Fast generation of Schrödinger cat states in a Kerr-tunable superconducting resonator

  1. X.L. He,
  2. Yong Lu,
  3. D.Q. Bao,
  4. Hang Xue,
  5. W.B. Jiang,
  6. Zhen Wang,
  7. A.F. Roudsari,
  8. Per Delsing,
  9. J. S. Tsai,
  10. and Z. R. Lin
Schrödinger cat states, quantum superpositions of macroscopically distinct classical states, are an important resource for quantum communication, quantum metrology and quantum computation.
Especially, cat states in a phase space protected against phase-flip errors can be used as a logical qubit. However, cat states, normally generated in three-dimensional cavities, are facing the challenges of scalability and controllability. Here, we present a novel strategy to generate and store cat states in a coplanar superconducting circuit by the fast modulation of Kerr nonlinearity. At the Kerr-free work point, our cat states are passively preserved due to the vanishing Kerr effect. We are able to prepare a 2-component cat state in our chip-based device with a fidelity reaching 89.1% under a 96 ns gate time. Our scheme shows an excellent route to constructing a chip-based bosonic quantum processor.

Ultrastrong tunable coupler between superconducting LC resonators

  1. T. Miyanaga,
  2. A. Tomonaga,
  3. H. Ito,
  4. H. Mukai,
  5. and J. S. Tsai
We investigate the ultrastrong tunable coupler for coupling of superconducting resonators. Obtained coupling constant exceeds 1 GHz, and the wide range tunability is achieved both antiferromagnetics
and ferromagnetics from -1086 MHz to 604 MHz. Ultrastrong coupler is composed of rf-SQUID and dc-SQUID as tunable junctions, which connected to resonators via shared aluminum thin film meander lines enabling such a huge coupling constant. The spectrum of the coupler obviously shows the breaking of the rotating wave approximation, and our circuit model treating the Josephson junction as a tunable inductance reproduces the experimental results well. The ultrastrong coupler is expected to be utilized in quantum annealing circuits and/or NISQ devices with dense connections between qubits.

Photon-Dressed Bloch-Siegert Shift in an Ultrastrongly Coupled Circuit Quantum Electrodynamical System

  1. Shuai-Peng Wang,
  2. Guo-Qiang Zhang,
  3. Yimin Wang,
  4. Zhen Chen,
  5. Tiefu Li,
  6. J. S. Tsai,
  7. Shi-Yao Zhu,
  8. and J. Q. You
A cavity quantum electrodynamical (QED) system beyond the strong-coupling regime is expected to exhibit intriguing quantum phenomena. Here we report a direct measurement of the photon-dressed
qubit transition frequencies up to four photons by harnessing the same type of state transitions in an ultrastrongly coupled circuit-QED system realized by inductively coupling a superconducting flux qubit to a coplanar-waveguide resonator. This demonstrates a convincing observation of the photon-dressed Bloch-Siegert shift in the ultrastrongly coupled quantum system. Moreover, our results show that the photon-dressed Bloch-Siegert shift becomes more pronounced as the photon number increases, which is a characteristic of the quantum Rabi model.

Tunable Microwave Single-photon Source Based on Transmon Qubit with High Efficiency

  1. Yu Zhou,
  2. Zhihui Peng,
  3. Yuta Horiuchi,
  4. O. V. Astafiev,
  5. and J. S. Tsai
Single-photon sources are of great interest because they are key elements in different promising applications of quantum technologies. Here we demonstrate a highly efficient tunable
on-demand microwave single-photon source based on a transmon qubit with the intrinsic emission efficiency more than 99%. To confirm the single-photon property of the source, we study the single-photon interference in a Hanbury-Brown-Twiss (HBT) type setup and measure the correlation functions of the emission field using linear detectors with a GPU-enhanced signal processing technique. The antibunching in the second-order correlation function is clearly observed. The theoretical calculations agree well with the experimental results. Such a high-quality single-photon source can be used as a building block of devices for quantum communication, simulations and information processing in the microwave regime.

Pseudo-2D superconducting quantum computing circuit for the surface code

  1. H. Mukai,
  2. K. Sakata,
  3. S.J. Devitt,
  4. R. Wang,
  5. Y. Zhou,
  6. Y. Nakajima,
  7. and J. S. Tsai
Of the many potential hardware platforms, superconducting quantum circuits have become the leading contender for constructing a scalable quantum computing system. All current architecture
designs necessitate a 2D arrangement of superconducting qubits with nearest neighbour interactions, compatible with powerful quantum error correction using the surface code. A major hurdle for scalability in superconducting systems is the so called wiring problem, where qubits internal to a chip-set become inaccessible for external control/readout lines. Current approaches resort to intricate and exotic 3D wiring and packaging technology which is a significant engineering challenge to realize, while maintaining qubit fidelity. Here we solve this problem and present a modified superconducting scalable micro-architecture that does not require any 3D external line technology and reverts back to a completely planar design. This is enabled by a new pseudo-2D resonator network that provides inter-qubit connections via airbridges. We carried out experiments to examine the feasibility of the newly introduced airbridge component. The measured quality factor of these new inter-qubit resonators is sufficient for high fidelity gates, below the threshold for the surface code, with negligible measured cross-talk. The resulting physical separation of the external wirings and the inter-qubit connections on-chip should reduce cross-talk and decoherence as the chip-set increases in size. This result demonstrates that a large-scale, fully error corrected quantum computer can be constructed by monolithic integration technologies without additional overhead and without special packaging know-hows.

