Thermal hysteresis in a micron-size Superconducting Quantum Interference Device (µ-SQUID), with weak links as Josephson junctions, is an obstacle for improving its performance for magnetometery. Following the "hot-spot" model of Skocpol et al. [J. Appl. Phys. 45, 4054 (1974)] and by incorporating the temperature dependence of thermal conductivity of superconductor using a linear approximation, we find a much better agreement with the observed temperature dependence of the retrapping current in short superconducting Nb-based weak links and µ-SQUIDs. In addition, using the temperature dependence of the critical current, we find that above a certain temperature hysteresis disappears. We analyze the current-voltage characteristics and the weak link temperature variation in both the hysteretic and non-hysteretic regimes. We also discuss the effect of the weak link geometry in order to widen the temperature range of hysteresis-free operation.

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Superconducting quantum circuits are potential candidates to realize a large-scale quantum computer. The envisioned large density of integrated components, however, requires a proper thermal management and control of dissipation. To this end, it is advantageous to utilize tunable dissipation channels and to exploit the optimized heat flow at exceptional points (EPs). Here, we experimentally realize an EP in a superconducting microwave circuit consisting of two resonators. The EP is a singularity point of the effective Hamiltonian, and corresponds to critical damping with the most efficient heat transfer between the resonators without back and forth oscillation of energy. We observe a crossover from underdamped to overdamped coupling across the EP by utilizing photonassisted tunneling as an in situ tunable dissipative element in one of the resonators. These methods can be used to obtain fast dissipation, for example, for initializing qubits to their ground states. In addition, these results pave the way for thorough investigation of parity-time symmetry and the spontaneous symmetry breaking at the EP in superconducting quantum circuits operating at the level of single energy quanta.

The ability to generate single photons is not only an ubiquitous tool for scientific exploration with applications ranging from spectroscopy and metrology 1,2 to quantum computing 3 , but also an important proof of the underlying quantum nature of a physical process 4 . In the microwave regime, emission of anti-bunched radiation has so far relied on coherent control of Josephson qubits 5-8 , where precisely calibrated microwave pulses are needed, and the achievable bandwidth is limited by the anharmonicity of the qubit. Here, we demonstrate the operation of a bright on-demand source of quantum microwave radiation capable of emitting anti-bunched photons based on inelastic Cooper pair tunneling and driven by a simple DC voltage bias. It is characterized by its normalized second order correlation function of g (2) (0) ≈ 0.43 corresponding to anti-bunching in the single photon regime. Our source can be triggered and its emission rate is tunable in situ exceeding rates obtained with current microwave single photon sources by more than one order of magnitude.

Niobium nitride (NbN) is widely used in high-frequency superconducting electronics circuits because it has one of the highest superconducting transition temperatures (Tc ∼ 16.5 K) and largest gap among conventional superconductors. In its thin-film form, the Tc of NbN is very sensitive to growth conditions and it still remains a challenge to grow NbN thin film (below 50 nm) with high Tc. Here, we report on the superconducting properties of NbN thin films grown by high-temperature chemical vapor deposition (HTCVD). Transport measurements reveal significantly lower disorder than previously reported, characterized by a Ioffe-Regel (kF ℓ) parameter of ∼ 14. Accordingly we observe Tc ∼ 17.06 K (point of 50 % of normal state resistance), the highest value reported so far for films of thickness below 50 nm, indicating that HTCVD could be particularly useful for growing high quality NbN thin films.Niobium nitride (NbN)thin films -thanks to their high T c ∼ 16.5 K, superconducting energy gap ∆ ∼ 2.5 meV, and upper critical field B c2 ∼ 40 T -have been the subject of intense research for the last few decades, both on application and fundamental grounds. The combination of high T c and small coherence length (ξ(0) ∼ 5 nm) allows one to fabricate very thin NbN films with reasonably high T c , which is essential for, e.g, Superconducting Single Photon Detectors (see e.g. [1,2]). NbN thin films are used as hot electron bolometers and superconducting radio frequency cavities. NbN has higher kinetic inductance to other S-wave superconductors [3], which this helps fabricating superconducting micro wave resonators with high characteristic impedance and microwave kinetic inductance detectors. On the fundamental level, the effects of disorder on superconducting and normal state properties have been studied in NbN thin films [4][5][6]. Nano-wires, made from NbN thin films, have demonstrated thermal and quantum phase slips [7]a phenomenon of great interest in understanding onedimensional superconductivity. Further, the large superconducting energy gap of NbN can be explored in designing circuit Quantum Electrodynamics experiments in the THz frequency range.Thus, there has been a growing demand of high quality NbN thin films. Reactive DC magnetron sputtering from an Nb target in an argon and nitrogen atmosphere is most commonly used to deposit NbN on various substrates [8][9][10]. The main difficulty in this process, arises from the creation of atomic level nitrogen vacancies and from the formation of non-superconducting Nb 2 N and hexagonal phases. Besides, in the optimal parameter range, the high sputtering rate (typically ∼ 1-5 nm/sec) makes it difficult to control the thickness below 10 nm. Some other methods, where the superconducting properties of NbN thin films were probed, include Pulsed Laser Deposition (PLD) [11,12], Molecular Beam Epitaxy (MBE) [13] and Atomic Layer Deposition (ALD) [14]. In this regard, de-position of superconducting NbN films by high temperature chemical vapor deposition (HTCVD) is rather rare. HTCVD, comp...

The interaction between propagating microwave fields and Cooper-pair tunneling across a DC voltage-biased Josephson junction can be highly nonlinear. We show theoretically that this nonlinearity can be used to convert an incoming single microwave photon into an outgoing n-photon Fock state in a different mode. In this process, the electrostatic energy released in a Cooper-pair tunneling event is transferred to the outgoing Fock state, providing energy gain. The created multi-photon Fock state is frequency entangled and highly bunched. The conversion can be made reflectionless (impedance-matched) so that all incoming photons are converted to n-photon states. With realistic parameters multiplication ratios n > 2 can be reached. By two consecutive multiplications, the outgoing Fock-state number can get sufficiently large to accurately discriminate it from vacuum with linear post-amplification and power measurement. Therefore, this amplification scheme can be used as single-photon detector without dead time. PACS numbers: 42.65.-k, 74.50.+r, 85.25.Cp, 85.60.Gzexhibits the strong nonlinearity of this light-charge interaction most clearly, due to the absence of quasi-particle excitations. This system is understood to be a bright and robust on-chip source of nonclassical microwave radiation, such as of antibunched photons [34,35], nonclassical photon pairs [24,28,31,36], and multi-photon Fock states [40,41].We explore theoretically a process which converts a propagating photon in one mode to n photons in another arXiv:1612.07098v2 [cond-mat.mes-hall] 1 Feb 2018where the Josephson in-Hamiltonian has the formand the Josephson out-HamiltonianThe two Josephson frequencies account for different voltage biases of the islands, ω in J = 2eV in and ω out J = 2eV out . The free evolution resonator Hamiltonian is now

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International audienceNano-Superconducting Quantum Interference Devices (nano-SQUIDs) are usually fabricated from a single layer of either Nb or Al. We describe here a simple method for fabricating suspended nano-bridges in Nb/Al thin-film bilayers. We use these suspended bridges, which act as Josephson weak links, to fabricate nano-SQUIDs which show critical current oscillations at temperatures up to 1.5 K and magnetic flux densities up to over 20 mT. These nano-SQUIDs exhibit flux modulation depths intermediate between all-Al and all-Nb devices, with some of the desirable characteristics of both. The suspended geometry is attractive for magnetic single nanoparticle measurements