[en] Micron-size superconducting quantum interfering device (μ - SQUID) is fabricated using Focused Ion Beam (FIB). Nb thin film of thickness 50 nm is used as base material. The I-V characteristic at lower temperature shows a good Josephson characteristic. Hysteresis is present at lower temperature and above a threshold temperature it disappears. The critical current shows oscillation with the external magnetic field. (author)

[en] This paper presents design and development of a broad-range magnetic (electron energy ∼15 MeV to 1 GeV) for laser wakefield electron accelerator (LWFA) with unique features such as simultaneous recording of the electron beam profile and spectrum with unambiguous determination of the undeflected electron beam position. Spectrograph was set up and characterized in LWFA experiment performed using 150TW Ti:Sapphire laser system and gas-jet target. Further, Geant-4 simulations were performed to study transport through various components of the magnetic spectrograph. (author)

[en] Laser driven plasma based electron acceleration is seen as a potential candidate for development of future compact accelerators. Generation of electron beams in excess of GeV energy has been demonstrated using laser Wakefield acceleration (LWFA) scheme. In LWFA electrons are accelerated through laser wakefield i.e. electric field associated with the nonlinear plasma wave. There is also possibility of direct energy transfer from the laser field to the electrons in the plasma which is known as direct laser acceleration (DLA). One such DLA mechanism is betatron resonance acceleration which could be applicable for relatively longer laser pulses. In this scheme electrons oscillate at betatron frequency in the self-generated static fields of the ion channel created by high-intensity laser pulse in underdense plasma. When this oscillation frequency matches with the laser frequency, resonant energy transfer occurs directly from the laser field to the electrons, which leads to generation of relativistic electron beams. In this paper, we report an experimental study on laser driven electron acceleration using 150TW, 25fs Ti: Sapphire laser system at RRCAT, Indore, interacting with He gas-jet target of 1.2 mm length. For the present study laser pulse was stretched to a duration of ∼ 200 fs, and focussed using an off axis parabolic mirror (focal spot size ∼ 15 μm x 30 μm) to an intensity of ∼2.1x10"1"8 W/cm"2

[en] We present a new process for fabricating vertical NbN–MgO–NbN Josephson junctions using self-aligned silicon nitride spacers. It allows for a wide range of junction areas from 0.02 to several 100 μm². At the same time, it is suited for the implementation of complex microwave circuits with transmission line impedances ranging from $<<!--\; <\; -->1\mathrm{\Omega <!--\; ??\; -->}$ to $><!--\; >\; -->1\mathrm{k}\mathrm{\Omega <!--\; ??\; -->}$. The constituent thin films and the finished junctions are characterized. The latter are shown to have high gap voltages ($><!--\; >\; -->4\mathrm{m}\mathrm{V}$) and low sub-gap leakage currents. (paper)

[en] Nano-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

[en] The design and development of a magnetic spectrograph covering a broad electron energy range (few MeV to 1 GeV) for laser plasma accelerators are presented. Two phosphor screens, one in the forward and other in the side direction, were used to record electrons spectra after the dispersion from the magnet. The third phosphor screen was kept in front of the magnet to simultaneously record the electron beam profile. GEANT4 simulations were performed to estimate extra divergence in the electron beam introduced by the phosphor screen kept in front of the magnet. In the magnetic spectrograph setup, the electron beam was transported through a Ti foil of thickness 54 μm from the vacuum chamber, therefore, GEANT4 simulation was performed to estimate the contribution of secondary particles emitted in the front phosphor signal to correctly measure the beam charge. Next, it was used to simulate and design a suitable electron beam dump. Finally, the spectrograph was used to characterize electron beams generated in laser plasma accelerator driven by 120 fs Ti: Sapphire laser pulse and He and Ar gas jet targets.

[en] Niobium nitride (NbN) is widely used in high-frequency superconducting electronics circuits because it has one of the highest superconducting transition temperatures ($T_{\mathrm{c}}\sim <!--\; ???\; -->16.5\mathrm{K}$) and largest gap among conventional superconductors. In its thin-film form, the T _c of NbN is very sensitive to growth conditions and it still remains a challenge to grow NbN thin films (below 50 nm) with high T _c. 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 parameter ($k_{\mathrm{F}}\mathrm{\ell <!--\; ???\; -->}$) ∼ 12. Accordingly we observe $T_{\mathrm{c}}\sim <!--\; ???\; -->17.06\mathrm{K}$ (point of 50% of normal state resistance), the highest value reported so far for films of thickness 50 nm or less, indicating that HTCVD could be particularly useful for growing high quality NbN thin films. (paper)

[en] An experimental study of laser driven electron acceleration in N₂ and N₂-He mixed gas-jet targets using laser pulses with durations of ∼60–70 fs is presented. Generation of relativistic electron beams with quasi-thermal spectra was observed at a threshold plasma density of ∼1.6 × 10¹⁸ cm⁻³ in the case of pure N₂. The threshold density was found to increase with increasing doping concentrations of He. At an optimum fraction of 50% of He in N_2, generation of quasi-monoenergetic electron beams was observed at a comparatively higher threshold density of ∼2 × 10¹⁸ cm⁻³, with an average peak energy of ∼168 MeV, average energy spread of ∼21%, and average total beam charge of ∼220 pC. The electron acceleration could be attributed to direct laser acceleration as well as the hybrid mechanism. The observation of an optimum fraction of He in N₂ (in turn threshold plasma density) for comparatively better quality electron beam generation can be understood in terms of the plasma density dependent variation in the dephasing rate of electrons with respect to the transverse oscillating laser field. Results are also supported by 2D particle-in-cell simulations performed using the code EPOCH. (paper)

[en] The generation of relativistic electron beams with quasi-thermal energy distribution (maximum energy: 30 MeV) by the interaction of a Ti:sapphire laser pulse of 200 fs duration, focussed to an intensity of ∼2.1 × 10¹⁸ W cm⁻², with an underdense (electron density ∼3.6 × 10¹⁹ to ∼1.1 × 10²⁰ cm⁻³) He gas-jet plasma was observed. We observed two stages of self-focusing of the laser pulse in the plasma. Two groups of accelerated electrons associated with these two stages of laser channeling were also observed, and are attributed to the betatron resonance acceleration mechanism. This is supported by 2D PIC simulations performed using the EPOCH code and a detailed theoretical analysis. Further, the generation of quasi-monoenergetic (ΔE/E ∼ 10%–20%) electron beams with a peak energy of ∼17–22 MeV was also observed, even with such long laser pulses, although with a low probability of occurrence. (paper)

[en] Superconducting quantum circuits hold great potential in providing revolutionizing practical applications such as quantum sensing or computing. However, in many cases noise limits the operation and the fidelity of these circuits. Here we introduce a concept that exploits noise instead of trying to reduce it. Our concept uses photon-assisted single-electron tunneling as a controlled source for dissipation in superconducting qubits. We show how the recently developed quantum-circuit refrigerator $[$1$]$, QCR, is suitable to control the dynamics of superconducting qubits. In our experiments, the QCR works as a voltage-controlled environmental bath for the qubit. The qubit-bath coupling strength can be tuned over several orders of magnitude on a nanosecond timescale. Such a tunable environment is promising for fast qubit reset and studies of dissipative open quantum circuits. Our highly integrable circuit architecture may prove useful in the initialization of qubit arrays and in dissipation-assisted quantum annealing.