Quantum processing elements and systems

Abstract

The present disclosure provides a quantum processing device comprising: one or more functional nanowires, each functional nanowire connected to at least one of a source and a drain; a sensing nanowire spaced from the one or more functional nanowires and connected to at least one of a source and a drain; one or more gate electrodes capacitively coupled to each of the one or more functional nanowires; one or more electrodes capacitively coupled to the sensing nanowire; a floating coupler positioned between the one or more functional nanowires and the sensing nanowire and electrostatically coupling the one or more functional nanowires to the sensing nanowire; and a controller connected to one or more gates of the sensing nanowires and one or more gates of the one or more functional nanowires.

Description

[Technical Field]

[0001] This application claims priority from Australian Provisional Patent Application No. 2020 / 900803, filed on 16 March 2020, the contents of which are incorporated herein by cross-reference.

[0002] Aspects of the present disclosure relate to charge sensing, and particularly, but not exclusively, to charge sensing arrangements for quantum dot arrays. [Background technology]

[0003] The developments described in this section are known to the inventors. However, unless otherwise indicated, it should not be assumed that any of the developments described in this section constitute prior art or that those developments were known to those skilled in the art merely by virtue of their inclusion in this section.

[0004] Large-scale quantum processing systems hold the promise of solving problems that are intractable with classical machines, and have the potential to revolutionize technology. Many different structures, materials, and architectures have been proposed to establish quantum processing systems and fabricate their basic information units (i.e., qubits).

[0005] One way to fabricate qubits is to use, for example, silicon quantum dots. Quantum dots are tiny systems confined in all three spatial dimensions, e.g., electrons in a semiconductor confined via a gate-defined electric field and an insulating material. In one example, a quantum dot: 28Quantum dots can be formed using an interface between a Si substrate and a dielectric material. A confinement arrangement is utilized to confine one or more electrons within the silicon substrate to form quantum dots, and a control arrangement (e.g., a gate) is formed on the dielectric material to control the confined electrons (e.g., by applying a voltage to adjust the electron spin resonance frequency of the confined electrons). One technique for fabricating such quantum dots and processing systems utilizing these quantum dots is described in International Patent Applications PCT / AU2014 / 000596 and PCT / AU2016 / 050713.

[0006] When a large number of such quantum dots are fabricated at a suitable distance from each other, quantum information can be transferred through the quantum dot array via spin-shuttle or exchange-mediated coupling of electrons.

[0007] Electron spin-based quantum dots exhibit high control fidelity and can leverage existing fabrication techniques for the production of metal-oxide-semiconductor field-effect transistors (MOSFETs), making them a popular choice for semiconductor-based qubits. Summary of the Invention [Means for solving the problem]

[0008] According to a first aspect, the present invention provides a quantum processing device comprising one or more functional nanowires, each functional nanowire connected to at least one of a source and a drain, one or more functional nanowires, a sensing nanowire spaced apart from the one or more functional nanowires and connected to at least one source and a drain, one or more gate electrodes capacitively coupled to each of the one or more functional nanowires, one or more gate electrodes capacitively coupled to the sensing nanowire, and one or more gate electrodes located between the one or more functional nanowires and the sensing nanowire and connected to at least one source and a drain, and a controller connected to one or more gates of the sensing nanowire and one or more gate electrodes of the one or more functional nanowires, wherein during operation the controller is configured to apply a bias voltage to bias the one or more gates of the sensing nanowire and the gate electrodes of the one or more functional nanowires such that a sensing quantum dot is formed in the sensing nanowire, one or more functional quantum dots are formed in the one or more functional nanowires, and the sensing quantum dot senses a change in charge in each of the one or more functional quantum dots.

[0009] According to some embodiments, a quantum processing system includes a functional nanowire and two gates associated with the functional nanowire.

[0010] In such an embodiment, the floating coupler includes two floating gates.

[0011] During operation, in some embodiments, a potential difference is maintained between the sensing nanowire and one or more functional nanowires to form one or more functional quantum dots in the one or more functional nanowires beneath the floating gate.

[0012] Furthermore, in some embodiments, the sensing nanowire is coupled to two gates, and the two gates are biased to the same voltage.

[0013] To form a functional quantum dot beneath the floating gate in the functional nanowire, in some embodiments, the voltages of the sensing nanowire and the functional nanowire are biased relative to each other.

[0014] In some other embodiments, the quantum processing system includes one functional nanowire that is coupled to multiple gates. In such cases, the floating coupler may be capacitively coupled at one end to the sensing nanowire, and at the other end, the floating coupler may include multiple branches, each branch forming a branch gate, and each branch capacitively coupled to a functional nanowire.

