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 their systems.

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, 30 and
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). 27 Complex
bench-top linear optical circuits are
capable of entangling eight-photon
qubit states, and have been shown to
perform nontrivial computation over
short timescales. 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. 21 Inte-
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. 19

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
of qubits. 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.

Josephson junctions
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.