Types of quantum
computer
There are at least four main
approaches towards building a quantum computer processor that have attracted
major commercial investment. These are known as silicon spin qubits,
superconducting qubits, ion trap qubits and topological qubits. Each brings
specific challenges.
Silicon spin qubits: can operate at 1.5 kelvin
Spin qubits in silicon are
excellent candidates for scalable quantum information processing, because they can
be made in their billions using the same tools and infrastructure that the
global computer industry already uses to make silicon chips for today’s computers
and smart phones. The physical properties of silicon also mean that the qubits
can be very stable and accurate. Now, thanks to Andrew Dzurak’s team, they can
be operated at substantially warmer temperatures (on the quantum scale) than
competing technologies.
Silicon spin qubits are the focus of several teams at UNSW.
Companies using this approach include Intel, Silicon Quantum Computing, Quantum
Motion and HRL Laboratories.
Superconducting qubits: require near absolute
zero temperatures
These qubits are created from
a loop of superconducting material, typically aluminium, combined with thin
insulating barriers through which pairs of electrons can tunnel. This approach
has produced the most advanced quantum computers to date, including a 53-qubit
machine developed by Google which was used last October to demonstrate ‘quantum
supremacy’ – meaning the quantum machine, for the first time, outperformed a
large supercomputer. The technology needed to control each qubit means that
scaled up machines are expected to be very large. Required operating
temperatures are one-tenth of a degree above absolute zero (or -273 degrees
Celsius).
Companies using this technology
include Google, IBM, Intel, Quantum Circuits, DWave, and Rigetti.
Ion trap qubits: require lasers to cool atoms
close to absolute zero
This approach uses one of the
electrons in an ion (of calcium, for instance) to create a qubit in two states,
defined either by the electron’s orbital state or its interaction with the
atom’s nucleus. The ions are suspended and moved around in free space using
electric and magnetic fields, which makes them easy to isolate from
interference, giving them considerable stability. They were the first type of
qubit to be studied more than two decades ago and there are now prototype
systems with up to 20 qubits in use. On the downside, the qubits have to be held
in ultra-high vacuums to prevent them interacting with other atoms, and they
need to be kept cool and controlled by sophisticated lasers. Optimal operating
temperatures are below a thousandth of a degree Celsius
above absolute zero.
The main company using this
approach is IonQ.
Topological qubits: require near absolute zero
temperatures
These qubits are typically
made from thin wires or sheets of a semiconductor coupled to a superconducting
circuit to produce exotic states of matter known as Majorana fermions. Such a
qubit has not yet been demonstrated, but it is predicted that they will be far
more immune to errors than other types, which is their main advantage. Like
superconducting qubits, however, topological qubits need to be cooled to
temperatures around one-tenth of a degree above absolute zero.
Microsoft is the main company
investing in this technology.
Timeline of the development of silicon CMOS quantum
computing
1994: Peter
Shor from Bell Labs (USA) shows that a quantum computer would be able to
decrypt Public Key Encrypted codes (at the heart of modern secure
communications) exponentially faster than today’s supercomputers. This triggers
massive interest in quantum computing worldwide, mainly from government
agencies.
1998:
UNSW paper outlines concept for silicon-based quantum computing
Bruce Kane, a postdoctoral
researcher at UNSW, publishes a paper in Nature outlining the concept
for a silicon-based quantum computer, in which the qubits are defined by single
phosphorus atoms in an otherwise ultra-pure silicon chip. This is the first
such scheme in silicon – the material used for all modern-day microprocessors.
Kane’s paper attracts great interest because: (a) silicon is ‘industrially
relevant’; (b) silicon electron ‘spins’ have very long ‘coherence times’ and,
hence, low error rates. This paper has now generated over 2,800 citations.
2000: UNSW
research team established
Bob Clark establishes the Australian
Research Council Special Research Centre for Quantum Computer Technology
(CQCT), headquartered at UNSW, with the task of developing the technologies
needed to build a quantum computer. The centre expanded in 2003 to become an
ARC Centre of Excellence, and since 2010 has been led by Michelle Simmons
(UNSW). It has more than 150 researchers in Australia, with major
collaborations worldwide. In 2000, Andrew Dzurak (UNSW), a founding
investigator in CQCT, begins his development of silicon device technologies for
building a silicon quantum computer.
2007: A
single electron device
Dzurak’s group develops a
variation of a silicon Metal-Oxide-Semiconductor
Field-Effect Transistor (MOSFET) device that can be reduced to the level
of a single electron. These devices can be manufactured using complementary metal-oxide-semiconductor
(CMOS) technology, from which most silicon chips for smart phones and computers
are made. The device is central to quantum computing breakthroughs by Dzurak
and colleagues to this day.
2010: A
qubit reader
Andrea Morello and Dzurak publish in Nature a paper describing the first
step of measuring a silicon qubit.
2012: A qubit
writer
Another paper in Nature
by the Morello group, “A single-atom electron spin qubit in silicon”, describes
the crucial step of writing information on an electron to operate the first
silicon qubit.
2014: A silicon
CMOS quantum bit
Dzurak, Menno Veldhorst and Henry
Yang patent the concept of a gate-addressable quantum bit based on the modified
silicon MOSFET transistor.
