A superconductor free of quasiparticles for seconds

  1. E. T. Mannila,
  2. P. Samuelsson,
  3. S. Simbierowicz,
  4. J.T. Peltonen,
  5. V. Vesterinen,
  6. L. Grönberg,
  7. J. Hassel,
  8. V. F. Maisi,
  9. and J P Pekola
Superconducting devices, based on the Cooper pairing of electrons, are of outstanding importance in existing and emergent technologies, ranging from radiation detectors to quantum computers.
Their performance is limited by spurious broken Cooper pairs also known as quasiparticle excitations. In state-of-the-art devices, the time-averaged number of quasiparticles can be on the order of one. However, realizing a superconductor with no excitations remains an outstanding challenge. Here, we experimentally demonstrate a superconductor completely free of quasiparticles up to seconds. The quasiparticle number on a mesoscopic superconductor is monitored in real time by measuring the charge tunneling to a normal metal contact. Quiet excitation-free periods are interrupted by random-in-time events, where one or several Cooper pairs break, followed by a burst of charge tunneling within a millisecond. Our results vindicate the opportunity to operate devices without quasiparticles with potentially improved performance. In addition, our present experiment probes the origins of nonequilibrium quasiparticles in it; the decay of the Cooper pair breaking rate over several weeks following the initial cooldown rules out processes arising from cosmic or long-lived radioactive sources.

Thermodynamics in Single-Electron Circuits and Superconducting Qubits

  1. J P Pekola,
  2. and I. M. Khaymovich
Classical and quantum electronic circuits provide ideal platforms to investigate stochastic thermodynamics and they have served as a stepping stone to realize Maxwell’s demons
with highly controllable protocols. In this article we first review the central thermal phenomena in quantum nanostructures. Thermometry and basic refrigeration methods will be described as enabling tools for thermodynamics experiments. Next we discuss the role of information in thermodynamics which leads to the concept of Maxwell’s demon. Various Maxwell’s demons realized in single-electron circuits over the past couple of years will be described. Currently true quantum thermodynamics in superconducting circuits is in focus of attention, and we end the review by discussing the ideas and first experiments in this exciting area of research.

Coupled qubits as a quantum heat switch

  1. B. Karimi,
  2. J P Pekola,
  3. M. Campisi,
  4. and R. Fazio
We present a quantum heat switch based on coupled superconducting qubits, connected to two LC resonators that are terminated by resistors providing two heat baths. To describe the system
we use a standard second order master equation with respect to coupling to the baths. We find that this system can act as an efficient heat switch controlled by the applied magnetic flux. The flux influences the energy level separations of the system, and under some conditions, the finite coupling of the qubits enhances the transmitted power between the two baths, by an order of magnitude under realistic conditions. At the same time, the bandwidth at maximum power of the switch formed of the coupled qubits is narrowed.

Otto refrigerator based on a superconducting qubit – classical and quantum performance

  1. B. Karimi,
  2. and J P Pekola
We analyse a quantum Otto refrigerator based on a superconducting qubit coupled to two LC-resonators each including a resistor acting as a reservoir. We find various operation regimes:
nearly adiabatic (low driving frequency), ideal Otto cycle (intermediate frequency), and non-adiabatic coherent regime (high frequency). In the nearly adiabatic regime, the cooling power is quadratic in frequency, and we find substantially enhanced coefficient of performance ϵ, as compared to that of an ideal Otto cycle. Quantum coherent effects lead invariably to decrease in both cooling power and ϵ as compared to purely classical dynamics. In the non-adiabatic regime we observe strong coherent oscillations of the cooling power as a function of frequency. We investigate various driving waveforms: compared to the standard sinusoidal drive, truncated trapezoidal drive with optimized rise and dwell times yields higher cooling power and efficiency.

Multiplexing Superconducting Qubit Circuit for Single Microwave Photon Generation

  1. R E George,
  2. J Senior,
  3. O.-P. Saira,
  4. S. E. de Graaf,
  5. T. Lindstrom,
  6. J P Pekola,
  7. and Yu A Pashkin
We report on a device that integrates eight superconducting transmon qubits in lambda/4 superconducting coplanar waveguide resonators fed from a common feedline. Using this multiplexing
architecture, each resonator and qubit can be addressed individually thus reducing the required hardware resources and allowing their individual characterisation by spectroscopic methods. The measured device parameters agree with the designed values and the resonators and qubits exhibit excellent coherence properties and strong coupling, with the qubit relaxation rate dominated by the Purcell effect when brought in resonance with the resonator. Our analysis shows that the circuit is suitable for generation of single microwave photons on demand with an efficiency exceeding 80%.