Editing Kane quantum computer

{{primary sources|date=October 2015}}
'''The Kane quantum computer''' is a proposal for a scalable [[quantum computer]] proposed by [[Bruce Kane]] in 1998,<ref name="nature">Kane, B.E. (1998)"[http://www.nature.com/nature/journal/v393/n6681/abs/393133a0.html A silicon-based nuclear spin quantum computer ]", ''[[Nature (journal)|Nature]]'', '''393''', p133</ref> 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 atom]]s embedded in a pure [[silicon]] lattice. Both the nuclear [[Spin (physics)|spins]] of the donors and the spins of the donor [[electron]]s 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 [[qubit]]s may be individually addressed by electrical means.

==Description==
[[Image:Kane QC.png|right]]
The original proposal calls for phosphorus donors to be placed in an array with a spacing of 20&nbsp;[[nanometre|nm]], approximately 20&nbsp;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 <sup>31</sup>P, which have a nuclear [[Spin (physics)|spin]] of 1/2. The silicon substrate is isotopically pure <sup>28</sup>Si which has nuclear spin 0. Using the nuclear spin of the P donors as a method to encode [[qubit]]s has two major advantages. Firstly, the state has an extremely long [[decoherence]] time, perhaps on the order of 10<sup>18</sup> 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&nbsp;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 [[quantum tunneling|tunneling]] of an electron to transform two neutral donors to a D<sup>+</sup>&ndash;D<sup>&ndash;</sup> state, that is, one where two electrons associate with the same donor. The charge excess is then detected using a [[single-electron transistor]]. This method has two major difficulties. Firstly, the D<sup>&ndash;</sup> 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<sup>&ndash;</sup> state has a sufficiently long lifetime to allow for readout&mdash;the electron tunnels into the [[conduction band]].

==Development==
Since Kane's proposal, under the guidance of [[Robert Clark (physicist)|Robert Clark]] and now [[Michelle Simmons]], pursuing realisation of the Kane quantum computer has become the primary quantum computing effort in [[Australia]].<ref>[http://www.cqc2t.org/ Centre for Quantum Computation & Communication Technology]</ref> Theorists have put forward a number of proposals for improved readout. Experimentally, atomic-precision deposition of phosphorus atoms has been achieved using a [[scanning tunneling microscope]] (STM) technique in 2003.<ref>Schofield, S. R. Atomically precise placement of single dopants in Si. {{ArXiv|cond-mat/0307599}} 2003</ref> 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.<ref>O'Gorman, J. A silicon-based surface code quantum computer. {{ArXiv|1406.5149}} 2014</ref>

In 2020, [[Andrea Morello]] and others demonstrated that an antimony nucleus (with eight spin states) embedded in silicon could be controlled using an electric field, rather than a magnetic field.<ref>{{cite news |last1=Cho |first1=Adrian |title=Chance discovery brings quantum computing using standard microchips a step closer |url=https://www.science.org/content/article/chance-discovery-brings-quantum-computing-using-standard-microchips-step-closer |access-date=13 March 2020 |work=Science {{!}} AAAS |date=11 March 2020 |language=en}}</ref>

==See also==
*[[Spin qubit quantum computer]]
*[[Nuclear magnetic resonance quantum computer]]

==References==
{{Reflist}}

{{quantum_computing}}

[[Category:Quantum computing]]
[[Category:Quantum dots]]