Methods and apparatus for modifying properties of luminescence centres

Abstract

Methods and apparatus for modifying a property of a luminescence centre is provided. The method involves obtaining a value of the property of the luminescence centre based on a measurement of the luminescence centre and, responsive to the property value failing to satisfy a criterion, heating the luminescence centre such that a thermal energy of the luminescence centre during the heating is less than an energy associated with destruction of the luminescence centre.

Description

METHODS AND APPARATUS FOR MODIFYING PROPERTIES OF LUMINESCENCE CENTRESCross-Reference to Related Application

[0001] This application claims priority from US application No. 63 / 647229 filed 14 May 2024 and entitled METHODS AND APPARATUS FOR MODIFYING PROPERTIES OF LUMINESCENCE CENTRES which is hereby incorporated herein by reference for all purposes. For purposes of the United States of America, this application claims the benefit under 35 U.S.C. §119 of US application No. 63 / 647229 filed 14 May 2024 and entitled METHODS AND APPARATUS FOR MODIFYING PROPERTIES OF LUMINESCENCE CENTRES which is hereby incorporated herein by reference for all purposes.Technical Field

[0002] This disclosure relates to luminescence centres and, in particular, to methods and apparatus for modifying properties of luminescence centres.Background

[0003] Luminescence centres are a promising candidate for storing quantum information. Although existing technologies allow for creating arrays of luminescence centres, current production methods result in a significant variations in the properties of the luminescence centres. This can create challenges for using luminescence centres in quantum devices. For example, it may be challenging to optically address luminescence centres, since the wavelength of the optical transitions of the luminescence centres in the array may vary between different luminescence centres. In addition, the optical transitions of some of the luminescence centres may not be sufficiently bright to allow for reliable manipulation and measurement of the luminescence centres.Summary

[0004] In a first aspect, the present disclosure provides a method for modifying a property of a luminescence centre in a substrate. The method involves: a) obtaining a value of the property of the luminescence centre based on a measurement of the luminescence centre and, b) responsive to the property value failing to satisfy a criterion, heating the luminescence centre such that a thermal energy of the luminescence centre during the heating is less than an energy associated with destruction of the luminescence centre.

[0005] Steps a) and b) may be repeated until the criterion is satisfied or a first maximum number of iterations is reached.

[0006] The value of the property may be based on the measurement of the luminescence centre when illuminated by an optical beam. The measurement of the luminescence centre may include a measurement of an optical intensity of the luminescence centre when illuminated by the optical beam.

[0007] The measurement of the optical intensity of the luminescence centre when illuminated by the optical beam may include a first emission spectrum of the luminescence centre obtained by, for each wavelength in a first range of optical wavelengths, measuring a respective first intensity of the luminescence centre when illuminated by the optical beam having that wavelength.

[0008] The criterion may include a minimum intensity for the emission spectrum. The criterion may include a minimum intensity for a peak in the emission spectrum.

[0009] The criterion may have been determined based on a set of optical intensity measurements obtained by two or more iterations of measuring and heating the luminescence centre such that the thermal energy of the luminescence centre during the heating was less than the energy associated with destruction of the luminescence centre. Each of the set of optical intensity measurements may include a second emission spectrum of the luminescence centre. The second emission spectrum may have been obtained by, for each wavelength in a second range of optical wavelengths, measuring a respective second intensity of the luminescence centre when illuminated by the optical beam having that wavelength. The two or more iterations of measuring and heating the luminescence centre may have comprised iteratively measuring and heating the luminescence centre until a same emission spectrum was obtained twice, or a second maximum iteration number was reached.

[0010] The criterion may include one or more characteristics identifying a target emission spectrum. The one or more characteristics may comprise at least one of: an intensity of the target emission spectrum at a particular wavelength, a wavelength of a maximum intensity of the target emission spectrum, a minimum intensity of the target emission spectrum, a centre wavelength of the target emission spectrum, a linewidth of the target emission spectrum, and a lifetime associated with the target emission spectrum.

[0011] The measurement of the optical intensity of the luminescence centre when illuminated by an optical beam may include a measurement of a number of photons emitted by the luminescence centre when illuminated by the optical beam.

[0012] Heating the luminescence centre may include illuminating the luminescence centre with a first laser. The optical beam may be emitted by a second laser. The second laser may include a tunable laser. The second laser may be different to the first laser. An optical path between the first laser and the luminescence centre may be distinct from an opticalpath between the second laser and the luminescence centre. Heating the luminescence centre may involve heating the luminescence centre with a PIN diode. The luminescence centre may be positioned in an intrinsic region of the PIN diode. Heating the luminescence centre with the PIN diode may involve operating the PIN diode in a forward bias.

[0013] The substrate may comprise a semiconductor substrate. The semiconductor substrate may consist principally of silicon. The luminescence centre may, for example, include a T centre, an I centre or an M centre. The semiconductor substrate may include (e.g. consist principally of) silicon carbide or diamond.

[0014] The luminescence centre may be in an optical cavity in the substrate.

[0015] The substrate may form part of a quantum device.

[0016] In another aspect, a non-transitory processor-readable storage medium is provided. The non-transitory processor-readable storage medium contains instructions which, when executed by a processor, cause the processor to perform the method according to the first aspect. The instructions may be processor-executable. In yet a further aspect, an apparatus configured to perform the method according to the first aspect is provided.

[0017] In another aspect, an apparatus for modifying a property of a luminescence centre in a substrate is provided. The apparatus includes an interface and a processor. The processor is configured to obtain a value of the property of the luminescence centre, and responsive to the property value failing to satisfy a criterion, send instructions, via the interface, to heat the luminescence centre such that a thermal energy of the luminescence centre during the heating is less than an energy associated with destruction of the luminescence centre. The apparatus may be configured to perform the method according to the first aspect.Brief Description of the Drawings

[0018] Embodiments of the disclosure are described with reference to, by way of example only, the following drawings:

[0019] Figure 1 shows emission spectra for an example luminescence centre according to embodiments of the disclosure;

[0020] Figure 2 shows an illustration of an example system according to embodiments of the disclosure;

[0021] Figure 3 shows a flowchart illustrating an example method according to embodiments of the disclosure;

[0022] Figures 4 and 5 show examples of substrates according to embodiments of the disclosure; and

[0023] Figure 6 shows a flowchart illustrating an example method according to embodiments of the disclosure.Detailed Description

[0024] Luminescence centres are defects that have at least one optical transition. Such optical transitions can be used to, for example, excite a luminescence centre from a ground state to an excited state. The luminescence centre will then decay from the excited state to a ground state, potentially via one or more intermediate states. This excitation and subsequent decay are particularly useful for quantum computing and quantum communication, in which the ground states of luminescence centres may be used to store information (e.g. may function as qubits). The existence of an optical transition, in particular a state-selective optical transition, between the ground and excited states of a luminescence centre allows for optical manipulation and / or measurement of qubit(s) associated with the luminescence centre. However, reliably manipulating and / or measuring the state of qubit associated with a luminescence centre using an optical transition requires that that the luminescence centre can be reliably excited to the excited state and that the excited state will reliably decay via that optical transition. In practice, decay from an excited state is often probabilistic. There may be multiple possible decay paths and, in many cases, the optical transition between the excited state and the ground state may not be the most probable decay path.

[0025] A further advantage of luminescence centres is that qubits associated with two different luminescence centres may be entangled via optical manipulation of the luminescence centres. However, the success of existing protocols for creating entanglement between luminescence centres relies on the indistinguishability of photons emitted by the luminescence centres. Although there are techniques fortuning the emission properties of luminescence centres such as Purcell enhancement (which may be used to, for example, improve the Zero Phonon Line, ZPL, efficiency) and the application of a strain, electric field and / or magnetic field for wavelength tuning, these may be insufficient to overcome the large variations in optical properties between luminescence centres.

[0026] Aspects of the present disclosure seek to address these and other problems. In an aspect, a method for modifying a property of a luminescence centre in a substrate is provided. The method involves heating a luminescence centre one or more times until a property of the luminescence centre satisfies a criterion. During heating, the thermal energy of the luminescence centre remains below the energy associated with destruction (or formation) of the luminescence centre. This allows for inducing physical changes thatchange the properties of the luminescence centre, without risking destroying the luminescence centre itself.

