silicon crystal lattice cell size. In contrast, although the atoms themselves
are of course tiny, ion traps are limited
to inter-atom spacing of perhaps tens
of microns for RF and optical control.
Nanophotonic systems will require
components tens of microns across,
to accommodate the 1.5µm
wavelength light that is desirable for telecommunications and silicon optics.
Superconducting flux qubits require
a current ring microns across. All of
these technologies result in qubit devices that are macroscopic, or nearly
so, with areal densities a million times
less than computer chips. This fact
will have enormous impact on large-scale architectures, as we will see.
First steps in quantum architecture. At this lowest level, the job of
quantum architecture is to determine
how individual qubits or qubit blocks
interconnect and communicate
in order to process data. There are
three main areas where experimental
groups have begun to consider architectural implications in designing
Heterogeneity. Some researchers
have been investigating technological
heterogeneity by using combinations
of electron spin, nuclear spin, magnetic flux, and photon polarization in
a single system.
5 It is, however, equally
important to consider structural heterogeneity, both in operational capability and in interconnect speed or
fidelity. Martinis’s group has recently
demonstrated a chip with a functional
distinction between two flux qubits
and memory storage elements, leading them to refer to it as a quantum
von Neumann architecture.
24 In many
technologies, including some forms
of quantum dots and Josephson junction qubits, measurement of a qubit
requires an additional physical device,
consuming die space and making layout of both classical and quantum
components more complex.
Integration and classical control.
Increasing the number of on-chip de-
vices will require improving on-chip
integration of control circuits and
multiplexing of I/O pins to get away
from multiple rack-mount units for
controlling each individual qubit. Ear-
ly efforts at considering the on-chip
classical control overhead as systems
grow include Oskin’s design,
and controlled and entangled. Ions
are trapped in a vacuum by electro-
magnetic potentials, and 14-qubit
entanglement has been demonstrat-
ed (the largest entangled state in
any form shown to date).
bench-top linear optical circuits are
capable of entangling eight-photon
qubit states, and have been shown to
perform nontrivial computation over
40 Both of these ap-
proaches do not scale in those forms,
but scalable approaches are also un-
der development. Groups headed by
Wineland, Monroe, Chuang, and oth-
ers have demonstrated the necessary
building blocks for ion traps.
grated nanophotonics (using photons
as qubits) made on silicon chips pro-
vides the route to getting optics off of
the lab bench and into easily scalable
systems, and is making substantial
progress in groups such as O’Brien’s.
Solid-state electronic devices for
gate-based computation, while cur-
rently trailing in terms of size of en-
tangled state demonstrated, hold out
great promise for mass fabrication
19 By trapping a single extra
electron in a 3D block of semicon-
ducting material, a quantum dot has a
quantum spin relative to its surround-
ing that can hold a qubit of data. For
flux qubits, the state variable is the
quantum state of the magnetic flux
generated by a micron-scale ring of
current in a loop of superconducting
wire (Figure 3).
In solid-state technologies, the ex-
perimental focus has been on improv-
ing the control of individual qubits
rather than growing their numbers, but
that focus has begun to shift. Jaw-Shen
Tsai has noted that superconducting
qubit memory lifetime has more than
doubled every year since 1999, and has
now reached the point where quantum
error correction becomes effective.
Overall, the prospects are very good
for systems consisting of tens of qubits
to appear in multiple technologies over
the next few years, allowing experimental confirmation of the lower reaches of
the scaling behavior of quantum algorithms and the effectiveness of quantum error correction.
However, one factor that is often unappreciated when looking at
these qubit technologies is the physical scale of the devices, particularly
in comparison with classical digital
technologies. Many people associate
quantum effects with tiny objects, but
most of these technologies use devices that are enormous compared to
the transistors in modern computer
chips. Transistors really are reaching
down to atomic scales, with vendors
having shipped chips fabricated with
a 22-nanometer process at the end of
2011. In these chips, the smallest features will be only about 40 times the
figure 3. the Josephson junction superconducting flux qubit.
force current to be quantized
Qubit is state of single magnetic flux quantum
Loop of super-conducting wire
We can use a counterclockwise current (referred to as spin “up,” as shown by the large arrow)
to be a zero, and a clockwise current (spin “down”) to be a one.