appearance in the last few years of excellent reviews on many architecture-relevant subfields. 1, 3, 4, 9, 13, 19, 28

Qubit technologies

At the lowest level of our Figure 2, we have the technological building blocks for the quantum computer. The first significant attempt to characterize the technology needed to build a computer came in the mid- 1990s, when DiVincenzo listed criteria that a viable quantum computing technology must have: ( 1) a two-lev-el physical system to function as a qubit;a ( 2) a means to initialize the qubits into a known state; ( 3) a universal set of gates between qubits; ( 4) measurement; and ( 5) long memory lifetime. 10 These criteria were later augmented with two communication criteria, the ability to convert between stationary and “flying” qubits, and the ability to transmit the latter between two locations.

In any qubit technology, the first
criterion is the most vital: What is the
state variable? Equivalent to the elec-
trical charge in a classical computer,
what aspect of the physical system
encodes the basic “0” or “ 1” state?
The initialization, gates, and measure-
ment process then follow from this
basic step. Many groups worldwide
are currently working with a range of
state variables, from the direction of
a DiVincenzo’s original criteria was written with
qubits, on which we concentrate in this article,
but tri-level qutrits or higher-dimension qudit
data elements are also possible.
quantum spin of an electron, atom,
nucleus, or quantum dot, through the
magnetic flux of a micron-scale cur-
rent loop, to the state of a photon or
photon group (its polarization, posi-
tion or timing).
The accompanying table lists a
selection of qubit technologies that
have been demonstrated in the labo-
ratory, with examples of the mate-
rial and the final device given for each
state variable.
Controlling any kind of physical
system all the way down to the quan-
tum level is difficult, interacting qubits
with each other but not anything else
is even harder, and control of systems
for quantum computing over useful
lifetimes is an immense experimental
challenge. While experimental prog-
ress has been impressive in the last
decade, moving away from one- and
two-qubit demonstrations, we are still
a very long way away from entangling,
storing, and manipulating qubits on
anything like the scale of classical
computing and bits. Here, we are able
to discuss only selected examples of
the various technologies on offer; for
more information, we recommend the
recent survey by Ladd et al. 19

Ion trap devices and optical systems currently lead in the number of qubits that can be held in a device,

figure 2. Quantum computer architecture among some sub-fields of quantum computation. QeC is quantum error correction; ft is fault tolerant.

Quantum computing theory

Quantum complexity theory

Quantum algorithms
QEC theory
Architecture-aware
algorithm implementation
Programming languages
System Organization
QEC and FT implementation
Classical
control
Interconnection topologies
and floor planning
Qubit interconnect technologies
Qubit storage and gate technologies
Quantum
programming
Quantum
computer
microarchitecture
Technology
building blocks
Quantum
computer
architecture

A few of the many types of qubit technologies available. Different technologies rely on the same or different state variables to hold quantum data, implemented in different materials and devices. the equivalent technology for standard classical computing is given.

type
scaled Cmos
(classical) ion trap Quantum dot optical circuit
Gate-based
superconducting
circuit
superconducting
circuit (adiabatic
computation)
State variable Electrical charge Ion spin Electron spin, energy
level, or position
Photon polarization,
time, or position
Magnetic flux, charge,
or current phase
Magnetic flux
Material Doped silicon Atoms in free-space
electromagnetic field
Solid-state
semi-conductor
at cryogenic
temperatures
Optical waveguides,
for example, etched in
silicon
Superconducting
Josephson junction at
cryogenic temperatures
Superconducting
Josephson junction at
cryogenic temperatures
Device gate MOSFET laser- or vibrational-
mediated interaction
laser- or electrically-
driven exchanges and
rotations
beam splitters and
photon detectors
Electrically-driven
exchanges and
rotations
Electrically-controlled
couplers
Maximum
demonstrated
variables
> 109 transistors
per chip, ≈ 1016 per
supercomputing
system

14 3 8 2 full qubits + 2 special-purpose memories

8 coupled, 50 functional?