Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit

  1. Marta Pita-Vidal,
  2. Arno Bargerbos,
  3. Rok Žitko,
  4. Lukas J. Splitthoff,
  5. Lukas Grünhaupt,
  6. Jaap J. Wesdorp,
  7. Yu Liu,
  8. Leo P. Kouwenhoven,
  9. Ramón Aguado,
  10. Bernard van Heck,
  11. Angela Kou,
  12. and Christian Kraglund Andersen
Spin qubits in semiconductors are currently one of the most promising architectures for quantum computing. However, they face challenges in realizing multi-qubit interactions over extended
distances. Superconducting spin qubits provide a promising alternative by encoding a qubit in the spin degree of freedom of an Andreev level. Such an Andreev spin qubit could leverage the advantages of circuit quantum electrodynamic, enabled by an intrinsic spin-supercurrent coupling. The first realization of an Andreev spin qubit encoded the qubit in the excited states of a semiconducting weak-link, leading to frequent decay out of the computational subspace. Additionally, rapid qubit manipulation was hindered by the need for indirect Raman transitions. Here, we exploit a different qubit subspace, using the spin-split doublet ground state of an electrostatically-defined quantum dot Josephson junction with large charging energy. Additionally, we use a magnetic field to enable direct spin manipulation over a frequency range of 10 GHz. Using an all-electric microwave drive we achieve Rabi frequencies exceeding 200 MHz. We furthermore embed the Andreev spin qubit in a superconducting transmon qubit, demonstrating strong coherent qubit-qubit coupling. These results are a crucial step towards a hybrid architecture that combines the beneficial aspects of both superconducting and semiconductor qubits.

Mitigation of quasiparticle loss in superconducting qubits by phonon scattering

  1. Arno Bargerbos,
  2. Lukas Johannes Splitthoff,
  3. Marta Pita-Vidal,
  4. Jaap J. Wesdorp,
  5. Yu Liu,
  6. Peter Krogstrup,
  7. Leo P. Kouwenhoven,
  8. Christian Kraglund Andersen,
  9. and Lukas Grünhaupt
Quantum error correction will be an essential ingredient in realizing fault-tolerant quantum computing. However, most correction schemes rely on the assumption that errors are sufficiently
uncorrelated in space and time. In superconducting qubits this assumption is drastically violated in the presence of ionizing radiation, which creates bursts of high energy phonons in the substrate. These phonons can break Cooper-pairs in the superconductor and, thus, create quasiparticles over large areas, consequently reducing qubit coherence across the quantum device in a correlated fashion. A potential mitigation technique is to place large volumes of normal or superconducting metal on the device, capable of reducing the phonon energy to below the superconducting gap of the qubits. To investigate the effectiveness of this method we fabricate a quantum device with four nominally identical nanowire-based transmon qubits. On the device, half of the niobium-titanium-nitride ground plane is replaced with aluminum (Al), which has a significantly lower superconducting gap. We deterministically inject high energy phonons into the substrate by voltage biasing a galvanically isolated Josephson junction. In the presence of the low gap material, we find a factor of 2-5 less degradation in the injection-dependent qubit lifetimes, and observe that undesired excited qubit state population is mitigated by a similar factor. We furthermore turn the Al normal with a magnetic field, finding no change in the phonon-protection. This suggests that the efficacy of the protection in our device is not limited by the size of the superconducting gap in the Al ground plane. Our results provide a promising foundation for protecting superconducting qubit processors against correlated errors from ionizing radiation.

Singlet-doublet transitions of a quantum dot Josephson junction detected in a transmon circuit

  1. Arno Bargerbos,
  2. Marta Pita-Vidal,
  3. Rok Žitko,
  4. Jesús Ávila,
  5. Lukas J. Splitthoff,
  6. Lukas Grünhaupt,
  7. Jaap J. Wesdorp,
  8. Christian K. Andersen,
  9. Yu Liu,
  10. Leo P. Kouwenhoven,
  11. Ramón Aguado,
  12. Angela Kou,
  13. and Bernard van Heck
We realize a hybrid superconductor-semiconductor transmon device in which the Josephson effect is controlled by a gate-defined quantum dot in an InAs/Al nanowire. Microwave spectroscopy
of the transmon’s transition spectrum allows us to probe the ground state parity of the quantum dot as a function of gate voltages, external magnetic flux, and magnetic field applied parallel to the nanowire. The measured parity phase diagram is in agreement with that predicted by a single-impurity Anderson model with superconducting leads. Through continuous time monitoring of the circuit we furthermore resolve the quasiparticle dynamics of the quantum dot Josephson junction across the phase boundaries. Our results can facilitate the realization of semiconductor-based 0−π qubits and Andreev qubits.

Controlling the charge dispersion of a nearly-open superconducting island

  1. Arno Bargerbos,
  2. Willemijn Uilhoorn,
  3. Chung-Kai Yang,
  4. Peter Krogstrup,
  5. Leo P. Kouwenhoven,
  6. Gijs de Lange,
  7. Bernard van Heck,
  8. and Angela Kou
Isolation from the environment determines the extent to which charge is confined on an island. This confinement leads to an energy cost for adding an electron onto the island, which
manifests experimentally through Coulomb oscillations such as charge dispersion. In superconducting circuits, the link to the environment has typically been formed from tunnel junctions where the charge dispersion can be tuned by changing the ratio between the Josephson energy EJ and the charging energy Ec. If, instead, a transparent ballistic junction forms the link between the superconducting island and the environment, the charge dispersion is predicted to be suppressed far beyond the dependence on the EJ/Ec ratio due to imaginary-time Landau-Zener tunneling between Andreev bound states. Here we investigate the charge dispersion of a nanowire transmon hosting a quantum dot in the junction. We observe rapid suppression of the charge dispersion consistent with the predicted scaling law incorporating diabatic transitions between Andreev bound states. We also observe greatly improved qubit coherence times at the point of highest charge dispersion suppression. Our observations further our fundamental understanding of charging effects in superconductors and suggest novel approaches for building charge-insensitive qubits.

A gate-tunable, field-compatible fluxonium

  1. Marta Pita-Vidal,
  2. Arno Bargerbos,
  3. Chung-Kai Yang,
  4. David J. van Woerkom,
  5. Wolfgang Pfaff,
  6. Nadia Haider,
  7. Peter Krogstrup,
  8. Leo P. Kouwenhoven,
  9. Gijs de Lange,
  10. and Angela Kou
Circuit quantum electrodynamics, where photons are coherently coupled to artificial atoms built with superconducting circuits, has enabled the investigation and control of macroscopic
quantum-mechanical phenomena in superconductors. Recently, hybrid circuits incorporating semiconducting nanowires and other electrostatically-gateable elements have provided new insights into mesoscopic superconductivity. Extending the capabilities of hybrid flux-based circuits to work in magnetic fields would be especially useful both as a probe of spin-polarized Andreev bound states and as a possible platform for topological qubits. The fluxonium is particularly suitable as a readout circuit for topological qubits due to its unique persistent-current based eigenstates. In this Letter, we present a magnetic-field compatible hybrid fluxonium with an electrostatically-tuned semiconducting nanowire as its non-linear element. We operate the fluxonium in magnetic fields up to 1T and use it to observe the φ0-Josephson effect. This combination of gate-tunability and field-compatibility opens avenues for the exploration and control of spin-polarized phenomena using superconducting circuits and enables the use of the fluxonium as a readout device for topological qubits.