[0015] Furthermore, in such an embodiment, a potential difference is maintained between the sensing nanowire and the functional nanowire to form a functional quantum dot under one or more of the branch gates of the floating coupler.

[0016] In some other embodiments, the quantum processing system includes multiple functional nanowires, each functional nanowire coupled to two or more gates. In such cases, the floating coupler includes a stem and multiple branches, each branch having one or more leaf gates, and the stem end is coupled to a sensing nanowire while the leaf gates are coupled to a functional nanowire.

[0017] Furthermore, in such an embodiment, a potential difference is maintained between the sensing nanowire and each of the functional nanowires to form functional quantum dots under one or more of the leaf gates of the floating coupler.

[0018] According to another aspect of the present disclosure, there is provided a method for controlling operation of a quantum processing system including one or more functional nanowires, a sensing nanowire spaced apart from the one or more functional nanowires, one or more gate electrodes capacitively coupled to each of the one or more functional nanowires, one or more electrodes capacitively coupled to the sensing nanowire, a floating coupler positioned between and capacitively coupling the one or more functional nanowires and the sensing nanowire, and a controller connected to one or more gates of the sensing nanowires and one or more gates of the one or more functional nanowires. The method includes, in a controller, receiving a current reading from a sensing nanowire, determining a bias voltage to be applied to one or more gates of the sensing nanowire based on the received current reading, determining a bias voltage to be applied to one or more gates of one or more functional nanowires based on the received current reading, applying the determined bias voltage to one or more gates of the sensing nanowire, and applying the determined bias voltage to one or more gates of the one or more functional nanowires, wherein the bias voltage is determined such that a sensing quantum dot is formed in the sensing nanowire, one or more functional quantum dots are formed in the one or more functional nanowires, and the sensing quantum dot senses a charge transition event in each of the one or more functional quantum dots.

[0019] In some embodiments, the quantum processing system includes one functional nanowire, the functional nanowire coupled to two gates, the floating coupler comprising two floating gates, and the method of the second aspect maintains a potential difference between the sensing nanowire and the one or more functional nanowires to form one or more functional quantum dots in the one or more functional nanowires beneath the floating gates.

[0020] In such an embodiment, the sensing nanowire may be coupled to two gates, and the method includes determining the same bias voltage to be applied to the two gates of the sensing nanowire and causing the same bias voltage to be applied to the two gates of the sensing nanowire.

[0021] Further, in some embodiments, the method of the second aspect includes biasing the voltages of the sensing nanowire and the functional nanowire relative to one another to form one or more functional quantum dots within the one or more functional nanowires.

[0022] In some embodiments, the quantum processing system includes a functional nanowire, the functional nanowire coupled to a plurality of gates, a floating coupler coupled at one end to a sensing nanowire, and at the other end, the floating coupler includes a plurality of branches, each branch coupled to a functional nanowire, and the method of the second embodiment includes biasing the voltages of the sensing nanowire and the functional nanowire relative to each other to form functional quantum dots under one or more of the branches of the floating coupler.

[0023] In yet another embodiment, the quantum processing system includes a plurality of functional nanowires, each functional nanowire including two or more gates, and the floating coupler includes a stem and a plurality of branches, each branch having one or more leaf gates, the end of the stem being coupled to the sensing nanowire while the leaf gates are coupled to the functional nanowire, and the method of the second aspect includes biasing a voltage of each of the functional nanowires with respect to the sensing nanowire to form functional quantum dots under one or more of the leaf gates of the floating coupler. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is an exemplary prior art arrangement for sensing charge events on a collection of quantum dots using a single-electron transistor dot. [Figure 2]1 is an exemplary prior art arrangement for sensing quantum dot charge events using a sensing quantum dot and coupler. [Figure 3A] FIG. 1 is a perspective view of a device architecture for sensing charge events of multiple quantum dots using sensing quantum dots, according to some aspects of the present disclosure. [Figure 3B] FIG. 3B is a cross-sectional side view of the device architecture of FIG. 3A. [Figure 3C] FIG. 3B is another cross-sectional side view of the device architecture of FIG. 3A. [Figure 4] 10 is a graph illustrating the measured differential conductance of a SET for increasing gate voltage. [Figure 5] FIG. 3B is a schematic diagram illustrating the simultaneous formation of two functional quantum dots in one nanowire and a sensing quantum dot in another nanowire in the device architecture of FIG. 3A, according to some embodiments of the present disclosure. [Figure 6] 10 is a plot of the charge sensing response of a sensing quantum dot as a function of gate voltage applied to a remote nanowire. [Figure 7] FIG. 3B is a schematic diagram illustrating the formation of four functional quantum dots in one nanowire and a sensing quantum dot in the other nanowire in the device architecture of FIG. 3A according to some embodiments of the present disclosure. [Figure 8] 8 is a plot illustrating charge-sensitive spectroscopy of the four quantum dots of FIG. 7. [Figure 9] 3B is a schematic diagram illustrating the formation of two functional quantum dots in one nanowire and a sensing quantum dot in the other nanowire in the device architecture of FIG. 3A according to some embodiments of the present disclosure. [Figure 10] 10 is a plot illustrating charge-sensitive spectroscopy of the two functional quantum dots of FIG. 9. [Figure 11] FIG. 10 is a schematic diagram of another embodiment of a device architecture for sensing charge events of multiple quantum dots using a sensing quantum dot, in accordance with some aspects of the present disclosure. [Figure 12] FIG. 10 is a schematic diagram of yet another embodiment of a device architecture for sensing charge events of multiple quantum dots using a sensing quantum dot, in accordance with some aspects of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0025] While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular forms disclosed. The intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

