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In quantum information processing, a tension between two different tasks occurs: while qubits' states can be preserved by isolating them, quantum gates can be realized only through qubit-qubit interactions. In arrays of qubits, weak coupling leads to states being spatially localized and strong coupling to delocalized states. Here, we study the average energy level spacing and the relative entropy of the distribution of the level spacings (Kullback-Leibler divergence from Poisson and Gaussian Orthogonal Ensemble) to analyze the crossover between localized and delocalized (chaotic) regimes in linear arrays of superconducting qubits. We consider both transmons as well as capacitively shunted flux qubits, which enables us to tune the qubit anharmonicity. Arrays with uniform anharmonicity, comprising only transmons or flux qubits, display remarkably similar dependencies of level statistics on the coupling strength. In systems with alternating anharmonicity, the localized regime is found to be more resilient to the increase in qubit-qubit coupling strength in comparison to arrays with a single qubit type. This result supports designing devices that incorporate different qubit types to achieve higher performances., Comment: 11 pages, 9 figures

Superconducting circuits constitute a promising platform for future implementation of quantum processors and simulators. Arrays of capacitively coupled transmon qubits naturally implement the Bose-Hubbard model with attractive on-site interaction. The spectrum of such many-body systems is characterised by low-energy localised states defining the lattice analog of bright solitons. Here, we demonstrate that these bright solitons can be pinned in the system, and we find that a soliton moves while maintaining its shape. Its velocity obeys a scaling law in terms of the combined interaction and number of constituent bosons. In contrast, the source-to-drain transport of photons through the array occurs through extended states that have higher energy compared to the bright soliton. For weak coupling between the source/drain and the array, the populations of the source and drain oscillate in time, with the chain remaining nearly unpopulated at all times. Such a phenomenon is found to be parity dependent. Implications of our results for the actual experimental realisations are discussed.

Not so long ago, thermoelectricity in superconductors was believed to be possible only by breaking explicitly the particle-hole symmetry. Recently, it has been theoretically predicted that a superconducting tunnel junction can develop bipolar thermoelectric phenomena in the presence of a large thermal gradient owing to non-equilibrium spontaneous PH symmetry breaking. The experimental realization of the first thermoelectric Josephson engine then followed. Here, we give a more extended discussion and focus on the impact of the Josephson contribution on thermoelectricity modulating the Cooper pairs transport in a double-loop SQUID. When the Cooper pairs current prevails on the quasiparticle one, the Josephson contribution short-circuits the junction thereby screening the thermoelectric effect. We demonstrate that the thermoelectric generation due to the pure quasiparticle transport is phase-independent, once Josephson contribution is appropriately removed from the net current measured. At the same time, we investigate an additional metastable state at V\simeq0 determined by the presence of the Josephson coupling, which peculiarly modifies the hysteretic behavior of our thermoelectric engine realized. At the end, we also discuss how the current-voltage characteristics are affected by the presence of multiple thermoelectric elements, which improve the generated output power., Comment: 16 pages, 11 figures

We investigate the thermoelectric properties of a hybrid nanodevice composed by a 2D carbon based material and a superconductor. This system presents nonlinear bipolar thermoelectricity as induced by the spontaneous breaking of the Particle-Hole (PH) symmetry in a tunnel junction between a BiLayer Graphene (BLG) and a Bardeen-Cooper-Schrieffer (BCS) superconductor. In this scheme, the nonlinear thermoelectric effect, predicted and observed in SIS' junctions is not affected by the competitive effect of the Josephson coupling. From a fundamental perspective, the most intriguing feature of this effect is its bipolarity. The capability to open and control the BLG gap guarantees improved thermoelectric performances, that reach up to 1 mV/K regarding the Seebeck coefficient and a power density of 1 nW/$\mu$m$^2$ for temperature gradients of tens of Kelvins. Furthermore, the externally controlled gating can also dope the BLG, which is otherwise intrinsically PH symmetric, giving us the opportunity to investigate the bipolar thermoelectricity even in presence of a controlled suppression of the PH symmetry. The predicted robustness of this system could foster further experimental investigations and applications in the near future, thanks to the available techniques of nano-fabrication.

Designing the spatial profile of the superconducting gap -- gap engineering -- has long been recognized as an effective way of controlling quasiparticles in superconducting devices. In aluminum films, their thickness modulates the gap; therefore, standard fabrication of Al/AlOx/Al Josephson junctions, which relies on overlapping a thicker film on top of a thinner one, always results in gap-engineered devices. Here we reconsider quasiparticle effects in superconducting qubits to explicitly account for the unavoidable asymmetry in the gap on the two sides of a Josephson junction. We find that different regimes can be encountered in which the quasiparticles have either similar densities in the two junction leads, or are largely confined to the lower-gap lead. Qualitatively, for similar densities the qubit's excited state population is lower but its relaxation rate higher than when the quasiparticles are confined; therefore, there is a potential trade-off between two desirable properties in a qubit., Comment: Revised version. To be published in PRX Quantum

Thermoelectric effects in metals are typically small due to the nearly-perfect particle-hole (PH) symmetry around their Fermi surface [1, 2]. Despite being initially considered paradoxical [3], thermophase effects [4-8] and linear thermoelectricity [9] in superconducting systems were identified only when PH symmetry is explicitly broken [10-14]. Here, we experimentally demonstrate that a superconducting tunnel junction can develop a very large bipolar thermoelectric effect in the presence of a nonlinear thermal gradient thanks to spontaneous PH symmetry breaking [15]. Our junctions show a maximum thermovoltage of $\pm150\; \mu$ V at $\pm650$ mK, directly proportional to the superconducting gap. Notably, the corresponding Seebeck coefficient of $\pm300\; \mu$V/K is roughly $10^5$ times larger than the one expected for a normal metal at the same temperature [16, 17]. Moreover, by integrating our junctions into a Josephson interferometer, we realize a bipolar thermoelectric Josephson engine (BTJE) [18] with phase-coherent thermopower control [19]. When connected to a generic load, the BTJE generates a phase-tunable electric power up to about 140 mW/m$^2$ at subKelvin temperatures. In addition, our device implements the prototype for a persistent thermoelectric memory cell, written or erased by current injection [20]. We expect that our findings will trigger thermoelectricity in PH symmetric systems, and will lead to a number of groundbreaking applications in superconducting electronics [21], cutting-edge quantum technologies [22-24] and sensing [25]., Comment: 18 pages, 10 figures