Patent Description:
The idea of the quantum computer has been around since the early days of today's classic computers. In a quantum computer, the classical bits of a computer are represented by qubits. In accordance with quantum physics, the qubits can be in several states at one time. Conventional spin qubits are based on a heterostructure in e.g. Si(<NUM>-x)Ge(x)/Ge or AlGaAs/GaAs using electrostatic confinement. A <NUM>-dimensional electron gas (2DEG) is used to create a potential energy landscape by applying a voltage to gate electrodes. This is, for example, known from patent publications <CIT> and <CIT>. Qubits using physical quantum dots are also known, for example, from patent publication <CIT>. To be able to realize quantum computers, there is still a need for improvement.

It is an object of the present inventive concept to provide a spin qubit transistor being suitable for scalability. It is a further objective to provide a quantum computer employing a plurality of the spin qubit transistor. These and other objectives of the inventive concept are at least partly met by the invention as defined in the independent claims.

In this text, abbreviations for the natural elements will be used in accordance with the periodic table. For example, Al means aluminum, Ga means gallium, In means indium, N means nitrogen, Si means silicon.

According to a first aspect, there is provided an electron-hole spin qubit transistor comprising:.

It is a realization that a qubit may comprise physical semiconductor islands. Thus, the qubits may be realized without the use of <NUM>-dimensional electron gas. The inventor has realized that it is advantageous to encode quantum information in the total angular momentum of an electron-hole qubit. Hence, the qubit may be a singlet (S)-triplet (T) two-level qubit that physically comprises of two semiconductor islands with a tunnel junction in between. In other words, the qubit may be in a singlet (<NUM>,<NUM>) or triplet (<NUM>,<NUM>) state. Thus, the first qubit comprises the first computing semiconductor island and the first readout semiconductor island having a tunnel junction in between. The second qubit comprises the second computing semiconductor island and the second readout semiconductor island with a tunnel-junction in between. A qubit according to the present invention, comprising physical semiconductor islands, has less cross talk than qubits defined by <NUM>-dimentional electron gas. It should be realized that the spin qubit transistor may comprise more than two qubits. Thus, the spin qubit transistor may comprise two, five, ten, one hundred or thousands of qubits. More than two qubits may be called quantum dits or qudits. The fidelity is improved when using quantum dits. It is an advantage of the present inventive concept that the number of qubits is scalable.

The qubit can be described as an electromagnetic resonator, with a resonant frequency ω<NUM> = (E<NUM> - E<NUM>)/ℏ associated with typically a high-quality-factor better than <NUM><NUM>. Resonant driving of a qubit causes sinusoidal oscillations of the probabilities |α<NUM>|<NUM> and |α<NUM>|<NUM> (where α<NUM> and α<NUM> are the coefficients of the superposition of two quantum states |<NUM>〉 back to |<NUM>〉) in time which are called Rabi oscillations. Resonance occurs when the frequency of the oscillating field is equal to the difference in energy between the Zeeman energy levels for the qubit. The qubit has a spontaneous transition time from the quantum states |S〉 back to |T〉.

Semiconductor islands according to the present invention can be regarded as artificial atoms of an ideal isolated quantum mechanical one-level system. Such is possible due to the fact that the semiconductor islands are formed by strain-relaxation during e.g. Metalorganic vapor phase epitaxy (MOVPE). The inventor has realized that in particular double semiconductor islands according to the present invention can be configured to behave as a two-level quantum Singlet-Triplet system with the energy difference ΔE >> kBT. At room temperature kBT is <NUM> eV. With this, it is possible to drive the qubits fast. In particular, this does not excite the qubit to a third or fourth quantum state. The size of the semiconductor islands may be tuned for excited states with ΔE >> kT. The inventor has realized that the double semiconductor islands according to the present invention are advantageous for the computational entangled state of a quantum computer using electron-hole spin qubits.

A strong spin-orbit coupling enhances the quantization of total angular momentum. The spin-orbit coupling in the present invention is the interaction of a magnetic field that arises e.g. from the orbiting of the nucleus around an unpaired electron creating a ring of current. This gives rise to a magnetic field. The energy contribution from spin-orbit coupling that gives an energy shift, Eso ~ Z<NUM> , of the quantum state depends on the size of the nuclei. Larger atoms have larger atomic numbers, Z, and the magnetic field from the nucleus is typically larger with Z from relativistic effects, with a larger degree of spin-orbit coupling.

According to the invention, it is advantageous to use a wide band gap semiconductor. Thus, InN may be a good choice as the band gap of the semiconductor islands may be chosen as to be the highest for a potential energy well with GaN for an electron-hole. By this, the spin-orbit coupling of the semiconductor island reaches its maximum value for an infinite potential well height, (<NPL>).

The single heavy electron-hole in the semiconductor island populate the ground state, whereas the energy difference between the ground state and the excited states becomes larger and more separated according to the Schrödinger equation. Using holes as charge carrier of the semiconductor decreases the error rates compared to using electrons as charge carrier. The small size of the semiconductor island is such that excitation to higher levels than the Zeeman splitting can be ignored. Coulomb blockade provides a single positive charge or electron-hole on each of the semiconductor islands. The self-capacitance of the semiconductor island to be charged with a single positive charge +e becomes <MAT> according to theory of Coulomb blockade, which in this case can be determined from the magnetic field strength B<NUM>.

