We theoretically put forward the concept of a phase-controlled superconducting heat-flux quantum modulator. Its operation relies on phase-dependent heat current predicted to occur in temperature-biased Josephson tunnel junctions. The device behavior is investigated as a function of temperature bias across the junctions, bath temperature, and junctions asymmetry as well. In a realistic Al-based setup the structure could provide temperature modulation amplitudes up to ∼50 mK with flux-to-temperature transfer coefficients exceeding ∼125 mK/Φ0 below 1 K, and temperature modulation frequency of the order of a few MHz. The proposed structure appears as a promising building-block for the implementation of caloritronic devices operating at cryogenic temperatures.
We theoretically put forward the concept of a phase-controlled superconducting heat-flux quantum modulator. Its operation relies on phase-dependent heat current predicted to occur in temperature-biased Josephson tunnel junctions. The device behavior is investigated as a function of temperature bias across the junctions, bath temperature, and junctions asymmetry as well. In a realistic Al-based setup the structure could provide temperature modulation amplitudes up to ∼50 mK with flux-to-temperature transfer coefficients exceeding ∼125 mK/Φ0 below 1 K, and temperature modulation frequency of the order of a few MHz. The proposed structure appears as a promising building-block for the implementation of caloritronic devices operating at cryogenic temperatures.
We introduce a superconducting transconductance amplifier based on the thermal modulation of a SINIS (Superconductor-Insulator-Normal metal-Insulator-Superconductor) configuration. The device is composed of a normal metal island interfaced with two superconducting leads through tunnel barriers, establishing a voltage-biased symmetric SINIS setup. An additional NIS junction connects the island to a third superconducting lead, which serves as input. When the input voltage surpasses the superconducting gap, the resultant injection of quasiparticles increases the electronic temperature of the island, thereby modulating the SINIS current. We perform numerical analyzes of the device performance, influenced by input voltage, frequency, and bath temperature. At bath temperatures below 250 mK, the device shows a transconductance exceeding 4 mS and a current gain exceeding 45 dB. Both gain and transconductance maintain their levels up to 1 MHz, but decrease at higher frequencies, with a -3 dB cutoff around 10 MHz, and an average power dissipation of approximately 5 nW. Our simulations reveal a fully voltage-controlled, three-terminal superconducting amplifier characterized by high transconductance and gain, achieved through thermally mediated signal transduction. This architectural design presents a promising avenue for cryogenic amplification with reduced power dissipation and compatibility with current superconducting electronic systems.
We theoretically study the behavior of the critical current of a thermally biased tunnel Josephson junction with a particular design, in which the electrodes of the junction are enclosed in two different superconducting loops pierced by independent magnetic fluxes. In this setup, the superconducting gaps can be modified independently through the magnetic fluxes threading the loops. We investigate the response of the device as a function of the magnetic fluxes, by changing the asymmetry parameter, i.e., the ratio between the zero-temperature superconducting gaps $\ensuremath{\delta}={\mathrm{\ensuremath{\Delta}}}_{10}/{\mathrm{\ensuremath{\Delta}}}_{20}$ and the temperatures of the two rings. We show a magnetically controllable steplike response of the critical current, which emerges even in a symmetric junction, $\ensuremath{\delta}=1$. Finally, we discuss the optimal working conditions and the high response of the critical current to small changes in the magnetic flux, reporting good performance of a magnetic flux-to-critical current transducer, with a high transfer function that depends on the operating point, the temperature gradient, and the quality of the junction.
The research work discussed in this thesis deals with the study of superconducting Josephson qubits. Superconducting qubits are solid-state artificial atoms which are based on lithographically defined Josephson tunnel junctions properties. When sufficiently cooled, these superconducting devices exhibit quantized states of charge, flux or junction phase depending on their design parameters. This allows to observe coherent evolutions of their states. The results presented can be divided into two parts. In a first part we investigate operations of superconducting qubits based on the quantum coherence in superconducting quantum interference devices (SQUID). We explain experimental data which has been observed in a SQUID subjected to fast, large-amplitude modifications of its effective potential shape. The motivations for this work come from the fact that in the past few years there have been attempts to interpret the supposed quantum behavior of physical systems, such as Josephson devices, within a classical framework. Moreover, we analyze the possibility of generating GHZ states, namely maximally entangled states, in a quantum system made out of three Josephson qubits. In particular, we investigate the possible limitations of the GHZ state generation due to coupling to bosonic baths. In the second part of the thesis we address a particular cause of decoherence of flux qubits which has been disregarded until now: thermal gradients, which can arise due to accidental non equilibrium quasiparticle distributions. The reason for these detrimental effects is that heat currents flowing through Josephson tunnel junctions in response to a temperature gradient are periodic functions of the phase difference between the electrodes. The phase dependence of the heat current comes from Andreev reflection, namely an interplay between the quasiparticles which carry heat and the superconducting condensate which is sensitive to the superconducting phase difference. Generally speaking, the flux qubit states are characterized by different values of the phase difference through their Josephson junctions. Consequently, the phase-dependent thermal current through a device subject a temperature gradient is related to the phase-dependent qubit states. We study how the thermal currents change according to the state of the qubits hence yielding a measurement of the qubit state. This in turn leads to an impact of temperature gradient on the dynamics of the system. We show that flux qubits in the Delft qubit design can have limitations of the decoherence time to the order of microseconds as a result of this newly discovered source of decoherence. In contrast, the fluxonium qubit is found to be well protected due to its superinductance.
Abstract Superconducting circuits represent a versatile playground to explore novel quantum phenomena and a platform to realize quantum bits (qubits). Superconducting qubits hinge on the coherent tunneling of Cooper pairs across the insulating barrier of a Josephson junction. This qubit implementation is, however, plagued with the variability of the fabricated junction, along with the need for magnetic tunability of the qubit transition frequency. Alternatively, a dual to the Josephson effect—referred to as quantum phase slips—allows for a coherent coupling of flux states and has been used to build phase-slip qubits. In this paper, we propose coupling flux states of a superconducting ring using a bias voltage. By solving the time-dependent Ginzburg–Landau equations, we demonstrate that the bias voltage locally suppresses the density of Cooper pairs, thereby forming a weak link where phase slips occur. Moreover, we show that a voltage-biased superconducting ring serves as an electrically-controlled magnetic memory. The work presented in this paper is an important step towards realizing electrically-tunable superconducting devices, such as qubits and SQUIDs.
A bistable superconducting quantum interference device (SQUID) exclusively comprising Josephson junctions (JJs) is proposed. The design integrates two 2Φ-JJs within the loop structure, while the state of the SQUID is monitored through a conventional 2π-periodic JJ (0-JJ). This configuration eliminates the need for geometric inductors, leveraging the nonlinear phase dynamics for bistability. Extensive simulations confirm robust bistable operation under controlled external flux bias, positioning the proposed SQUID as an area-efficient building block for superconducting memory cells, sensors for various applications, and SFQ-compatible logic elements.