Method for producing colour centres with integrated optics

Abstract

The invention relates to a method for producing individual spin quantum bits (spin qubits) in crystals, such as silicon carbide (SiC) or diamond. The aim of the invention is to significantly improve the coupling efficiency of light emitted by colour centres and to enable a substantially more precise, simpler and more scalable production of spin qubits with integrated optics. Furthermore, in the production method claimed here, the production of miniaturised coupling-out optics with the generation of the colour centre in directly successive process steps is to be made possible, which does not require a relative adjustment between the colour centre and the optics. According to the invention, said aim is achieved by the features of claim 1.

Description

[0001] 1465-10.24-01 1 / 19 TH Nuremberg

[0002] Claims

[0003] 1. Method for producing individual color centers (2) in a substrate (1) consisting of a diamond or silicon carbide crystal, comprising the following process steps: a) providing a substrate (1) contaminated or doped with foreign atoms, containing or consisting of a diamond or SiC crystal, b) providing a device for producing a microlens system (10) on the substrate (1), c) providing a laser device for generating a high-energy laser beam (9), d) producing the microlens system (10) on the substrate (1), e) passing the laser beam (9) over the previously produced microlens system (10) focusing on an area located at a depth t below the substrate surface (13) and generating at least one lattice defect (vacancy center) in this area.

[0004] 2. Method according to claim 1, characterized in that the substrate (1) is thermally annealed to form a color center (2) wherein at least one foreign atom (4) diffuses to a lattice site adjacent to the lattice defect (5).

[0005] 3. Method according to claim 1 or 2, characterized in that the laser source for the laser beam (9) is a short pulse laser, in particular an ultrashort pulse laser.

[0006] 4. Method according to claim 1, 2 or 3, characterized in that the microlens system (10) comprises an immersion medium.

[0007] 5. The method of claim 4, characterized in that the immersion medium is a solid immersion lens (8). 1465-10.24-01 2 / 19 TH Nuremberg

[0008] 6. Method according to claim 5, characterized in that the solid-state immersion lens (8) is a semi-hyperboloid or a Weitrass lens with conical constant k, e.g. a hemisphere (k=0) or a semi-ellipsoid (-l <k<0) ist.

[0009] 7. Method according to claim 5 or 6, characterized in that the solid immersion lens (8) is applied (mounted) onto the substrate (1) or is built up on the substrate surface (13) by additive manufacturing.

[0010] 8. Method according to claim 5 or 6, characterized in that the solid immersion lens (8) is machined from the substrate (1) by subtractive processing (surface structuring).

[0011] 9. Method according to at least one of claims 1 to 8, characterized in that the microlens system (10) has an on / off coupling optic (6) with at least one refractive element (11).

[0012] 10. Method according to claim 9, characterized in that the refractive element (11) is built up on the substrate (1) or the solid immersion lens (8) by an additive manufacturing process.

[0013] 11. Method according to claim 10, characterized in that the additively manufactured refractive element (11) and / or a support structure (14) for its fixation is produced by partial curing (crosslinking) of a liquid polymer or ormocer material, which is heated for this purpose by a guided laser beam.

[0014] 12. Method according to one of claims 9, 10 or 11 characterized in that the input / output coupling optics (6) comprises at least one further refractive element (12).

[0015] 13. Method according to claim 12, characterized in that the at least one further refractive element (12) is additively manufactured.

[0016] 14. The method of claim 12, characterized in that the at least one further refractive element (12) is mounted. 1465-10.24-01 3 / 19 TH Nuremberg

[0017] 15. Method according to claim 12, 13 or 14, characterized in that at least two refractive elements (11, 12) differ from each other by their material.

[0018] 16. Method according to claim 15, characterized in that at least one refractive element (11, 12) , in particular the second refractive element (12) , consists of glass.

[0019] 17. Method according to at least one of the preceding claims, characterized in that the microlens system (10) or parts of the microlens system serve for optical excitation of the color center (2) or for coupling out fluorescence light from the color center.

[0020] 18. Method according to at least one of the preceding claims, characterized in that the color center (2) is a nitrogen defect center (NV center).

[0021] 19. Method according to at least one of the preceding claims, characterized in that the color center (2) is a silicon defect center (SiV center).

[0022] 20. Method according to one of the preceding claims for the manufacture of quantum sensors.

[0023] 21. Method according to one of the preceding claims for the manufacture of quantum computers, in particular quantum processors.

