Apparatus for generating and detecting photons

Abstract

An apparatus 100 having a silicon substrate (104, Fig. 1) comprises a first surface and an opposing second surface. On the first surface is a lithium niobate waveguide 102 configured to guide photons

Description

TECHNOLOGICAL FIELD Examples of the disclosure relate to apparatus for generating and detecting photons. Some relate to apparatus for generating and detecting photons for use in quantum computing. BACKGROUND A photonic based quantum computer comprises a quantum source, a quantum circuit and quantum detectors. The quantum source generates single photons or photon pairs (entangled photons). These photons can be used to create a complex photonic quantum state known as a cluster state. The photonic states or cluster states can then be manipulated or measured by using quantum circuits. The quantum circuits can be interferometer circuits or any other means that enables implementation of an algorithm. After the implementation of the algorithm, the photons are then detected by quantum detectors. BRIEF SUMMARY According to various, but not necessarily all, examples of the disclosure there is provided an apparatus comprising: a silicon substrate; a lithium niobate waveguide provided on a first surface of the silicon substrate and configured to convert a pump photon into a photon pair and the waveguide is also configured to guide photons from the waveguide to the silicon substrate; and at least one photon detector where the photon detector is provided on a second surface of the silicon substrate wherein the second surface opposes the first surface. The pump photon may be converted into a photon pair via three wave mixing. The photons in the photon pair may have orthogonal polarisation. The pump photons may have a wavelength of 780nm and the photons in the photon pair may have a wavelength of 1560nm. The waveguide may comprise a periodically poled waveguide. The waveguide may comprise one or more grating couplers configured to direct photons towards the second surface of the silicon substrate. The waveguide may be configured to guide pump photons and converted photons into the silicon substrate. The apparatus may comprise at least one first photon detector for detecting idler photons and at least one second photon detector for detecting signal photons. The at least one photon detector may comprise a cavity-based superconducting nanowire single photon detector. The apparatus may comprise one or more interferometers. The apparatus may comprise multiple waveguides and multiple photon detectors on a silicon substrate. According to various, but not necessarily all, examples of the disclosure there is provided a method comprising: forming a lithium niobate waveguide on a surface of a silicon substrate such that the waveguide is arranged to guide photons from the waveguide to the silicon substrate; and forming at least one photon detector on an opposing surface of the silicon substrate such that the photon detector is arranged to detect photons that have passed through the silicon substrate. While the above examples of the disclosure and optional features are described separately, it is to be understood that their provision in all possible combinations and permutations is contained within the disclosure. It is to be understood that various examples of the disclosure can comprise any or all of the features described in respect of other examples of the disclosure, and vice versa. Also, it is to be appreciated that any one or more or all of the features, in any combination, may be implemented by / comprised in / performable by an apparatus, a method, and / or computer program instructions as desired, and as appropriate. BRIEF DESCRIPTION Some examples will now be described with reference to the accompanying drawings in which: FIG. 1 shows an example apparatus; FIG. 2 shows an example apparatus; FIG. 3 shows an example apparatus; FIG. 4 shows an example method; FIGS. 5A to 5D show an example method; FIGS. 6A to 6D show an example method; FIGS. 7 A to 7C show an example method; FIG. 8 shows transmission of photons through silicon; and FIG. 9 shows photon detection efficiency. The figures are not necessarily to scale. Certain features and views of the figures can be shown schematically or exaggerated in scale in the interest of clarity and conciseness. For example, the dimensions of some elements in the figures can be exaggerated relative to other elements to aid explication. Corresponding reference numerals are used in the figures to designate corresponding features. For clarity, all reference numerals are not necessarily displayed in all figures. DETAILED DESCRIPTION Photonic based quantum devices require means for generating photons, means for manipulating the generated photons and means for detecting the photons. The low efficiency of photon sources is problematic for practical quantum computing. Examples of the disclosure relate to an apparatus that can be used to efficiently generate and detect photons. The photons can be for use in quantum computing. In examples of the disclosure the source of the photons and the photon detectors can be provided on the same substrate. Fig. 1 schematically shows an example apparatus 100 that can be used in some examples of the disclosure. The apparatus 100 comprises a waveguide 102, a silicon substrate 104, and at least one photon detector 106. The waveguide 102 is a lithium niobate waveguide 102. The lithium niobate waveguide can be thin film lithium niobate waveguide. A thin film lithium niobate waveguide could be up to 600nm thick. The thin film lithium