IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 35, NO. 5, AUGUST 2025 1500305
Measurable Improvement in Multi-Qubit Readout
Using a Kinetic Inductance Traveling Wave
Parametric Amplifier
M. A. Castellanos-Beltran ,L.Howe , A. Giachero , M. R. Vissers, D. Labranca ,J.N.Ullom,
and P. F. Hopkins
, Senior Member, IEEE
Abstract—Increasing the size and complexity of quantum infor-
mation systems requires highly-multiplexed readout architectures,
as well as amplifier chains operating near the quantum limit (QL)
of added noise. While documented prior efforts in KI-TWPA inte-
gration in quantum systems are scarce, in this work we demonstrate
integration of a KI-TWPA with a multiplexed-qubit device. To
quantify the system noise improvement we perform an ac Stark
shift calibration to precisely determine noise power levels on-chip
(at each cavity’s reference plane) and the total system gain. We then
characterize the qubit state measurement fidelity and the corre-
sponding signal-to-noise ratio (SNR). To conduct the most faithful
measurement of the benefits offered by the KI-TWPA we perform
these measurements for readout chains where the high electron
mobility transistor (HEMT) amplifier is the first-stage amplifier
(FSA) – with none of the external hardware required to operate
the KI-TWPA – and with the KI-TWPA as the FSA. While some
readout cavities fall outside the KI-TWPA bandwidth, for those
inside the bandwidth we demonstrate a maximum improvement in
the state measurement SNR by a factor of 1.45, and increase the
fidelity from 96.2% to 97.8%. These measurements demonstrate
a system noise below 5 quanta referenced on-chip and we bound
the KI-TWPA excess noise to be below 4 quanta for the six cavities
inside its bandwidth. These results show a promising path forward
Received 25 September 2024; revised 19 December 2024; accepted 21 De-
cember 2024. Date of publication 2 January 2025; date of current version 16
January 2025. This work was supported in part by the National Aeronautics
and Space Administration (NASA) under Grant NNH18ZDA001N-APRA, in
part by the Department of Energy (DOE) Accelerator and Detector Research
Program under Grant 89243020SSC000058, and DARTWARS a project, in part
by the European Union’s H2020-MSCA under Grant 101027746, and in part
by the Italian National Quantum Science and Technology Institute through the
PNRR MUR Project under Grant PE0000023-NQSTI. (Corresponding author:
A. Giachero.)
M. A. Castellanos-Beltran and P. F. Hopkins are with the RF Technology
Division, National Institute of Standards and Technology, Boulder, CO 80305
USA.
L. Howe and J. N. Ullom are with the Quantum Sensors Division, National
Institute of Standards and Technology, Boulder, CO 80305 USA, and also with
the Department of Physics, University of Colorado, Boulder, CO 80309 USA.
A. Giachero is with the Quantum Sensors Division, National Institute of
Standards and Technology, Boulder, CO 80305 USA, also with the Department
of Physics, University of Colorado, Boulder, CO 80309 USA, and also with
the Department of Physics, University of Milano-Bicocca, 20126 Milan, Italy
(e-mail: andrea.giachero@colorado.edu).
M. R. Vissers is with the Quantum Sensors Division, National Institute of
Standards and Technology, Boulder, CO 80305 USA.
D. Labranca is with the RF Technology Division, National Institute of Stan-
dards and Technology, Boulder, CO 80305 USA, and also with the Department
of Physics, University of Milano-Bicocca, 20126 Milan, Italy.
Color versions of one or more figures in this article are available at
https://doi.org/10.1109/TASC.2024.3525451.
Digital Object Identifier 10.1109/TASC.2024.3525451
for realizing quantum-limited readout chains in large qubit systems
using a single parametric amplifier.
Index Terms—Kinetic inductance, multi-qubit, noise, quantum-
limit, quantum computing, traveling wave parametric amplifier.
I. INTRODUCTION
S
UPERCONDUCTING parametric amplifiers are critical
components for the precise readout of qubits [1]. To achieve
fast, high-fidelity readout [2], [3] it is paramount to amplify
weak signals without introducing excessive noise, i.e the first-
stage amplifier (FSA) must operate very close to the quantum
limit (QL) of added noise [4]. Josephson parametric amplifiers
(JPAs) [5], [6] have been a mainstay solution due to their
low-noise performance [7], [8]. However, as the number of
superconducting qubits increases the need for simultaneous
readout [9], [10] of a large number of devices severely hampers
feasible scaling using JPAs. Notably, JPAs are very narrowband
and these Josephson-junction-based amplifiers typically have an
input dynamic range below −100 dBm [11], [12]. Impedance
transformers and parallel distribution of t he amplification can
be implmented to modestly increase bandwidth and dynamic
range, however, this comes at the cost of significant added
complexity [13]. An alternate technology for achieving broad-
band, quantum-limited readout is that of the Traveling Wave
Parametric Amplifier (TWPA) [14]. Superconducting TWPAs
exploit the non-linearity of inductive elements embedded in a
long transmission line (TL) to amplify weak microwave signals
with minimal added noise [15]. TWPAs based on Josephson
junctions [16] (JTWPAs) are a potential solution for multiplexed
qubit readout.
Another solution is a TWPA based on the non-linearity of ki-
netic inductance in disordered materials [17], [18] (KI-TWPAs).
Although proof-of-principle demonstrations in qubit readout
exist [19], KI-TWPAs have not enjoyed the same extensive i nte-
gration as J osephson-junction based parametric amplifiers. This
is contrary to the most attractive features of KI-TWPAs, relative
to JPAs and JTWPAs: they are simpler to fabricate [20] and
can provide a dynamic range orders of magnitude higher [21],
[22]. Further, they have demonstrated power gains above 15 dB
over 3 GHz of bandwidth [23], [24], they can operate at higher
temperatures [25], and are resilient to high magnetic fields [26],
[27] – reducing the amount of shielding required.
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