Quantum Superconducting Devices

Our research involves studying quantum mechanical effects in superconducting circuits at very low temperatures
(T ~ 20 mK). The circuits that we are currently studying are transmon devices, which consist of a small Josephson
junction shunted by a capacitor. The Josephson junction gives the circuit a little bit of anharmonicity so that the
lower-lying photonic or energy states can be individually excited and coherently controlled using resonant pulses of microwaves. To measure the state of the transmon, both quasi-lumped element resonators and 3D cavities are
used in our laboratory.

   

Device Images.  (Left) Colorized optical micrograph of a transmon device coupled to a lumped element resonator.
(Right) Transmon device embedded in a 3D copper cavity. The transmon devices are fabricated from a
thin film of superconducting aluminum on a sapphire substrate.

   

Quantum State Tomography Measurements of a state consisting of a superposition of
zero and one photon stored in the transmon device.


Coherence

A major effort with quantum superconducting devices is to understand and mitigate sources of decoherence,
since fidelity of gate operations can be limited by coherence times. The two main sources of decoherence (T2)
are energy relaxation processes (T1) and dephasing processes (TΦ). Given our previous results with a Cooper-pair
box[4,5] we find it useful to use our T1 measurements to also estimate the spectral density of noise (or in most
cases the bound in the noise) that gives rise to relaxation processes. The plot to the right shows bounds in
measurements of the extrapolated spectral density of charge noise for a number of transmon and Cooper-pair
box devices at different transition frequencies.

   
 

Spectroscopic measurements of a 3D transmon.

Extrapolated bound in the spectral density of charge
noise for a variety of transmon and Cooper-pair box
devices at different transition frequencies.

 


Quantum Interference
For the past few years, we have been exploring quantum interferometric effects using multiple microwave tones in conjunction with multiple levels of a transmon/cavity device. Using a 3D transmon, our group has reported on
the observations of the Autler-Townes splitting using the first two excited states of a transmon [2] (|g,0>, |e,0>
and |f,0> in the energy diagram) and the Autler-Townes splitting using a combination of cavity photon states and
transmon states (|g,0>, |g,1> and |e,1> in the energy diagram).[1]

By engineering the decay rates of the system, we are currently working on demonstrating coherent population
trapping and electromagnetically induced transparency in this system.

 
 

Measurements of Autler-Townes Doublet versus probe detuning and coupler power.


Energy level diagram of cavity transmon system.


List of Publications

  1. B. Suri, Z. K. Keane, R. Ruskov, Lev S. Bishop, C. Tahan, S. Novikov, J. E. Robinson, F. C. Wellstood, and B. S. Palmer, “Observation of Autler–Townes effect in a dispersively dressed Jaynes–Cummings system,” New Journal of Physics 15, 125007 (2013).
  2. S. Novikov, J. E. Robinson, Z. K. Keane, B. Suri, F. C. Wellstood, and B. S. Palmer, “Autler-Townes splitting in a three-dimensional transmon superconducting qubit,” Physical Review B 88 0605 03 (2013).
  3. V. Zaretskey, B. Suri, S. Novikov, F. C. Wellstood, and B. S. Palmer, “Spectroscopy of a Cooper-pair box coupled to a two-level system via charge and critical current,” Physical Review B 87, 174522 (2013).
  4. V. Zaretskey, S. Novikov, B. Suri, Z. Kim, F. C. Wellstood, and B. S. Palmer, “Decoherence in a pair of long-lived Cooper-pair boxes,” Journal of Applied Physics 114, 094305 (2013).
  5. Z. Kim, B. Suri, V. Zaretskey, S. Novikov, K. D. Osborn, A. Mizel, F. C. Wellstood, and B. S. Palmer, “Decoupling a Cooper-Pair Box to Enhance the Lifetime to 0.2 ms,” Physical Review Letters 106, 120501 (2011).
  6. Z. Kim, V. Zaretskey, Y. Yoon, J. F. Schneiderman, M. D. Shaw, P. M. Echternach, F. C. Wellstood, and B. S. Palmer, “Anomalous avoided level crossings in a Cooper-pair box spectrum,” Physical Review B 78, 144506 (2008).

Dissertations

  1. Vitaley Zaretskey, “Decoherence and Defects in Cooper-Pair Boxes,” Ph.D. thesis, University of Maryland, College Park, 2013.
  2. Zaeill Kim, “Dissipative and Dispersive Measurements of a Cooper-Pair Box,” Ph.D. thesis, University of Maryland, College Park, 2010.

 


For further information please contact:
Benjamin S. Palmer
Laboratory for Physical Sciences
8050 Greenmead Road
College Park, MD 20740
Phone: 301-935-6727
bpalmer at lps.umd.edu