the gap voltage and fit well to the universal scaling law (IcRn is proportional to the square root of Jc) found for GBJs fabricated from the hole doped HTS. This suggests that the symmetry of the order parameter, which most likely is different for the electron and the hole doped HTS has little influence on the characteristic properties of symmetrical [001] tilt GBJs.

cond-mat.supr-con↗