niobate waveguide could be between 300 to 600nm. The waveguide 102 is provided on a first surface of the silicon substrate 104. The silicon substrate 104 can have a high resistivity. One or more intervening layers can be between the silicon substrate 104 and the waveguide 102. For instance, an oxide layer could be provided between the silicon substrate 104 and the waveguide 102. The waveguide 102 is configured to convert a pump photon 108 into a photon pair and the waveguide 102 is also configured to guide photons 110 from the waveguide 102 to the silicon substrate 104. The waveguide 102 can be configured so that a pump photon 108 is converted into a photon pair via three wave mixing. The photon pair can be generated by a second order nonlinear process. The photon pair can be generated by a parametric down conversion process that is not supported in silicon. In some examples the photon pair can be generated by a X2 parametric down conversion process. In a parametric process a single input photon is split into two generated photons. The only restriction n the combination of frequencies of the generated photons is that energy must be 5 conserved however only communications of photon frequencies that are phase matched will be efficiently generated. The photons 110 in the generated photon pair can have orthogonal polarisation. The photons 110 in the generated photon pair can have a different wavelength to the pump photons 108. The pump photons 108 can have a wavelength such that the pump photons 108 do not pass through the silicon substrate 104 while the generated photons 110 can have a wavelength that does allow them to pass through the silicon substrate 104. This can enable the silicon substrate 104 to act as a filter for the pump photons 108. The pump photons 108 can have a wavelength of 780nm and the photons 110 in the photon pair can have a wavelength of 1560nm. The waveguide 102 can have any suitable structure. In some examples the waveguide 102 can comprise a periodically poled waveguide. In some examples the waveguide 102 comprises one or more grating couplers configured to direct photons 110 towards the second surface of the silicon substrate 104. The waveguide 102 can be configured to guide pump photons 108 and converted photons 110 into the silicon substrate 104. The photon detector 106 is provided on a second surface of the silicon substrate 104 wherein the second surface opposes the first surface. The photon detector 106 is provided on a second surface of the silicon substrate 104 such that a photon 110 from the waveguide 102 passes through the silicon substrate 104 to get to the photon detector 106. The photon detector 106 is configured to detect photons 110 that have passed through the silicon substrate 104. One or more intervening layers can be between the photon detector 106 and the waveguide 102. For instance, an anti reflective layer could be provided between the silicon substrate 104 and the photon detector 106. The photon detector 106 can comprise any suitable type of photon detector. In some examples the photon detector 106 can comprise a cavity-based superconducting nanowire single photon detector. The photon detector 106 can be configured to absorb photons and generate a voltage pulse upon detecting a photon. The photon detector 106 does not need to be mode matched to the waveguide 102 or other parts of the apparatus 100. The photon detector 106 can have a size larger than the mode size of the photons generated by the waveguide 102. The apparatus 100 can comprise multiple photon detectors 106. In some examples the apparatus 100 can comprise at least one first photon detector 106 for detecting idler photons and at least one second photon detector 106 for detecting signal photons. The signal photons can be used for computing, for transmitting signals, or for any other suitable purpose. In some examples the apparatus 100 can comprise additional components that are not shown in Fig. 1. For instance, the apparatus 100 can comprise one or more interferometers. The interferometers can be configured to manipulate the signal photons or for any other purpose. The interferometers can be used as a quantum circuit to enable an algorithm to be implemented, or for any other suitable purpose. In the schematic example of Fig. 1 only one waveguide 102 and one photon detector 106 are shown. In other examples the apparatus 100 can comprise multiple waveguides 102 and multiple photon detectors 106 on the silicon substrate 104. Fig. 2 shows another example apparatus 100. This example apparatus 100 also comprises a waveguide 102, a silicon substrate 104, and at least one photon detector 106. The silicon substrate 104 is not shown in Fig. 2. In some examples a layer of silicon dioxide can be provided on the first surface of the silicon substrate 104. The waveguide 104 can be formed on the layer of silicon dioxide. The silicon dioxide layer can be thinner than the silicon substrate 104. The waveguide 104 can be formed from lithium niobate. The waveguide 104 can be formed from thin film lithium niobate. The waveguide 104 can have any suitable structure. The waveguide 104 can be a periodically poled waveguide 104. In the periodically poled waveguide 104 the orientation of the lithium niobate crystal is periodically inverted. The inverted portions provide generated photons that are orthogonal to a generated photon that would have been generated at the point if the orientation had not been inverted. The periodicity of the poling of the waveguide 104 can be selected so that the newly generated photons can, at least partially, interfere