Quantum Computing with Superconducting Flux Qubits

S. Han, University of Kansas, DMR-0325551

SEM micrograph of a SQUID flux qubit. The qubit has a second order gradiometer configuration which makes it immune to fluctuations of homogeneous magnetic field. Experiments show the qubit has a relaxation time of ~7 s, corresponding to a maximum decoherence time of ~14 s, a figure that is very promising for realization of quantum computing..

We have designed, fabricated, and characterized SQUID flux qubits aiming for scalable quantum computation. Energy relaxation time (T1) between the “0” and “1” states of the qubit has been measured using a high-precision time-domain technique. The result of T1~7 s is very encouraging. Temperature dependence of relaxation time agrees very well with the spin-boson model.

1.0

0.5

0.0Popu

latio

n of

"1"

Sta

te

2520151050

Time (s)

data fit

T1=6.9 s

| 0

|1

DetectorQubit

Bias control

Supercurrent

‘0’ ‘1’

Epitaxial Barrier NbN/AlN/NbN Tunnel Junctionsfor Quantum Information Applications

Z. Wang, Kobe Advanced ICT Research Center, NICT, JAPAN, DMR-0325551

200

020

220

200

020

220

200

020

220

5nm

NbN

NbN

AlN

MgO

200

020

220

200

020

220

200

020

220

5nm5nm

NbN

NbN

AlN

MgO

TEM micrograph of the junction cross section (center and left) and electron diffraction patterns (right) for an NbN/AlN/NbN junction with a Jc of 400 A/cm2.

We fabricated high-quality NbN tunnel junctions with epitaxial AlN barrier by reactive dc-magnetron sputtering. The junctions show excellent properties in a wide range of Jc, from 100 A/cm2 to 20 kA/cm2, suitable for quantum information applications.

TEM and electron diffraction show atomically smooth and sharp interfaces between the superconducting NbN electrodes and the insulating AlN tunnel barrier

ASC2006-1EG01

The epitaxial tunnel barrier greatly reduce the number of defects (microscopic two-level fluctuators) which have been identified as the dominant decoherence mechanism in all types of solid-state Josephson superconducting qubits.

Mission of KARC     

Is acting as a base for basic research that plays an important role in the National Institute of Information and Communications Technology, Incorporated Administrative Agency.We are conducting a variety of bio, brain, nanotechnology, superconductor, quantum and laser, etc. research, aiming at the creation of knowledge and technological breakthroughs, and dream of contributing to future, affluent info-communications.    

The Superconducting Electronics Group web sitehttp://www-karc.nict.go.jp/102/E_index.html contains our research subjects, activities, and recent research presentations.

Equipments and facilitiesWe have a 600 m2 area clean room (class 100 and class 1,000) for fabrication of superconducting junctions and circuits. Various equipments are available both for device fabrication and analysis in the clean room. We have rf and dc reactive sputtering systems, RIE, ECR, laser ablation, ion-beam deposition, i-line stepper, e-beam writer, x-ray diffraction, SEM, AFM, etc.

Epitaxial Barrier NbN/AlN/NbN Tunnel Junctionsfor Quantum Information Applications

Z. Wang, Kobe Advanced ICT Research Center, NICT, JAPAN, DMR-0325551

Quantum Computer: Need to Reduce Decoherence Great Josephson Junctions Required

J. E. Lukens, Stony Brook University, DMR-0325551

0.1 1 10 10010

100

1000

Spectral Density of Critical Current Noise (1/f)

E F10 F11 F18

Data Theoretical Johnson noise Stony Brook junctions Average for other labs

SI (

10-2

4 A2 /H

z)

Frequency (Hz)

T = 4.2 K

Stony Brook Superconducting DevicesDesign and Fabrication

Vijay Patel & Wei Chen

Superconducting Qubit

JJs– 1mx1m

• 1/f fluctuations in Ic lead todecoherence.

• Stony Brook Nb junctions are 100 times quieter than typical—the best available.

Data from Shawn Pottorf