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.

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.

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.

Correlated emission lasing in harmonic oscillators coupled via a single three-level artificial atom

  1. Z.H. Peng,
  2. Yu-xi Liu,
  3. J.T. Peltonen,
  4. T. Yamamoto,
  5. J. S. Tsai,
  6. and O. Astafiev
A single superconducting artificial atom provides a unique basis for coupling electromagnetic fields and photons hardly achieved with a natural atom. Bringing a pair of harmonic oscillators
into resonance with transitions of the three-level atom converts atomic spontaneous processes into correlated emission dynamics. We demonstrate two-mode correlated emission lasing on harmonic oscillators coupled via the fully controllable three-level artificial atom. Correlation of two different color emissions reveals itself as equally narrowed linewiths and quench of their mutual phase-diffusion. The mutual linewidth is more than four orders of magnitude narrower than the Schawlow-Townes limit. The interference between the different color lasing fields demonstrates the two-mode fields are strongly correlated.