[0027] An example of this phenomenon is described with respect to Figure 1 , which shows example emission spectra that were obtained in accordance with methods described herein. The emission spectra are for a T centre in a silicon substrate. A T centre is an example of a luminescence centre for which the methods described herein may be used.

[0028] Figure 1 includes one spectrum for each of eight different runs that were performed in sequence, with each run represented by a different marker. The values for each run include photon counts for wavelengths in the range 1325.90nm to 1326.1 Onm. Each value in a given run was obtained by illuminating the T centre with an optical beam emitted by a tuneable laser at the respective wavelength illustrated in the Figure. Between and during the runs, the silicon substrate hosting the T centre was housed in a cryostat that cooled the substrate to 1 ,5K. Between runs, the T centre was illuminated by a different, heating laser that emitted 20 nJ pulses with a beam diameter of 50 microns to heat the T centre to a temperature below the temperature associated with destruction of the T centre.

[0029] As shown in Figure 1 , the emission spectrum changed significantly between runs: each spectrum has a different peak position, linewidth, and intensity. The emission spectrum for each run was measured after heating was ceased, indicating that this change was persistent after the source of heat was removed.

[0030] Figure 1 thus illustrates how the optical emission properties of a luminescence centre may change after heating the luminescence centre such that the thermal energy of the luminescence centre remains below the energy associated with destruction of the luminescence centre. Optical emission properties are just one example of the properties of luminescence centres that may be modified by this heating. Aspects of the present disclosure make use of this phenomenon to change the properties of luminescence centres such that the change in the property is persistent after heating is ceased. This allows for repeatedly heating a luminescence centre until its property reaches an optimal or sufficient value.

[0031] In addition to these methods, an apparatus is disclosed that is configured to perform the methods disclosed herein. For example, the apparatus may comprise a processor configured to directly perform (or instruct the apparatus to perform) the method steps. The apparatus may be a controller, for example. In yet another aspect, a non-transitory processor-readable medium is provided. The non-transitory processor-readable medium stores processor-executable instructions that, when executed by a processor of an apparatus (such as a controller), cause the apparatus to the perform operations of the methods disclosed herein.

[0032] The present disclosure thus provides methods and apparatus for modifying the properties of luminescence centres by applying low energy heating (i.e. at energies lower than the energy associated with destruction of the luminescence centre) until the desired properties are achieved. One particular implementation of this approach may involve iteratively heating and measuring a luminescence centre until a property of the luminescence centre satisfies a criterion. In each iteration, the measurement value may be used to obtain a value of the property for comparison to the criterion. In this way, iterative heating and measurement can be used to tune the property of the luminescence centre.

[0033] The methods and apparatus disclosed herein are expected to be particularly advantageous forquantum applications. The method may be used, for example, to calibrate luminescence centres in a quantum computer or quantum communication device to improve the probability of optical excitation, the intensity (brightness) of a particular optical transition (that is, the probability of a particular decay path) and / or the indistinguishability of photons emitted by pairs of luminescence centres etc. This can enable more reliable measurement and manipulation of luminescence centres. Improving the indistinguishability of photons emitted by luminescence centres can, in particular, increase the rate of successful entanglement attempts for the quantum computer or quantum communication device. Since entanglement is expected to be a key resource for both quantum computation and quantum communication, this can increase the performance of both quantum computers and quantum communication devices.Example System

[0034] Figure 2 shows an example of a system 200 according to embodiments of the disclosure. The system 200 may be used for quantum computing, quantum communication or quantum sensing. In some embodiments, the system 200 may form part of a larger system that may be used for quantum computing, quantum communication or quantum sensing. Thus, some or all of the system 200 may form part of a quantum device.

[0035] The system 200 includes a luminescence centre 202 in a substrate 204. The substrate 204 may form part of a quantum device, such as a quantum processor, quantum computing, quantum repeater, quantum hub, orquantum client etc.. The substrate 204 may be a crystalline substrate. That is, the substrate 204 may comprise a crystal structure. In some examples, the substrate 204 may comprise a semiconductor or an insulator. The substrate 204 may, for example, comprise silicon, silicon carbide, diamond or any other suitable material. For example, the substrate 204 may be a semiconductor substrate that consists principally of silicon (e.g. is >80% silicon by mass). In some examples, the substrate 204 may comprise natural silicon. In other examples, the substrate 204 maycomprise silicon that has been isotopically purified to silicon-28. That is, the substrate 204 may have been purified to enrich the isotopic concentration of silicon-28. Enriching the isotopic concentration of silicon-28 improves optical inhomogeneities and spin coherence times of T centres in a silicon substrate.

[0036] The luminescence centre 202 is a defect in the substrate 204 that can emit (e.g. can be stimulated to emit) an optical photon. As such, the luminescence centre 202 has at least one optical transition. The term “optical” is used herein to refer to electromagnetic radiation with a wavelength within the range of infrared to ultraviolet (e.g. light having wavelengths in the range of about 200 nm to about 2000 pm). Optical wavelengths include wavelengths within the optical telecommunication bands (O, E, S, C, L and U bands) which collectively span about 1260 nm to 1675 nm. In the present disclosure, luminescence centres may alternatively be referred to as colour centres.

[0037] It will be appreciated that the type of luminescence centre depends on the substrate. Example luminescence centres in silicon include the T centre, the I centre, the M centre and the G centre. Example luminescence centres in silicon carbide include the negatively charged silicon vacancy (Vsr), the neutral divacancy (W°), the carbon vacancy (Vc), vanadium dopant (V) and chromium dopant (Cr). Example luminescence centres in diamond include the NV centre and the group VI vacancy centres. Rare earth dopants, such as erbium, may be comprised in luminescence centres formed in any of a variety substrates.

[0038] In some embodiments, the luminescence centre 202 may comprise an interstitial defect. The interstitial defect may comprise a single interstitial atom or may form a larger interstitial complex. The methods described herein may be particularly advantageous for luminescence centres 202 that comprise interstitial defects because the heating described herein may be used to change the relative position of the interstitial particle in the luminescence centre 202 without destroying the luminescence centre 202.

[0039] In some embodiments, the luminescence centre 202 may be of a type that can have any of a plurality of distinct orientations with respect to the substrate 204. The methods described herein may be particularly advantageous for such luminescence centres 202 because the heating described herein may be used to change the luminescence centre 202 from a first orientation to a second orientation within the substrate 204. This change in orientation may impact one or more other properties of the luminescence centre 202, such as the luminescence centre properties described herein.

[0040] In some embodiments, the luminescence centre 202 may be in an optical cavity (not illustrated) in the substrate 204. The optical cavity may alternatively be referred to as an optical resonator or resonating cavity. The optical cavity may comprise a photonic crystal(e.g. a one- or two-dimensional photonic crystal), a Bragg resonator (e.g. using heterostructures), a partial mirror, or a coated fiber etc.

[0041] Optical cavities have been shown to enhance optical emissions from luminescence centres via the Purcell effect. Aspects of the present disclosure may be particularly advantageous when applied to luminescence centres in optical cavities because the techniques derived herein may improve optical coupling between a luminescence centre and its associated cavity.

[0042] Although the system 200 only includes a single luminescence centre 202, it will be appreciated that in practice the substrate 204 may comprise many more luminescence centres 202. In general, the substrate may comprise one or more luminescence centres 202. Example substrates that comprise more than one luminescence centre 202 are described below with respect to Figures 4 and 5.

[0043] The system 200 also includes an optical source 208, an optical detector 210 and a heater 214.

[0044] The optical source 208 and the optical detector 210 are in optical communication with the luminescence centre 202 via respective optical paths 212a, 212b. The optical paths 212a, 212b may be provided by optical fibres, waveguides, free space etc., or some combination thereof. For example, the optical path 212a may be provided by an optical fibre connecting the optical source 208 to a grating coupler on the substrate (not illustrated) and one or more waveguides connecting the grating coupler to the luminescence centre 202. In some examples, the optical paths 212a, 212b may at least partially overlap. The optical paths 212a, 212b may be reconfigurable e.g. the controller 206 may be configured to send (e.g. via the interface 218) instructions to one or more components (e.g. one or more switches) to create the optical paths 212a, 212b.

