This application claims priority from Australian provisional patent application number 2022900615 filed on 14 Mar. 2022 and Australian patent application number 2022203834 filed on 2 Jun. 2022; the entire contents of both are incorporated herein in their entirety.
Aspects of the present disclosure are directed to systems and methods for fabricating quantum processing systems using dopant atoms in semiconductors.
Universal quantum computing is a potentially revolutionary technology that could be applied in certain domains to solve problems that are intractable when running the currently known best classical algorithms on state-of-the-art classical computers. Examples of domains in which universal quantum computers are known to provide an advantage include classes of optimization problems, advanced chemistry simulations, and finding prime factors of large numbers, which would defeat the most common classical encryption protocols. For some of these applications, such as finding prime factors of large numbers, quantum computers should be exponentially faster than their classical counterparts. Quantum computing may also be useful in certain machine learning applications.
Quantum computers use the properties of quantum physics to store data and perform operations required for computation. In a quantum computer, the basic unit of memory is a quantum bit or a qubit. Engineering qubits in quantum processing elements is integral to building a universal quantum computer. To date, a number of different structures, materials, and architectures have been proposed to implement quantum processing systems and fabricate their basic information units (or quantum bits).
According to a first aspect of the present disclosure, there is provided a quantum processing element comprising: a semiconductor substrate; a dielectric material forming an interface with the semiconductor substrate; and a donor molecule comprising a plurality of dopant dots embedded in the semiconductor, each dopant dot comprising one or more dopant atoms and one or more electrons/holes confined to the dopant dots, where a distance between the dopant dots is between 3 and 9 nanometres.
According to a second aspect of the present disclosure, there is provided a method of fabricating an engineered quantum processing element, the method comprising: exposing a semiconductor substrate to atomic hydrogen to form a monolayer of hydrogen and passivating a surface of the semiconductor substrate; selectively desorbing hydrogen atoms from the passivated surface by application of appropriate voltages and tunnelling currents to an STM tip, and forming a plurality of patches in the hydrogen monolayer, wherein a distance between adjacent patches is between 3 and 9 nanometres; and incorporating one or more donor atoms in each of the plurality of patches in the hydrogen monolayer, to form a donor molecule.
According to a third aspect of the present disclosure, there is provided a quantum processing system, comprising: a semiconductor substrate; a dielectric material forming an interface with the semiconductor substrate; and a plurality of donor molecules embedded in a plane in the semiconductor substrate. Each donor molecule includes a plurality of dopant dots, each dopant dot includes one or more dopant atoms, and one or more electrons/holes confined to the dopant dots. A distance between adjacent dopant dots in a donor molecule is between 3 and 9 nanometres.
Features and advantages of the present invention will become apparent from the following description of embodiments thereof, by way of example only, with reference to the accompanying drawings, in which:
One type of quantum computing system is based on spin states of individual qubits where the qubits are electron and/or nuclear spins localized inside a semiconductor quantum chip. These electron and/or nuclear spins are confined either in gate-defined quantum dots or on donor atoms that are positioned in a semiconductor substrate.
Donors in silicon offer many benefits compared to gate-defined quantum dots. Firstly, confinement of an electron spin qubit arises naturally from the presence of a donor potential, so that heterostructure engineering and surface confinement gates are not necessary to isolate single electrons. Secondly, atom qubits allow for electron spin qubits to be hosted within a crystalline environment, which has been shown to reduce the electrical noise coupling from the environment to qubits. Furthermore, the confinement potential of donors is much stronger than in the case of gate-defined quantum dots, resulting in long qubit life-times (approximately 30 seconds), and high-fidelity fast readout of the qubits.
To date, two main approaches to fabricating donor qubits in silicon have been pursued: donors can either be implanted into the silicon crystal lattice via ion implantation techniques or placed with atomic precision using hydrogen lithography scanning tunnelling microscopy (STM). The latter approach allows for atomic engineering of the confinement potential by placing either a single P atom or multiple P atoms in close proximity (<3 nm) to form multi-donor dots. It has been shown that these multi-donor dots offer several benefits including tunability of the exchange interaction and improved addressability of individual qubits leading to lower gate error rates. Moreover, the confinement potential of multi-donor dots is stronger compared to single donors, yielding longer electron spin life-times (˜30 s) and ability to load multiple electrons (<5) onto a single multi-donor dot.
Aspects of the present disclosure present a novel type of electron-donor system in a semiconductor substrate. As referred to herein, the presently disclosed system is called a donor molecule system. The donor molecule system includes n separate donor sites, each including at least one donor atom. For the system to be considered a donor molecule, separation between the n donor dot sites is within a specific range—approximately 3-9 nm. The lower bound of this range in defined by the Bohr radius of a confined electron—i.e., the separation between any two donor dot sites must be larger than the electron Bohr radius. Otherwise, the created system (between those two donor sites) would be considered a single donor site. The upper bound of this range is defined by the exchange coupling between the electrons loaded onto these two separate donor sites, i.e., the exchange coupling between the electrons loaded onto the two donor dot sites must be larger than the Zeeman energy of the individual electrons). Otherwise, the created system would be considered as two separate electron-donor systems.