The symmetry of the electron-hole ground state wave function is π-symmetry (i.e. not σ-symmetry) with spin quantum number of <NUM>/<NUM>. Electron qubits have the drawback of a trade-off between creating qubits with long decoherence time with slow gate times, or short decoherence times with fastest possible gate time. Electron qubits require a weak spin-orbit coupling in order to provide a long decoherence time. Electron-hole qubits behave differently than electron qubits in that a higher driving speed and high spin-orbit coupling provides longer decoherence times. Thus, it is an advantage of the present invention that it uses confined electron-holes.

The base layer may comprise an alloy of AlGaN. The base layer may comprise Al(y)Ga(<NUM>-y)N, where y > <NUM> or where y = <NUM>. The base layer may comprise an alloy of InGaN. The base layer may comprise In(z)Ga(<NUM>-z)N, where z > <NUM> or where z = <NUM>. The base layer may be arranged below each individual semiconductor island, having a lateral dispersion only relating to a single semiconductor island. The base layer may be arranged under more than one semiconductor island. Thus, the base layer may be arranged under a pair of semiconductor islands configured into a qubit. The base layer may be arranged under more than one qubit. The base layer is configured such that it forms a heterojunction with the semiconductor islands. Thus, there is a strain in the surface between the base layer and the semiconductor island. The base layer may be a monolayer. The base layer may be several monolayers arranged on top of each other.

The computing semiconductor island is defined as a physical island. It should be understood that by physical it is meant that it has a miniaturized size at the nanoscale causing each of said semiconductor islands to exhibit <NUM>-dimensional quantum confinement of a single electron-hole. Thus, the semiconductor islands are grown physically and not held together by electrostatic means. The computing semiconductor island is configured for a single electron-hole. Thus, it is configured to have one single electron-hole as charge carrier.

The readout semiconductor island is defined as a physical island. It should be understood that by physical it is meant that it has a size causing each of said semiconductor islands to exhibit <NUM>-dimensional quantum confinement of a single electron-hole. Thus, the semiconductor islands are grown physically and not held together by electrostatic means. The readout semiconductor island is configured for a single electron-hole. Thus, it is configured to have one single electron-hole as charge carrier.

The semiconductor islands could be interpreted as a "quantum dot". Thus, the semiconductor island may be configured as a quantum dot to exhibit <NUM>-dimensional quantum confinement of a single electron-hole.

Spin-blockade is used to configure a non-interacting state between the respective computing semiconductor island and the respective readout semiconductor island of one qubit. The Zeeman effect is used for splitting an electron-hole ground state with a magnetic field strength B<NUM> in order to configure a deterministic excited state with an energy difference ΔE i.e. that is known beforehand based on the magnetic field strength. Barriers between the plurality of readout semiconductor islands may be significantly high to prevent cross-talk of the readout. The respective readout semiconductor island with a respective second and fourth gate may be used for readout of the quantum state, (<NUM>,<NUM>) or (<NUM>,<NUM>), of the respective computing semiconductor island.

In one embodiment, the compute semiconductor island and readout semiconductor island may be cubic that is confined by an infinite potential energy well in three dimensions. Hence, according to the Schrödinger equation, the wavefunction solutions are eigenfunctions, <MAT> with energy levels <MAT> where En is the energy of the quantum state, nx , ny, nz are quantum numbers, mh is the mass of an electron-hole and a is the cubic width of the potential energy well. Thus the ground state energy E<NUM> for a GaN semiconductor island is given by the Schrödinger equation.

However, it should be understood that the potential energy well is not infinite. The GaN semiconductor island may be a cube of width <NUM>b surrounded by AlGaN with the potential energy barrier V<NUM>. (In practice the GaN semiconductor island may be grown on Al<NUM>Ga<NUM>N and overgrown by the same. ) The band gap of the AlGaN alloy composition is given by the expression Eg(AlxGa<NUM>-xN) = xEg(AlN) + (<NUM> - x)Eg(GaN). Thus, the barrier height is V<NUM>= Eg(AlxGa<NUM>-xN) - Eg(GaN). Instead, given by the potential energy well of width <NUM>b in one dimension, the solution to the even parity energy levels En, are given by the expression: <MAT> and the solution to the odd parity energy levels En, are given by the expression: <MAT>.

Negative values of the energy are bound states for the electron-holes. For an AIN-composition of <NUM>%, the potential energy barrier height may be <NUM> eV as provided by the AlGaN alloy and the cubic width of the GaN semiconductor island may be <NUM> giving a ground state of -<NUM> eV with even parity. The entanglement between compute semiconductor islands are associated with a tunneling or transmission probability which may be configured by the surrounding AlGaN alloy. The surrounding AlGaN alloy may hence be <NUM> thick. Thus the potential energy barrier V<NUM>, may be <NUM> eV for a <NUM> thick Al<NUM>Ga<NUM>N surrounding the GaN semiconductor island as a shell layer. The transmission probability T, is given by the expression <MAT>. The transmission probability may be configured to be <NUM>% at the ground state energy V<NUM> - E<NUM> < <NUM> eV. A transmission probability of <NUM>% occurs very close to the top of the finite potential energy barrier i.e. at ~<NUM> meV from its top. The transmission probability may be increased by applying a bias to the barrier electrode and thereby lowering the potential energy barrier between compute semiconductor islands. The tunneling probability between compute semiconductor islands and readout semiconductor islands may similarly be configured to be <NUM>% in the ground state by having the shell layer surrounding the GaN semiconductor island to be Al<NUM>Ga<NUM>N. A fractional occupancy e. g <NUM>% of the quantum states |S〉 back to |T〉 may be sufficient to correlate the electron-holes for providing an entangled quantum state between qubits.