[0045] The optical source 208 is configured to illuminate the luminescence centre 202 with an optical beam via the optical path 212a. The optical source 208 may comprise, for example, an optical laser. In some examples, the optical source 208 may comprise a tunable optical laser. That is, the optical source 208 may include a laser with a wavelength that can be changed.

[0046] The detector 210 is configured to measure an optical intensity of the luminescence centre 202. The detector 210 may be integrated with the substrate 204 or may be provided separately. The detector 210 may measure the optical intensity of the luminescence centre 202 by detecting (optical) photons emitted by the luminescence centre 202. The detector 210 may thus comprise a light sensor (a photodetector). In some embodiments, the detector 210 may comprise a single photon detector (a photon counter) such as, for example, a superconducting nanowire single-photon detector (SNSPD), a photomultiplier,a single-photon avalanche counter, a transition edge centre, a charge-coupled device, a scintillation counter etc. Thus, measuring the optical intensity of the luminescence centre 202 may comprise counting a number of photons emitted by the luminescence centre 202.

[0047] The heater 214 is for heating the luminescence centre 202. The heater 214 may comprise a heating laser configured to heat the luminescence centre 202 with an optical beam. The heating laser may be directed towards the luminescence centre 202 e.g. may be in optical communication with the luminescence centre 202. Alternatively, the heating laser may be directed towards the substrate 204 and may indirectly heat the luminescence centre 202 by heating the substrate 204. In some examples, a single laser is used to heat and optically excite the luminescence centre 202. That is, the optical source 208 may also function as the heater 214. In other examples, the heater 214 may be distinct from the optical source 208. For example, the system 200 may comprise two lasers: one laser for use as an optical source and another laser for use as a heater. Using one laser for heating and another for optical excitation may advantageously allow for closely controlling the heating of the luminescence centre 202 whilst also maximizing optical coupling of the excitation laser to the luminescence centre 202.

[0048] Laser heating is more effective when the optical beam produced by the laser is strongly absorbed by the substrate 204. This can be achieved by using an above-bandgap heating laser. That is, in some embodiments, the photon energy of the heating laser may be greater than the bandgap of the substrate 204. Equivalently, the wavelength of the heating laser may be less than the wavelength corresponding to the bandgap of the substrate 204. For example, a heating laser with a wavelength that is shorter than 1100nm may be used for a silicon substrate. In some embodiments, the wavelength of the heating laser may be much less than the wavelength corresponding to the bandgap of the substrate. Generally, higher photon energy (lower wavelength) lasers will result in better absorption and thus better conversion of laser power to heat. In some examples, the heating laser may have a wavelength in the range 400-600nm. For example, the heating laser may have a wavelength of 450nm or 532nm. Additionally or alternatively, other means for increasing absorption of the heating laser by the substrate 204 may be used such as directing the heating laser towards an optical cavity in the substrate 204, and / or placing a material adjacent to the substrate 204 and / orthe luminescence centre 202 that has a higher optical absorption than substrate etc. These other means may be particularly effective when the heater 214 comprises a below bandgap laser e.g. when the heater 214 comprises a laser with a wavelength that is greater than the wavelength corresponding to the bandgap of the substrate 204.

[0049] In examples that use one laser for heating (a heating laser) and another for optical excitation (an excitation laser), the two lasers may use distinct optical paths. For example, each laser may be connected by (e.g. in optical communication with the luminescence centre 202 and / or the substrate 204) a respective optical fibre, a respective grating coupler and respective waveguide(s). As another example, the excitation laser may be in communication with the luminescence centre 202 by an optical fibre, a grating coupler and zero or more (e.g. one or more) waveguides, whilst the heating laser may in communication with the substrate 204 and / or the luminescence centre 202 by a free space optical path. Using distinct optical paths can result in smaller losses than, for example, using a beamsplitter. In other examples, at least part of the optical path 212a may be used by the beam emitted by the heating laser.

[0050] A laser is just one example implementation of the heater 214. Alternative implementations of the heater 214 may include, for example, a resistor proximate to the luminescence centre 202 (e.g. such that supplying electrical current to the resistor heats the resistor and thus the luminescence centre 202), another electromagnetic emitter (e.g. a radio frequency emitter and / or a microwave emitter) or a laser configured to illuminate a photothermal material proximate to the luminescence centre 202.

[0051] In some embodiments, the heater comprises a PIN diode. The luminescence centre 202 may, for example, be positioned in an intrinsic region of a PIN diode. Since a PIN diode may already be provided to, for example, tune luminescence centres using the Stark effect, using a PIN diode to heat the luminescence centre 202 may enable implementing the invention without requiring additional components.

[0052] The luminescence centre 202 may be heated using a PIN diode by operating the PIN diode in a forward bias. Operating the PIN diode in a forward bias causes a current to pass through the intrinsic region of the PIN diode in which the luminescence centre 202 is positioned, thereby heating the luminescence centre 202. The PIN diode may be integrated in the substrate 204. For example, the substrate 204 may comprise a waveguide in which the luminescence centre 202 is embedded. A first side of the waveguide may have been doped with p-type dopants and a second, opposing, side of the waveguide may have been doped with n-type dopants to form a PIN diode with an intrinsic region, between the two sides, in which the luminescence centre 202 is positioned. The waveguide may, for example, comprise one or more nanostructures (e.g. holes) such that the waveguide functions as a photonic crystal.

[0053] In some embodiments, the heater 214 may be omitted and the luminescence centre 202 may be heated by ceasing cooling of the luminescence centre 202. This is discussed in more detail in the “Temperature Control” section below.

[0054] The system 200 also includes a controller 206. The controller 206 comprises a processor 216 and an interface 218. The controller 206 is configured to control one or more components that interface with (e.g. manipulate and / or measure) the luminescence centre 202.

[0055] The processor 216 may, for example, be implemented by one or more general purpose processors that execute instructions that may be stored in a processor-readable medium of the controller 206 (not illustrated). The processor-readable medium may be memory. The processor-readable medium may be non-transitory. The instructions, when executed, may cause the processor 216 to directly perform, or instruct the controller 206 to perform, the operations described herein. In other embodiments, the processor 216 may be implemented using dedicated circuitry, such as a programmed field programmable gate array (FPGA), a graphics processing unit (GPU), or an application specific integrated circuit (ASIC).

[0056] The controller 206 is configured to control the optical source 208, the heater 214 and the detector 210 via the interface 218. For example, the controller 200 may send one or more instructions to the optical source 208, the detector and / or the heater 214 via the interface 218. The interface 218 may, for example, comprise a port for a wired interface (e.g. a physical outlet to which an electrical wire or cable, an optical fibre etc. may be connected) and / or a wireless interface (e.g. a transmitter, receiver and / or transceiver). Although only a single interface 218 is shown in Figure 2, the controller206 may, in general, comprise one or more interfaces 218. In some embodiments, the interface 218 may include separate interfaces for the optical source 208, the heater 214 and the detector 210. For example, the interface 218 may comprise a dedicated interface for the optical source 208, a dedicated interface for the heater 214 and a dedicated interface for the detector 210.

[0057] Although Figure 2 shows only a single controller 206, it will be appreciated that, in some embodiments, the functionality of the controller 206 may be distributed over more than one device. Thus, references to the controller 206 may be understood to refer to one or more devices (entities) that perform the functionality of the controller 206.Example Methods

[0058] Figure 3 shows a flowchart of a method 300 for modifying a property of a luminescence centre in a substrate according to embodiments of the disclosure.

[0059] In the following description of the method 300, the steps are described as being performed by the controller 206, but this is only an example. In general, the method 300 may be performed by any suitable apparatus, which may or may not comprise a controller. The steps of the method 300 are also described with reference to the other components ofthe system 200. This is only an example. In general, the configuration of the system 200 may vary. For example, some of the components of the system 200 may integrated into a single device (e.g. the controller, optical source, detector and / or heater may form part of a single device). As another example, one or more other components may take the place of the components of the system 200.

[0060] At step 302, the controller 206 obtains a value of the property of the luminescence centre 202. The property is based on a measurement of an optical intensity of the luminescence centre 202 when illuminated by an optical beam. Thus, in step 302, the controller 206 may instruct the optical source 208 to illuminate the luminescence centre 208. That is, the controller 206 may send one or more instructions to the optical source 208 via the interface 218 to cause the optical source 208 to direct an optical beam towards the luminescence centre 202. The optical beam may, for example, comprise a plurality of laser pulses.

