arXiv subjects

Juha Hassel

Publications and source records attributed to Juha Hassel.

23 records · Page 2Linked to original sources

arXiv2014

Kinetic Inductance Magnetometer

Ultrasensitive magnetic field detection is utilized in the fields of science, medicine and industry. We report on a novel magnetometer relying on the kinetic inductance of superconducting material. The kinetic inductance exhibits a non-linear response with respect to DC current, a fact that is exploited by applying magnetic flux through a superconducting loop to generate a shielding current and a change in the inductance of the loop. The magnetometer is arranged into a resonator, allowing readout through a transmission measurement that makes the device compatible with radio frequency multiplexing techniques. The device is fabricated using a single thin-film layer of NbN, simplifying the fabrication process compared to existing magnetometer technologies considerably. Our experimental data, supported by theory, demonstrates a magnetometer having potential to replace established technology in applications requiring ultra-high sensitivity.

physics.ins-det↗

arXiv2013

Differential Bloch Oscillating Transistor Pair

We examine a Bloch Oscillating Transistor pair as a differential stage for cryogenic low-noise measurements. Using two oppositely biased, nearly symmetric Bloch Oscillating Transistors, we measured the sum and difference signals in the current gain and transconductance modes while changing the common mode signal, either voltage or current. From the common mode rejection ratio we find values $\sim 20$ dB even under non-optimal conditions. We also characterize the noise properties and obtain excellent noise performance for measurements having source impedances in the M$Ω$ range.

cond-mat.mes-hall↗

arXiv2013

Dynamics of Bloch oscillating transistor near bifurcation threshold

Tendency to bifurcate can often be utilized to improve performance characteristics of amplifiers or even to build detectors. Bloch oscillating transistor is such a device. Here we show that bistable behaviour can be approached by tuning the base current and that the critical value depends on the Josephson coupling energy $E_J$ of the device. We demonstrate record-large current gains for device operation near the bifurcation point at small $E_J$. From our results for the current gains at various $E_J$, we determine the bifurcation threshold on the $E_J$ - base current plane. The bifurcation threshold curve can be understood using the interplay of inter- and intra-band tunneling events.

cond-mat.mes-hall↗

arXiv2011

Dynamical Casimir effect in a Josephson metamaterial

Vacuum modes confined into an electromagnetic cavity give rise to an attractive interaction between the opposite walls. When the distance between the walls is changed non-adiabatically, virtual vacuum modes are turned into real particles, i.e. photons are generated out of the vacuum. These effects are known as the static and dynamical Casimir effect, respectively. Here we demonstrate the dynamical Casimir effect using a Josephson metamaterial embedded in a microwave cavity at 5.4 GHz. We achieve the non-adiabatic change in the effective length of the cavity by flux-modulation of the SQUID-based metamaterial, which results in a few percent variation in the velocity of light. We show that energy-correlated photons are generated from the ground state of the cavity and that their power spectra display a bimodal frequency distribution. These results are in excellent agreement with theoretical predictions, all the way to the regime where classical parametric effects cannot be of consequence.

cond-mat.mes-hall↗

arXiv2006

Characterization of a fabrication process for the integration of superconducting qubits and RSFQ circuits

In order to integrate superconducting qubits with rapid-single-flux-quantum (RSFQ) control circuitry, it is necessary to develop a fabrication process that fulfills at the same time the requirements of both elements: low critical current density, very low operating temperature (tens of milliKelvin) and reduced dissipation on the qubit side; high operation frequency, large stability margins, low dissipated power on the RSFQ side. For this purpose, VTT has developed a fabrication process based on Nb trilayer technology, which allows the on-chip integration of superconducting qubits and RSFQ circuits even at very low temperature. Here we present the characterization (at 4.2 K) of the process from the point of view of the Josephson devices and show that they are suitable to build integrated superconducting qubits.

cond-mat.supr-con↗