In one embodiment, the spin qubit transistor may comprise barrier electrodes arranged in between and above compute semiconductor islands and a readout semiconductor islands in order to reduce the effective potential energy barrier height.

The source is arranged in ohmic contact through the supporting material to the first qubit.

The electrical contact between the source terminal and the first computing semiconductor island, via the supporting material, may be a tunnel contact.

The drain is arranged in ohmic contact through the supporting material to the second qubit.

The electrical contact between the drain terminal and the second computing semiconductor island, via the supporting material, may be a tunnel contact.

The effect of the transistor is the transfer of one hole from source to drain with a measurable average current.

The first and third gate terminals are configured to modulate the respective first and second computing semiconductor islands. Thus, the first and third gate terminals are configured to change the quantum state of the computing semiconductor island. Modulation by the first and third gate leads to rotation of the spin of the first and second computing quantum dot, respectively. The effect of the transistor is the transfer of one hole from source to drain with a measurable average current. Modulation applied to the first and third gate is in order to flip the total angular momentum from |T〉 to |S〉. This spin rotation result in removing the restraint of spin blockade. Therefore, an electron-hole can tunnel from the first computing semiconductor island to the second computing semiconductor island. The resulting (<NUM>,<NUM>) state decays by emitting an electron-hole into the drain and a hole from the source is successively fed back to the first computing semiconductor island restoring the initial spin blockade in the (<NUM>,<NUM>) charge configuration.

The first and third gate are arranged on top of the first and second computing semiconductor island. By on top, it should be realized that the first and third gate are arranged on the supporting material at a position being related to the position of the first and second computing semiconductor islands, respectively. Thus, the first gate is arranged such that it can modulate the first computing semiconductor island. The third gate is arranged such that it can modulate the second computing semiconductor island.

The first and third gate are configured to modulate the respective semiconductor island. Under resonant microwave modulation, the qubit state undergoes so-called Rabi oscillations, where the probabilities |α<NUM>|<NUM> and |α<NUM>|<NUM> evolve sinusoidally in time. Consider a semiconductor island with energy splitting ΔE between E<NUM> and E<NUM>. Apply energy as a perturbation at frequency <MAT> by microwave modulation applied to the first or third gate. For example, if the quantum state is |T> in the respective computing semiconductor island and applying the perturbation by microwave power for short time, this develop a small amplitude to be in |S>. If continued power is applied the quantum state can be pumped entirely into |S> which is referred to as a π-pulse. According to the invention, this operation flips the respective readout semiconductor island. The readout of the first and second readout transistor island is performed by the second and fourth gate. It is widely understood that quantum computations can be realized using a combination of CNOT quantum gates and single qubit rotations CROT. The CNOT gate operates on a quantum register consisting of two qubits. The classical computational analog of the CNOT gate is a reversible XOR gate. The controlled CNOT-gate flips the readout semiconductor island only if the respective computing transistor island is in state |S〉 as was described above. Similarly, the CROT gate is equivalent to a CNOT gate except for a π/<NUM> rotation of the spin around the z-axis.

The supporting material is embedding each of the semiconductor islands. The supporting material may be a material configured for being charge carrier for electron-holes. Thus, the supporting material may be a p-doped material or in some embodiments p-doped GaN. The supporting material may be a dielectric material. The supporting material may be configured for heavy electron-holes. The heavy-hole state is ideal for a qubit as the total angular momentum of different qubits can be coupled at large distance by tunneling using a linear or 2D array of semiconductor islands. The electron-hole spin points in the same direction as the orbital angular momentum which in quantum mechanics is measurable by the total angular momentum operator J<NUM>. According to the invention a single heavy electron-hole quantum state may be configured by separation of a light hole sub-bands from the heavy electron-hole sub-band providing a pure heavy electron-hole state, where the lowest energy state at k=<NUM> of the wave vector is a heavy electron-hole quantum state. The heavy electron-hole quantum state may be configured by the physical size of the semiconductor island e.g. <NUM>-<NUM> and straining GaN by an underlayer of pAl(y)Ga(<NUM>-y)N as a hole reservoir or InN by an under layer of p-In(Z)Ga(<NUM>-z)N as a hole reservoir. The semiconductor islands may be configured for a single heavy electron-hole state i.e. without an external magnetic field. Such may be provided by larger confinement and therefore result in longer hole spin-relaxation times.

According to electric dipole spin resonance (EDSR) the effective mass m^ * and g-factors can be estimated as, <MAT> and <MAT>. The g-factors of InAs/GaAs quantum dots is described in <NPL>) as <MAT>. The g-factor approaches <NUM>,<NUM> for both bulk InN and GaN. Hence, it is advantageous according to the invention that the quantum dots are defined by a heterojunction of aluminium gallium nitride (AlGaN), and gallium nitride (GaN). The size of the quantum dots and the strain is used to configure the Rabi frequency.

By unique resonance frequency it should be realized that each of the computing semiconductor islands has a unique frequency. The frequency of one computing semiconductor island is not shared with any other computing semiconductor island or with any readout semiconductor island. It should be realized that two compute semiconductor islands may have the same frequency if they are separated by a distance such that they do not disrupt each other.

Each semiconductor island may comprise an Al(x)Ga(<NUM>-x)N alloy, where <NUM> > x > <NUM>. Each computing semiconductor island may comprise of a unique Al(x)Ga(<NUM>-x)N alloy. Thus, each computing semiconductor island may have a unique Al(x)Ga(<NUM>-x)N not shared with any other semiconductor island. The Al(x)Ga(<NUM>-x)N alloy is configured to give the semiconductor islands their frequency. Thus, a unique Al(x)Ga(<NUM>-x)N alloy will achieving computing semiconductor islands having said unique resonance frequency. The base layer may comprise Al(y)Ga(<NUM>-y)N, where <NUM> > y ≥ <NUM>. Thus, the base layer may be GaN or any alloy of Al(y)Ga(<NUM>-y)N.