[0061] The controller 206 may optionally also instruct the detector 210 to wait for a detection (e.g. by sending one or more instructions to the detector 210 via the interface 218). Alternatively, the detector 210 might not need explicit instructions e.g. the detector 210 may be “always on”.

[0062] The illumination of the luminescence centre 202 by the optical beam may excite the luminescence centre 202 to an excited state. The decay of the luminescence centre 202 from this excited state may involve the emission of an optical photon that is detected by the detector 210. The emission of a photon may be recorded as a photon count (i.e. indicating a specific number of detected photons), which is an example of an optical intensity measurement that may be made by the detector 210. The photon count may be recorded as a photon count rate e.g. photon counts per second. In practice, excitation and decay are likely to occur a large number of times, leading to multiple photon counts per second.

[0063] The controller 206 may, in step 302, receive the measurement from the detector 210 and determine the value of the property based on the measurement. Alternatively, the detector 210 (or another intermediate component) may determine the value of the property and send the value to the controller 206. As yet a further alternative, the value of the property may be obtained directly through the measurement e.g. without any additional calculation steps.

[0064] The property may include one or more of:• an (optical) intensity of the luminescence centre 202 when illuminated at a particular wavelength,• a lifetime of an excited state of the luminescence centre 202,• a branching ratio of the luminescence centre 202 between one or more excited states to one or more ground states, and• an autocorrelation of the intensity of the luminescence centre 202.

[0065] In some embodiments, the measurement of the optical intensity of the luminescence centre 202 comprises an emission spectrum of the luminescence centre 202. That is, the value of the property may be determined based on the luminescence centre’s emission spectrum. The emission spectrum may include wavelengths (or an equivalent such as frequencies or energies) and associated intensity values (e.g. photon counts). Examples of properties that may be derived from the emission spectrum of a luminescence centre 202 include, for example, one or more of the following:• a linewidth of an emission line of the luminescence centre 202,• a peak wavelength of the emission spectrum (e.g. indicating a wavelength of an optical transition of the luminescence centre 202),• presence or absence of hyperpolarisation in an emission spectrum of the luminescence centre 202,• an intensity of a peak in the emission spectrum of the luminescence centre 202, and• a ratio of the peak intensity to an intensity of the optical beam (e.g. to an intensity of a laser generating the optical beam).

[0066] The emission spectrum may be obtained using photoluminescence spectroscopy. That is, the emission spectrum may be obtained by measuring how the intensity of the luminescence centre varies when it is illuminated at different optical wavelengths. An example method for obtaining an emission spectrum is described below with respect to Figure 6.

[0067] The above example properties of the luminescence centre 202 are examples of optical properties (e.g. optical emission properties) of the luminescence centre 202. In other examples, the property of the luminescence centre 202 obtained in step 302 might not be an optical property. The property may, for example, be based on one or more optical properties, such as one or more of the aforementioned optical properties. That is, the property may be a derived or composite property determined based on one or more optical properties that are, in turn, based on the measurement of the optical intensity of the luminescence centre 202. For example, the property may comprise a Lande g-factor (referred to herein as a g-factor) of an excited state of the luminescence centre 202, which can be calculated based on an optical property of the luminescence centre 202. The calculation of the g-factor depends on the structure of the luminescence centre 202. A value of the g-factor for a T centre, for example, (or a range of potential values of the g-factor)may be indicated by the presence or absence of hyperpolarisation (multiple peaks) in the emission spectrum of the luminescence centre 202 in the presence of an applied magnetic field. Thus, the g-factor is an example of a property that may be derived from an optical property (e.g. the emission spectrum) of the luminescence centre. In general, the calculation of the g-factor depends on the structure of the luminescence centre 202.

[0068] In step 304, the controller 206 compares a value of the property to a criterion. The criterion may indicate a threshold value for the property. The threshold value may be a minimum value or a maximum value. For example, the criterion may comprise a minimum intensity for the emission spectrum. Thus, if the luminescence centre 202 is not sufficiently bright at any wavelength in the spectrum, the criterion is not satisfied. In another example, the criterion may comprise a maximum lifetime of an excited state of the luminescence centre 202. That is, the criterion may indicate a maximum amount of time for the luminescence centre 202 to radiatively decay after excitation. In some examples, the criterion may relate to a range of values (e.g. an acceptable range). In some examples, the criterion may involve a comparison to a value of the property obtained in a previous iteration of the method 300. For example, the criterion may require that the value of the property is the best (e.g. highest, lowest etc. depending on the property) obtained in two or more iterations of the method 300.

[0069] If the controller determines, in step 304, that criterion is satisfied, the method 300 may end at step 306. That is, the value of the property may be determined to be sufficient or optimized such that no change in the property is required. Thus, the method 300 may end responsive to the criterion being satisfied.

[0070] If the controller determines, in step 304, that criterion is not satisfied, the method 300 may proceed to step 308.

[0071] In some examples, the controller 206 may, in step 304, compare the value of the property to two or more criteria. This may occur when, for example, step 302 involves obtaining values of two or more properties of the luminescence centre 202 and / or when the two or more criteria indicate maximum and minimum values for a particular property. In such examples, the method 300 might only end at step 306 if all of the two or more criteria are satisfied. For example, step 304 may comprise the controller 206 comparing the value of the intensity of the peak of the emission spectrum to a minimum value and comparing the linewidth of the luminescence centre to a maximum value. If the linewidth is too broad (exceeds the maximum) value, the method 300 might not end at step 306 and may instead proceed to step 308.

[0072] Step 308 comprises heating the luminescence centre 202. That is, the controller 206 causes the heater 214 to heat the luminescence centre 202 e.g. by instructing theheater 214. The controller 206 may thus send instructions, via the interface 218, to the heater 214 to heat the luminescence centre 202. In other embodiments, the controller 206 may directly perform the heating itself. For example, the heater 214 and the controller 206 may form a single device.

[0073] During the heating in step 308, the thermal energy of the luminescence centre 202 remains less than an energy associated with destruction (or, equivalently, formation) of the luminescence centre 202. That is, the temperature of the luminescence centre 202 remains below the temperature required to destroy the luminescence centre. In this temperature range, the thermal energy of the substrate 204 is low enough that the luminescence centre 202 is persistent (e.g. its constituents do not dissociate). This temperature range is specific not only to the type (composition, structure) of the substrate but also to the type of luminescence centre 202. Heating at these temperatures has been shown to cause persistent changes to the properties of the luminescence centre 202. That is, the heating in step 308 may cause a change in a value of the property of the luminescence centre 202 that persists after the heating is ceased e.g. after the luminescence centre 202 is allowed to cool. This change in the property of the luminescence centre 202 does not degrade after the source of heat is removed e.g. further heating is not required to sustain the change in property.

[0074] In some examples, the temperature of the luminescence centre 202 may be kept far below the temperature associated with destruction of the luminescence centre 202 during heating in step 308. For a T centre, for example, the heating may be such that the temperature of the luminescence centre 202 is kept below 500K (e.g. 400K, 300K, 200K, 100K or 50K) whilst still causing a change in a value of a property. In practice, the exact temperature of the luminescence centre 202 during step 308 may not be known. Instead, the heater 214 may be configured to supply heat to the luminescence centre 202 such that the temperature of the luminescence centre 202 is expected to remain below the temperature associated with destruction of the luminescence centre 202.

[0075] In some examples, the temperature of the luminescence centre 202 during heating may be below the temperature associated with destruction of the luminescence centre 202 but higher than a temperature associated with destruction of another type of defect (e.g. another type of centre) that may form in the substrate 204. For example, step 308 may involve heating a T centre in a silicon substrate to a temperature that is below the temperature associated with destruction of the T centre but above the temperature(s) associated with destruction of one or more other silicon defects such as W or G centres. In this way, the heating in step 308 may initiate destruction (or formation) of other types of defect in the substrate 204, which may influence the luminescence centre 204. Thus, forexample, healing other defects in the substrate 204 may affect one or more properties of the luminescence centre 204.

[0076] In some examples in which the heater 214 comprises a laser, step 308 may involve illuminating the luminescence centre 202 with one or more laser pulses. The duration, power, and number of pulses may have been determined to maintain the luminescence centre 202 below the temperature associated with destruction of the luminescence centre 202.