[0024] 22. Method according to one of the preceding claims for the production of single-photon emitters. 1465-10.24-01 4 / 19 TH Nuremberg

[0025] Applicant: Nuremberg Institute of Technology Georg Simon Ohm

[0026] 90489 Nuremberg, Germany

[0027] Title: Method for producing color centers with integrated optics

[0028] Description

[0029] The invention relates to a method for producing individual spin quantum bits (spin qubits) in crystals such as silicon carbide (SiC) or diamond. Spin qubits represent a promising technology platform for quantum processors, quantum sensors, or single-photon emitters and are therefore increasingly becoming the focus of applied research and development.

[0030] Pure diamond crystals consist only of carbon atoms arranged in a regular carbon lattice. However, crystal lattices typically contain defects. These defects can take the form of carbon atoms being replaced by foreign atoms or by vacancies in the carbon lattice (vacancy centers V). Defects within solids determine their electrical, optical, and mechanical properties. Some defects are called color centers because they are optically active and can alter the absorption and emission spectra of the solid. Well over 100 such defects are known in diamonds, which can, for example, cause fluorescence effects. Pure diamonds are colorless and exhibit a very broad transparent frequency range from the deep ultraviolet to the far infrared.Therefore, color centers embedded in the regular diamond lattice can be easily optically excited and light emitted by the color centers can be reliably detected.

[0031] Nitrogen impurities are common in diamond lattices. These nitrogen atoms (N) are naturally stochastically distributed throughout the crystal lattice, but they can also be introduced intentionally through doping, e.g., by ion implantation, even during crystal formation. If a defect is located in the immediate vicinity of a nitrogen substituent, it is referred to as an NV center (NV). This forms a point defect with specific physical, especially optical, properties. The spin state of an NV can be optically read out, e.g., at room temperature and with resonant microwave excitation, depending on an external magnetic field. Therefore, an NV is suitable, e.g., as a magnetic field sensor, which allows for the investigation of molecular structures or irregularities in the structure of solids at the nanoscale.NV centers are also used to measure very low currents in integrated circuits. In most of these applications, the NV centers are used as probes for optically detected magneto-resonance (ODMR) measurements. In this process, the NV centers are optically excited and thereby activated. Shortly after excitation, the spontaneous emission of infrared radiation can be optically detected.

[0032] To produce NV centers, lattice defects are usually created in a substrate contaminated with nitrogen atoms, e.g. by ion beam bombardment. 1 or by deterministic generation of individual NV centers using short laser pulses 2Lattice defects often tend to cluster together, naturally forming double defects such as nitrogen vacancy centers. This natural process can be artificially accelerated by annealing. The resulting defects (holes) migrate until they remain in a stable state in the immediate vicinity of a nitrogen atom. This causes the magnetic moments of the spins of several individual electrons to combine into a tiny bar magnet. This magnet can only lock into two positions when placed in a magnetic field: parallel or antiparallel to the field. This quantum physical principle allows information to be stored in the form of a zero or a one. An NV center thus forms the smallest unit of storage, namely a bit, specifically a qubit.To operate a functioning quantum computer, it is sufficient for these stored quantum states to persist for only a few thousandths of a second; this is the case here. 1465-10.24-01 6 / 19 TH Nuremberg.

[0033] Naturally occurring NV centers can be used as magnetic field sensors, but this requires highly precise alignment of optical detectors. Particularly for applications in quantum computers, the controlled generation of multiple NV centers located close together, typically with a spacing of less than 20 nm, is necessary.

[0034] For industrial applications, the coupling efficiency of the light emitted by the NV centers is currently barely sufficient, lying in the single-digit percentage range, and would need to be improved by about an order of magnitude. To read out the state of the generated NV centers, which is essential for functionality in quantum processors, quantum sensors, or single-photon sources, their fluorescence light must be collected. This is typically done with macroscopic optical elements placed around the crystal. For industrial applications, miniaturization of the optical access to the individual NV centers is also necessary.

[0035] To avoid a large refractive index jump between an optical lens and a crystal, solid immersion lenses (SILs) are brought into direct contact with another optical element (here, diamond or a silicon carbide crystal). Alternatively, the air gap can be filled with an oil that has a similar refractive index to the crystal. In both cases, the numerical aperture is increased, allowing a larger proportion of the light emitted from a color center to be captured.

[0036] A new approach is to additively manufacture optical lenses directly on the crystal. When additively manufacturing lenses onto an existing NV center, the lens position must be adjusted with ultra-precision (±200 nm) relative to the NV center. This requires a very accurate determination of the NV center's location, marking of this location, and finally, precise positioning of the additively manufactured lens relative to this mark. 1465-10.24-01 7 / 19 TH Nuremberg

[0037] The object of the invention is to significantly improve the coupling efficiency of light emitted by color centers and to enable a considerably more precise, simpler, and scalable fabrication of spin qubits with integrated optics. Furthermore, the fabrication method claimed herein is intended to enable the production of a miniaturized output optics assembly with the generation of the color center in directly successive process steps, which does not require any relative alignment between the color center and the optics.