Each semiconductor island may comprise an In(z)Ga(<NUM>-z)N alloy, where <NUM> > z > <NUM>. Each computing semiconductor island may comprise of a unique In(z)Ga(<NUM>-z)N alloy. Thus, each computing semiconductor island may have a unique In(z)Ga(<NUM>-z)N not shared with any other semiconductor island. The In(z)Ga(<NUM>-z)N alloy is configured to give the semiconductor islands their frequency. Thus, a unique In(z)Ga(<NUM>-z)N alloy will achieving computing semiconductor islands having said unique resonance frequency. The base layer may comprise In(z)Ga(<NUM>-z)N, where <NUM> > z ≥ <NUM>. Thus, the base layer may be InN or any alloy of In(z)Ga(<NUM>-z)N.

Each semiconductor island may comprise a lower region comprising an Al(x)Ga(<NUM>-x)N alloy, and an upper region comprising GaN. The base layer may be GaN, wherein the heterojunction is formed at an interface of the lower region and the base layer.

Each semiconductor island may comprise a lower region comprising an In(z)Ga(<NUM>-z)N alloy, and an upper region comprising InN. The base layer may be InN, wherein the heterojunction is formed at an interface of the lower region and the base layer.

Each semiconductor island may have a unique size, thereby achieving computing semiconductor islands having said unique resonance frequency. Thus, the size of the semiconductor islands may be decisive of the frequency.

The semiconductor islands may be formed by epitaxial growth and exhibit <NUM>-dimensional quantum confinement. The strain of the semiconductor islands may be configured during growth of GaN semiconductor islands e.g. by selective area growth. The distance between quantum dots may be controlled by nanosize holes in a hard mask such as SiO<NUM>, Al<NUM>O<NUM> or Si<NUM>N<NUM> using selective area growth. Thus, the epitaxial growth may be selective area growth. This causes semiconductor islands to form during epitaxial growth with pattern control with a short but significant abruptness of the growth on the order of ~<NUM>. A <NUM>-dimensional hole gas (0DHG) forms within the quantum dot. The thickness of the AlGaN underlayer may be significantly thick to reduce Coulomb interaction between electrons and holes. In another embodiment of the invention AlGaN semiconductor islands are grown on GaN. The document <NPL>, describes selective area growth for the formation of quantum dots. Undoped GaN semiconductor islands that may be formed on InGaN becomes negatively polarized due to strain. The semiconductor islands may be formed by Stranski-Krastanov growth.

Each of the semiconductor islands may consist of a material having a wurtzite crystal structure. Gallium nitride and its alloys of In(z)Ga(<NUM>-z)N and of Al(x)Ga(<NUM>-x)N have wurtzite crystal structure that is non-centrosymmetric. Hence, it lacks inversion symmetry. Electron qubits in these alloys are subject to piezoelectric phonons that cause decoherence. Broken symmetry or inversion asymmetry can be invoked by straining the heterostructure quantum dots, causing splitting of the valence band in light holes and heavy electron-holes. The gate may be pulsed with a microwave modulation that reduces the tunneling barrier between two neighboring quantum dots. The coherence time of the quantum state should be longer than <MAT>.

The respective computing semiconductor island and readout semiconductor island is arranged with a distance in the range of <NUM>-<NUM> therebetween. The semiconductor islands may be arranged in a plane parallel to the base layer. The semiconductor islands may be separated from each other in said plane parallel to the base layer. The semiconductor islands may be laterally disposed with respect to each other.

The respective computing semiconductor islands and readout semiconductor islands of each qubit may be arranged and positioned such that an electron hole can be excited from the compute semiconductor island to the readout semiconductor island in each qubit.

The first qubit and the second qubit may be arranged and positioned to be entangled. The first computing semiconductor island and the second computing semiconductor island may be arranged and positioned to be entangled. The first qubit and the second qubit may be arranged such that the spin of the electron hole of the respective computing semiconductor islands are entangled. It should be understood that in embodiments comprising more than two qubits, all qubits are entangled. The inventor has realized that the double semiconductor islands, i.e. the qubits, formed in In(<NUM>-z)Ga(z)N and its alloys or in Al(<NUM>-x)Ga(x)N and its alloys are advantageous for the computational entangled state of a quantum computer using electron-hole spin qubits.

The first gate terminal may be configured to excite an electron hole to cause the first qubit to transition from a singlet state to a triplet state. The first gate terminal may be configured to electrostatically excite said electron hole to cause the first qubit to transition from said singlet state to said triplet state. The first gate may be configured for microwave modulation of the spin state of the first computing semiconductor island of the first qubit. The first gate may have ohmic contact with the base layer. The third gate terminal is configured to excite an electron hole to cause the first qubit to transition from a singlet state to a triplet state. The third gate terminal is configured to electrostatically excite said electron hole to cause the second qubit to move from said singlet state to said triplet state. The third gate may be configured for microwave modulation of the spin state of the second computing semiconductor island of the first qubit. The third gate may have ohmic contact with the base layer. The coherence time of the quantum state should be longer than ℏ/ΔE. The gate may be pulsed with a microwave modulation that reduces the tunneling barrier between two neighboring compute semiconductor islands. Microwave modulation applied to the first and third gate are used to rotate the spin of quantum dots i.e. a radio frequency electric field may be applied to the electrodes. The microwave modulation may be a burst of time τ_burst. A flipping between the |S〉 state to the |T〉 state can be accomplished by controlling the envelope amplitude and pulse duration of a microwave resonator gate where a rotation of the quantum state vector occurs of the electron-hole when supplied with energy according to the photoelectric effect, E= ℏω.