[0026] overview Noninvasive charge sensing is a valuable tool for studying electron charge and spin states in nanostructured devices. It has been used to distinguish electron occupancies down to the single-electron level, enabling one-shot readout of single electron spins confined in quantum dots. Furthermore, sensing devices are useful for detecting elementary charge transfer events occurring between quantum dots within multi-dot structures.

[0027] Generally speaking, the sensing device can be a remote electrometer. In some systems, charge sensing has been achieved by using quantum point contacts (QPCs) or single-electron transistors (SETs) fabricated near quantum dots as sensors. Both quantum point contacts (QPCs) and single-electron transistors (SETs) have high transconductance and are sensitive to their local electrostatic environments, making them excellent charge sensors.

[0028] QPCs can be conveniently integrated with lateral quantum dot structures formed in two-dimensional electron layers in GaAs / AlGaAs, as well as Si / SiGe and Si metal-oxide semiconductor devices. SETs have also been integrated with quantum dots in a variety of structures, including Ge / Si core / shell nanowires, carbon nanotubes, graphene, and Si MOS devices.

[0029] In some known systems, SETs are placed near quantum dots to induce capacitive coupling between the two. For example, one SET can be placed adjacent to one quantum dot, such that each SET senses an individual quantum dot. However, because SETs can often be larger than the quantum dots they sense, individual SETs are less meaningful for quantum processing systems containing tens, hundreds, or even thousands of quantum dots (because fabricating an individual sensing device for each quantum dot would essentially increase the quantum processing chip size by a factor of two or three).

[0030] To overcome some of these challenges, some systems may utilize a common SET to sense changes in the charge of multiple quantum dots (e.g., three quantum dots). One such system is depicted in FIG. 1. Specifically, FIG. 1 illustrates an exemplary system in which three quantum dots 104 are formed within a region 106 where electrons are laterally confined. The quantum dots 104 are formed by applying a bias to gate 102 that allows electrons to accumulate. The lateral confinement can be created by an electric field resulting from a confinement barrier gate or insulating material around the nanowire. A top gate 108 accumulates an electron gas below the gate so that current can be measured following the path defined by gate 108. Gate 110 is then set to a voltage that forms a barrier to the electron gas, which defines a small region through which electrons can tunnel in and out, defining a single SET. SET 112 is located near the confinement region 106, such that one SET is adjacent to the linear arrangement of multiple quantum dots and is capacitively coupled to each of the quantum dots. Although this architecture allows one SET to sense multiple quantum dots, it has drawbacks.

[0031] As the distance between the quantum dots and the SET increases, the capacitive coupling between them decreases, and therefore the sensitivity of the SET to multiple quantum dots within the same nanowire or confinement region is lost, as there is an upper limit to the number of quantum dots that can be sensed by a single SET using this architecture, fixed based on the quantum dot spacing and the sensitivity of the SET device.

[0032] To avoid the trade-off between SET sensitivity and distance, some designs utilize a piece of floating conductive material between the quantum dot and the sensor, thereby allowing the SET to be placed at a greater distance from the quantum dot than is allowed by the system of FIG.

[0033] FIG. 2 illustrates this alternative system design. As shown in FIG. 2, a floating conductive material 206 is positioned between a quantum dot 204 and a SET 212. The quantum dot 204 is formed by biasing a gate 202. A top gate 208 accumulates an electron gas below the gate so that current can be measured following the path defined by the gate 208. A gate 210 is then set to a voltage that forms a barrier to the electron gas, which defines a small region through which electrons can tunnel in and out, which defines a single SET. Increasing the length of the floating conductive material 206 allows for a longer distance between the quantum dot and the SET without reducing the capacitive coupling between the quantum dot 204 and the SET 212. However, this design also suffers from several drawbacks.

