Unimon qubit

  1. Eric Hyyppä,
  2. Suman Kundu,
  3. Chun Fai Chan,
  4. András Gunyhó,
  5. Juho Hotari,
  6. Olavi Kiuru,
  7. Alessandro Landra,
  8. Wei Liu,
  9. Fabian Marxer,
  10. Akseli Mäkinen,
  11. Jean-Luc Orgiazzi,
  12. Mario Palma,
  13. Mykhailo Savytskyi,
  14. Francesca Tosto,
  15. Jani Tuorila,
  16. Vasilii Vadimov,
  17. Tianyi Li,
  18. Caspar Ockeloen-Korppi,
  19. Johannes Heinsoo,
  20. Kuan Yen Tan,
  21. Juha Hassel,
  22. and Mikko Möttönen
Superconducting qubits are one of the most promising candidates to implement quantum computers. The superiority of superconducting quantum computers over any classical device in simulating
random but well-determined quantum circuits has already been shown in two independent experiments and important steps have been taken in quantum error correction. However, the currently wide-spread qubit designs do not yet provide high enough performance to enable practical applications or efficient scaling of logical qubits owing to one or several following issues: sensitivity to charge or flux noise leading to decoherence, too weak non-linearity preventing fast operations, undesirably dense excitation spectrum, or complicated design vulnerable to parasitic capacitance. Here, we introduce and demonstrate a superconducting-qubit type, the unimon, which combines the desired properties of high non-linearity, full insensitivity to dc charge noise, insensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. We measure the qubit frequency, ω01/(2π), and anharmonicity α over the full dc-flux range and observe, in agreement with our quantum models, that the qubit anharmonicity is greatly enhanced at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13-ns single-qubit gates on two qubits with (ω01,α)=(4.49 GHz,434 MHz)×2π and (3.55 GHz,744 MHz)×2π, respectively. The energy relaxation time T1≲10 μs is stable for hours and seems to be limited by dielectric losses. Thus, future improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible quantum advantage with noisy systems.

Broadband Tunable Phase Shifter For Microwaves

  1. Jinli Zhang,
  2. Tianyi Li,
  3. Roope Kokkoniemi,
  4. Chengyu Yan,
  5. Wei Liu,
  6. Matti Partanen,
  7. Kuan Yen Tan,
  8. Ming He,
  9. Lu Ji,
  10. Leif Grönberg,
  11. and Mikko Möttönen
We implement a broadly tunable phase shifter for microwaves based on superconducting quantum interference devices (SQUIDs) and study it both experimentally and theoretically. At different
frequencies, a unit transmission coefficient, |S21|=1, can be theoretically achieved along a curve where the phase shift is controllable by magnetic flux. The fabricated device consists of three equidistant SQUIDs interrupting a transmission line. We model each SQUID embedded at different positions along the transmission line with two parameters, capacitance and inductance, the values of which we extract from the experiments. In our experiments, the tunability of the phase shift varies from from 0.07×π to 0.14×π radians along the full-transmission curve with the input frequency ranging from 6.00 to 6.28~GHz. The reported measurements are in good agreement with simulations, which is promising for future design work of phase shifters for different applications.

Fast control of dissipation in a superconducting resonator

  1. Vasilii Sevriuk,
  2. Kuan Yen Tan,
  3. Eric Hyyppä,
  4. Matti Silveri,
  5. Matti Partanen,
  6. Máté Jenei,
  7. Shumpei Masuda,
  8. Jan Goetz,
  9. Visa Vesterinen,
  10. Leif Grönberg,
  11. and Mikko Möttönen
We report on fast tunability of an electromagnetic environment coupled to a superconducting coplanar waveguide resonator. Namely, we utilize a recently-developed quantum-circuit refrigerator
(QCR) to experimentally demonstrate a dynamic tunability in the total damping rate of the resonator up to almost two orders of magnitude. Based on the theory it corresponds to a change in the internal damping rate by nearly four orders of magnitude. The control of the QCR is fully electrical, with the shortest implemented operation times in the range of 10 ns. This experiment constitutes a fast active reset of a superconducting quantum circuit. In the future, a similar scheme can potentially be used to initialize superconducting quantum bits.

Quantum Gates for Propagating Microwave Photons

  1. Roope Kokkoniemi,
  2. Tuomas Ollikainen,
  3. Russell E. Lake,
  4. Sakari Saarenpää,
  5. Kuan Yen Tan,
  6. Janne I. Kokkala,
  7. Ceren B. Dağ,
  8. Joonas Govenius,
  9. and Mikko Möttönen
We report a generic scheme to implement transmission-type quantum gates for propagating microwave photons, based on a sequence of lumped-element components on transmission lines. By
choosing three equidistant superconducting quantum interference devices (SQUIDs) as the components on a single transmission line, we experimentally implement a magnetic-flux-tunable phase shifter and demonstrate that it produces a broad range of phase shifts and full transmission within the experimental uncertainty. Together with previously demonstrated beam splitters, these phase shifters can be utilized to implement arbitrary single-qubit gates. Furthermore, we theoretically show that replacing the SQUIDs by superconducting qubits, the phase shifter can be made strongly nonlinear, thus introducing deterministic photon–photon interactions. These results critically complement the previous demonstrations of on-demand single-photon sources and detectors, and hence pave the way for an all-microwave quantum computer based on propagating photons.