IEEE Solid-States Circuits Magazine - Spring 2021 - 55

Ti/Au
ISQD
NbTiN
500 nm
B B B B
LP MP RP
(b)
(a)
(c)
(d)
FIGURE 1: A few candidates for solid-state qubits that are available today. (a) A spin qubit, with semiconductor quantum dots. (b) A topological
qubit, showing semiconductor-superconductor hybrids. (c) A superconductive qubit. (d) Nitrogen vacancy centers. SQD: semiconductor
quantum dot; Ti: titanium; Au: gold; Nb: niobium; TiN: Ti nitride; InSb: indium antimonide.
controller is generally operated at
RT and the qubits at a few tens of
milli-Kelvin.
As a result of the large temperature
gradient, the interconnect between
classical and quantum devices
poses a major challenge. To keep the
phonon flow to a minimum while
ensuring fast and compact electrical
connections, several thermalization
stages are needed, each attenuating
the control signal so that the output
impedance of the signal generates
only the noise corresponding to the
lower temperature. Thus, the signals
at RT are large and generally have an
output impedance of 50 Ω so as to
achieve the needed signal amplitudes
at deep-cryogenic temperatures with
the same output impedance. RT control
has worked well so far, whereas
the number of qubits has been lower
than 100. Scaling up the number of
qubits requires a new approach, one
that can ensure a compact, possibly
integrated, design while satisfying all
thermal requirements and improving
the reliability and debuggability
of the overall system. To address
scalability, in 2016, we proposed
cryogenic electronics and, in particular,
cryo-CMOS as a technology that
could be placed near the qubits at
temperatures comparable to those at
which qubits operate [4].
Due to the lack of thermalization
requirements, cryo-CMOS electronics
could generate control signals at amplitudes
that are far smaller than at
RT. Readout circuits could also take
advantage of low temperatures to reduce
thermal noise, and the proximity
in space and temperature to qubits
could reduce the complexity of the
quantum computer by enabling the
use of a superconductive interconnect,
which could maximize thermal isolation
with virtually zero losses. This
could be simplified further if qubits
could operate at elevated temperatures,
as recently demonstrated in
the literature [5]-[7]. In the remainder
of the article, we focus on these
types of qubits, or electron/hole
spin qubits, although superconducting
qubits, or transmons, could also
be compatible with the circuits and
systems described here.
Since a cryo-CMOS controller operates
at temperatures close, and ideally
equal, to those of the qubits, the main
limitation is the power dissipation of
Control
Quantum
Processor
(<< 1 K)
Readout
Classical
Controller
FIGURE 2: The classical control of a quantum processor represented by an array of qubits.
IEEE SOLID-STATE CIRCUITS MAGAZINE
SPRING 2021
55
InSb
Nanowire

IEEE Solid-States Circuits Magazine - Spring 2021

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