constructively with previously generated photons. This can improve the efficiency of the conversion of input photons to generated photons. In the example of Fig. 2 the waveguide 104 is a type-ll phase-matched periodically poled waveguide 104. The waveguide 102 is configured to convert a pump photon 108 into a photon pair. The waveguide 102 is configured to convert a pump photon 108 into a photon pair via three wave mixing. The waveguide 102 can be driven with pump photons 108 having a wavelength of 780nm and can generate photon pairs having wavelengths of 1560nm. In the example of Fig. 2 the waveguide 102 comprises a beam splitter 200. The beam splitter 200 can be configured to split the beam of generated photons into two beams. The beam splitter 200 can be configured to separate idler photons 202 from signal photons 206. In this example the photon pairs generated by the waveguide 102 have orthogonal polarisation. The beam splitter 200 can be a polarisation beam splitter or any other suitable type of beam splitter. The waveguide 102 comprises multiple grating couplers 204, 208, 210. Some of the grating couplers 204, 210 can be configured to guide photons into the silicon substrate 104. A first grating coupler 210 can be configured to guide unconverted pump photons 108 into the silicon substrate 104. In this case pump photons 108 have a wavelength such that they do not pass through the silicon substrate 104. The pump photons 108 are blocked by the silicon substrate 104. The first grating coupler 210 can have any suitable structure. In some examples the first grating coupler 210 can comprise an apodized grating coupler cladded with an oxide layer and a top metal reflector. The beam splitter 200 can be configured so that idler photons 202 are directed towards a second grating coupler 204 and signal photons 206 are directed towards to a third grating coupler 208. The idler photons 202 can be photons with transverse electric polarization and the signal photons 206 can be photons with transverse magnetic polarization. The second grating coupler 204 can be configured to guide the idler photons 202 into the silicon substrate 104. The second grating coupler 204 can have any suitable structure. In some examples the second grating coupler 204 can comprise an apodized back-illuminated grating coupler cladded with an oxide layer and a top metal reflector. The idler photons 202 have a wavelength such that they do pass through the silicon substrate 104. The idler photons 202 are not blocked by the silicon substrate 104. The idler photons 202 can therefore pass through to the opposing surface of the silicon substrate 104. At least one photon detector 106 is provided on the second surface of the silicon substrate 104. The photon detector 106 can be configured to detect the idler photons 202 that have passed through the silicon substrate 104. The photon detector 106 can comprise any suitable structure. In some examples the photon detector 106 can comprise a superconducting nanowire single photon detector. For example, the photon detector 106 can comprise a superconducting niobium nitride based cavity-coupled nanowire detector. The third grating coupler 208 can be configured to guide the signal photons 206 to one or more quantum circuits. In some examples the apparatus 100 can comprise one or more interferometers that can provide the quantum circuits. The third grating coupler 208 can have any suitable structure. In some examples the third grating coupler 208 can comprise an apodized grating coupler cladded with an oxide layer and a top metal reflector. The apparatus 100 can comprise one or more further photon detectors 106. The further photon detectors 106 can be configured to detect the signal photons 206 after they have passed through the quantum circuits. The further photon detectors 106 can also be provided on the second surface of the silicon substrate 104. The signal photons 206 can be guided from the quantum circuitry, through the silicon substrate 104 to the further photon detectors 106. The example apparatus 100 of Fig. 2 shows one waveguide 102 and one photon detector 106. In implementations of the disclosure multiple waveguides 102 can be provided on the first surface of the silicon substrate 104 and / or multiple photon detectors 106 can be provided on the second surface of the silicon substrate 106. Fig. 3 shows a cross section of an example apparatus 100 such as the apparatus 100 of Fig. 2. Corresponding reference numerals are used for corresponding features. This example apparatus 100 also comprises a waveguide 102, a silicon substrate 104, and at least one photon detector 106. The waveguide 102 is provided on a first surface of the silicon substrate 104 and the photon detector 106 is provided on the second surface of the silicon substrate 104. The silicon substrate 104 can have high resistivity. The silicon substrate 104 has a thickness tsu The thickness tsi can be selected so that pump photons 108 that are used to drive the waveguide 102 are blocked by the silicon substrate 104. An oxide layer 300 is provided on the first surface of the silicon substrate 104. The oxide can be silicon dioxide or any other suitable material. The oxide layer 300 has a thickness of tsra. The thickness tscc of the oxide layer 300 can be less than the thickness tsi of the silicon substrate 104. The waveguide 102 is provided on the layer of silicon dioxide 300. The waveguide 102 comprises lithium niobate. The waveguide 102 can comprise thin film lithium niobate. The waveguide 102 has a thickness of tf. The thickness tf could be between 300 and 600nm. The waveguide 