[0077] During heating in step 308, the luminescence centre 202 may be reconfigured. That is, the heating may cause the configuration of the luminescence centre 202 to change. This reconfiguration may be persistent e.g. the configuration of the luminescence centre 202 after heating has ceased may be different to the configuration of the luminescence centre 202 before heating began. Reconfiguration may involve, for example, a change in position of a constituent of the luminescence centre 202 and / or a change in the orientation of the luminescence centre 202 (e.g. relative to the substrate 204).

[0078] The luminescence centre 202 may comprise one or more constituents such as, for example, a vacancy, an interstitial particle (e.g. an interstitial atom) and / or a substitutional particle (e.g. a substitutional atom). Thus, a change in the position of a constituent of the luminescence centre 202 during the heating in step 308 may comprise a change in a position of one or more of: a vacancy, an interstitial particle and a substitutional particle in the luminescence centre 202. The heating in step 308 may thus allow a constituent of the luminescence centre 202 to move within the substrate 204 whilst still remaining part of the luminescence centre 202.

[0079] The change in the constituent’s position may involve a local change in position of the luminescence centre 202 e.g. a change in position that requires less energy than the energy required remove the constituent from the luminescence centre 202. In some examples, the change in the position might not significantly change the geometry of the luminescence centre 202. For a luminescence centre 202 with multiple constituents, for example, a local change in the position of a constituent may involve the constituent moving by a distance that is less than the minimum distance between constituents of the luminescence centre 202.

[0080] Reorientation of the luminescence centre 202 may involve the luminescence centre 202 changing from a first orientation with respect to the substrate to a second orientation with respect to the substrate 204. This may be illustrated by considering the T centre as an example. A T centre is a luminescence centre that includes a carbon-carbon pair sharing a substitutional site in a silicon substrate. One of the substitutional carbon atoms is bonded with a hydrogen atom while the other carbon atom contains an unpaired electron in theground state of a dangling bond. Within a silicon substrate, the T centre has 24 symmetry elements corresponding to 24 possible orientations. Heating a T centre to a temperature below the temperature associated with destruction of the T centre may result in reorienting the T centre from one of these orientations to another.

[0081] Reconfiguration may, additionally or alternatively, involve reconfiguration of a bond between constituents of the luminescence centre 202.

[0082] After heating the luminescence centre 202 in step 308, the method returns to step 302 in which the controller 206 obtains another value of the property based on a new measurement of the luminescence centre 202. The controller 206 then compares the other (second) value of the property to the criterion in step 304 to determine whether further heating is required. Iterations of the method 300 may continue until the criterion is satisfied. In each iteration of the method 300, the luminescence centre 202 is measured again to obtain a new, updated value of the property in step 302. In this way, the luminescence centre 202 may be heated one or more times until a value of the property is optimized or sufficient (e.g. to meet operating requirements).Calibrating Heating

[0083] As described above, during heating of the luminescence centre 202 in step 308, the thermal energy of the luminescence centre remains less than an energy associated with destruction. In some embodiments, the heating of the luminescence centre 202 in step 308 may be calibrated empirically (e.g. rather than being based on a known temperature or energy associated with destruction during heating).

[0084] As described above in respect of Figure 1 , when a luminescence centre is heated at temperatures lower than the temperature associated with its destruction, an optical property of the luminescence centre may change after each round of heating. It has been observed that this process exhibits a periodicity, in which the same property (e.g. the same emission spectrum) can be achieved again through further rounds of heating. This periodicity indicates that the luminescence centre is persistent (has not been destroyed) throughout the rounds of heating. As such, in some embodiments, the controller 206 may configure the heating applied in step 308 (e.g. the configuration of the heater 214 used in step 308) based on whether a same property (e.g. optical emission property such as an optical emission spectrum) may be obtained after successively performing steps 302 and steps 308 of the method 300. If the property does not change from one iteration of steps 302 and 308 to the next, it may be determined that the heating applied in step 308 was insufficient to induce a persistent change in the luminescence centre 202 or substrate 204. Thus, the controller 206 may determine to increase the heating applied in the step 308when performing the method 300. If a property of the luminescence centre 202 changes from one iteration to the next, but no periodicity is observed (e.g. the same value of a property is not observed more than once after multiple iterations of the method 300), it may be determined that too much heat was supplied in step 308. As such, it may be determined to supply less heat in step 308 when performing the method 300 (e.g. for another luminescence centre of the same type). In this way, the heating of the luminescence centre 202 in step 308 may be calibrated empirically.Alternative Termination Criterion

[0085] In the description of the method 300 above, the method 300 continues until a criterion relating to a value of a property is satisfied in step 304. In some embodiments, the method 300 may terminate after a maximum number of iterations is reached. The maximum number of iterations may be, for example, 5, 10, 15, 20, 25, 30, etc.Multiple Luminescence Centres

[0086] In the aforementioned description, the method 300 is described with respect to a single luminescence centre 202. In practice, the substrate 204 is likely to contain more than one luminescence centre 202. Thus, the controller 206 may perform the method 300 for one or more luminescence centres 202 in the substrate 204. The controller 26 may perform the method 300 for different luminescence centres 202 sequentially (in series) or in parallel (e.g. simultaneously). For example, the controller 206 may perform the method 300 for a first luminescence centre (e.g. the luminescence centre 202) in a plurality of luminescence centres before performing the method 300 for a second luminescence centre in a plurality of luminescence centres. The same criterion may be used for all of the luminescence centres. Alternatively, a first criterion may be used for a first luminescence centre and a second criterion may be used for a second luminescence centre. This may be particularly appropriate when, for example, a quantum information device (e.g. quantum computer or quantum communication device) is required to meet a particular performance metric for only a fraction of its qubits.

[0087] Figure 4 shows an example of a substrate 404 that comprises a plurality of luminescence centres 202. The substrate 404 may be the same as the substrate 204 except that it contains more than one luminescence centre. Each of the luminescence centres 202 may be the same type of luminescence centre (e.g. the substrate 404 may comprise T centres and no other type of luminescence centre) or the substrate 404 may comprise different types of luminescence centres. In some examples, each of the luminescence centres 202 in the substrate 204 may be associated with a respective optical cavity (not shown). In Figure 4, the luminescence centres 202 are arranged in a repeatingtwo-dimensional pattern. In general, the luminescence centres 202 may be in any suitable arrangement. The controller 206 may, for example, perform the method 300 in respect of each of the luminescence centres 202. The controller 206 may perform the method 300 for each luminescence centre 202 in sequence (in series) or in parallel (e.g. simultaneously). For example, beamsplitters could be used to split the optical beam used in step 302 into a plurality of beams, such that each of the plurality of beams may be used to illuminate a different luminescence centre 202. The system 200 may comprise a respective detector 210 for each luminescence centre 202, enabling the optical intensity of multiple luminescence centres 202 to be measured in parallel.

[0088] To enable the method 300 to be performed in sequence for multiple luminescence centres 202, the heater 214 may be configured to heat a first luminescence centre 202 in the plurality of luminescence centres 202 without heating a second luminescence centre 202 in the plurality of luminescence centres 202. This may be referred to as locally heating the luminescence centres 202. Local heating can be achieved through, for example, using a heating laser with a beam size (e.g. beam diameter) selected based on the distance between luminescence centres 202. Locally heating the luminescence centres 202 may also be advantageous when performing the method 300 in parallel for multiple luminescence centres 202 because different luminescence centres 202 may satisfy the criterion in step 304 after different numbers of iterations. That is, more iterations of the method 300 may be required for some luminescence centres 202 than others. Local heating may also be advantageous for a single luminescence centre 202 on a substrate 504 since it may allow for performing the method 300 in respect of the luminescence centre 202 without risking damaging components that are proximate to the luminescence centre 202 (e.g. other components on and / or in the substrate 204).

