The Kane quantum computer is a proposal for
a scalable quantum computer proposed by Bruce
Kane in 1998, who was then at the University
of New South Wales.
Often thought of as a hybrid between quantum
dot and nuclear magnetic resonance (NMR) quantum
computers, the Kane computer is based on an
array of individual phosphorus donor atoms
embedded in a pure silicon lattice.
Both the nuclear spins of the donors and the
spins of the donor electrons participate in
the computation.
Unlike many quantum computation schemes, the
Kane quantum computer is in principle scalable
to an arbitrary number of qubits.
This is possible because qubits may be individually
addressed by electrical means.
== Description ==
The original proposal calls for phosphorus
donors to be placed in an array with a spacing
of 20 nm, approximately 20 nm below the surface.
An insulating oxide layer is grown on top
of the silicon.
Metal A gates are deposited on the oxide above
each donor, and J gates between adjacent donors.
The phosphorus donors are isotopically pure
31P, which have a nuclear spin of 1/2.
The silicon substrate is isotopically pure
28Si which has nuclear spin 0.
Using the nuclear spin of the P donors as
a method to encode qubits has two major advantages.
Firstly, the state has an extremely long decoherence
time, perhaps on the order of 1018 seconds
at millikelvin temperatures.
Secondly, the qubits may be manipulated by
applying an oscillating magnetic field, as
in typical NMR proposals.
By altering the voltage on the A gates, it
should be possible to alter the Larmor frequency
of individual donors.
This allows them to be addressed individually,
by bringing specific donors into resonance
with the applied oscillating magnetic field.
Nuclear spins alone will not interact significantly
with other nuclear spins 20 nm away.
Nuclear spin is useful to perform single-qubit
operations, but to make a quantum computer,
two-qubit operations are also required.
This is the role of electron spin in this
design.
Under A-gate control, the spin is transferred
from the nucleus to the donor electron.
Then, a potential is applied to the J gate,
drawing adjacent donor electrons into a common
region, greatly enhancing the interaction
between the neighbouring spins.
By controlling the J gate voltage, two-qubit
operations are possible.
Kane's proposal for readout was to apply an
electric field to encourage spin-dependent
tunneling of an electron to transform two
neutral donors to a D+–D– state, that
is, one where two electrons orbit the same
donor.
The charge excess is then detected using a
single-electron transistor.
This method has two major difficulties.
Firstly, the D– state has strong coupling
with the environment and hence a short decoherence
time.
Secondly and perhaps more importantly, it's
not clear that the D– state has a sufficiently
long lifetime to allow for readout—the electron
tunnels into the conduction band.
== Development ==
Since Kane's proposal, under the guidance
of Robert Clark and now Michelle Simmons,
pursuing realisation of the Kane quantum computer
has become the primary quantum computing effort
in Australia.
Theorists have put forward a number of proposals
for improved readout.
Experimentally, atomic-precision deposition
of phosphorus atoms has been demonstrated,
using a scanning tunneling microscope (STM)
technique.
Detection of the movement of single electrons
between small, dense clusters of phosphorus
donors has also been achieved.
The group remains optimistic that a practical
large-scale quantum computer can be built.
Other groups believe that the idea needs to
be modified