102 comprises one or more grating couplers 302. Only one grating coupler 302 is shown in Fig. 3 but the waveguide 102 can comprise any number of grating couplers 302. The grating coupler 302 can be configured to guide photons from the waveguide 102 into the silicon substrate 104. The grating coupler 302 has a periodicity Pg, a grating height of tw and a grating size of dj. A cladding layer 304 is provided over the waveguide 102. The cladding layer 304 can comprise an oxide such as silicon dioxide or any other suitable material. The cladding layer 304 has a thickness of tciad. A reflective coating 306 is provided over the cladding layer 304. The reflective coating 306 can comprise gold or any other suitable material. The reflective coating 304 has a thickness of tri. The photon detector 106 is provided on an opposing surface of the silicon substrate 104 to the waveguide 102. The photon detector 106 is positioned on the silicon substrate 104 so that photons from the waveguide 102 pass through the silicon substrate 104 to reach the photon detector 106. In the example of Fig. 3 the photon detector 106 is a superconducting nanowire single photon detector. A reflective coating 308 is provided on some of the second surface of the silicon substrate 104. The reflective coating 308 can be configured to reflect photons back into the silicon substrate 104. The reflective coating 308 can cover, or at least partially cover, areas of the second surface on which no photon detector 106 is provided. The reflective coating 308 can be made of any suitable material such as gold. An anti-reflective coating 310 is provided on the second surface of the silicon substrate 104. The antireflective coating 310 can cover the second surface of the silicon substrate 104 including the areas in which the reflective coating is provided 308. The anti-reflective coating 308 can be configured enable photons to be guided out of the silicon substrate 104. The reflective coating 308 and the antireflective coating 310 are arranged so that an anti reflective coating 310, but no reflective coating 308, is provided in areas where the photon detector 106 is positioned. This can enable photons to be guided from the silicon substrate 104 to the photon detector 106. The photon detector 106 comprises a superconductor 312. The superconductor 312 could comprise niobium nitrate or any other suitable type of superconductor 312. The superconductor 312 can be arranged in any suitable geometry. The superconductor 312 can have a width of WNbN. The superconductor 312 can be arranged so that it doubles back on itself in an oscillating arrangement. The periodicity of adjacent portions of the superconductor 312 is Ps as shown in Fig. 3. The superconductor 312 can be formed on a cavity layer 314. The cavity layer 314 can be formed from silicon dioxide or any other suitable material. The cavity layer 314 has a thickness of tc. A further reflective coating 316 is provided on the cavity layer 314. The further reflective coating 310 can be made of any suitable material such as gold. The further reflective coating 316 has a thickness of fo. When the apparatus 100 such as the example apparatus 100 of Figs. 1 to 3 are in use the waveguide 100 is driven by pump photons 108. The pump photons 108 are converted into a pair of photons by the waveguide by a three wave mixing process. The pump photons have a wavelength such that the pump photons are blocked by the silicon substrate 104. For example, the pump photons 108 could have a wavelength of 780nm. Therefore, any excess pump photons 108 are filtered by the silicon substrate 104 before they reach the photon detector 106. The generated photons can be coupled into the silicon substrate 104 by the one or more coupling gratings 302. The generated idler photons can be coupled into the silicon substrate 104 without undergoing processing by one or more quantum circuits. In some examples the generated signal photons can be coupled into the silicon substrate 104 after undergoing processing by one or more quantum circuits. The generated photons have a wavelength such that they are not blocked by the silicon substrate. For example, the generated photons could have a wavelength of 1560nm. This enables the generated photons to pass though the silicon substrate 104 as indicated by the arrow 318 in Fig. 3. The photons that pass through the silicon substrate 104 can then be detected by the one or more photon detectors 106. Fig. 4 shows an example method that can be used to fabricate apparatus 100 according to examples of the disclosure. The example method could be used to fabricate apparatus 100 such as the apparatus 100 shown in Figs. 1 to 3. At block 400 the method comprises forming a lithium niobate waveguide 100 on a surface of a silicon substrate 104. The lithium niobate waveguide 100 can be formed such that the waveguide 102 is arranged to guide photons from the waveguide 100 to the silicon substrate 104. The waveguide 102 can be formed from thin film lithium niobate. The waveguide can comprise one or more grating couplers 300. The grating couplers 300 can be configured to guide pump photons 108 and generated photons into the silicon substrate 108. The waveguide 102 and grating couplers 300 can be configured so that idler photons 202 can be guided into the silicon substrate 104 without passing through a quantum circuit. The waveguide 102 and grating couplers 300 can be configured so that signal photons 206 can be guided into the silicon substrate 104 after passing through a quantum circuit. The quantum circuit can comprise one or more interferometers. In some examples