[0089] Local heating may alternatively be achieved by using a first type of heater to heat both the first luminescence centre 202 and the second luminescence centre 202 to a temperature (or, equivalently, thermal energy) at which no persistent change in the property of the first and second luminescence centres occurs, and using a second type of heater to selectively heat one of the first and second luminescence centres 202 to a temperature at which a persistent change in the property occurs. The first and second types of heater may be any suitable heaters, such as those discussed above. Implementing local heating using a first type of heater and a second type of heater may be particularly advantageous when one instance of the first type of heater is used to heat both the first and second luminescence centres, or when respective instances of the first type of heater for the first and second luminescence centres are not individually controllable. For example, the first and second luminescence centres 202 may be in respective PIN diodes that share a controlline. The PIN diodes may be jointly controlled to heat the first and second luminescence centres 202 to a first temperature at which no persistent change in the property of the first and second luminescence centres 202 occurs. At the same time, another type of heater, such as a laser, may be used to selectively heat one of the first luminescence centre 202 and the second luminescence centre 202 to a higher temperature at which a persistent change occurs. This may enable, for example, locally heating the luminescence centres 202 using a lower power laser (e.g. a below bandgap laser) and / or shorter laser pulses. The laser may be the same laser used for excitation of the luminescence centre, for example.

[0090] Figure 5 shows another example of a substrate 504 that contains a plurality of luminescence centres 202. The substrate 504 may be the same as the substrate 204 except that it contains more than one luminescence centre. In this example, the luminescence centres 202 are clustered into groups 506 of the luminescence centres 202. Although eighteen groups are shown in Figure 4, in general the substrate 404 may comprise one or more (e.g. two or more) groups 506.

[0091] In Figure 5, the groups 506 of luminescence centres 202 are arranged in a repeating two-dimensional pattern and each group 506 contains three luminescence centres 202. In general, each group 506 may contain one or more (e.g. two or more) luminescence centres 202 and the groups 506 may be arranged in any suitable way. Each group 506 may contain the same type of luminescence centre 202. Different groups 506 may contain different types orthe same type of luminescence centre 202. In some examples, the luminescence centres 202 in each group 506 may share an optical cavity, for example. That is, one optical cavity may be provided for each group.

[0092] It may be particularly advantageous to cluster luminescence centres 202 into one or more groups 506 as shown in Figure 5 because it can be challenging to fabricate luminescence centres 202 with the desired optical properties. Even with the application of techniques described herein, it might not be possible to achieve the desired or required optical properties for each luminescence centre 202. Clustering the luminescence centres 506 into groups 506 increases the chances that at least one luminescence centre 202 in each group 506 will be able to achieve the desired or required optical properties using the techniques described herein.

[0093] In some embodiments, the method 300 may be performed for each of the groups 506 of luminescence centres 202. The controller 206 may, for example, perform the method 300 for groups of luminescence centres 202 in sequence (in series) or in parallel (e.g. simultaneously). In step 302, for example, the optical intensity of a group 506 of luminescence centres 202 may be measured when the group 506 is illuminated by anoptical beam. The value of the property obtained in step 302 may be forthe group 506 (e.g. ratherthan for an individual luminescence centre 202 within the group 506). The beam size (e.g. beam diameter) of the optical source 208 may be based on the size of the group 506 and the distance between groups 506. For example, the beam diameter may be larger than the size of the group 506 and less than twice the distance between neighbouring groups 506. Step 308 may involve heating the group 506 of luminescence centres 202. This may involve locally heating each group 506 of luminescence centres 202 e.g. heating a first group 506 in the plurality of groups 506 without heating a second group 506 in the plurality of groups 506.Optical Cavities

[0094] When a luminescence centre is incorporated in an optical cavity with a resonant wavelength that corresponds to a wavelength of a particular transition of the luminescence centre, the emission efficiency of that particular transition is enhanced through the Purcell enhancement. In some embodiments, the luminescence centre 202 may be in an optical cavity and the method 300 may be used to tune the wavelength of a particular transition of the luminescence centre 202 to correspond to a resonant wavelength of the optical cavity. This may be achieved by, for example, using a criterion in step 304 that relates to a resonant wavelength (or equivalently, resonant frequency) of the optical cavity.

[0095] In practice, the optical cavity may have one or more resonant wavelengths, each of which may be tunable through a variety of means. Thus, the cavity may have a range of achievable resonant wavelengths e.g. potential resonant wavelengths that the cavity could be tuned to have. Thus, in some examples, the criterion in step 304 may relate to a range of achievable resonant wavelengths for the optical cavity. For example, the criterion may specify that the peak wavelength of the emission spectrum of the luminescence centre 202 must fall within the range of achievable resonant wavelengths for the optical cavity.Tunable Emission Spectrum

[0096] The emission spectrum of a luminescence centre 202 may be tunable through other techniques such as, for example, the application of a strain, electric field and / or magnetic field to the luminescence centre 202. As a result, after the method 300 has been performed for the luminescence centre 202, the emission spectrum of the luminescence centre 202 may be further modified using one or more (other) spectrum tuning techniques such as the application of a strain, electric field and / or magnetic field to the luminescence centre 202.

[0097] In some embodiments, the criterion in step 304 may account for the extent to which the emission spectrum of the luminescence centre 202 may be tuned after the method 300 has been performed. In some examples, the criterion may relate to a range of achievablepeak wavelengths of the emission spectrum of the luminescence centre 202, in which the range of achievable peak wavelengths of the emission spectrum of the luminescence centre 202 is determined based on the peak wavelength of the emission spectrum (e.g. as measured in step 302) and the change in the peak wavelength that can be achieved through emission spectrum tuning (e.g. through application of an electric field, magnetic field and / or strain). The change in the peak wavelength that can be achieved through emission spectrum tuning may be a predetermined value e.g. may be determined through theory or prior experimentation and stored for retrieval during the determination of the criterion. For example, the criterion may specify that the range of achievable peak wavelengths of the emission spectrum of the luminescence centre 202 must overlap with the range of achievable resonant wavelengths for the optical cavity.Criterion Determination

[0098] Prior to heating a luminescence centre 202, it might not be possible to predict how heating may change a value of a property of the luminescence centre 202. For example, the property value might not continue to improve with each round of heating. Indeed, in the example described with respect to Figure 1 , the emission spectrum peaked at run 3, with lower intensities obtained for all subsequent runs. In addition, different luminescence centres behave differently e.g. have different peaks in their emission spectra (both in wavelength and intensity), cycle through spectra differently etc. This means that, when iterating through the method 300, it might not be possible to determine whether re-heating the luminescence centre 202 may further improve the property value or whether the value of the property value has already been optimized.

[0099] In seeking to address this and other problems, in some embodiments, a set of (optical) intensity measurements for the luminescence centre 202 may be used to determine the criterion used in step 304. These intensity measurements may be referred to as preliminary intensity measurements and may indicate the range of intensities that may be achievable for the luminescence centre 202. It has been observed that, although the properties of luminescence centres do not necessarily improve with each round of heating, the same property can be achieved again through further rounds of heating. That is, there is some periodicity to the process. This periodicity might not be predictable e.g. it might not be apparent how many rounds of heating may be needed in orderto obtain a property value for a second time. The periodicity might not be sequential e.g. the first three iterations of heating may result in values A, B, C and the next three iterations may result in values C, B, A. Despite this, the periodicity may be used to initially identify a target value for the property and then iteratively heat the luminescence centre until that target value is achieved again.

[0100] In the following description, the controller 206 obtains the preliminary intensity measurements and determines the criterion. In general, this need not be the case. One or more other entities (devices) may obtain the preliminary intensity measurements and / or determine the criterion.

[0101] The controller 206 may obtain the preliminary intensity measurements by iteratively heating and measuring the luminescence centre 202. That is, the preliminary intensity measurements may comprise a plurality of intensity measurements obtained by illuminating the luminescence centre 202 with an optical beam (e.g. from the optical source 208), measuring the intensity of the luminescence centre 202 (e.g. using the detector 210) and then heating the luminescence centre 202 (e.g. with the heater 214) before repeating the illumination and measurement steps. The illumination, measurement and heating may be performed as described above in the method 300.

[0102] The preliminary intensity measurements may comprise intensities measured at a single wavelength. Alternatively, each of the set of intensity measurements may comprise an emission spectrum. The emission spectrum may be obtained in accordance with the method 600 described below, for example.

[0103] The controller 206 may determine the criterion based on the preliminary intensity measurements by identifying a target intensity, or a target emission spectrum, to be achieved.