[0034] In this design, one coupler 206 is placed between one sensor and one quantum dot. Therefore, if the quantum processing element includes n quantum dots, n sensing devices or SETs may be required in this design. This again affects the footprint of the quantum processing element and increases the chip size. Furthermore, the capacitive coupling between the quantum dots 204 and the floating conductive material 206 is a function of the distance between the tip of the floating conductive material and the quantum dot. Therefore, in such a design, the quantum dot must be placed in close proximity to the floating conductive material, thereby placing high constraints on such a system architecture.

[0035] Embodiments of the present disclosure are directed to novel and inventive device architectures for sensing the charge state of multiple quantum dots. Specifically, embodiments of the present disclosure are directed to charge sensing of quantum dots formed in a CMOS multi-gate nanowire device by electrostatically coupling the quantum dots via a floating gate to a remote single-electron transistor (SET) formed in an adjacent nanowire. By biasing the gate and / or the nanowire itself in the remote SET relative to the nanowire hosting the quantum dots, some embodiments of the present disclosure can form auxiliary quantum dots under the floating gate, and the SET can be used to sense charge transitions in all quantum dots, i.e., the initial set and the auxiliary set.

[0036] The disclosed system thus increases the number of quantum dots sensed through a single floating coupler, leading to strong capacitive coupling between the SET and each of the quantum dots. Furthermore, the disclosed device overcomes limitations associated with measurements based on tunneling transport through the dots, enabling sensing of all charge transitions down to the last electron within each quantum dot.

[0037] These and other aspects of the disclosure are described in detail in the following sections.

[0038] Exemplary Device Architecture 3 illustrates an exemplary device architecture 300 for sensing charge changes in multiple quantum dots via a remote SET, in accordance with some embodiments of the present disclosure. Specifically, FIG. 3A depicts a perspective view of device architecture 300, FIG. 3B illustrates a cross-sectional side view of the device architecture along the x-z axis, and FIG. 3C illustrates a cross-sectional side view of the device architecture along the y-z axis.

[0039] The device architecture 300 includes two parallel silicon nanowires 302A and 302B. In one embodiment, the silicon nanowires 302A and 302B are fabricated on a silicon substrate using silicon-on-insulator (SOI) technology, and more specifically, fully-depleted silicon-on-insulator (FD-SOI) technology. For example, the silicon nanowires are fabricated on an oxide layer that is then formed on the silicon substrate. 28 It can be formed by etching Si material, this oxide material is known as a buried oxide layer.

[0040] In device architecture 300, gate electrode 304A and gate electrode 304B are proximate to nanowire 302A, and gate electrode 304C and gate electrode 304D are proximate to nanowire 302B. Specifically, gate electrode 304A, gate electrode 304B, gate electrode 304C, and gate electrode 304D may be wrapped around the edges of corresponding nanowire 302A and nanowire 302B such that an oxide layer separates the gates from the corresponding nanowires and electrically insulates the gates from the nanowires.

[0041] All four gate electrodes are connected to a DC voltage source for electrostatic control of the qubits formed under the gates. In addition to these four gates, device architecture 300 includes non-contact gates 306A and 306B that are in close proximity to (e.g., partially wrapped around) both nanowires in a manner similar to gates 304A-304D. Non-contact gates 306A and 306B act as floating couplers between nanowires 302A and 302B, enhancing the electrostatic coupling between the nanowires.

[0042] In particular embodiments, gate electrodes 304A-304D and gate electrodes 306A-B may be made of polysilicon, however, it will be understood that the gates may similarly be made of other suitable materials, such as aluminum, tantalum, tantalum nitride, tungsten, titanium nitride, palladium, and platinum, without departing from the scope of the present disclosure.

[0043] The two silicon nanowires 302A and 302B can be connected to separate sources (e.g., source 308A and source 308B) and separate drains (e.g., drain 310A and drain 310B). Furthermore, the electrostatic potential of each of the two nanowires is controlled by a respective gate, with nanowire 302A being controlled by gate 304A and gate 304B, and nanowire 302B being controlled by gate 304C and gate 304D.

[0044] During operation, a sensing quantum dot or SET is formed in one of the nanowires and a functional quantum dot is formed in the other nanowire. For ease of reference, it will be assumed that the sensing quantum dot is formed in nanowire 302B and the functional quantum dot is formed in nanowire 302A. However, it will be understood that this is merely an example and that the locations of the sensing quantum dot and the functional quantum dot can be swapped, such that the sensing quantum dot is formed in nanowire 302A and the functional quantum dot is formed in nanowire 302B.