The spin qubit transistor may further comprise a control electrode arrangement configured to modify a tunneling property between the first computing semiconductor island and the second computing semiconductor island. Alternatively, the spin qubit transistor may further comprise a control electrode arrangement configured to modify a tunneling probability between the first computing semiconductor island and the second computing semiconductor island. The control electrode arrangement may be arranged in the same plane, parallel to the plane of the semiconductor island, where the first, second, third and fourth gates are arranged. The control electrode arrangement may be arranged between the first and the third gate. The control electrode arrangement may be configured to modify a region of material between the first computing semiconductor island and the second computing semiconductor island. The control electrode arrangement may be configured to reduce a potential barrier between the compute semiconductor islands. The region of material may be of the same material as the supporting material. The region may be a barrier region of a different material. The region may be a barrier region being n-doped.

The spin qubit transistor may further comprise a control electrode arrangement configured to modify a tunneling property between the first computing semiconductor island and the first readout semiconductor island. The control electrode arrangement may be arranged between the first gate and the second gate. The spin qubit transistor may further comprise a control electrode arrangement configured to modify a tunneling property between the second computing semiconductor island and the second readout semiconductor island The control electrode arrangement may be arranged between the third gate and the fourth gate. Alternatively, the spin qubit transistor may further comprise a control electrode arrangement configured to modify a tunneling probability between the first computing semiconductor island and the first readout semiconductor island. Alternatively, the spin qubit transistor may further comprise a control electrode arrangement configured to modify a tunneling probability between the second computing semiconductor island and the second readout semiconductor island. The control electrode arrangement may be arranged in the same plane, parallel to the plane of the semiconductor island, where the first, second, third and fourth gates are arranged. The control electrode arrangement may be configured to modify a region of material between the first computing semiconductor island and the first readout semiconductor island. The control electrode arrangement may be configured to modify a region of material between the second computing semiconductor island and the second readout semiconductor island. The control electrode arrangement may be configured to reduce a potential barrier between the respective compute semiconductor island and readout semiconductor island. The region of material may be of the same material as the supporting material. The region may be a barrier region of a different material. The region may be a barrier region being n-doped.

The supporting material may be p-doped GaN. Thus, the supporting material may act as a hole reservoir for the semiconductor islands.

The supporting material may be a dielectric material.

The band structure of the semiconductor islands, including compute and readout semiconductor islands, are configured with heavy electron-holes at the origin in k-space by the composition of the base layer.

Another aspect of the present inventive concept is a method for forming an electron-hole spin qubit transistor comprising the steps of:.

It should be realized that there may be more optional steps between the steps of the method.

The method provides with a method for forming a spin qubit transistor. The method allows for forming <NUM>-dimensional semiconductor islands, such as physically determined semiconductor islands. The method allows for forming compute semiconductor islands of unique alloys, giving them unique frequency.

The distance between the first compute semiconductor islands may be <NUM>-<NUM>. The distance between the second compute semiconductor islands may be <NUM>-<NUM>. The semiconductor reactive species forming the first compute semiconductor islands may be a different alloy than the semiconductor reactive species forming the second compute semiconductor islands.

The method may comprise additional steps of forming of hard mask layers, such that they cover the first and second semiconductor islands respectively before forming the second and third plurality of nano size cavities.

The plurality of first compute semiconductor islands and the plurality of second compute semiconductor islands may be configured for a lattice-mismatch to the base layer for providing a Rabi frequency. Thus, semiconductor reactive species may for a strain with the base layer.

The plurality of first compute semiconductor islands, the plurality of second compute semiconductor island, and the plurality of readout semiconductor islands may form a matrix of qubits. Thus, the plurality of first compute semiconductor islands, the plurality of second compute semiconductor island, and the plurality of readout semiconductor islands may form a qudit. Thus, the method may be used for the production of a large size quantum computer.

The method may further comprise forming a cap layer on each of the plurality of first compute semiconductor islands and each of the plurality of second compute semiconductor islands to configure a heterostructure. Thus, the first compute semiconductor islands and the second semiconductor islands may each comprise of two alloys.

As an alternative, a third aspect of the present inventive concept is a method for forming an electron-hole spin qubit transistor comprising the steps of:.

It is to be understood that the step of removing all hard mask layers by wet etching or plasma etching may be performed as a single step, or alternatively as several steps throughout the disclosed method. Such several steps of removing a hard mask layer may be performed at various stages of the disclosed method, as will be appreciated by the person skilled in the art.

Thus, by the third aspect of the inventive concept the unique frequency of each of the compute semiconductor islands is decided by the unique depth of the semiconductor islands.

A fourth aspect of the present inventive concept is a quantum computer comprising at least one spin qubit transistor according to above. The computer may comprise a plurality of spin qubit transistors. Each of the spin qubit transistors of the quantum computer may comprise a plurality of qubits according to the above. The inventor has realized that the double semiconductor islands formed in Al(<NUM>-x)Ga(x)N or In(<NUM>-x)Ga(x)N and its alloys are advantageous for the computational entangled state of a quantum computer using electron-hole spin qubits. A qubit processor of N qubits may be used. Further parallelization with multiple N-qubit processors may be used on the same chip. The scalability of quantum computers requires long distance coupling methods between the physical realization of qubits.

