The Kane quantum computer is a proposal for a scalable

quantum computer A quantum computer is a computer that exploits quantum mechanical phenomena. On small scales, physical matter exhibits properties of both particles and waves, and quantum computing takes advantage of this behavior using specialized hardware. ...

proposed by Bruce Kane in 1998,Kane, B.E. (1998
A silicon-based nuclear spin quantum computer
, ''

Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...

'', 393, p133 who was then at the

University of New South Wales The University of New South Wales (UNSW) is a public research university based in Sydney, New South Wales, Australia. It was established in 1949. The university comprises seven faculties, through which it offers bachelor's, master's and docto ...

. Often thought of as a hybrid between

quantum dot Quantum dots (QDs) or semiconductor nanocrystals are semiconductor particles a few nanometres in size with optical and electronic properties that differ from those of larger particles via quantum mechanical effects. They are a central topic i ...

and

nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...

(NMR) quantum computers, the Kane computer is based on an array of individual

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

donor atom A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or c ...

s embedded in a pure

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

lattice. Both the nuclear

spins The spins (as in having "the spins") is an adverse reaction of Substance intoxication, intoxication that causes a state of vertigo and nausea, causing one to feel as if "spinning out of control", especially when lying down. It is most commonly as ...

of the donors and the spins of the donor

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

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 In quantum computing, a qubit () or quantum bit is a basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical syste ...

s 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 Spin or spinning most often refers to: * Spin (physics) or particle spin, a fundamental property of elementary particles * Spin quantum number, a number which defines the value of a particle's spin * Spinning (textiles), the creation of yarn or thr ...

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

s has two major advantages. Firstly, the state has an extremely long

decoherence Quantum decoherence is the loss of quantum coherence. It involves generally a loss of information of a system to its environment. Quantum decoherence has been studied to understand how quantum systems convert to systems that can be expla ...

time, perhaps on the order of 10¹⁸ seconds at

millikelvin List of orders of magnitude for temperature Detailed list for 100 K to 1000 K Most ordinary human activity takes place at temperatures of this order of magnitude. Circumstances where water naturally occurs in liquid form are shown in light g ...

temperatures. Secondly, the qubits may be manipulated by applying an

oscillating Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

, as in typical NMR proposals. By altering the voltage on the A gates, it should be possible to alter the

Larmor frequency Sir Joseph Larmor (; 11 July 1857 – 19 May 1942) was an Irish mathematician and physicist who made breakthroughs in the understanding of electricity, dynamics, thermodynamics, and the electron theory of matter. His most influential work was ...

of individual donors. This allows them to be addressed individually, by bringing specific donors into

resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

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 associate with the same donor. The charge excess is then detected using a

single-electron transistor A single-electron transistor (SET) is a sensitive electronic device based on the Coulomb blockade effect. In this device the electrons flow through a tunnel junction between source/drain to a quantum dot (conductive island). Moreover, the electric ...

. 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 In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in ...

Development

Since Kane's proposal, under the guidance of

Robert Clark Robert, Bob, or Bobby Clark may refer to: Television and film *Robert Clark (actor) (born 1987), American-born Canadian television actor *Bob Clark (1939–2007), Canadian filmmaker *Bob Clark (television reporter) (1922–2015), American televisi ...

and now

Michelle Simmons Michelle Yvonne Simmons (born 14 July 1967) is an Australian quantum physicist, recognised for her foundational contributions to the field of atomic electronics. She is founding director of the Australian Research Council's Centre of Excelle ...

, pursuing realisation of the Kane quantum computer has become the primary quantum computing effort in

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. 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 A scanning tunneling microscope (STM) is a type of scanning probe microscope used for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer, then at IBM Zürich, the Nobel Prize in ...

(STM) technique in 2003. 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. In 2020,

Andrea Morello Andrea Morello (born 26 June 1972, in Pinerolo, Italy) is the Scientia Professor of Quantum Engineering in the School of Electrical Engineering and Telecommunications at the University of New South Wales, and a Program Manager at the ARC Centre o ...

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.

References

{{quantum_computing Quantum computing Quantum dots

Description

Development

See also

References