Kane quantum computer
Encyclopedia
The Kane quantum computer is a proposal for a scalable quantum computer
proposed by Bruce Kane in 1998, then at the University of New South Wales
. Often thought of as a hybrid between quantum dot
and 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 electron
s participate in the computation.
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 qubit
s 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
.
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.
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
. Currently, about 100 researchers work on the problem. Theorists have put forward a number of proposals for improved readout. Experimentally, atomic-precision deposition of phosphorus atoms has been demonstrated, using an 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.
Quantum computer
A quantum computer is a device for computation that makes direct use of quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. Quantum computers are different from traditional computers based on transistors...
proposed by Bruce Kane in 1998, then at the University of New South Wales
University of New South Wales
The University of New South Wales , is a research-focused university based in Kensington, a suburb in Sydney, New South Wales, Australia...
. Often thought of as a hybrid between quantum dot
Quantum dot
A quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei...
and NMR
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
quantum computers, the Kane computer is based on an array of individual phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
donor atoms embedded in a pure silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
lattice. Both the nuclear spins
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
of the donors and the spins of the donor electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s participate in the computation.
The original proposal calls for phosphorus donors to be placed in an array with a spacing of 20 nm
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
, 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
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
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 qubit
Qubit
In quantum computing, a qubit or quantum bit is a unit of quantum information—the quantum analogue of the classical bit—with additional dimensions associated to the quantum properties of a physical atom....
s has two major advantages. Firstly, the state has an extremely long decoherence time, perhaps on the order of 1018 seconds at millikelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
temperatures. Secondly, the qubits may be manipulated by applying an oscillating magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
, 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
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
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
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
.
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.
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
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. Currently, about 100 researchers work on the problem. Theorists have put forward a number of proposals for improved readout. Experimentally, atomic-precision deposition of phosphorus atoms has been demonstrated, using an STM
Scanning tunneling microscope
A scanning tunneling microscope is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer , the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and...
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.