[0038] This problem is solved according to the invention by the features of claim 1. For the fabrication of individual color centers, a substrate contaminated or doped with nitrogen atoms, containing or consisting of a diamond or SiC crystal, a fabrication device for micro-optics (multiphoton lithography or direct laser writing), and one or more laser devices for generating a high-energy laser beam are first provided. A microlens system (10) is built up on the surface of the substrate (1). The laser beam (9) is then focused via the microlens system (10) into a region located at a depth t (a few nm to several pm) below the substrate surface (13). There, it creates at least one lattice defect (5) – also called a vacancy center.Since the entire microlens system (10) is fabricated before the generation of a color center (2), a laser beam (9) can be automatically focused to the location where the color center (2) is to be introduced during subsequent generation of a color center (2). This eliminates the need for complex lateral and vertical adjustment steps to position optical elements on an existing color center (2) in the substrate (1) (crystal). In other words, a laser beam (9) focused on a narrowly defined area within the crystal creates exactly one lattice defect as a prerequisite for the formation of a color center (2), with the laser beam (9) being guided through a previously fabricated microlens system. Diffusion processes create pairs consisting of a foreign atom and a defect, which together form a color center. 1465-10.24-01 8 / 19 TH Nuremberg.

[0039] These color centers can be optically stimulated and then emit fluorescent light.

[0040] Further developments of the invention are described in the dependent claims. As a rule, the aforementioned impurities are statistically distributed with foreign atoms in the crystal lattice. Lattice defects (5) cannot therefore be selectively created next to a foreign atom (4). To stimulate and accelerate the formation of color centers (2), it is proposed to thermally anneal the substrate (1), whereby foreign atoms (4) preferably diffuse to a lattice site adjacent to a lattice defect (5). The energy for the annealing can also be locally introduced into the substrate using laser beams. Temperatures above 850 °C are required for this process.

[0041] It is advantageous to use a short-pulse laser, especially an ultrashort-pulse laser, to deliver high amounts of energy to narrowly defined areas. Ultrashort-pulse lasers include lasers that emit pulsed laser light with pulse durations in the picosecond or femtosecond range.

[0042] To detect light rays, they must first leave the diamond and reach a collecting optic. For this, the light typically has to overcome several material or phase boundaries, such as between the diamond substrate and air. In materials with high refractive indices, like diamonds, only those light rays that strike the interface between the diamond and air at angles between 0 degrees and approximately 25 degrees (half an angle) are coupled out of the crystal. At emission angles within this range, the rays subsequently refracted at the interface between the crystal and air form a significantly larger angle with the optical axis and can—depending on the numerical aperture of the collecting optic—just barely be captured by it. For angles greater than 25 degrees, total internal reflection (TIR) ​​occurs, and the rays no longer leave the crystal.Due to the large differences between the refractive indices of diamond (~2.5) and air (~1) and the associated TIR, a maximum of approximately 5% of the fluorescence light could be collected in the case of a smooth diamond surface (1465-10.24-01 9 / 19 TH Nuremberg) and a collecting lens with a high numerical aperture positioned above it. Therefore, according to an advantageous further development of the method, the microlens system (10) incorporates an immersion medium. This immersion medium serves to collect as much fluorescence light as possible from the color center (2) during operation and thus increase its defectivity.

[0043] In principle, an immersion oil is suitable as an immersion medium, but for the present application a solid immersion lens (8) (SIL) is more suitable.

[0044] To increase the collection efficiency for the fluorescence light of the color center, it is proposed to place an optically active element in the form of a solid immersion lens (SIL) directly onto the diamond surface above the color center. A number of geometries are suitable for the SIL, such as a semi-hyperboloid or a Weierstrass lens with a conical constant k, e.g., a hemisphere (k = 0) or a semi-ellipsoid (-1 < k < 0). The most suitable geometries for the specific application can be selected from these options. In principle, the SIL can be made of the substrate material or a different material.

[0045] With a SIL adapted to the position (depth) of the color center in the crystal, light rays emitted at angles greater than 25 degrees from the color center can also be collected by the collecting optics. Geometrically, the upper limit of the collecting efficiency is 50%, which represents a significant improvement compared to a smooth diamond surface.

[0046] When using different materials for the SIL and the substrate, it is possible, for example, to place and integrate a SIL (half-hyperboloid or Weierstrass lens) onto the substrate or to additively build it up as a polymer or ormocer lens onto the surface of the diamond. However, the resulting interfaces can impair the optical path, for example by creating an offset (see 1465-10.24-01 10 / 19 TH Nuremberg). Furthermore, each mounting method requires additional effort for positioning and fastening.