Hybrid rf SQUID qubit based on high kinetic inductance

  1. J.T. Peltonen,
  2. P. C. J. J. Coumou,
  3. Z.H. Peng,
  4. T. M. Klapwijk,
  5. J. S. Tsai,
  6. and O. V. Astafiev
We report development and microwave characterization of rf SQUID (Superconducting QUantum Interference Device) qubits, consisting of an aluminium-based Josephson junction embedded in
a superconducting loop patterned from a thin film of TiN with high kinetic inductance. Here we demonstrate that the systems can offer small physical size, high anharmonicity, and small scatter of device parameters. The hybrid devices can be utilized as tools to shed further light onto the origin of film dissipation and decoherence in phase-slip nanowire qubits, patterned entirely from disordered superconducting films.

Vacuum induced Aulter-Townes splitting in a superconducting artificial atom

  1. Z.H. Peng,
  2. J.H. Ding,
  3. Y. Zhou,
  4. L.L. Ying,
  5. Z. Wang,
  6. L. Zhou,
  7. L.M. Kuang,
  8. Yu-xi Liu,
  9. O. Astfiev,
  10. and J. S. Tsai
We study experimentally a vacuum induced Aulter-Townes doublet in a superconducting three-level artificial atom strongly coupled to a coplanar waveguide resonator and simultaneously
to a transmission line. The Aulter-Townes splitting is observed in the reflection spectrum of the three-level atom when the transition between two excited states is resonant with the resonator. By varying an amplitude of the driving field applied to the resonator, we observe quantum-to-classical transition of the Aulter-Townes splitting. Our results may pave the way for the control of microwaves by single photons.

Method for identifying electromagnetically induced transparency in a tunable circuit quantum electrodynamics system

  1. Qi-Chun Liu,
  2. Tie-Fu Li,
  3. Xiao-Qing Luo,
  4. Hu Zhao,
  5. Wei Xiong,
  6. Ying-Shan Zhang,
  7. Zhen Chen,
  8. J. S. Liu,
  9. Wei Chen,
  10. Franco Nori,
  11. J. S. Tsai,
  12. and J. Q. You
Electromagnetically induced transparency (EIT) has been realized in atomic systems, but fulfilling the EIT conditions for artificial atoms made from superconducting circuits is a more
difficult task. Here we report an experimental observation of the EIT in a tunable three-dimensional transmon by probing the cavity transmission. To fulfill the EIT conditions, we tune the transmon to adjust its damping rates by utilizing the effect of the cavity on the transmon states. From the experimental observations, we clearly identify the EIT and Autler-Townes splitting (ATS) regimes as well as the transition regime in between. Also, the experimental data demonstrate that the threshold ΩAIC determined by the Akaike information criterion can describe the EIT-ATS transition better than the threshold ΩEIT given by the EIT theory.

Multi-photon sideband transitions in an ultrastrongly-coupled circuit quantum electrodynamics system

  1. Zhen Chen,
  2. Yimin Wang,
  3. Tiefu Li,
  4. Lin Tian,
  5. Yueyin Qiu,
  6. Kunihiro Inomata,
  7. Fumiki Yoshihara,
  8. Siyuan Han,
  9. Franco Nori,
  10. J. S. Tsai,
  11. and J. Q. You
Ultrastrong coupling in circuit quantum electrodynamics systems not only provides a platform to study the quantum Rabi model, but it can also facilitate the implementation of quantum
logic operations via high-lying resonator states. In this regime, quantum manifolds with different excitation numbers are intrinsically connected via the counter-rotating interactions, which can result in multi-photon processes. Recent experiments have demonstrated ultrastrong coupling in superconducting qubits electromagnetically coupled to superconducting resonators. Here we report the experimental observation of multiphoton sideband transitions of a superconducting flux qubit coupled to a coplanar waveguide resonator in the ultrastrong coupling regime. With a coupling strength reaching about 10% of the fundamental frequency of the resonator, we obtain clear signatures of higher-order red-sideband transitions and the first-order blue-sideband transition in a transmission spectroscopic measurement. This study advances the understanding of driven ultrastrongly-coupled systems.

Coherent dynamics and decoherence in a superconducting weak link

  1. J.T. Peltonen,
  2. Z.H. Peng,
  3. Yu. P. Korneeva,
  4. B. M. Voronov,
  5. A. A. Korneev,
  6. A. V. Semenov,
  7. G. N. Gol'tsman,
  8. J. S. Tsai,
  9. and O. V. Astafiev
We demonstrate coherent dynamics of quantized magnetic fluxes in a superconducting loop with a weak link – a nanobridge patterned from the same thin NbN film as the loop. The
bridge is a short rounded shape constriction, close to 10 nm long and 20 – 30 nm wide, having minimal width at its center. Quantum state control and coherent oscillations in the driven time evolution of the tunnel-junctionless system are achieved. Decoherence and energy relaxation in the system are studied using a combination of microwave spectroscopy and direct time-domain techniques. The effective flux noise behavior suggests inductance fluctuations as a possible cause of the decoherence.