A feature described in relation to one aspect may also be incorporated in other aspects, and the advantage of the feature is applicable to all aspects in which it is incorporated.

Further, the use of terms "first", "second", and "third", and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. All references to "a/an/the [element, device, component, means, step, etc]" are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise.

The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description of the present inventive concept, with reference to the appended drawings, wherein:.

The figures are not necessarily to scale, and generally only show parts that are necessary in order to elucidate the inventive concept, wherein other parts may be omitted or merely suggested.

In cooperation with attached drawings, the technical contents and detailed description of the present invention are described thereinafter according to preferable embodiments, being not used to limit the claimed scope. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

<FIG> illustrates a cross sectional view of an electron-hole spin qubit transistor <NUM>. The illustrated spin qubit transistor <NUM> comprises a first qubit <NUM> and a second qubit <NUM>. The first qubit <NUM> comprises a first computing semiconductor island <NUM>. The first qubit <NUM> further comprises a first readout semiconductor island <NUM> (not shown in <FIG>). The second qubit <NUM> comprises a second semiconductor island <NUM>. The second qubit <NUM> further comprises a second readout semiconductor island <NUM> (not shown in <FIG>). The first computing semiconductor island <NUM> and the second computing semiconductor island <NUM> are configured to each have a unique resonance frequency.

The semiconductor islands <NUM>, <NUM>, <NUM>, <NUM> has a size causing each of said semiconductor islands <NUM>, <NUM>, <NUM>, <NUM> to exhibit <NUM>-dimensional quantum confinement of a single electron hole. Thus, each semiconductor island has a size corresponding to only one electron hole. The semiconductor islands <NUM>, <NUM>, <NUM>, <NUM> has dimensions of <NUM>. The semiconductor islands <NUM>, <NUM>, <NUM>, <NUM> may comprise an Al(x)Ga(<NUM>-x)N alloy, where <NUM> > x > <NUM>. Each of the computing semiconductor islands <NUM>, <NUM> may comprise a unique Al(x)Ga(<NUM>-x)N alloy. Each semiconductor islands <NUM>, <NUM> may comprise an In(z)Ga(<NUM>-z)N alloy, where <NUM> > z > <NUM>. Each computing semiconductor islands <NUM>, <NUM> may comprise of a unique In(z)Ga(<NUM>-z)N alloy. Thus, each computing semiconductor islands <NUM>, <NUM> may have a unique In(z)Ga(<NUM>-z)N alloy. Thereby, each computing semiconductor island <NUM>, <NUM> achieves said unique resonance frequency. The first computing semiconductor island <NUM> is entangled to the second computing semiconductor island <NUM>.

The semiconductor islands <NUM>, <NUM>, <NUM>, <NUM> are epitaxially grown on top of a base layer <NUM>. The semiconductor islands <NUM>, <NUM>, <NUM>, <NUM> have an axial direction normal to the base layer <NUM>. Each of said semiconductor islands <NUM>, <NUM>, <NUM>, <NUM> forms a heterojunction with the base layer <NUM>. The base layer <NUM> is illustrated as if it in one single piece underlines the entire spin qubit transistor <NUM>. However, the base layer <NUM> may be arranged only below each semiconductor island <NUM>, <NUM>, <NUM>, <NUM>. The epitaxial growth of the semiconductor islands <NUM>, <NUM>, <NUM>, <NUM> on the base layer <NUM> creates a strain between the materials needed to create an electron hole. The base layer <NUM> may comprise Al(y)Ga(<NUM>-y)N, where <NUM> > y ≥ <NUM>. The base layer <NUM> may comprise In(z)Ga(<NUM>-z)N, where <NUM> > z ≥ <NUM>.

The spin qubit transistor <NUM> further comprises a supporting material <NUM>. The supporting material <NUM> is arranged on top of and embedding each of the semiconductor islands <NUM>, <NUM>, <NUM>, <NUM>. The supporting material may be p-doped, thereby acting as a hole reservoir for the semiconductor islands <NUM>, <NUM>, <NUM>, <NUM>. In addition, the supporting material <NUM> may comprise Al(y)Ga(<NUM>-y)N. The supporting material <NUM> may be dielectric.

As illustrated in <FIG>, the spin qubit transistor <NUM> further comprises a source terminal S. The source terminal S is in electrical contact with the first computing semiconductor island <NUM> via the supporting material <NUM>. The electrical contact may be a tunnel contact. Further, the spin qubit transistor <NUM> comprises a drain terminal D. The drain terminal D is in electrical contact with the second computing semiconductor island <NUM> via the supporting material <NUM>. The electrical contact may be a tunnel contact. The effect of the transistor is the transfer of one hole from source S to drain D with a measurable average current.

The spin qubit transistor <NUM> further comprises a first gate terminal G1, a second gate terminal G2 (not shown in <FIG>), a third gate terminal G3 and a fourth gate terminal G4 (not show in <FIG>). The first gate terminal G1 is arranged over the first computing semiconductor island <NUM>. The first gate terminal G1 is configured for microwave modulation of the spin state of the first computing semiconductor island <NUM>. The second gate terminal G2 arranged over the first readout semiconductor island <NUM>. The second gate terminal G2 is configured for readout of a quantum state of the first readout semiconductor island <NUM>. The third gate terminal G3 is arranged over the second computing semiconductor island <NUM>. The third gate terminal G3 is configured for microwave modulation of the spin state of the second computing semiconductor island <NUM>. The fourth gate terminal G4 is arranged over the second readout semiconductor island <NUM>. The fourth gate terminal G4 is configured for readout of a state of the second readout semiconductor island <NUM>.

