Jian Li

High-fidelity, high-scalability two-qubit gate scheme for superconducting qubits

  1. Yuan Xu,
  2. Ji Chu,
  3. Jiahao Yuan,
  4. Jiawei Qiu,
  5. Yuxuan Zhou,
  6. Libo Zhang,
  7. Xinsheng Tan,
  8. Yang Yu,
  9. Song Liu,
  10. Jian Li,
  11. Fei Yan,
  12. and Dapeng Yu
High-quality two-qubit gate operations are crucial for scalable quantum information processing. Often, the gate fidelity is compromised when the system becomes more integrated. Therefore,
a low-error-rate, easy-to-scale two-qubit gate scheme is highly desirable. Here, we experimentally demonstrate a new two-qubit gate scheme that exploits fixed-frequency qubits and a tunable coupler in a superconducting quantum circuit. The scheme requires less control lines, reduces crosstalk effect, simplifies calibration procedures, yet produces a controlled-Z gate in 30ns with a high fidelity of 99.5%. Error analysis shows that gate errors are mostly coherence-limited. Our demonstration paves the way for large-scale implementation of high-fidelity quantum operations.
arxiv pdf

Hybrid circuit cavity quantum electrodynamics with a micromechanical resonator

  1. J. M. Pirkkalainen,
  2. S. U. Cho,
  3. Jian Li,
  4. G. S. Paraoanu,
  5. P. J. Hakonen,
  6. and M. A. Sillanpää
Hybrid quantum systems with inherently distinct degrees of freedom play a key role in many physical phenomena. A strong coupling can make the constituents loose their individual character
and form entangled states. The properties of these collective excitations, such as polaritons of light and phonons in semiconductors, can combine the benefits of each subsystem. In the emerging field of quantum information control, a promising direction is provided by the combination between long-lived atomic states and the accessible electrical degrees of freedom in superconducting cavities and qubits. Here we demonstrate the possibility to integrate circuit cavity quantum electrodynamics with phonons. Besides coupling to a microwave cavity, our superconducting transmon qubit interacts with a resonant phonon mode in a micromechanical resonator, allowing the combination of long lifetime, strong tunable coupling, and ease of access. We measure the phonon Stark shift, as well as the splitting of the transmon qubit spectral line into motional sidebands representing transitions between electromechanical polaritons formed by phonons and the qubit. In the time domain, we observe coherent sideband Rabi oscillations between the qubit states and phonons. This advance may allow for storage of quantum information in long-lived phonon states, and for investigations of strongly coupled quantum systems near the classical limit.
arxiv pdf

Dielectric losses in multi-layer Josephson junction qubits

  1. David Gunnarsson,
  2. Juha-Matti Pirkkalainen,
  3. Jian Li,
  4. Gheorghe Sorin Paraoanu,
  5. Pertti Hakonen,
  6. Mika Sillanpää,
  7. and Mika Prunnila
We have measured the excited state lifetimes in Josephson junction phase and transmon qubits, all of which were fabricated with the same scalable multi-layer process. We have compared
the lifetimes of phase qubits before and after removal of the isolating dielectric, SiNx, and find a four-fold improvement of the relaxation time after the removal. Together with the results from the transmon qubit and measurements on coplanar waveguide resonators, these measurements indicate that the lifetimes are limited by losses from the dielectric constituents of the qubits. We have extracted the individual loss contributions from the dielectrics in the tunnel junction barrier, AlOx, the isolating dielectric, SiNx, and the substrate, Si/SiO2, by weighing the total loss with the parts of electric field over the different dielectric materials. Our results agree well and complement the findings from other studies, demonstrating that superconducting qubits can be used as a reliable tool for high-frequency characterization of dielectric materials. We conclude with a discussion of how changes in design and material choice could improve qubit lifetimes up to a factor of four.
arxiv pdf

Motional Averaging in a Superconducting Qubit

  1. Jian Li,
  2. M. P. Silveri,
  3. K. S. Kumar,
  4. J.-M. Pirkkalainen,
  5. A. Vepsäläinen,
  6. W. C. Chien,
  7. J. Tuorila,
  8. M. A. Sillanpää,
  9. P. J. Hakonen,
  10. E. V. Thuneberg,
  11. and G. S. Paraoanu
Superconducting circuits with Josephson junctions are promising candidates for developing future quantum technologies. Of particular interest is to use these circuits to study effects
that typically occur in complex condensed-matter systems. Here, we employ a superconducting quantum bit (qubit), a transmon, to carry out an analog simulation of motional averaging, a phenomenon initially observed in nuclear magnetic resonance (NMR) spectroscopy. To realize this effect, the flux bias of the transmon is modulated by a controllable pseudo-random telegraph noise, resulting in stochastic jumping of the energy separation between two discrete values. When the jumping is faster than a dynamical threshold set by the frequency displacement of the levels, the two separated spectral lines merge into a single narrow-width, motional-averaged line. With sinusoidal modulation a complex pattern of additional sidebands is observed. We demonstrate experimentally that the modulated system remains quantum coherent, with modified transition frequencies, Rabi couplings, and dephasing rates. These results represent the first steps towards more advanced quantum simulations using artificial atoms.
arxiv pdf

Dynamical decoupling of superconducting qubits

  1. Jian Li,
  2. and G. S. Paraoanu
We show that two superconducting qubits interacting via a fixed transversal coupling can be decoupled by appropriately-designed microwave feld excitations applied to each qubit. This
technique is useful for removing the effects of spurious interactions in a quantum processor. We also simulate the case of a qubit coupled to a two-level system (TLS) present in the insulating layer of the Josephson junction of the qubit. Finally, we discuss the qubit-TLS problem in the context of dispersive measurements, where the qubit is coupled to a resonator.
arxiv pdf

Dynamical Autler-Townes control of a phase qubit

  1. Jian Li,
  2. G. S. Paraoanu,
  3. Katarina Cicak,
  4. Fabio Altomare,
  5. Jae I. Park,
  6. Raymond W. Simmonds,
  7. Mika A. Sillanpaa,
  8. and Pertti J. Hakonen
Routers, switches, and repeaters are essential components of modern information-processing systems. Similar devices will be needed in future superconducting quantum computers. In this
work we investigate experimentally the time evolution of Autler-Townes splitting in a superconducting phase qubit under the application of a control tone resonantly coupled to the second transition. A three-level model that includes independently determined parameters for relaxation and dephasing gives excellent agreement with the experiment. The results demonstrate that the qubit can be used as a ON/OFF switch with 100 ns operating time-scale for the reflection/transmission of photons coming from an applied probe microwave tone. The ON state is realized when the control tone is sufficiently strong to generate an Autler-Townes doublet, suppressing the absorption of the probe tone photons and resulting in a maximum of transmission.
arxiv pdf