Quantum information processing with tunable and low-loss superconducting circuits
Doktorsavhandling, 2020

The perhaps most promising platform for quantum information processing is the circuit-QED architecture based on superconducting circuits representing quantum bits. These circuits must be made with low losses so that the quantum information is retained for as long as possible. We developed fabrication processes achieving state-of-the-art coherence times of over 100 µs. We identified the primary source of loss to be parasitic two-level systems by studying fluctuations of qubit relaxation times.

Using our high-coherence circuits, we implemented a quantum processor built on fixed-frequency qubits and frequency-tunable couplers. The tunable couplers were lumped-element LC resonators, where the inductance came from a superconducting quantum interference device (SQUID). We achieved a controlled-phase gate with a fidelity of 99% by parametric modulation of the coupler frequency. Using this device, and another similar to it, we demonstrated two different quantum algorithms, the quantum approximate optimization algorithm, and density matrix exponentiation. We achieved high algorithmic fidelities, aided by our carefully calibrated gates.

Additionally, we researched parametric oscillations using frequency-tunable resonators. Previously, degenerate parametric oscillations have been demonstrated by modulation of the resonant frequency at twice that frequency. We use this phenomenon to implement a readout method for a superconducting qubit with a fidelity of 98.7%.

We demonstrated correlated radiation in nondegenerate parametric oscillations by modulating at the sum of two resonant frequencies of a multimode resonator. We showed an excellent quantitative agreement between the classical properties of the oscillations with a theoretical model. Moreover, we studied higher-order modulation at up to five times their resonant frequencies. These types of parametric oscillation states might be used as a quantum resource for continuous-variable quantum computing.

quantum information

circuit quantum electrodynamics

superconducting circuits

high coherence

parametric modulation

Opponent: Dr. Hanhee Paik, IBM T. J. Watson Research Center, USA


Andreas Bengtsson

Chalmers, Mikroteknologi och nanovetenskap (MC2), Kvantteknologi

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Y. Lu, A. Bengtsson, J. J. Burnett, E. Wiegand, B. Suri, P. Krantz, A. F. Roudsari, A. F. Kockum, S. Gasparinetti, G. Johansson, and P. Delsing. Characterizing decoherence rates of a superconducting qubit by direct microwave scattering

M. Kjaergaard, M. E. Schwartz, A. Greene, G. O. Samach, A. Bengtsson, M. O'Kee e, C. M. McNally, J. Braumuller, D. K. Kim, P. Krantz, M. Marvian, A. Melville, B. M. Niedzielski, Y. Sung, R. Winik, J. Yoder, D. Rosenberg, K. Obenland, S. Lloyd, T. P. Orlando, I. Marvian, S. Gustavsson, and W. D. Oliver, A quantum instruction set implemented on a superconducting quantum processor

A. Bengtsson, P. Vikstal, C.Warren, M. Svensson, X. Gu, A. F. Kockum, P. Krantz, C. Krizan, D. Shiri, I.-M. Svensson, G. Tancredi, G. Johansson, P. Delsing, G. Ferrini, and J. Bylander, Quantum approximate optimization of the exact-cover problem on a superconducting quantum processor

Today's computer can perform many extraordinary tasks. However, there are problems that not even the most powerful computers in the world are able to solve. To overcome some of the fundamental issues with ordinary computers, it has been suggested to use quantum mechanical systems to store and process information. Quantum computers are highly specialized machines that promise an enormous speed-up for specific problems. Calculations that would take thousands of years could instead be performed in minutes. Currently, prototypes of such computers are being built and researched. This thesis touches on many different topics related to how to build and characterize quantum computers using superconducting circuits.

Information stored in quantum systems is fragile and can easily be lost. In superconducting circuits, one source of loss is defects on the surface of the device. In this thesis, we have developed fabrication processes that minimize the number of defects and yield superconducting qubits with state-of-the-art performance. By careful analysis of the performance, we identified that our circuits are still limited by defects, which means that even better processes are needed in the future.

Nevertheless, we use our low-loss circuits to demonstrate two quantum algorithms. We show that a quantum optimization algorithm can be used to solve logistics problems for airlines trying to optimize their routes and personal assignment. Even if our quantum processor is still too small to outperform a standard computer, our demonstration shows that the algorithm works and that it could be used for real-world applications once we have large enough quantum processors.

Finally, we implement frequency-tunable circuits. By modulating the circuits at specific multiples of a fundamental frequency, we observe splittings of one high-energy light-particle (a photon) into many lower-energy photons. While we mostly investigate some basic properties, such photon-splittings could provide useful resources for quantum computing in the future.

Fotonkondensat och relativistisk fysik på ett mikrochip

Knut och Alice Wallenbergs Stiftelse, 2015-01-01 -- 2019-12-31.


Nanovetenskap och nanoteknik (SO 2010-2017, EI 2018-)


Annan fysik


Den kondenserade materiens fysik



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4720


Chalmers tekniska högskola


Opponent: Dr. Hanhee Paik, IBM T. J. Watson Research Center, USA

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