[0045] In either case, the sensing quantum dots are formed by applying a positive voltage to gates 304C and 304D, causing a large number of electrons to accumulate under gates 304C and 304D. The accumulation of electrons under gates 304C and 304D forms sensing quantum dots under gates 304C and 304D.

[0046] The sensitivity of the sensing quantum dot is a function of the transconductance of the sensing nanowire 302B and is adjusted by biasing the SET gates, eg, gate 304D and gate 304C, relative to the nanowire drain 310B.

[0047] Thus, in operation, the gate voltages of 304C and 304D are set to result in larger changes in current with smaller changes in the electrostatic environment of the sensing dots. Figure 4 is a graph 400 illustrating the differential conductance (μS (microsiemens)) in nanowire 302B for increasing voltages on gates 304C and 304D from 0.5V to 0.65V. As can be seen in this graph, when the gate voltage is approximately 0.6V, subthreshold Coulomb oscillations occur in the quantum dots under gates 304C and 304D, indicating dot formation. The squares in graph 400 represent the current I versus changes in dot chemical potential, which can be used for charge sensing of any quantum dots formed in nanowire 302A. SDR The point of optimum sensitivity is shown.

[0048] Additionally, in some embodiments, a constant bias of approximately 1 mV (or 0-2 mV) is applied between the source 308B and drain 310B of the sensor nanowire 302B. Such a small bias ensures that the current in the sensing quantum dot always flows in the same direction.

[0049] To form a functional quantum dot on nanowire 302A, a voltage is applied to gate 304A and gate 304B, drawing electrons under gate 304A and gate 304B. In some embodiments, the voltage applied to gate 304A and gate 304B can range from 0.1 to 1 V.

[0050] 5 is a schematic diagram illustrating the simultaneous formation of quantum dots 502 and 504 under gates 304A and 304B of nanowire 302A, and a large sensing quantum dot in nanowire 302B under both gates 304C and 304D, when appropriate bias voltages are applied to all gates of device 300. As illustrated in FIG. 5, controller 508 can be used to apply bias voltages to gates 304A, 304B, 304C, and 304D. Controller 508 also controls the flow of current (I SDR) value. In some embodiments, the controller 508 may control the bias voltage of the gate based on the input current value. The controller 508 may be a software controller or a hardware controller.

[0051] Under this operating condition, any change in the charge distribution within nanowire 302A affects the chemical potential of sensing quantum dot 506 within nanowire 302B through floating capacitive coupled gate 306A and floating capacitive coupled gate 306B. Charge transitions are observed either through transport spectroscopy (measuring the source-drain current through 302A) or charge sensing (SET current through 302B). This allows monitoring of quantum dots 502 and 504 down to the last electron. In other words, as the gate voltages of gates 304A and / or 304B vary, the number of electrons accumulated under the corresponding gate changes; that is, as the voltage increases, more electrons are drawn under the corresponding gate, and as the voltage decreases, fewer electrons are accumulated under the corresponding gate. Each change in the number of electrons under gate 304A or gate 304B shifts the charge-sensing operating point of sensing quantum dot 506. Thus, any change in the charge stored in nanowire 302A can be detected through a corresponding change in the current experienced by sensing quantum dot 506 in nanowire 302B.

[0052] Furthermore, from the readings of the charge sensor response to changes in gate voltage at gates 304A and 304B, a determination can be made as to whether the change in sensitivity of the sensor quantum dot 506 is responsive to changes in quantum dot 502 or quantum dot 504. For example, FIG. 304A and gate voltage V 304B3A is a plot of the charge-sensing response of a sensing quantum dot 506 as a function of V. More vertical lines in this plot correspond to electrons added to quantum dot 502, while more horizontal lines correspond to electrons added to quantum dot 504. In either case, the addition or removal of electrons from the quantum dot in nanowire 302A is detected via a change in the current through sensor dot 506. A decrease in the visibility of certain charge transitions can be seen when the tunneling rate between the quantum dot and the reservoir (source or drain) becomes too slow, as can be seen, for example, in the portion of the plot corresponding to 0 to 0.5 V on the x- and y-axes.

[0053] In some embodiments, during operation, two additional quantum dots can be formed on nanowire 302A. These two additional quantum dots are formed under floating gate 306A and floating gate 306B by creating a potential difference between nanowire 302A and nanowire 302B. Furthermore, when the electrostatic potential of the sensing nanowire (i.e., nanowire 302B) is higher than the electrostatic potential of the functional nanowire (i.e., nanowire 302A), the sensing nanowire can induce electrons to accumulate under gates 306A and 306B, such that the gates are capacitively coupled to functional nanowire 302A, thereby forming quantum dots under gates 306A and 306B of functional nanowire 302A.