[0104] For example, the controller 206 may obtain a set of 10 intensity measurements at an illumination wavelength of 1326nm by repeatedly heating and measuring the luminescence centre 202. The measurement values may be in the range of 5000 photon counts per second to 40,000 photon counts per second. The controller 202 may identify the maximum intensity measurement value of 40,000 photon counts per second as the target intensity. Since measurement values may vary within a tolerance, the controller 206 may set the criterion to require that the measured intensity exceeds a particular fraction of the target intensity value e.g. 70, 80 or 90% of the target intensity value. Thus, the controller 206 may determine that the criterion is satisfied in step 304 when the intensity of the luminescence centre 202 is at least 80% of the maximum intensity measurement value (40,000 photon counts per second) obtained from a set of preliminary intensity measurements. Thus, the criterion used in step 304 of the method 300 may be based on a target intensity identified from a set of intensity measurements.

[0105] In some embodiments, the criterion may be based on one or more characteristics that identify a target emission spectrum. That is, a target emission spectrum may be identified from the preliminary intensity measurements and the method 300 may be performed until the target emission spectrum is obtained again (e.g. is obtained twice). Thecontroller 206 may identify when the target emission spectrum is obtained again using a characteristic that identifies the target emission spectrum. That is, the characteristic is a feature of the target emission spectrum that distinguishes the target emission spectrum from the other emission spectra that may be produced by the luminescence centre 202. The characteristic may comprise one or more of: an intensity (e.g. a particular intensity) of the target emission spectrum at a particular wavelength, a wavelength of a maximum intensity of the target emission spectrum, a minimum intensity of the target emission spectrum, a centre wavelength of the target emission spectrum (e.g. a centre of a full width half maximum of the target emission spectrum), a linewidth ofthe target emission spectrum, a lifetime associated with the target emission spectrum, and any other suitable characteristic of the target emission spectrum. It may be particularly advantageous to characterize the target emission spectrum by the intensity at a particular wavelength because this may allow the target emission spectrum to be identified by a measurement at a singular wavelength.

[0106] Returning to the emission spectra shown in Figure 1 as an example, the controller 206 may obtain the emission spectra for runs 1 -8 as preliminary intensity measurements. The controller 206 may identify the particular spectrum from run 3 as the target emission spectrum because it has the highest peak. The controller 206 may determine that the emission spectrum from run 3 is characterized by (e.g. is distinguished from the emission spectra of the other runs by) having an intensity above 25,000 counts per second at a wavelength of 1325.9875nm. Thus, the controller206 may determine the criterion to require that the luminescence centre 202 has an intensity of at least 25,000 photon counts per second at a wavelength of 1325.9875nm.Temperature Control

[0107] It will be appreciated that, depending on the substrate 204 and / or the luminescence centre 202, the temperature associated with destruction of the luminescence centre 202 may be below room temperature. As such, temperature control may be required to maintain the temperature of the luminescence centre 202 below the temperature associated with destruction of the luminescence centre 202 during the method 300. In some embodiments, the system 200 may further comprise a cooler (not illustrated) that cools the luminescence centre 202 (and optionally the substrate 204) to a temperature below the temperature associated with destruction of the luminescence centre. The cooler may comprise, for example, a cryostat, laser cooling apparatus etc. The cooler may be controlled by the controller 206 to perform the functionality ofthe cooler described herein. Thus, for example, the controller 206 may send instructions to the cooler via the interface 218.

[0108] In embodiments in which step 308 of the method 300 involves heating the luminescence centre 202 to above a minimum thermal energy (and an associated minimum temperature), the cooler may cool the luminescence centre 202 (and optionally the substrate 204) such that such that the temperature of the luminescence centre 202 only exceeds the minimum temperature during step 308. After the method 300 ends in step 306, the cooler may maintain the thermal energy of the luminescence centre 204 below the minimum thermal energy. This may ensure that, for example, the luminescence centre 202 retains its desired property during operation of a quantum computer, quantum communication device or quantum sensor comprising the substrate 204 and the luminescence centre 202.

[0109] For example, in embodiments in which the luminescence centre 202 comprises a T centre, the substrate 204 may be cooled to temperatures below 2K (e.g. 1.5K), 4K, 5K, 10K, 15K or 20K during the method 300. The substrate 204 may be cooled by housing the substrate 204 in a cryostat, for example.

[0110] In some embodiments, step 308 (the heating step) of the method 300 may involve temporarily halting cooling of the luminescence centre 202 by the cooler. This may be instead of, or in addition to, actively heating the luminescence centre 202.Other Environmental Controls

[0111] Depending on the type of luminescence centre 202, other aspects of the environment of the luminescence centre 202 and / or the substrate 204 may controlled during the method 300. This may be in addition to or instead of the temperature control discussed above. For example, some luminescence centres only have state-selective optical transitions under the application of an applied magnetic field. The T centre is a luminescence centre with this property, as it exhibits a spin-selective optical transition between ground and excited states in the presence of a magnetic field. Thus, the method 300 may be performed for a T centre when the T centre is held in a magnetic field. The strength of the magnetic field may be configured according to the broadening of the optical transition for the T centre in question. Thus, for example, the magnetic field may have a field strength at or around 10mT, 10OmT, 1 T, 10T etc. In some examples, the environmental controls may maintain an oxygen-free environment, such as a vacuum, in which the luminescence centre 202 is located during the method 300. In general, the method 300 may, in some embodiments, involve controlling one or more aspects of the environment of the luminescence centre 202 to allow the value of the property to be measured in step 302.Alternatives to Optical Intensity Measurements

[0112] In the above description of the method 300, a value of a property of the luminescence centre 202 is obtained in step 302 based on an optical intensity measurement of the luminescence centre 202 when illuminated by an optical beam. Using optical intensity measurements may be particularly advantageous for quantum devices because the optical source 208 and detector 210 used to measure the optical intensity of the luminescence centre 202 may also be used during operation of the quantum device. That is, the optical source 208 and detector 210 may be used for calibration of the luminescence centre 202 (using the method 300) and also for manipulation and / or measurement of the luminescence centre 202 at runtime.

[0113] In some embodiments, the value of the property of the luminescence centre 202 obtained in step 302 might not be based on an optical intensity measurement of the luminescence centre 202. For example, the value of the property of the luminescence centre 202 may be based on a measurement of a polarization of one or more (optical) photons emitted by the luminescence centre 202 when illuminated by an optical beam (e.g. provided the optical source 208). This measurement may be used to obtain, in step 302, a value of an optical dipole orientation of the luminescence centre 202. Thus, the method 300 may be modified to, in step 302, obtain a value of the optical dipole orientation of the defect based on a measurement of a polarization of a photon (an optical photon) emitted by the luminescence centre 202 when illuminated by an optical beam. This may be instead of, or in addition to, obtaining a value of a property based on an intensity of the luminescence centre 202.

[0114] As another example, the value of the property of the luminescence centre 202 may be based on an electrical measurement, such as a measurement of a charge in the substrate 204. For some luminescence centres 204, the presence or absence of a charge (such as a hole or an electron) may be correlated with a property of the luminescence centre 202. Thus, step 302 may involve obtaining, based on a measurement of a charge in the substrate 204 (e.g. in the luminescence centre 202), a value of the property of the luminescence centre 202. The charge may be measured using a charge sensor such as a single electron transistor. For some luminescence centres 202, the presence of the charge might require optical excitation. As such, the electrical measurement might be performed whilst the luminescence centre 202 is illuminated by an optical beam. Electrical measurement may be particularly advantageous because electrical readout typically has a higher signal to noise ratio than optical readout, enabling more efficient measurements. This approach may also be easier to scale to larger numbers of luminescence centres.

[0115] The skilled person will appreciate that there are various types of measurements that may be used to obtain a value of a property in step 302 such as, for example, emission intensity measurements (such as optical intensity measurements), polarization measurements, electrical measurements (such as charge measurements), electron spin resonance measurements (e.g. to measure spin properties) etc. As such, the system 200 may, in general, comprise a measurement apparatus for measuring the luminescence centre 202. One or more of the optical source 208 and the detector 210 may form (part of) the measurement apparatus, or one or both of the optical source 208 and the detector 210 may be omitted from the system 200.Obtaining an Emission Spectrum

[0116] Figure s shows a method 600 for obtaining an emission spectrum of a luminescence centre. The method 600 may be used to obtain one or more of the emission spectra referred to above in respect of the method 300.