2014: A
high accuracy quantum dot qubit in silicon
A paper in Nature
Nanotechnology by Dzurak’s group, with lead author Veldhorst, describes an
addressable quantum dot qubit with fault-tolerant control-fidelity, which is a
high accuracy qubit based on the modified CMOS transistor developed by Dzurak’s
team in 2007.
2015: A
two-qubit logic gate in silicon
Dzurak’s group (again with
lead author Veldhorst) publishes paper in Nature titled “A two-qubit
logic gate in silicon”. This is the first ever demonstration of a calculation
between two qubits in silicon, meaning that all of the logic requirements for
silicon quantum computing are now in place.
2017:
Design for a full-scale silicon quantum computer chip
Dzurak’s group publishes a paper
in Nature Communications titled “Silicon CMOS architecture for a
spin-based quantum computer”. This sets out a detailed design of how computer
chip manufacturers can use their existing fabrication plants build a quantum
chip with millions of qubits.
2019: Most
accurate electron qubit in silicon
Dzurak’s group (with lead
author Yang) publishes a paper in Nature Electronics demonstrating the
most accurate electron spin qubit ever, based on his silicon CMOS technology,
with an error rate of only 0.04%. Such low rates are vital for full-scale
quantum computers, since otherwise errors quickly accumulate and destroy any
calculations.
2019:
First accuracy measurement of a two-qubit logic gate in silicon
Dzurak’s group (with lead
author Wister Huang) publishes paper in Nature titled “Fidelity
benchmarks for two-qubit gates in silicon”, which provides the first measure of
accuracy for silicon two-qubit logic. The results show that silicon is now near
the threshold of accuracy needed for fault-tolerant quantum computing.
2020:
Silicon CMOS qubit breaks the one Kelvin barrier
Dzurak’s group (with lead
author Yang) publishes apaper in Nature titled “Silicon quantum
processor unit cell operation above one Kelvin”. This is a temperature at which
most other qubit types cannot operate. It also means that far cheaper cooling
systems can be used, valued at thousands, rather than millions, of dollars.
Glossary
Qubits are the basic units of
information in a quantum computer, equivalent to the ‘bits’ in everyday
computers. Like a bit, a qubit stores a binary code of a ‘0’ or a ‘1’, but
unlike a conventional bit, a qubit can also be in a ‘superposition’ of both
‘0’ and ‘1’ at the same time. Every time you add one qubit to a quantum
computer, the amount of information it can store and compute with doubles. This
gives quantum computers an exponential increase in processing power (or speed)
compared with conventional computers for a wide range of calculations (or
algorithms).
Quantum
superposition is a basic
principle of quantum mechanics, which means that quantum systems can exist in
multiple states at the same time, until you try and measure the system. A qubit
is a system that has only two basic states, equivalent to ‘0’ or ‘1’,
which can exist in a superposition of those, until you measure it, after which it
is forced to randomly choose which one of the two states it is in, via a
process known as ‘wavefunction collapse’. This principle of quantum
superposition is what gives rise to the concept of Schrödinger’s cat, which can be thought of as simultaneously
alive and dead, until you look inside the closed box it exists in.
Quantum coherence is related to the fact that all objects
have wave-like properties. In the presence of noise, the wave-like properties
get washed out, over a time known as the coherence time. To be useful
for computing, you must be able to complete calculations on the qubits in a
quantum computer within this coherence time, otherwise the information gets
scrambled.
Quantum dots are devices or particles which are so
small that they can behave like ‘artificial atoms’, and the electrons inside
the quantum dot exist in specifically allowed quantum states. They can be used
to create qubits.
Quantum tunnelling is
the term used to describe a quantum function that has no equivalent in
classical physics. Because of the wave-like properties of subatomic particles
they are able to pass through seemingly impenetrable barriers even when they
don’t have the energy to do so. What’s more, they can do so instantaneously. It
sounds weird, but is well understood and is a key part of many human
endeavours, including electronics and some types of microscopy.
Spin is a quantum property of subatomic particles,
such as electrons, which is related to the particle’s angular momentum,
analogous to its rotation. The spin of an electron also produces a tiny
magnetic field, which will either line up in the same direction as (spin ‘up’)
or opposite to (spin ‘down’) an externally applied magnetic field. An electron
spin is an ideal qubit, whereby the spin up represents a ‘1’ and a spin down a
‘0’.
Silicon chips are used as computer memory or processing
units in computers, smart phones, and other electrical items. They are
typically about one centimetre square, and have tiny electrical integrated
circuits inscribed on them, typically with billions of CMOS transistors, or
memory cells. Silicon is a chemical element that is the second most abundant in
the earth’s crust.
Silicon CMOS (or complementary metal-oxide-semiconductor)
is the technology used to make and manufacture the transistors used for
information storage and processing on silicon chips. Every mobile phone has one
or more silicon CMOS chips inside it.
Silicon CMOS
qubit is a qubit made using
standard CMOS manufacturing technology, in which the spin of one or more
electrons is used to encode the quantum information (‘0’ or ‘1’). The electrons that make up the qubit are
electrically confined inside a silicon quantum dot, which is just a
modified conventional transistor, as found on everyday silicon chips.