[0054] 7 is a schematic diagram illustrating the formation of four functional quantum dots in nanowire 302A and the formation of a sensing quantum dot in nanowire 302B. As can be seen in this figure, the four functional quantum dots include quantum dots 502 and 504, and quantum dots 702 and 704, formed under coupling gates 306A and 306B.

[0055] 8 is a plot 800 illustrating charge-sensing spectroscopy of four quantum dots, dot 502, dot 504, dot 702, and dot 704. As can be seen in the plot, all charge transitions are distinguishable due to their different electrostatic coupling to gates 304A and 304B and sensor quantum dot 506, leading to different angles and contrast in the color plot (the color scale is the same in both plots). The transitions at more vertical and more horizontal angles, which have less contrast in charge sensing, indicate a change in the number of electrons in the dots under gate 304A (horizontal) and gate 304B (vertical). The transitions at intermediate angles, which have higher contrast, indicate the addition of electrons to quantum dot 702 (more horizontal) and quantum dot 704 (more vertical). The transitions corresponding to the quantum dots under gates 304A and 304B are nearly perpendicular to their respective axes because the quantum dots under these gates have stronger coupling to the bias gate, while the transitions corresponding to the quantum dots under coupling gates 306A and 306B have intermediate angles because these quantum dots have weaker coupling to the bias gate. Second, because quantum dots 702 and 704 have stronger coupling to sensor nanowire 302B, again mediated by floating gates 306A and 306B, the transitions of quantum dots 702 and 704 have a higher signal-to-noise ratio compared to the transitions of quantum dots 502 and 504. The white areas of plot 800 represent transitions in which electrons move away from the SET sensor rather than toward it. This can occur, for example, when electrons move from quantum dot 704 to quantum dot 504, effectively moving a negative charge away from the sensor. For all other darker transitions, an electron is added from the source or drain reservoir, effectively moving a negative charge towards the sensor. This is a common technique known in the art for identifying the direction of charge transfer.

[0056] In general, any form of orbital or valley degeneracy is detrimental to quantum computing, potentially leading to rapid spin relaxation and reduced exchange coupling between spins. In the context of spin qubit readout, the additional degree of freedom can prevent spin blockade. To a priori guarantee that unwanted degeneracy is absent, knowledge of the electronic structure of the dot well is necessary, which can be a very challenging task for many electron qubits. In the specific case of very small, highly symmetric quantum dots, it is possible to recognize the shell structure of the dot and view the electronic interactions as small perturbations, similar to those found in atomic physics. In more general cases, such as corner dots in nanowires, the electronic structure and excitation spectrum may not have easily recognizable labels in terms of quantum numbers. In these conditions, it is desirable to operate at low electron occupancy.

[0057] With this in mind, in some embodiments, instead of forming four quantum dots on nanowire 302A, two quantum dots can be formed, for example, under gate 304A and gate 306B. The formation of two such quantum dots is shown in FIG. 9. In some embodiments, this can be achieved by fully depleting the quantum dots formed under gate 304B and gate 306A, and accumulating a small number of electrons under gate 304A and gate 306B. The charge stability diagram for this configuration is illustrated in FIG. 10, showing the formation of well-defined double quantum dots with distinguishable charge configurations for various dot occupancies of interest for quantum computing.

[0058] Figures 3-10 illustrate two nanowire structures in which one sensing quantum dot on one of the nanowires can be used to sense changes in the charge of up to four functional quantum dots on the other nanowire. However, it will be understood that this concept of electrostatically coupling nanowires via a floating coupler and sensing a functional quantum dot on one nanowire via a sensing quantum dot on the other nanowire can be extended to more quantum dots. Figures 11 and 12 illustrate two such exemplary structures.

[0059] Alternative Architectures FIG. 11 shows an exemplary device architecture 1100 for an extended quantum processing system. Similar to device architecture 300 shown in FIG. 3, this device architecture includes two nanowires: a sensing nanowire 1102 and a functional nanowire 1104. Each nanowire includes an independent source and drain. Additionally, sensing nanowire 1102 includes one or more gates. FIG. 11 shows one gate, gate 1105, while functional nanowire 1104 includes multiple gates 1107A-1107N. The two nanowires are capacitively coupled to each other via a floating coupler 1106. Floating coupler 1106 includes a floating gate that spans between sensing nanowire 1102 and functional nanowire 1104. One end of the floating coupler is coupled to sensing nanowire 1102, while the other end of floating coupler 1106 has multiple branches 1108A-N that wrap around the edge of functional nanowire 1104. In one embodiment, these branches 1108 of the floating coupler 1106 can be in the same number as the gates 1107A-N of the functional nanowire 1104. Furthermore, in some embodiments, the branches 1108 can be wrapped around the edge of the functional nanowire 1104 at approximately the same longitudinal position as the gates 1107. They can be wrapped around the edge of the functional nanowire 1104 at a different longitudinal position in other embodiments.