[0117] In the method 600, the steps are described as being performed by the controller 206, but this is only an example. In general, the method 600 may be performed any suitable apparatus, which may or may not be a controller. The steps of the method 600 are also described with reference to the other components of the system 200. This is only an example. In general, the configuration of the system 200 may vary. For example, some of the components of the system 200 may integrated into a single device (e.g. the controller, optical source, detector and / or heater may form part of a single device). As another example, one or more other components may take the place of the components of the system 200.

[0118] In step 602, the controller206 causes the luminescence centre 202 to be illuminated with an optical beam of a particular wavelength in a range of optical wavelengths. That is, the controller 206 may send, via the interface 218, one or more instructions to the optical source 208 to cause the optical source 208 to direct an optical beam of that particular wavelength towards the luminescence centre 202. The optical source 208 may be, for example, a tunable laser with an operating range that includes the range of optical wavelengths.

[0119] In step 604, the controller 206 measures the intensity of the luminescence centre 202. Measuring the intensity may involve instructing the detector 210 to wait for a detection and receiving an intensity measurement from the detector 210. Alternatively, the controller 206 may receive the intensity measurement without sending any instructions to the detector 210.

[0120] In step 606, the controller 206 stores the intensity measurement and the associated wavelength in a processor-readable medium (e.g. a memory). The processor-readable medium may form part of the controller 206 (e.g. may be a same or different to the processor-readable medium that stores instructions executed by the processor 216) or may be distinct.

[0121] In step 608, the controller 206 determines whether or not measurements have been obtained for each wavelength in the range of wavelengths. If measurements have been obtained for each wavelength in the range of wavelengths, the method 600 may end in step 610. If measurements have not been obtained for each wavelength in the range of wavelengths, the method 600 proceeds to step 612.

[0122] In step 612, the controller 206 changes the wavelength of the optical source 208 to a second wavelength in the range of optical wavelengths. That is, the controller 208 may send instructions to the optical source 208 to change the wavelength of its optical beam. The method 600 then returns to step 602, in which the controller 206 illuminates the luminescence centre 202 with an optical beam with the second wavelength. In this way, the controller 206 may obtain an emission spectrum for the luminescence centre 202.

[0123] In some embodiments, steps 602 and 612 may be performed in a single step. For example, the one or more instructions sent by the controller 206 in step 602 may indicate the wavelength of the optical beam.Other Defects

[0124] Although the present disclosure has focussed on modifying the properties of luminescence centres, it will be appreciated that the methods and apparatus described herein may be adapted for other defects, such as defects that emit at other wavelengths (e.g. at microwave wavelengths). In general, the methods and apparatus described herein (such as, for example, the system 200 and / or the method 300) may be applied for any quantum emitter that comprises a defect in a (crystalline) substrate. The methods and apparatus described herein may be adapted depending on the properties of the defect. For example, the value of the property obtained in step 302 may be based on a measurement of an intensity of the luminescence centre when illuminated by a beam with a wavelength corresponding to a transition of the defect. The methods and apparatus described herein may be particularly advantageous for interstitial defects, for example, or for a defect in a substrate that has a plurality of distinct orientations with respect to the substrate.Concluding Remarks

[0125] It should be noted that the above-mentioned examples illustrate ratherthan limit the disclosure, and that those skilled in the art will be able to design many alternative exampleswithout departing from the scope of the appended statements. Moreover, those skilled in the art will be able to combine features from different examples in the disclosure, including the statements below, even when a particular combination is not explicitly recited. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several units, and the function of a single processor or unit may be distributed across multiple processors or other units. Where the terms, “first”, “second” etc. are used they are to be understood merely as labels for the convenient identification of a particular feature. In particular, they are not to be interpreted as describing the first or the second feature of a plurality of such features (i.e. the first or second of such features to occur in time or space) unless explicitly stated otherwise. Steps in the methods disclosed herein, including in the following statements, may be carried out in any order unless expressly otherwise stated.

Claims

CLAIMS1 . A method for modifying a property of a luminescence centre in a substrate, the method comprising: a) obtaining a value of the property of the luminescence centre based on a measurement of the luminescence centre; and b) responsive to the property value failing to satisfy a criterion, heating the luminescence centre such that a thermal energy of the luminescence centre during the heating is less than an energy associated with destruction of the luminescence centre.

2. The method of claim 1 , wherein steps a) and b) are repeated until the criterion is satisfied or a first maximum number of iterations is reached.

3. The method of claim 1 or claim 2, wherein the value of the property is based on the measurement of the luminescence centre when illuminated by an optical beam.

4. The method of claim 3, wherein the measurement of the luminescence centre comprises a measurement of an optical intensity of the luminescence centre when illuminated by the optical beam.

5. The method of claim 4, wherein the measurement of the optical intensity of the luminescence centre when illuminated by the optical beam comprises: a first emission spectrum of the luminescence centre obtained by, for each wavelength in a first range of optical wavelengths, measuring a respective first intensity of the luminescence centre when illuminated by the optical beam having that wavelength.

6. The method of claim 5, wherein the criterion comprises a minimum intensity for the emission spectrum.

7. The method of claim 6, wherein the criterion comprises a minimum intensity for a peak in the emission spectrum.

8. The method of any one of claims 4-7, wherein the criterion had been determined based on a set of optical intensity measurements obtained by two or more iterations of measuring and heating the luminescence centre such that the thermal energy of the luminescence centre during the heating was less than the energy associated with destruction of the luminescence centre.

9. The method of claim 8, wherein, each of the set of optical intensity measurements comprises: a second emission spectrum of the luminescence centre obtained by, for each wavelength in a second range of optical wavelengths, measuring a respective second intensity of the luminescence centre when illuminated by the optical beam having that wavelength.

10. The method of claim 9, wherein the two or more iterations of measuring and heating the luminescence centre comprised iteratively measuring and heating the luminescence centre until: a same emission spectrum was obtained twice; or a second maximum iteration number was reached.

11. The method of claim 9 or claim 10, wherein the criterion comprises one or more characteristics identifying a target emission spectrum.

12. The method of claim 11 , wherein the one or more characteristics comprise at least one of: an intensity of the target emission spectrum at a particular wavelength; a wavelength of a maximum intensity of the target emission spectrum; a minimum intensity of the target emission spectrum; a centre wavelength of the target emission spectrum; a linewidth of the target emission spectrum; and a lifetime associated with the target emission spectrum.

13. The method of any one of claims 4-12, wherein the measurement of the optical intensity of the luminescence centre when illuminated by the optical beam comprises a measurement of a number of photons emitted by the luminescence centre when illuminated by the optical beam.

14. The method of any one of claims 4-13, wherein heating the luminescence centre comprises illuminating the luminescence centre with a first laser.

15. The method of claim 14, wherein the optical beam is emitted by a second laser.

16. The method of claim 15, wherein the second laser comprises a tunable laser.

17. The method of claim 15 or 16, wherein the second laser is different to the first laser.

18. The method of any one of claims 15-17 wherein an optical path between the first laser and the luminescence centre is distinct from an optical path between the second laser and the luminescence centre.

19. The method of any one of the preceding claims, wherein heating the luminescence centre comprises heating the luminescence centre with a PIN diode.

20. The method of any one of the preceding claims, wherein the substrate comprises a semiconductor substrate.21 . The method of claim 20, wherein the semiconductor substrate consists principally of silicon.

22. The method of claim 21 , wherein the luminescence centre comprises a T centre, an I centre or an M centre.

23. The method of claim 20, wherein the semiconductor substrate comprises silicon carbide or diamond.

24. The method of any one of the preceding claims, wherein the luminescence centre is in an optical cavity in the substrate.

25. The method of any one of the preceding claims, wherein the substrate forms part of a quantum device.

26. A non-transitory processor-readable storage medium containing instructions which, when executed by a processor, cause the processor to perform the method of any one of claims 1 to 25.

27. An apparatus configured to perform the method of any one of claims 1 to 25.

28. An apparatus for modifying a property of a luminescence centre in a substrate, the apparatus comprising: an interface; and a processor configured to: a) obtain a value of the property of the luminescence centre; and b) responsive to the property value failing to satisfy a criterion, send instructions, via the interface, to heat the luminescence centre such that a thermal energy of theluminescence centre during the heating is less than an energy associated with destruction of the luminescence centre.

Images