2,247 research outputs found
Improving wafer-scale Josephson junction resistance variation in superconducting quantum coherent circuits
Quantum bits, or qubits, are an example of coherent circuits envisioned for
next-generation computers and detectors. A robust superconducting qubit with a
coherent lifetime of (100 s) is the transmon: a Josephson junction
functioning as a non-linear inductor shunted with a capacitor to form an
anharmonic oscillator. In a complex device with many such transmons, precise
control over each qubit frequency is often required, and thus variations of the
junction area and tunnel barrier thickness must be sufficiently minimized to
achieve optimal performance while avoiding spectral overlap between neighboring
circuits. Simply transplanting our recipe optimized for single, stand-alone
devices to wafer-scale (producing 64, 1x1 cm dies from a 150 mm wafer)
initially resulted in global drifts in room-temperature tunneling resistance of
30%. Inferring a critical current variation from this
resistance distribution, we present an optimized process developed from a
systematic 38 wafer study that results in 3.5% relative standard deviation
(RSD) in critical current () for 3000 Josephson junctions (both single-junctions and
asymmetric SQUIDs) across an area of 49 cm. Looking within a 1x1 cm moving
window across the substrate gives an estimate of the variation characteristic
of a given qubit chip. Our best process, utilizing ultrasonically assisted
development, uniform ashing, and dynamic oxidation has shown = 1.8% within 1x1 cm, on average,
with a few 1x1 cm areas having 1.0% (equivalent to 0.5%). Such stability would drastically improve the yield of
multi-junction chips with strict critical current requirements.Comment: 10 pages, 4 figures. Revision includes supplementary materia
1/f noise of Josephson-junction-embedded microwave resonators at single photon energies and millikelvin temperatures
We present measurements of 1/f frequency noise in both linear and
Josephson-junction-embedded superconducting aluminum resonators in the low
power, low temperature regime - typical operating conditions for
superconducting qubits. The addition of the Josephson junction does not result
in additional frequency noise, thereby placing an upper limit for fractional
critical current fluctuations of (Hz) at 1 Hz for
sub-micron, shadow evaporated junctions. These values imply a minimum dephasing
time for a superconducting qubit due to critical current noise of 40 -- 1400
s depending on qubit architecture. Occasionally, at temperatures above 50
mK, we observe the activation of individual fluctuators which increase the
level of noise significantly and exhibit Lorentzian spectra
Cavity-assisted quantum bath engineering
We demonstrate quantum bath engineering for a superconducting artificial atom
coupled to a microwave cavity. By tailoring the spectrum of microwave photon
shot noise in the cavity, we create a dissipative environment that autonomously
relaxes the atom to an arbitrarily specified coherent superposition of the
ground and excited states. In the presence of background thermal excitations,
this mechanism increases the state purity and effectively cools the dressed
atom state to a low temperature
Quantum State Sensitivity of an Autoresonant Superconducting Circuit
When a frequency chirped excitation is applied to a classical high-Q
nonlinear oscillator, its motion becomes dynamically synchronized to the drive
and large oscillation amplitude is observed, provided the drive strength
exceeds the critical threshold for autoresonance. We demonstrate that when such
an oscillator is strongly coupled to a quantized superconducting qubit, both
the effective nonlinearity and the threshold become a non-trivial function of
the qubit-oscillator detuning. Moreover, the autoresonant threshold is
sensitive to the quantum state of the qubit and may be used to realize a high
fidelity, latching readout whose speed is not limited by the oscillator Q.Comment: 5 pages, 4 figure
Transition Metal Complexes of a-Naphthylamine Dithiocarbamate
a-Naphthylamine dithiocarbamate and its complexes with
Co(II), Ni(U), Cu(II), Ru(III) , Rh(III), Pd(II), Pt(IV), Zn(II), Cd(II)
and Hg(II) have been prepared and characterized by chemical
analysis, IR - and reflectance spectral studies and magnetic
susceptibili ty measurements. In all these complexes the dithiocarbamato
moiety acts as a chelate. The Ni(II), Cu(II) and Pd(Il)
complexes have been found to be square planar while those of
Ru(III), Rh(III) and Pt(IV) were proposed to be octahedral in
nature. The Co(II) ion seems to have a tetrahedral geometry, unlike
the other known square planar dithiocarbamato complexes of
Co(II). No definite structure, however, could be proposed for Zn(II),
Cd(II) and Hg(II) on the basis of limited studies
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