Strong environmental coupling in a Josephson parametric amplifier

  1. Josh Mutus,
  2. Ted White,
  3. Rami Barends,
  4. Yu Chen,
  5. Zijun Chen,
  6. Ben Chiaro,
  7. Andrew Dunsworth,
  8. Evan Jeffrey,
  9. Julian Kelly,
  10. Anthony Megrant,
  11. Charles Neill,
  12. Peter O'Malley,
  13. Pedram Roushan,
  14. Daniel Sank,
  15. Amit Vainsencher,
  16. James Wenner,
  17. Kyle Sundqvist,
  18. Andrew Cleland,
  19. and John Martinis
We present a lumped-element Josephson parametric amplifier designed to operate with strong coupling to the environment. In this regime, we observe broadband frequency dependent amplification
with multi-peaked gain profiles. We account for this behaviour using the „pumpistor“ model which allows for frequency dependent variation of the external impedance. Using this understanding, we demonstrate control over gain profiles through changes in the environment impedance at a given frequency. With strong coupling to a suitable external impedance we observe a significant increase in dynamic range, and large amplification bandwidth up to 700 MHz giving near quantum-limited performance.

Fabrication and Characterization of Aluminum Airbridges for Superconducting Microwave Circuits

  1. Zijun Chen,
  2. Anthony Megrant,
  3. Julian Kelly,
  4. Rami Barends,
  5. Joerg Bochmann,
  6. Yu Chen,
  7. Ben Chiaro,
  8. Andrew Dunsworth,
  9. Evan Jeffrey,
  10. Joshua Mutus,
  11. Peter O'Malley,
  12. Charles Neill,
  13. Pedram Roushan,
  14. Daniel Sank,
  15. Amit Vainsencher,
  16. James Wenner,
  17. Theodore White,
  18. Andrew Cleland,
  19. and John Martinis
Superconducting microwave circuits based on coplanar waveguides (CPW) are susceptible to parasitic slotline modes which can lead to loss and decoherence. We motivate the use of superconducting
airbridges as a reliable method for preventing the propagation of these modes. We describe the fabrication of these airbridges on superconducting resonators, which we use to measure the loss due to placing airbridges over CPW lines. We find that the additional loss at single photon levels is small, and decreases at higher drive powers.

Design and characterization of a lumped element single-ended superconducting microwave parametric amplifier with on-chip flux bias line

  1. Josh Mutus,
  2. Ted White,
  3. Evan Jeffery,
  4. Daniel Sank,
  5. Rami Barends,
  6. Joerg Bochmann,
  7. Yu Chen,
  8. Zijun Chen,
  9. Ben Chiaro,
  10. Andrew Dunsworth,
  11. Julian Kelly,
  12. Anthony Megrant,
  13. Charles Neill,
  14. Peter O'malley,
  15. Pedram Roushan,
  16. Amit Vainsencher,
  17. Jim Wenner,
  18. Irfan Siddiqi,
  19. Rajamani Vijayaraghavan,
  20. Andrew Cleland,
  21. and John Martinis
We demonstrate a lumped-element Josephson Parametric Amplifier (LJPA), using a single-ended design that includes an on-chip, high-bandwidth flux bias line. The amplifier can be pumped
into its region of parametric gain through either the input port or through the flux bias line. Broadband amplification is achieved at a tunable frequency $\omega/2 \pi$ between 5 to 7 GHz with quantum-limited noise performance, a gain-bandwidth product greater than 500 MHz, and an input saturation power in excess of -120 dBm. The bias line allows fast frequency tuning of the amplifier, with variations of hundreds of MHz over time scales shorter than 10 ns.

Sputtered TiN films for superconducting coplanar waveguide resonators

  1. Shinobu Ohya,
  2. Ben Chiaro,
  3. Anthony Megrant,
  4. Charles Neill,
  5. Rami Barends,
  6. Yu Chen,
  7. Julian Kelly,
  8. David Low,
  9. Josh Mutus,
  10. Peter O'Malley,
  11. Pedram Roushan,
  12. Daniel Sank,
  13. Amit Vainsencher,
  14. James Wenner,
  15. Theodore C. White,
  16. Yi Yin,
  17. B. D. Schultz,
  18. Chris J Palmstrøm,
  19. Benjamin A. Mazin,
  20. Andrew N. Cleland,
  21. and John M. Martinis
We present a systematic study of the properties of TiN films by varying the deposition conditions in an ultra-high-vacuum reactive magnetron sputtering chamber. By increasing the deposition
pressure from 2 to 9 mTorr while keeping a nearly stoichiometric composition of Ti(1-x)N(x) (x=0.5), the film resistivity increases, the dominant crystal orientation changes from (100) to (111), grain boundaries become clearer, and the strong compressive strain changes to weak tensile strain. The TiN films absorb a high concentration of contaminants including hydrogen, carbon, and oxygen when they are exposed to air after deposition. With the target-substrate distance set to 88 mm the contaminant levels increase from ~0.1% to ~10% as the pressure is increased from 2 to 9 mTorr. The contaminant concentrations also correlate with in-plane distance from the center of the substrate and increase by roughly two orders of magnitude as the target-substrate distance is increased from 88 mm to 266 mm. These contaminants are found to strongly influence the properties of TiN films. For instance, the resistivity of stoichiometric films increases by around a factor of 5 as the oxygen content increases from 0.1% to 11%. These results suggest that the sputtered TiN particle energy is critical in determining the TiN film properties, and that it is important to control this energy to obtain high-quality TiN films. Superconducting coplanar waveguide resonators made from a series of nearly stoichiometric films grown at pressures from 2 mTorr to 7 mTorr show an increase in intrinsic quality factor from ~10^4 to ~10^6 as the magnitude of the compressive strain decreases from nearly 3800 MPa to approximately 150 MPa and the oxygen content increases from 0.1% to 8%. The films with a higher oxygen content exhibit lower loss, but the nonuniformity of the oxygen incorporation hinders the use of sputtered TiN in larger circuits.

Computing prime factors with a Josephson phase qubit quantum processor

  1. Erik Lucero,
  2. Rami Barends,
  3. Yu Chen,
  4. Julian Kelly,
  5. Matteo Mariantoni,
  6. Anthony Megrant,
  7. Peter O'Malley,
  8. Daniel Sank,
  9. Amit Vainsencher,
  10. James Wenner,
  11. Ted White,
  12. Yi Yin,
  13. Andrew N. Cleland,
  14. and John M. Martinis
. Compiled versions of Shor’s algorithm have been demonstrated"]on ensemble quantum systems[2] and photonic systems[3-5], however this has yet to be shown using solid state quantum bits (qubits). Two advantages of superconducting qubit architectures are the use of conventional microfabrication techniques, which allow straightforward scaling to large numbers of qubits, and a toolkit of circuit elements that can be used to engineer a variety of qubit types and interactions[6, 7]. Using a number of recent qubit control and hardware advances [7-13], here we demonstrate a nine-quantum-element solid-state QuP and show three experiments to highlight its capabilities. We begin by characterizing the device with spectroscopy. Next, we produces coherent interactions between five qubits and verify bi- and tripartite entanglement via quantum state tomography (QST) [8, 12, 14, 15]. In the final experiment, we run a three-qubit compiled version of Shor’s algorithm to factor the number 15, and successfully find the prime factors 48% of the time. Improvements in the superconducting qubit coherence times and more complex circuits should provide the resources necessary to factor larger composite numbers and run more intricate quantum algorithms.