The first gate terminal G1 may be configured to excite an electron hole to cause the first qubit <NUM> to transition from a singlet state to a triplet state. The state of the first qubit <NUM> is read by the second gate G2. The electron hole may tunnel to the second qubit <NUM>. An electron hole from the second qubit <NUM> may tunnel to the drain.

The spin qubit transistor may further comprise a control electrode arrangement B. The control electrode arrangement B is configured to modify a tunneling property between the first computing semiconductor island <NUM> and the second computing semiconductor island <NUM>. Alternatively, the control electrode arrangement B is configured to modify a tunneling probability between the first computing semiconductor island <NUM> and the second computing semiconductor island <NUM> The control electrode arrangement B may be arranged between the first G1 and the third gate G3. The control electrode arrangement B may be configured to modify a region of material <NUM> between the first computing semiconductor island <NUM> and the second computing semiconductor island <NUM>. The region of material <NUM> may be of the same material as the supporting material <NUM>. The region of material <NUM> may be a barrier region of a different material. The region of material <NUM> may be a barrier region being n-doped.

Referring now to <FIG>, the spin qubit transistor <NUM> is viewed cross sectionally from above. Many features are common with the one of <FIG> and are for means of simplicity not repeated.

In <FIG>, the arrangement of the first readout semiconductor island <NUM> in relation to the first computing semiconductor island <NUM> can be seen. The first computing semiconductor island <NUM> and the first readout semiconductor island <NUM> are laterally disposed along the axis A. The distance between the first computing semiconductor island <NUM> and the first readout semiconductor island <NUM> is <NUM>-<NUM>. The same arrangement is valid for the second semiconductor island <NUM> and the second readout semiconductor island <NUM>, which are laterally disposed along the axis B. The distance between the axis A and the axis B may vary between <NUM> and <NUM>.

Viewed from above, it can be understood that the first, second, third and fourth gate G1, G2, G3, G4 are arranged on top of the respective first computing semiconductor island <NUM>, first readout semiconductor island <NUM>, second computing semiconductor island <NUM> and second readout semiconductor island <NUM>.

The control electrode arrangement B may be located between the first gate G1 and the third gate G3 and between the second gate G2 and the further gate G4.

<FIG> illustrates the spin qubit transistor viewed cross sectionally from two opposite sides. Many features are common with the one of <FIG> and are for means of simplicity not repeated.

In <FIG> the arrangement of the first computing semiconductor island <NUM> in relation to the first gate G1, the first readout semiconductor island <NUM> and to the second gate G2 can be seen. The first gate G1 is arranged vertically above the first compute semiconductor island <NUM>. The second gate G2 is arranged vertically above the first readout semiconductor island <NUM>. In <FIG> the arrangement of the second computing semiconductor island <NUM> in relation the third gate G3, the second readout semiconductor island <NUM> and to the fourth gate G4 can be seen. The third gate G3 is arranged vertically above the second compute semiconductor island <NUM>. The fourth gate G4 is arranged vertically above the second readout semiconductor island <NUM>. The gates G1, G2, G3, G4 are electrically connected to the respective semiconductor island <NUM>, <NUM>, <NUM>, <NUM> via the supporting material <NUM>.

Referring now to <FIG>, the spin qubit transistor <NUM> is viewed cross sectionally from the side. Many features are common with the one of <FIG> and are for means of simplicity not repeated.

the compute semiconductor islands <NUM>, <NUM> has a cap layer <NUM>. The cap layer <NUM> may cover a part of the compute semiconductor island <NUM>, <NUM>. The cap layer <NUM> may comprise Al(y)Ga(<NUM>-y)N.

In <FIG> the compute semiconductor islands <NUM>, <NUM> has a shell layer <NUM>. The shell layer <NUM> may cover the entire semiconductor island <NUM>, <NUM>. The shell layer <NUM> may comprise Al(y)Ga(<NUM>-y)N.

The cap layer <NUM> or the shell layer <NUM> may also be applied to readout semiconductor islands <NUM>, <NUM>. The cap layer <NUM> or shell layer <NUM> as presented in <FIG> may be used together with any of the embodiments described herein, such as the embodiment of <FIG>, <FIG>, and <FIG>.

Now referring to <FIG>, sixteen pairs of a first qubit <NUM> and a second qubit <NUM> forming a qudit <NUM> are viewed from above. The qudit <NUM> comprises several spin qubit transistors as described in accordance with <FIG>.

A method <NUM> for forming a spin qubit transistor <NUM> according to the inventive concept will now be described with reference to <FIG>, <FIG>. For clarity and simplicity, the method will be described in terms of 'steps'. It is emphasized that steps are not necessarily processes that are delimited in time or separate from each other, and more than one 'step' may be performed at the same time in a parallel manner.

The method <NUM> for forming an electron-hole spin qubit transistor <NUM> comprises the steps of:.

During step <NUM> a hard mask <NUM> is formed on the base layer <NUM>. The hard mask is further during step <NUM> provided with a first plurality of nano size cavities <NUM>. The plurality of nano size cavities may be made by etching.

In step <NUM> of the method <NUM> a plurality of first compute semiconductor islands <NUM> are formed by exposing the base layer <NUM>, through the hard mask <NUM>, to a first semiconductor reactive species. The number of first compute semiconductor islands <NUM> formed may be at least <NUM>, such as <NUM>, <NUM>, <NUM>, <NUM> and so on. As can be seen in <FIG> the plurality of first compute semiconductor islands are arranged at a distance from each other. The distance may be selected such that two of the plurality of first compute semiconductor islands having the same frequency do not disrupt or interfere with each other.