[0060] In operation, functional quantum dots 1110 may be formed under one or more of gates 1107 and 1108, as described above with respect to device architecture 300. Additionally, sensing quantum dots 1112 may be formed under gate 1105. In this manner, device architecture 1100 allows a single SET charge sensor 1112 to be capacitively coupled to many functional quantum dots 1110 over long distances.

[0061] FIG. 12 illustrates another exemplary device architecture 1200 for an extended quantum processing system.

[0062] This device architecture includes multiple functional nanowires, see nanowire 1202, nanowire 1204, and nanowire 1206, and sensing nanowire 1208. Each nanowire may have its own independent source and drain. Additionally, like sensing nanowire 302B and sensing nanowire 1102, sensing nanowire 1208 includes one or more gates 1210. Each of functional nanowires 1202-1206 includes multiple gates 1211. The nanowires are capacitively coupled via floating gates 1212 that extend between sensing nanowire 1208 and the multiple functional nanowires 1202-1206.

[0063] In this device architecture, the floating coupler 1212 has a tree structure, including a trunk, branches, and leaves. The trunk end of the floating coupler 1212 is wrapped around the edge of the sensing nanowire 1208, and the floating coupler's multiple (three in this embodiment) branches are wrapped around the edges of the three functional nanowires. Each branch terminates in a leaf gate 1213, which may correspond in number and be aligned with the gates of the functional nanowires. However, this may not always be the case, and in some embodiments, the number or position of the leaf gates may not correspond to the number and / or position of the gates of the corresponding functional nanowires.

[0064] In operation, sensing quantum dots 1214 may be formed under the gate of sensing nanowire 1208, and functional quantum dots 1216 may be formed under one or more of gate 1211 and leaf gate 1213 (e.g., using the processes described with respect to FIGS. 3-10 ). Furthermore, sensing quantum dots 1214 may be electrostatically coupled to each of functional quantum dots 1216 such that any charge event (i.e., change in charge) in such functional dots can be sensed. Thus, in this embodiment, device architecture 1200 allows for coupling of a sensing quantum dot to multiple quantum dots present in multiple nanowires.

[0065] The architectures described thus far utilize floating couplers to capacitively couple a sensing nanowire to one or more functional nanowires. In other embodiments, floating couplers can be utilized to couple two or more functional nanowires to one another. For example, a floating coupler can be utilized between two functional nanowires (each nanowire having one or more functional quantum dots) to capacitively couple the two functional nanowires. In another example, a floating coupler can be utilized to couple two clusters of quantum nanowires. In this example, a floating coupler is utilized between the ends of two clusters, allowing quantum dots from one cluster to be electrostatically coupled to quantum dots from the other cluster.

[0066] It will be appreciated that the architectures described in the above embodiments utilize electrons in quantum dots. In other embodiments, the quantum dots may be configured to confine holes instead of electrons.

[0067] As used herein, the term "comprises" (and grammatical variations thereof) is used in the inclusive sense of "having" or "including" and not in the sense of "consisting only of."

[0068] It will be understood by those skilled in the art that many 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.

Claims

1. 1. A quantum processing system comprising: one or more functional nanowires, each functional nanowire connected to at least one of a source and a drain; a sensing nanowire spaced apart from the one or more functional nanowires and connected to at least one of a source and a drain; one or more gate electrodes capacitively coupled to each of the one or more functional nanowires; one or more electrodes capacitively coupled to the sensing nanowire; a floating coupler located between the one or more functional nanowires and the sensing nanowire, and electrostatically coupling the one or more functional nanowires and the sensing nanowire, the floating coupler partially overlapping each of the one or more functional nanowires and the sensing nanowire; a controller connected to the one or more gates of the sensing nanowire and the one or more gates of the one or more functional nanowires; A quantum processing system, wherein during operation, the controller is configured to apply a bias voltage to bias the one or more gates of the sensing nanowire and the one or more gates of the one or more functional nanowires so that a sensing quantum dot is formed in the sensing nanowire, one or more functional quantum dots are formed in the one or more functional nanowires, and the sensing quantum dot senses changes in charge in each of the one or more functional quantum dots.

2. The quantum processing system of claim 1 , wherein the quantum processing system includes one functional nanowire, the functional nanowire including two gates.

3. 3. The quantum processing system of claim 2, wherein the floating coupler comprises two floating gates.