the method can comprise forming multiple waveguides 102. At block 402 the method comprises forming at least one photon detector 106 on an opposing surface of the silicon substrate 104 such that the photon detector 106 is arranged to detect photons 106 that have passed through the silicon substrate 104. The photon detector 106 can comprise a cavity-based superconducting nanowire single photon detector or any other suitable type of photon detector 106. In some examples the method can comprise forming multiple photon detectors 106. Figs. 5A to 5D show an example method of forming one or more waveguides 102. The method of Figs. 5A to 5D could be used to implement block 400 of Fig. 4. In Fig. 5A a silicon substrate 104 is provided. The silicon substrate 104 is flat or substantially flat. An oxide layer 300 is deposited on a first surface of the silicon substrate 104. A thin film of lithium niobate 500 can then be deposited on the oxide layer 300. In Fig. 5B the thin film of lithium niobate 500 is poled. The poling of the thin film of lithium niobate 500 can comprise periodically inverting the orientation of lithium niobate crystals in the thin film of lithium niobate 500. The lithium niobate crystals can be inverted by applying an electric field across the thin film of lithium niobate 500. This results in a periodically poled section 502 of the thin film of lithium niobate 500. In Fig. 5C the thin film of lithium niobate 500 is etched to provide the structure of the waveguide 102. The thin film of lithium niobate 500 can be etched to generate the grating couplers 302, the beam splitter and any other suitable parts of the waveguide 102. Once the waveguide 102 has been etched then, in Fig. 5D a cladding layer 304 is provided over the waveguide 102. The cladding layer 304 can comprise an oxide such as silicon dioxide or any other suitable material. A reflective coating 306 is provided over the cladding layer 304. The reflective coating 306 can comprise gold or any other suitable material. An anti-reflective coating 310 is provided on the second surface of the silicon substrate 104. Figs. 6A to 6D show an example method of forming one or more photon detectors 106. The method of Figs. 6A to 6D could be used to implement block 402 of Fig. 4. In Fig. 6A a silicon substrate 600 is provided. In Fig 6B a reflective coating 316 is deposited on the silicon substrate 600. The reflective coating 316 can be formed from gold or any other suitable material. A cavity layer 314 is deposited over the reflective coating 316. The cavity layer 314 can be formed from an oxide such as silicon dioxide. In Fig. 6C a superconductor material is deposited onto the cavity layer 314 and patterned to create a photon detector 106. The superconductor material can comprise niobium bitrate or any other suitable material. In Fig. 6D an antireflective coating 310 is deposited over the photon detector 106. The antireflective coating 310 can comprise aluminum oxide or any other suitable type of material. Figs. 7A to 7C show an example method of forming an apparatus 100 by bonding a chip comprising a waveguide 102 to a chip comprising a photon detector. In Fig. 7A a chip comprising a waveguide 102 is provided. The chip can comprise a silicon substrate 104 with a waveguide 102 formed on a first surface of the silicon substrate 104. An antireflective coating 310 is provided on the second surface of the silicon substrate 104. The chip can be fabricated using a method as shown in Figs. 5A to 5D or any other suitable method. In Fig. 7B a chip comprising a photon detector 106 is provided. The chip can comprise a silicon substrate 600 with a photon detector 106 formed on a first surface of the silicon substrate 600. An antireflective coating 310 is provided over the photon detector 106. The chip can be fabricated using a method as shown in Figs. 6A to 6D or any other suitable method. In Fig. 7C the chip comprising a waveguide 102 is bonded to the chip comprising a photon detector 106. In this example the antireflective coating 310 on the second surface of the chip comprising a waveguide 102 can be bonded to the antireflective coating 310 of the chip comprising a photon detector 106. This bonds the photon detector 106 to the second surface of the silicon substrate 104. In the examples of Figs. 7A to 7C only one waveguide and one photon detector 106 are shown. However, any number of waveguides 102 and photon detectors 106 could be bonded together to fabricate an apparatus 100 in other examples. Fig. 8 is plots that show the transmission of photons through silicon. The first plot 800 shows the propagation distance inside silicon for photons with a wavelength of 780nm. This can be the wavelength of pump photons 108 that are used to drive the waveguide 102 in examples of the disclosure. The wavelength of the pump photons, 780nm, is below the bandgap of the silicon substrate 104. Therefore, the silicon substrate 104 acts as an absorptive filter for the pump photons 108. The second plot 802 shows the propagation distance inside silicon for photons with a wavelength of 1560nm. This can be the wavelength of photons that are generated in the waveguide 102 in examples of the disclosure. Silicon is transparent to the photons at 1560nm. Therefore, the photons generated in the waveguide 102 can be guided to the opposing surface of the silicon substrate 104 with high efficiency. Fig. 9 shows photon detection efficiency for generated photons from a waveguide 102 as a function of parameters of the photon detector 106. The results are obtained for an apparatus 100 as shown in Fig. 3. The plot of Fig. 9 