Technological progress goes hand in hand with incessant advances in computing power. Modern personal electronic devices have the computational power of a supercomputer from just a decade ago. Computer-aided designs, logistics, data analysis and cognitive computing have become an essential part of modern life. However, current progress in down-scaling transistors reaches physical limits when approaching atomic scales, and heat dissipation becomes a severe issue with increasing transistor densities.

Above all, certain complex physical problems such as computing energy spectra, correlations or time dynamics in molecular and condensed matter systems are beyond the reach of classical computers. These computations require exponential resources. The reason for this is the exponential growth of Hilbert space with the number of particles, preventing the computation of systems with more than a modest number of 50 particles. With a few more particles, even future supercomputers are destined to fail.

In contrast, a quantum computer has the potential to compute ground-state energies, energy spectra, time dynamics and correlations of such systems efficiently. Moreover, it is expected that certain types of optimization problems with application in logistics, time-scheduling and others can be solved more efficiently with the help of quantum effects.

“The goal is to build a quantum computing and simulation platform based on superconducting qubits to explore and potentially overcome the limits of classical computation.”—IBM scientist Stefan Filipp

## Tools and methods

In experiments, we use fixed-frequency superconducting qubits, which can be manipulated on short time scales with respect to their coherence times within a cryogenic environment. Thanks to the relatively simple and reliable fabrication there exists a clear path towards a scalable architecture to realize the building blocks of a future universal quantum computer and for practical quantum simulation applications. This effort is in close collaboration with our colleagues at the IBM Thomas J. Watson Research Center in Yorktown Heights, USA.

We are exploring qubit–qubit coupling schemes based on parametrically driven tunable couplers and geometric phases to achieve high-fidelity interactions between highly coherent superconducting qubits. In particular, the tunable coupler architecture has the potential to act as a quantum bus between multiple qubits in a qubit lattice.

Within the QuSCo European training network, we are exploring optimal control and calibration schemes for single and two-qubit devices. Numerical simulations of multi-qubit systems provide important insights regarding the quantum dynamics and the achievable gate fidelities. To increase the coherence of the superconducting qubits, we are operating a UHV apparatus for specific treatments of surfaces and interfaces.

Mixing chamber plate of the dilution refrigerator with mounted sample enclosure.

Superconducting qubit device with two transmon-type qubits.

Scheme of a superconducting qubit lattice (blue dots) coupled via 4-way coupling devices (pink rectangles).

Numerical simulation of qubit-qubit transitions induced by a parametric modulation of the tunable coupler frequency.

UHV sealing and surface preparation.

## Nano- and microfabrication of superconducting qubits

The cleanroom of the Binnig and Rohrer Nanotechnology Center gives us an ideal environment to explore new materials and fabrication methods and to develop chip designs for a scalable quantum computing architecture.

## Ask the experts

## Funding sources

Peter Müller

IBM Research scientist

**IARPA.** Intelligence Advanced Research Projects Activity. Project SLEEQ (Superconducting Logically Encoded Extensible Qubit) within the LogiQ Program.

QuSCo

EU H2020 MSCA-ITN Quantum-enhanced Sensing via Quantum Control

## Learn more

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