I think what I enjoy most
about quantum computers
is actually trying to build them.
I really like being hands-on
to really understand the device physics
and learn how to make the device
work for you,
how to make it do what it should do.
The story of this paper
really starts in 2017,
when a few groups
started to work on blueprints
of a quantum classical processor.
After a series of demonstrations,
which have shown that silicon-based
qubits are promising candidates
for large-scale quantum computing.
In this work recently published
on Nature Electronics,
we combined radio frequency
measurement techniques
with concepts of random access,
which is found in modern memory devices.
Using this, we addressed the challenges
of read-out of the large-scale devices,
by reading out one device
after another
using the same line.
To implement such a cryogenic
control circuit,
we decided, with out colleagues
at Hitachi Cambridge laboratory,
and CEA-LETI,
to use CMOS technology.
CMOS technology is the basis
for conventional processors.
It has driven the digital revolution
we've seen over the last decades,
due to reliable fabrication
of complex circuits
consisting of millions
and billions of transistors.
Now a proof of concept experiment,
we combine CMOS transistors
with CMOS quantum devices
all on the same chip.
First we had to check
that these transistors
actually still work
at cryogenic temperatures.
Because, for a selected qubit,
they should allow
the read-out signal
to be delivered without disturbance.
Additionally, for the selected qubit,
signals should be blocked,
and the qubit
should not be disturbed.
The transistors we chose
actually work quite well
and we showed sequential read-out
of two quantum devices.
However, the approach
can be easily extended
to a large number of devices,
where each individual cell consists of
a CMOS transistor and a qubit each.
Using this,
we can make a two-dimensional array
of quantum devices
which can be randomly addressed
similar to conventional memory chips
and then read out all of these devices
using a single line.
We start off with a big wafer,
which we cut into small pieces,
each still containing many transistor
and quantum devices.
To actually measure the device,
and bring it to life,
we need to make connections
from the nano-scale device
to the outside world.
For this, we take a chip and glue it
onto a printed circuit board.
We use wires thinner than a hair
to actually make the connection.
Now that we have the chip
wired to our PCB,
we take this PCB and connect it
to a cylindrical sample holder
which we can then attach to the coldest
part of our dilution refrigerator
which also makes
the electrical connection
from the temperature
to our device.
We're very proud of this work.
We managed to demonstrate
the CMOS technology
can help to solve the challenges
of a large-scale quantum computer.
Maybe quantum computer chips
will not look so different
from conventional processors.
From the circuit models
we have developed,
we could next go ahead and design
fully-optimised and integrated
multi-qubit circuits.
There's lots of exciting work ahead.