During step <NUM>, a second plurality of nano size cavities <NUM>. The plurality of nano size cavities may be made by etching.

In step <NUM>, a plurality of second compute semiconductor islands <NUM> are formed by exposing the base layer <NUM>, through the hard mask <NUM>, to a second semiconductor reactive species. The number of second compute semiconductor islands <NUM> formed may be at least <NUM>, such as <NUM>, <NUM>, <NUM>, <NUM> and so on. As can be seen in <FIG> the plurality of second compute semiconductor islands are at a distance from each other. The distance may be selected such that two of the plurality of second compute semiconductor islands having the same frequency do not disrupt or interfere with each other.

The first and second semiconductor reactive species differs from each other. The number of second compute semiconductor islands <NUM> equals the number of first compute semiconductor islands <NUM>.

In step <NUM> a plurality of first and second readout semiconductor islands <NUM>, <NUM> are formed by exposing the base layer <NUM>, through the hard mask <NUM>, to a third semiconductor reactive species.

The first, second and third semiconductor reactive species differs from each other. The first and second readout semiconductor islands <NUM>,<NUM> may be made by two different semiconductor reactive species, respectively. If the first and second readout semiconductor islands <NUM>, <NUM> are made of different semiconductor reactive species, they may be made in two different steps.

The number of first and second compute semiconductor islands <NUM>, <NUM> equals the number of first and second readout semiconductor islands <NUM>,<NUM>, respectively. The first readout semiconductor islands <NUM> forms a pair with the first compute semiconductor island <NUM>. The second readout semiconductor islands <NUM> forms a pair with the second compute semiconductor islands <NUM>.

The plurality of first compute semiconductor islands <NUM> and the plurality of second compute semiconductor islands <NUM> may be configured for a lattice-mismatch to the base layer <NUM> for providing a Rabi frequency.

The plurality of first compute semiconductor islands <NUM>, the plurality of second compute semiconductor island <NUM>, and the plurality of readout semiconductor islands <NUM>, <NUM> form a matrix of qubits.

The method may further comprise forming <NUM> a cap layer on each of the plurality of first compute semiconductor islands <NUM> and each of the plurality of second compute semiconductor islands <NUM> to configure a heterostructure.

In relation to <FIG>, <FIG>, an alternative method <NUM>, for forming an electron-hole spin qubit transistor <NUM> is disclosed, comprising the steps of:.

In relation to <FIG>, the layer structure of step <NUM> is shown. Further, in the layer structure compute semiconductor islands <NUM>, <NUM> are embedded. The different compute semiconductor islands are present at different depth in relation to full layers of the semiconductor layers. It should be understood that the number of layer may be more than the four depicted in <FIG>. Moreover, it should be understood that the compute semiconductor islands may have a larger or smaller height distance between each other. In other words, as illustrated, the first compute semiconductor island <NUM> may be arranged at a first semiconductor layer, and the second semiconductor island <NUM> may be arranged at a second semiconductor layer, wherein an intermediate semiconductor layer or other layer is arranged between the first semiconductor layer and the second semiconductor layer. Such other layer may for example be an etch stop layer.

In <FIG>, the Triplet (<NUM>,<NUM>) charge states of the qubit is shown. In <FIG>, the Singlet (<NUM>,<NUM>) charge states of the qubit is shown.

Claim 1:
An electron-hole spin qubit transistor (<NUM>) comprising:
a base layer (<NUM>);
a first qubit (<NUM>) comprising:
a first computing semiconductor island (<NUM>) and a first readout semiconductor island (<NUM>) arranged with a distance in the range of <NUM>-<NUM> therebetween;
a second qubit (<NUM>) comprising:
a second computing semiconductor island (<NUM>) and a second readout semiconductor island (<NUM>) arranged with a distance in the range of <NUM>-<NUM> therebetween;
wherein each of said semiconductor islands has a size causing each of said semiconductor islands to exhibit <NUM>-dimensional quantum confinement of a single electron hole, and wherein each of said semiconductor islands forms a heterojunction with the base layer;
wherein the spin qubit transistor (<NUM>) further comprises:
a supporting material (<NUM>) arranged on top of and embedding each of said semiconductor islands;
a source terminal (S) being in electrical contact with the first computing semiconductor island (<NUM>) via the supporting material (<NUM>);
a drain terminal (D) being in electrical contact with the second computing semiconductor island (<NUM>) via the supporting material (<NUM>);
a first gate terminal (G1) arranged over the first computing semiconductor island (<NUM>), the first gate terminal (G1) being configured for microwave modulation of the spin state of first computing semiconductor island (<NUM>);
a second gate terminal (G2) arranged over the first readout semiconductor island (<NUM>), the second gate terminal (G2) being configured for readout of a state of the first readout semiconductor island (<NUM>);
a third gate terminal (G3) arranged over the second computing semiconductor island (<NUM>), the third gate terminal (G3) being configured for microwave modulation of the spin state of second computing semiconductor island (<NUM>);
a fourth gate terminal (G4) arranged over the second readout semiconductor island (<NUM>), the fourth gate terminal (G4) being configured for readout of a state of the second readout semiconductor island (<NUM>);
wherein said first computing semiconductor island (<NUM>) and said second computing semiconductor island (<NUM>) are configured to have a unique resonance frequency respectively.