4. 4. The quantum processing system of claim 3, wherein during operation, a potential difference is maintained between the sensing nanowire and the one or more functional nanowires to form one or more functional quantum dots in the one or more functional nanowires below the floating gate.

5. 5. The quantum processing system of claim 1, wherein the sensing nanowire includes two gates, the two gates being biased to the same voltage.

6. 5. The quantum processing system of claim 4, wherein the sensing nanowire and the functional nanowire are voltage biased relative to each other to form the functional quantum dot below the floating gate in the functional nanowire.

7. The quantum processing system of claim 1 , wherein the quantum processing system comprises a functional nanowire, the functional nanowire comprising a plurality of gates.

8. 8. The quantum processing system of claim 7, wherein the floating coupler is coupled at one end to the sensing nanowire and at the other end the floating coupler includes multiple branches, each branch being coupled to the functional nanowire.

9. 9. The quantum processing system of claim 8, wherein a potential difference is maintained between the sensing nanowire and the functional nanowire to form a functional quantum dot under one or more of the branches of the floating coupler.

10. The quantum processing system of claim 1 , wherein the quantum processing system comprises a plurality of functional nanowires, each functional nanowire comprising two or more gates.

11. 11. The quantum processing system of claim 10, wherein the floating coupler comprises a trunk and a plurality of branches, each branch having one or more leaf gates, the end of the trunk being coupled to the sensing nanowire while the leaf gates being coupled to the functional nanowire.

12. 12. The quantum processing system of claim 11, wherein a potential difference is maintained between the sensing nanowire and each of the functional nanowires to form a functional quantum dot under one or more of the leaf gates of the floating coupler.

13. 1. A method for controlling the operation of a quantum processing system, the quantum processing system comprising: one or more functional nanowires; a sensing nanowire spaced from the one or more functional nanowires; one or more gate electrodes capacitively coupled to each of the one or more functional nanowires; one or more electrodes capacitively coupled to the sensing nanowire; a floating coupler located between the one or more functional nanowires and the sensing nanowire and electrostatically coupling the one or more functional nanowires to the sensing nanowire, the floating coupler partially overlapping each of the one or more functional nanowires and the sensing nanowire; and a controller connected to the one or more gates of the sensing nanowire and the one or more gates of the one or more functional nanowires, the method comprising: receiving, at the controller, a current reading from the sensing nanowire; determining a bias voltage to be applied to the one or more gates of the sensing nanowire based on the received current reading; determining a bias voltage to be applied to the one or more gates of the one or more functional nanowires based on the received current readings; applying the determined bias voltage to the one or more gates of the sensing nanowire; applying the determined bias voltage to the one or more gates of the one or more functional nanowires, wherein the bias voltage is determined such that a sensing quantum dot is formed in the sensing nanowire, one or more functional quantum dots are formed in the one or more functional nanowires, and the sensing quantum dot senses a charge transition event in each of the one or more functional quantum dots.

14. 14. The method of claim 13, wherein the quantum processing system includes a functional nanowire, the functional nanowire includes two gates, and the floating coupler comprises two floating gates.

15. 15. The method of claim 14, further comprising maintaining a potential difference between the sensing nanowire and the one or more functional nanowires to form one or more functional quantum dots in the one or more functional nanowires beneath the floating gate.

16. A method according to any one of claims 13 to 15, wherein the sensing nanowire includes two gates, and the method comprises determining the same bias voltage to be applied to the two gates of the sensing nanowire and applying the same bias voltage to the two gates of the sensing nanowire.

17. 17. The method of any one of claims 15 to 16, further comprising biasing the voltages of the sensing nanowire and the functional nanowire relative to each other to form the one or more functional quantum dots within the one or more functional nanowires.

18. 14. The method of claim 13, wherein the quantum processing system includes one functional nanowire, the functional nanowire including multiple gates, the floating coupler coupled at one end to the sensing nanowire, and the floating coupler at the other end including multiple branches, each branch coupled to the functional nanowire.

19. 20. The method of claim 18, comprising biasing the voltages of the sensing nanowire and the functional nanowire relative to one another to form a functional quantum dot under one or more of the branches of the floating coupler.

20. 14. The method of claim 13, wherein the quantum processing system comprises a plurality of functional nanowires, each functional nanowire comprising two or more gates, the floating coupler comprising a trunk and a plurality of branches, each branch comprising one or more leaf gates, the end of the trunk being coupled to the sensing nanowire while the leaf gates are coupled to the functional nanowire.

21. 21. The method of claim 20, further comprising biasing a voltage of each of the functional nanowires relative to the sensing nanowire to form a functional quantum dot under one or more of the leaf gates of the floating coupler.

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