The third regime, also called the weak coupling regime, occurs when inter-donor separations are greater than Bohr radium, (d>rB) and the exchange coupling J is less than the Zeeman energy EZ. In this regime, the electron wavefunctions of the two dots are spatially separated and have a relatively small overlap. This relatively small overlap between electron wavefunctions, gives rise to the exchange coupling J being smaller than the Zeeman energy EZ of the individual electron spins. Consequently, the |↓ |↑
state is the lowest energy state in this regime.
Finally, in the intermediate regime, also called the strong coupling regime, the inter-donor separations are greater than the Bohr radius but the exchange coupling J is greater than the Zeeman energy EZ. In this regime, the interdot distances are 3 nm<d<9 nm. This regime 140 includes two separated confinement potentials. As a result, it is energetically favourable for two electron spins to form a singlet state between the dots, such that the singlet
is the ground state. This gives rise to a unique molecular regime where, due to the strong exchange interaction, electrons are effectively shared between the spatially separate donor sites.
It is this intermediate regime 140 that defines a donor molecule according to aspects of the present disclosure.
The donor molecule 202 includes a plurality of dopant dots embedded in the semiconductor substrate 204. In this example, the donor molecule 202 includes two dopant dots, 208A, and 208B. Each dopant dot 208 includes one or more dopant atoms 110. In this example, the left dopant dot 208A has one donor atom 210A and the right dopant dot 208B has two donor atoms 210B and 210C. The distance between the dopant dot sites 108 is between 3-9 nanometers. In one example, the distance is 8 nanometers.
Further, one or more gates (e.g., gate 211) and an antenna 214 may be located on the dielectric 205 in a region above the quantum dot 201. Voltages may be applied to gate 211 to confine one or more electrons 212 in the donor molecule 202. These electrons 212 may be confined to the dopant dots. In this example, two electrons 212 are confined in the donor molecule 202. The 1P-2P molecule 202 shown in
In certain embodiments, the donor dots 208 are placed in the silicon substrate 204 with atomic-scale precision using scanning tunnelling lithography techniques. In particular, during fabrication, two separate lithographic patches can be defined in the semiconductor substrate that are separated by 3-9 nanometers. A predetermined number of donor atoms 210 can then be placed in the two lithographic patches. In some examples, the donor dots 208 may be located approximately 50 nm below the surface 206. In the example shown in
Despite the relatively large separation, the electron spins in the donor dots 208 form singlet states across both donor sites as the exchange energy J exceeds the electron Zeeman splitting.
The device 200 as shown in
Initially, a clean silicon (Si) 2×1 surface is formed in an ultra-high-vacuum (UHV) by heating to near the melting point. This surface has a 2×1 unit cell and consists of rows of σ-bonded silicon dimers with the remaining dangling bond on each silicon atom forming a weak π-bond with the other silicon atom of the dimer of which it comprises.
Method 300 commences at step 302 (i.e., monohydride deposition), where the clean Si 2×1 surface is exposed to atomic hydrogen H to break the weak silicon n-bonds, allowing H atoms to bond to the silicon dangling bonds. Under controlled conditions, a monolayer of H can be formed with one H atom bonded to each Si atom, satisfying the reactive dangling bonds, effectively passivating the surface.
Next, at step 304 (i.e., hydrogen desorption), an STM tip is used to selectively desorb H atoms from the passivated surface by the application of appropriate voltages and tunnelling currents, forming a pattern in the H resist.
It will be appreciated that H atoms are desorbed from precise locations where donor dot sites are to be placed. For example, if the donor molecule 202 is to include two donor atom sites that have a spacing of anywhere between 3-9 nanometers, the H atoms are desorbed in such a manner as to create two lithographic patches that are 3-9 nanometers apart. Further, the size of the lithographic patches created by the hydrogen desorption may depend on the number of donor atoms that are required to be placed in each of the donor dot sites. In one example, if 1 donor atom is to be positioned in one of the patches and two donor atoms are to be positioned in the other patch, the STM tip may be utilized to desorb 6 hydrogen atoms in a first location to create a first patch and 15 hydrogen atoms may be desorbed in a second location 3-9 nanometers apart to create a second larger patch. Similarly, if larger number of donor atoms are to be placed in the patches, more hydrogen atoms can be desorbed to create lithographic patches of larger sizes. In other examples, the sizes of the patches may be smaller or larger than those described in the example above. Further still, in some examples, machine learning techniques may be utilized to control the number of donor atoms placed in any lithographic patch.