shows that varying the cavity thickness for the photon detector 106 affects the efficiency of the photon detector 106. Examples of the disclosure provide an apparatus that can efficiently generate photons and guide generated photons to a photon detector. The use of thin film lithium niobate to form the waveguide enables the photons to be generated using a nonlinear process. This enables the silicon substrate 104 to be transparent to the generate photons but act as an absorptive filter for the pump photons 108. The apparatus 100 achieves high-efficiency for photon detection because the photons do not need to be coupled out of the photonic chip. There is no need for any mode matching between the source, fibres and the photon detectors. The apparatus 100 can be fabricated as an integrated apparatus with both the waveguide 102 and the photon detector 106 on the same silicon substrate 104. The methods that can be used to fabricate such chips are scalable and can form part of more complex designs. The term ‘comprise’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use ‘comprise’ with an exclusive meaning then it will be made clear in the context by referring to “comprising only one...” or by using “consisting”. In this description, the wording ‘connect’, ‘couple’ and ‘communication’ and their derivatives mean operationally connected / coupled / in communication. It should be appreciated that any number or combination of intervening components can exist (including no intervening components), i.e., so as to provide direct or indirect connection / coupling / communication. Any such intervening components can include hardware and / or software components. As used herein, the term "determine / determining" (and grammatical variants thereof) can include, not least: calculating, computing, processing, deriving, measuring, investigating, identifying, looking up (for example, looking up in a table, a database or another data structure), ascertaining and the like. Also, "determining" can include receiving (for example, receiving information), accessing (for example, accessing data in a memory), obtaining and the like. Also, "determine / determining" can include resolving, selecting, choosing, establishing, and the like. In this description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term ‘example’ or ‘for example’ or ‘can’ or ‘may’ in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus ‘example’, ‘for example’, ‘can’ or ‘may’ refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub-class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a feature described with reference to one example but not with reference to another example, can where possible be used in that other example as part of a working combination but does not necessarily have to be used in that other example. Although examples have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the claims. Features described in the preceding description may be used in combinations other than the combinations explicitly described above. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain examples, those features may also be present in other examples whether described or not. The term ‘a’, ‘an’ or ‘the’ is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising a / an / the Y indicates that X may comprise only one Y or may comprise more than one Y unless the context clearly indicates the contrary. If it is intended to use ‘a’, ‘an’ or ‘the’ with an exclusive meaning then it will be made clear in the context. In some circumstances the use of ‘at least one’ or ‘one or more’ may be used to emphasis an inclusive meaning but the absence of these terms should not be taken to infer any exclusive meaning. The presence of a feature (or combination of features) in a claim is a reference to that feature or (combination of features) itself and also to features that achieve substantially the same technical effect (equivalent features). The equivalent features include, for example, features that are variants and achieve substantially the same result in substantially the same way. The equivalent features include, for example, features that perform substantially the same function, in substantially the same way to achieve substantially the same result. In this description, reference has been made to various examples using adjectives or adjectival phrases to describe characteristics of the examples. Such a description of a characteristic in relation to an example indicates that the characteristic is present in some examples exactly as described and is present in other examples substantially as described. The above description describes some examples of the present disclosure however those of ordinary skill in the art will be aware of possible alternative structures and method features which offer equivalent functionality to the specific examples of such structures and features described herein above and which for the sake of brevity and clarity have been omitted from the above description. Nonetheless, the above description should be read as implicitly including reference to such alternative structures and method features which provide equivalent functionality unless such alternative structures or method features are explicitly excluded in the above description of the examples of the present disclosure. Whilst endeavoring in the foregoing specification to draw attention to those features believed to be of importance it should be understood that the Applicant may seek protection via the claims in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not emphasis has been placed thereon. l / we claim:

