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Coherent Superconducting Quantum Circuits

Working Group 2.42


The focus of our group is on coherent superconducting quantum circuits. Such circuits are built up from just four basic ingredients: resistors, capacitors, inductors, and Josephson junctions. But arguably the one, which opens a plethora of applications is the Josephson junction. In its essence being a non-linear inductor, it allows us to build superconducting qubits, parametric amplifiers with added noise close to the quantum limit, and many more circuits, in which current and voltage exhibit quantum mechanical behavior.

All our circuits have characteristic energies in the microwave frequency range, so we can control them using rf-signals. In our research we combine FEM and numerical simulation, nanofabrication in Opens internal link in current windowPTB’s clean room center, milli-Kelvin cryogenics, and microwave measurements at femto-Watt powers.

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We currently work on three main topics: superconducting quantum bits, Josephson parametric amplifiers, and the physics of small Josephson junctions in very high impedance environments. Besides the fundamental device physics, we are interested in working towards metrology for, and metrological applications of such circuits.

Superconducting quantum bits

Superconducting quantum bits - or qubits for short - are promising building blocks for the realization of a fault-tolerant, universal quantum computer. Over the last two decades the understanding of the physics of these devices increased greatly and the technology has matured. However, there remain plenty of questions and avenues to explore on the way to useful quantum information processing devices. Within the national large-scale project QSolid (www.q-solid.de, BMBF Grant No. 13N16158) we aim to improve the control and readout of superconducting qubits, as well as using these capabilities to sense rf-power in an cryogenic environment with unprecedented accuracy.

 Josephson parametric amplifiers

Typical signal powers for the readout of superconducting qubits are on the order of femto Watts, a few photons in the language of circuit quantum electrodynamics (cQED). Fortunately, the Josephson junction does not only allow to build superconducting qubits, but it also enables the design and realization of specially tailored metamaterials. These metamaterials give rise to three- and four wave mixing effects. Both effects are well known from quantum optics, but here we realize them in the microwave frequency domain. Specifically, we work on traveling-wave parametric amplifiers based on rf-SQUIDS for the qBriqs project (BMBF Grant No. 13N15949), the next generation of traveling-wave parametric amplifiers together with European partners in the project TruePA (www.truepa.eu, Horizon Europe), and so-called dimer Josephson-junction array amplifiers for the initial stages of QSolid. The common denominator in all projects is the question of what limits the noise performance, saturation power, and attainable bandwidth – and how to overcome these limits.

Owing to the accelerating commercialization of superconducting quantum circuits the standardization of measurements to determine the characteristics of superconducting parametric amplifiers is more and more becoming a metrological question – a task we work on within the European Qu-Test consortium (Horizon Europe Project 101080035).

Bloch Oscillations in small Josephson junctions

Small Josephson junctions (JJ) with dimensions below 100 nm show intriguing DC-transport properties, if embedded in a bias circuitry with characteristic impedance larger than the superconducting resistance quantum RQ = h/4e2 ≈ 6.5 kΩ. The bias current I in such devices can give rise to so-called Bloch oscillations whose frequency f is fundamentally proportional to the current via the elementary charge e, I = 2e f: the effect of interest for electrical metrology applications. In an ongoing project led by Dr. Sergey Lotkhov (DFG Grant No. 445530728), we developed a circuit layout consisting of a small Al/AlOx/Al-Josephson junction SQUID and high characteristic impedance bias circuitry fabricated from granular aluminum and oxidized titanium. DC-transport measurements show a clear signature of Bloch oscillations: a back-bending area in the voltage-current characteristics. We are aiming to couple these intrinsic oscillations to an external GHz-frequency source to obtain, and investigate so-called dual Shapiro current steps.

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If you are curious about our research, metrology involving coherent superconducting quantum circuits, or interested in collaborating, don't hesitate to get in touch.

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