This process is repeated to create positions for other donor atom sites if the molecule 202 includes addition donor dots. Otherwise, the process may be repeated to create positions for other donor-molecules 202. In this way, regions of bare, reactive Si atoms are exposed along dimer rows, allowing the subsequent adsorption of reactive species directly to the Si surface.
Returning to
Subsequent heating of the STM patterned surface for crystal growth causes the dissociation of the phosphine molecules and results in the incorporation of P into the first layer of Si at step 308. It is therefore exposure of an STM patterned H passivated surface to PH3 that produces the required donor molecules 202.
The hydrogen may then be desorbed, at step 310, before overgrowing the surface with silicon at room temperature, at step 312. An alternative to step 312 is to grow the silicon directly through the hydrogen layer, as shown in step 314.
At step 316, the surface is rapidly annealed.
Silicon is then grown on the surface at elevated temperature, shown in step 318. In one example, approximately 50±10 nm of epitaxial silicon is grown at a temperature of 250° C. In some cases, a barrier, also known as a locking layer, may be grown as shown in step 320. Finally, conductive gates 211 and a microwave antenna 214 may be aligned on the surface, as shown in step 322 using electron beam lithography. Using registration markers, such as evaporated metal markers, the antenna 214 may be aligned at a lateral distance of 300±50 nm from the buried donor molecules to produce an oscillating magnetic field B1 perpendicular to the substrate at the donors' position.
Control structures, 211A, 211B, 211C, 211D shown in the bright regions, become electronically active upon dosing with phosphine and incorporation of P atoms into Si crystal lattice. The device 200 consists of four electrostatic gates (left 211A, middle 211B, right 211C and the SET gate 211D), a single electron transistor (SET) charge sensor 404 is tunnel coupled to the source 406 and the drain 408, and the three donor dots 410A, 410B and 410C marked with white circles. During operation, bias voltages VL, VC, VR, VB applied to the gates 211 provide electrostatic control over the charge state, and the SET 404, which may be biased at VSET≈300 μV, permits single-shot charge detection by monitoring the drain current ISET. The SET can also act as an electron reservoir for the donor sites. In one example, the SET 404 is located at a distance of 18.5 nm from the donor molecule 200.
The central device region, indicated with the dashed rectangle 402, is enlarged in
In particular, a single lithographic opening 412A is defined on the left-hand side of the STM image in
Collectively, these two sites Rα and Rβ, separated by 8 nm, form a 1P-2P molecular state, which can bind up to five electrons. Importantly, the separation between the two donor sites (8 nm) is larger than the Bohr radius of the donor-bound electrons (typically <3 nm).
In particular,
In this measurement, a fixed voltage of 750 mV is applied to the middle gate and a fixed voltage of 300 mV is applied on the SET gate. The diagonal dark lines in
The bottom-left corner of the charge stability diagram 510 corresponds to all donor dots being ionised, with no donor dot transitions observed below (at negative voltages). Within the achievable gate range, 6 electron charge transitions are identified, meaning that the upper-right corner of the charge stability diagram 500 corresponds to 6 electrons residing within the three donor dots. Additionally, three inter-site transitions are observed, marked in solid lines 508 in
and |↑
energy states of the electron spin qubit are below the Fermi level of the SET and an electron is loaded onto a donor molecule. The point marked as U indicates the gate voltages for which |↓
and |↑
energy states of the electron spin qubit are above the Fermi level of the SET, and an electron is unloaded from a donor molecule. Finally, the readout point marked as R is swept along the detuning axis to find the position where the Fermi level of the SET is in-between the |↓
and |↑
energy states of the electron spin qubit, allowing for spin-to-charge conversion.
Here it is observed that there is no spin signature for D− readout, while a characteristic ‘spin-tail’ is observed for D+, indicating that there is an even electron number below the transition and odd above it.
In some examples, coherent control of the donor molecule 200 is achieved by varying the pulse duration for fixed microwave power and frequency and extracting the spin-up fraction using a single-shot spin readout method. The single-shot readout method includes a four-level pulse sequence based on controlled tunnelling to and from the SET.
The methods and systems are described with reference to a 1P-2P system. It will be appreciated that this is merely an example device and that the methods and systems can be used with other number of donor dot based donor molecules without departing from the scope of the present disclosure. Further, instead of donor atoms, the molecule described above may be formed of other dopant atoms such as acceptor atoms where the spin qubits are formed from unpaired holes in the quantum dots.
The term “comprising” (and its grammatical variations) as used herein are used in the inclusive sense of “having” or “including” and not in the sense of “consisting only of”.
It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.
Number | Date | Country | Kind |
---|---|---|---|
2022900615 | Mar 2022 | AU | national |
2022203834 | Jun 2022 | AU | national |