[0047] These limitations can be avoided if the SIL, e.g., in the form of a semi-hyperboloid, is made of the same material as the substrate, e.g., diamond or ray, and is subtractively written into the diamond surface using an ultrashort pulse laser; in this process, the diamond material around the respective hyperboloid is removed. Depending on the application and the distances between adjacent color centers, optically unused substrate areas can be left unprocessed to minimize manufacturing effort.

[0048] To guide the collected light, which emerges from the SIL as a diverging light beam originating from a point source in the substrate (1), it is necessary to convert this beam into a path that is as parallel as possible. A refractive element (11) serving this purpose can be manufactured directly on the substrate (1) by additive manufacturing. A suitable method for manufacturing, e.g., polymer lenses, involves focusing a guided laser beam onto those areas in a transparent polymer material that are to be cured by heat. This method allows for the simple production of individual lenses or multiple refractive elements (11, 12), as well as support structures (14) or spacers (15) between the lenses, in a single operation. The support structures (14) establish a rigid connection between the substrate (1) and the (first) refractive element (11).Excess polymer material can then be removed by means of cutouts. By using several refractive elements (11, 12), the beam path can be controlled even more precisely. The refractive elements (11, 12) together form the aforementioned collimating lens, which serves as the input and output coupling optics (6) for exciting the color center and for receiving the light emitted from it. The diverging light from the color center is converted into a parallel beam path in the output coupling optics (6) (collimation). The light collimated by the input and output coupling optics (6) can then be directed, for example, into a 1465-10.24-01 11 / 19 TH Nuremberg.

[0049] Fiber optic cable can be coupled in or directly imaged onto a detector.

[0050] The number of refractive elements and the materials from which they are made can vary and depend primarily on the requirements of the specific application. The additional refractive elements can also be additively manufactured. Alternatively, pre-assembled lenses can be used. These can also be made of a different material, such as glass. Support structures or spacers used to mount the individual elements can, as mentioned above, also be additively manufactured, including in the form of mounts for the pre-assembled lenses.

[0051] The central idea of ​​the invention described here is to fabricate the entire microlens system (10), consisting of the SIL (8) and the refractive elements (11, 12) above it, prior to the creation of a color center (2), in order to then have the possibility of precisely generating the color center where it is optimally positioned for the input and output coupling optics. Crucially, a parallel ultrashort pulse laser beam can be focused via the previously fabricated microlens system into the diamond or the SiC crystal to generate the color center. The energy of the laser beam causes the formation of a defect (vacancy center) in the crystal lattice at a depth t below the substrate surface (13).

[0052] Advantageously, the microlens system (10) or parts thereof are suitable for optically exciting the color center (2) or for coupling out fluorescence light from the color center (2); the microlens system (10) can therefore be used in both directions. This also eliminates the previously necessary, complex adjustment steps for aligning optical elements with the lateral and vertical position of existing color centers in the crystal. The method according to the invention is particularly suitable for color centers with nitrogen as an impurity atom, so-called NV centers. 1465-10.24-01 12 / 19 TH Nuremberg

[0053] The described method can be used to manufacture, among other things, quantum sensors, quantum computers, especially quantum processors or single-photon emitters, etc.

[0054] Exemplary embodiments of the invention are explained in more detail below with reference to the drawing. The drawing shows:

[0055] Fig. 1 shows an NV center in a diamond grid.

[0056] Fig. 2 a stylized microlens system,

[0057] Fig. 3 a microlens system,

[0058] Fig. 4 shows an example of the beam path with and without an immersion lens.

[0059] Note: Reference numerals with an index and corresponding reference numerals without an index denote identical details in the drawings and the drawing description. This refers to the use of a different embodiment, the prior art, and / or the detail being a variant. For the sake of simplicity, the claims, the introductory description, the list of reference numerals, and the summary contain only reference numerals without an index.

[0060] Fig. 1 shows a nitrogen f-vacancy center 2a in a substrate 1a, in particular a diamond lattice, with carbon atoms 3a arranged at their lattice sites, a lattice defect 5a (vacancy center) in the lattice and a nitrogen fatom 4a on a substitutional lattice site.

