Microwave quantum diode

  1. Rishabh Upadhyay,
  2. Dmitry S. Golubev,
  3. Yu-Cheng Chang,
  4. George Thomas,
  5. Andrew Guthrie,
  6. Joonas T. Peltonen,
  7. and Jukka P. Pekola
The fragile nature of quantum circuits is a major bottleneck to scalable quantum applications. Operating at cryogenic temperatures, quantum circuits are highly vulnerable to amplifier
backaction and external noise. Non-reciprocal microwave devices such as circulators and isolators are used for this purpose. These devices have a considerable footprint in cryostats, limiting the scalability of quantum circuits. We present a compact microwave diode architecture, which exploits the non-linearity of a superconducting flux qubit. At the qubit degeneracy point we experimentally demonstrate a significant difference between the power levels transmitted in opposite directions. The observations align with the proposed theoretical model. At -99 dBm input power, and near the qubit-resonator avoided crossing region, we report the transmission rectification ratio exceeding 90% for a 50 MHz wide frequency range from 6.81 GHz to 6.86 GHz, and over 60% for the 250 MHz range from 6.67 GHz to 6.91 GHz. The presented architecture is compact, and easily scalable towards multiple readout channels, potentially opening up diverse opportunities in quantum information, microwave read-out and optomechanics.

A Cooper-Pair Box Architecture for Cyclic Quantum Heat Engines

  1. Andrew Guthrie,
  2. Christoforus Dimas Satrya,
  3. Yu-Cheng Chang,
  4. Paul Menczel,
  5. Franco Nori,
  6. and Jukka P. Pekola
Here we present an architecture for the implementation of cyclic quantum thermal engines using a superconducting circuit. The quantum engine consists of a gated Cooper-pair box, capacitively
coupled to two superconducting coplanar waveguide resonators with different frequencies, acting as thermal baths. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at GHz frequencies. By terminating the resonators of the measured structure with normal-metal resistors whose temperature can be controlled and monitored, a quantum heat engine or refrigerator could be realized. Furthermore, we numerically evaluate the performance of our setup acting as a quantum Otto-refrigerator in the presence of realistic environmental decoherence.

Robust strong coupling architecture in circuit quantum electrodynamics

  1. Rishabh Upadhyay,
  2. George Thomas,
  3. Yu-Cheng Chang,
  4. Dmitry S. Golubev,
  5. Andrew Guthrie,
  6. Azat Gubaydullin,
  7. Joonas T. Peltonen,
  8. and Jukka P. Pekola
We report on a robust method to achieve strong coupling between a superconducting flux qubit and a high-quality quarter-wavelength coplanar waveguide resonator. We demonstrate the progression
from the strong to ultrastrong coupling regime by varying the length of a shared inductive coupling element, ultimately achieving a qubit-resonator coupling strength of 655 MHz, 10% of the resonator frequency. We derive an analytical expression for the coupling strength in terms of circuit parameters and also discuss the maximum achievable coupling within this framework. We experimentally characterize flux qubits coupled to superconducting resonators using one and two-tone spectroscopy methods, demonstrating excellent agreement with the proposed theoretical model.

Characterizing Low-Quality-Factor Dissipative Superconducting Resonators

  1. Yu-Cheng Chang,
  2. Bayan Karimi,
  3. Jorden Senior,
  4. Alberto Ronzani,
  5. Joonas T. Peltonen,
  6. Hsi-Sheng Goan,
  7. Chii-Dong Chen,
  8. and Jukka P. Pekola
Characterizing superconducting microwave resonators with highly dissipative elements is a technical challenge, but a requirement for implementing and understanding the operation of
hybrid quantum devices involving dissipative elements, e.g. for thermal engineering and detection. We present experiments on λ/4 superconducting niobium coplanar waveguide (CPW) resonators, shunted at the antinode by a dissipative copper microstrip via aluminium leads, yielding a quality factor unresolvable from the typical microwave environment. By measuring the transmission both above and below this transition, we are able to isolate the resonance. We then experimentally verify this method with copper microstrips of increasing thicknesses, from 50 nm to 150 nm, and measure quality factors in the range of 10∼67 in a consistent way.