Claims

1. An apparatus comprising:a silicon substrate;a lithium niobate waveguide provided on a first surface of the silicon substrate and configured to convert a pump photon into a photon pair and the waveguide is also configured to guide photons from the waveguide to the silicon substrate; andat least one photon detector where the photon detector is provided on a second surface of the silicon substrate wherein the second surface opposes the first surface.

2. An apparatus as claimed in claim 1 wherein the pump photon is converted into a photon pair via three wave mixing.

3. An apparatus as claimed in any preceding claim wherein the photons in the photon pair have orthogonal polarisation.

4. An apparatus as claimed in any preceding claim wherein pump photons have a wavelength of 780nm and the photons in the photon pair have a wavelength of 1560nm.

5. An apparatus as claimed in any preceding claim wherein the waveguide comprises a periodically poled waveguide.

6. An apparatus as claimed in any preceding claim wherein the waveguide comprises one or more grating couplers configured to direct photons towards the second surface of the silicon substrate.

7. An apparatus as claimed in any preceding claim wherein the waveguide is configured to guide pump photons and converted photons into the silicon substrate.

8. An apparatus as claimed in any preceding claim wherein the apparatus comprises at least one first photon detector for detecting idler photons and at least one second photon detector for detecting signal photons.

9. An apparatus as claimed in any preceding claim wherein the at least one photon detector comprises a cavity-based superconducting nanowire single photon detector.

10. An apparatus as claimed in any preceding claim wherein the apparatus comprises one or more interferometers.

11. An apparatus as claimed in any preceding claim comprising multiple waveguides and multiple photon detectors on a silicon substrate.

12. A method comprising:forming a lithium niobate waveguide on a surface of a silicon substrate such that the waveguide is arranged to guide photons from the waveguide to the silicon substrate; andforming at least one photon detector on an opposing surface of the silicon substrate such that the photon detector is arranged to detect photons that have passed through the silicon substrate.

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