[0061] Fig. 2 shows a stylized microlens system 10b, consisting of a coupling optic 6b comprising a refractive element 11b, in particular a converging lens, and an immersion medium 8b in the form of a solid immersion lens (SIL) with a total focal length fge S for imaging an NV center 2b (light source) at a depth t below a substrate surface 13b of a substrate 1b, in particular a diamond crystal, wherein a first collimated laser beam 1465-10.24-01 13 / 19 TH Nuremberg

[0062] The laser beam 9b is focused along an optical axis 7b onto the NV center 2b. The principal plane of the refractive element 11b is at a distance d from the substrate surface 13b. The SIL has the shape of a hemisphere with radius r; here, it is directly formed from the substrate surface 13b by surface structuring using the laser beam 9b. The position of the NV center 2b does not necessarily coincide with the spherical center of the SIL; likewise, the focal length f of the refractive element 11b differs from the overall focal length f. geaThe coupling optics 6b were applied to the SIL using additive manufacturing. The special feature here is that the NV center is only created after the SIL and the additively manufactured coupling optics 6b have been produced. This is achieved by a short-pulse laser, which is coupled into the microlens system 10b parallel to its optical axis 7b as a sequence of one or more laser pulses. The laser beam 9b initially creates only a lattice defect in the diamond lattice. Subsequently, thermal annealing or thermal treatment using the pulsed laser causes a nitrogen atom, previously present as an impurity in the diamond substrate, to diffuse (heal) to a lattice site adjacent to the defect, thereby forming a technically usable NV center. This healing process is also accomplished by the laser beam 9b.Since the solid immersion lens 8b (SIL) is hemispherical or nearly hemispherical in shape, and its center is located near the NV center, all light rays emanating from the color center leave the SIL 8b without or with almost no refraction. This allows significantly more light rays to be captured than would be the case with a flat substrate surface.

[0063] Fig. 3 shows a microlens system 10c consisting of a coupling optic 6c and a SIL 8c, wherein the coupling optic comprises two refractive elements 11c and 12c (lenses). The SIL 8c is applied to the diamond crystal and is shaped as a semi-ellipsoid. Due to the interface between the diamond crystal and the SIL, the beam path is refracted. This is illustrated by an angular displacement of the rays. The greater the difference in the refractive indices of the material pairings, the greater the angular displacement.

Claims

1465-10.24-01 14 / 19 TH Nürnberg The refractive element 11c (dotted line) adjacent to SIL 8c is additively manufactured, while the second refractive element 12c (diagonal hatching) shown here is a mounted glass lens. Support structures 14c, used to fix the first refractive element 11c to the crystal, are only indicated. Spacers 15c, which maintain a distance between the first refractive element 11c and the second refractive element 12c, are also indicated. The support structures 14c and spacers 15c are located outside the optically used area. The support structures 14c and spacers 15c can be rod-shaped, cylindrical, or truss-like. Different refraction of light rays in the center of a lens and in its peripheral regions results in aberrations.The second lens therefore serves to further optimize the extraction and collimation of the fluorescence light (solid black lines) from the NV center 2c. Fig. 4 shows the influence of a solid immersion lens 8d (SIL) on the aperture angle of an optical system. On the left, a solid immersion lens 8d (SIL) is indicated, which was machined from the substrate Id by subtractive processing. Here, the SIL has the same refractive index as the adjacent substrate Id. On the right, no SIL is present. With a SIL, more light rays from a color center 2d enter the coupling optics 6d from a larger aperture angle. Without a SIL, some of the light rays emanating from the light source (color center 2d) in the substrate Id are refracted in such a way that they either bypass the coupling optics 6d or are totally reflected at the substrate surface 13d (totally reflected ray 16d). Thus, a solid immersion lens 8d increases the light efficiency. This fluorescent light is needed for state detection.The more light that can be detected absolutely, the higher the signal-to-noise ratio and the faster a high-quality state determination is possible, which in turn increases the frequency of the application (clock rate of the quantum computer or spatial and temporal resolution of the quantum sensor). 1465-10.24-01 15 / 19 TH Nuremberg Reference symbol list 1 substrate 2 Color center 3 Carbon atoms 4 foreign atom (e.g. nitrogen) 5 grids missing parts 6. Coupling optics 7 optical axis 8 Immersion medium 9 Laser beam 10 microlens system (input / output coupling optics + SIL) 11 (first) refractive element 12 second refractive element 13 Substrate surface 14 Support structure 15 spacers 16 Totally reflected beam 1465-10.24-01 16 / 19 TH Nürnberg Literaturliste 1) Smith, J. M., Meynell, S. A., Jayich, A. C. B. & Meijer, J. Colour centre generation in diamond for quantum technologies. Nanophotonics 8, 1889-1906 (2019) 2) Chen, Y.-C. et al. Laser writing of individual nitrogen-vacancy defects in diamond with near-unity yield. Optica, OPTICA 6, 662- 667 (2019)

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