Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-06-23
2002-04-09
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S024000, C324S071500
Reexamination Certificate
active
06369404
ABSTRACT:
TECHNICAL FIELD
Electrons and some nuclei possess a quantized unit of angular momentum called “spin”. This invention concerns electron devices for single electron and nuclear spin measurement.
The |↑↓> notation is used here to represent the electron spin state, and the |01 > notation the nuclear state. For simplicity, normalization constants are omitted.
In two electron systems, the electron spins may be aligned (total spin angular momentum =1) in triplet states (|↓↓>, |↑↑>, and |↑↓+↓↑>) or opposed (total spin angular momentum=0 ) in a singlet state (|↑↓−↓↑>). Similarly the nuclear spins may be aligned or opposed. In the |↓↓11> state, all spins point in the same direction.
BACKGROUND ART
In the laboratory, large numbers (≧10
23
) of electron and nuclear spins are regularly probed using traditional magnetic resonance techniques.
There are important applications for devices and techniques that can measure a single electron or nuclear spin. For example, magnetic resonance experiments could be performed on a single atom or molecule and the local environment (electric and magnetic fields) could be measured with great precision. Alternatively, single electron or nuclear spins could be used as qubits in a quantum computer. Single spin measuring devices would be required in the computer to initialize and to measure the single-spin qubits.
SUMMARY OF THE INVENTION
A first aspect of the invention is an electron device for single spin measurement, comprising:
A semiconductor substrate into which at least one donor atom is introduced to produce a donor nuclear spin electron system having large electron wave functions at the nucleus of the donor atom.
An insulating layer above the substrate.
A first conducting gate on the insulating layer above the donor atom to control the energy of the bound electron state at the donor.
A second conducting gate on the insulating layer adjacent the first gate to generate at least one electron in the substrate.
In use, a single electron is bound to the donor, and the donor atom is weakly coupled to the at least one electron in the substrate. The gates are biased so that the at least one electron in the substrate will move to the donor, but only if the spins of the at least one electron and the donor are in a relationship which permits the movement.
The arrangement is such that detection of current flow, or even movement of a single electron, in the device constitutes a measurement of a single spin.
The motion of a single electron may be detected by probing the system capacitively, for instance by using single electron capacitance probes, and any metallic lead can couple to the system, with no special requirement for spin-polarized electrons. Alternatively, the charge motion may be detected by single electron tunnelling transistor capacitance electrometry.
A first example of the invention is an electron device for single electron spin measurement, comprising:
A semiconductor substrate into which at least one donor atom is introduced to produce a donor nuclear spin electron system having large electron wave functions at the nucleus of the donor atom.
An insulating layer above the substrate.
A conducting A-gate on the insulating layer above the donor atom to control the energy of the bound electron state at the donor.
A conducting E-gate on the insulating layer on either side of the A-gate to generate a reservoir of spin polarised electrons at the interface between the substrate and the insulating layer.
In use the donor atom is weakly coupled to the two reservoirs of spin-polarized electrons, both reservoirs have the same polarisation, and a single electron, whose spin is to be determined, is bound to the donor. The E-gates are biased so that current will flow between them, but only if the spin on the donor is opposite to the spin polarization in the reservoirs. In this case one electron at a time from one of the reservoirs may join the same quantum state (with opposite spin) as the bound electron, and then depart the donor to the other reservoir. But when the electrons are all polarized in the same direction no current can flow since the electrons from the reservoir cannot enter the same quantum state as the bound electron.
In another example, there are two donors with ‘A-gates’ located above each of them, and an ‘E-gate’ located between them. Electrons are bound to the two, positively charged, donors, and the donors are spaced sufficiently close to each other so that electron transfer, or exchange coupling, between them is possible.
In use, an increasing potential difference is applied to the two A-gates and at some point it will become energetically favorable for both electrons to become bound to the same donor, but only if the electrons are in a mutual singlet state. The signature of the singlet state, charge motion between donors as a differential bias is applied to the A-gates, can be detected externally.
Another example of the invention is an electron device for single nuclear spin measurement, comprising:
A semiconductor substrate into which at least one donor atom is introduced to produce a donor nuclear spin electron system having large electron wave functions at the nucleus of the donor atom.
An insulating layer above the substrate.
A conducting A-gate on the insulating layer above the donor atom to control the energy of the bound electron state at the donor.
A conducting E-gate on the insulating layer on either side of the A-gate to generate a reservoir of electrons at the interface between the substrate and the insulating layer.
Where all the electron spins are polarized in the same direction, and the donor is a nucleus with spin, coupled to the electrons by the hyperfine interaction. The E-gates are biased so that current will flow between them, but only if the the nuclear spin is initially opposed to the electron spins. The process involves the electron coming from the reservoir and exchanging its spin with the spin of the nucleus so that its spin is then opposed to the donor electron and can form a singlet with it. The arrangement is such that detection of movement of a single electron in the device constitutes a measurement of the nuclear spin on the donor.
Alternatively, since the transport of an electron onto the donor and off again involves two spin flips, a current flow across the donor preserves nuclear spin polarisation, and current flow is turned on or off depending on the orientation of the nuclear spin on the donor.
The electrons may, for example, be polarised by being at low temperature in a large magnetic field.
The conducting E-gate on the insulating layer on either side of the A-gate may generate a 2-Dimensional electron gas at the interface between the substrate and the insulating layer.
In use, the E-gates may be biased so that only |↓> electrons are present on both sides of the donor atom. And the A-gate may be biased so that E
F
lies at the energy of the two electron bound state at the donor (the D
−
state).
The host may contain only nuclei with spin I=0, such as Group IV semiconductors composed primarily of I=0 isotopes and purified to contain only I=0 isotopes. Si is an attractive choice for the semiconductor host. The donor can be
31
P.
The gates may be formed from metallic strips patterned on the surface of the insulating layer. A step in the insulating layer over which the gates cross may serve to localise the gates electric fields in the vicinity of the donor atoms.
The state of a given spin system may be inferred from the measurement if the system is prepared by adiabatic changes to the spin state energies before the measurement takes place, to ensure that the measurement outcome is determined by the initial state of a given spin.
Another aspect of the invention is a procedure for the preparation of spin states in a two electron system, which comprises the following steps:
First, manipulate the A-g
Crane Sara
Rockey Milnamow & Katz Ltd.
Unisearch Limited
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