Vapour cell

Abstract

The present invention relates to 3D printed, ultra-high vacuum vapour cells, and methods of manufacturing such vapour cells. A method of manufacturing an ultra-high vacuum vapour cell (100) is provided, the method comprising: forming a glass cell (102) via 3D printing; generating an ultra-high vacuum within the glass cell (102); and depositing an atomic vapour (102) within the g lass cell (102).

Description

[0001] VAPOUR CELL

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to 3D printed, ultra-high vacuum vapour cells, and methods of manufacturing such vapour cells.

[0004] BACKGROUND

[0005] Atomic vapour cells (e.g., alkali vapour cells) are a vital component in a wide range of atomic devices that are used in many different applications. Such vapour cells are used as a frequency reference for laser systems ; in atomic clocks for timing / navigation purposes; and in magnetometers for providing precise measurement of nanotesla-scale magnetic fields. In future atomic vapour cells will likely be key in enabling a number of emerging quantum technologies , such as quantum memory, quantum imaging and quantum sensing.

[0006] Despite their importance and widespread use, current vapour cell technologies pose a number of obstacles. Many of these difficulties result from existing fabrication techniques (namely, precision glass blowing), which impose constraints on the cells themselves and their wider adoption in industry.

[0007] Micro-fabrication techniques have been proposed for producing alkali vapour cells, which are reviewed by Wang et al1. These micro-fabrication approaches rely on bonding of wafers that are patterned with a cavity (for example, by direct fusion bonding, anodic bonding, frit reflow bonding or metal thermocompression bonding).

[0008] The specialist manufacturing methods that are currently used mean that the cost of vapour cells is often prohibitively expensive (approximately > £3k per cell) for certain applications, with each cell taking many hours to manufacture using bespoke equipment. Glass blowing is also only able to produce cells above a certain size and with fairly simple geometries. Again, this means that current vapour cells are unsuitable for use in some devices, particularly as atomic / quantum devices continue to be miniaturised. Finally, state of the art vapour cells face issues with repeatability and reproducibility in their production, owing from the labour-intensive craftsmanship involved in glass

[0009] 1Wang, Xuelei, et al. "Recent Progress on Micro -Fabricated Alkali Metal Vapor Cells." Biosensors 12.3 (2022): 165. blowing. Variations may arise in the fabrication, which may reduce the precision of any system that utilises vapour cells and / or require the need for additional calibration.

[0010] Given the drawbacks presented by existing vapour cell manufacturing methods, and the potential for improved atomic vapour cells to enable / develop a multitude of cutting-edge technologies, it is desirable to address at least some of these issues.

[0011] SUMMARY

[0012] According to an aspect of the present invention, there is provided a method of manufacturing an ultra-high vacuum (UHV) vapour cell, the method comprising: forming a glass cell via 3D printing.

[0013] The method may further comprise generating an ultra-high vacuum within the glass cell; and depositing an atomic vapour within the glass cell.

[0014] The use of 3D printing / additive manufacturing techniques to produce the glass cell may provide a number of advantages over conventional fabrication techniques (e.g., glass blowing). 3D printing may allow for multiple glass cells to be printed simultaneously ; may reduce the amount of manual labour involved in the manufacturing process (3D printing may be automated); and / or may allow glass cells to be produced faster. These improvements over conventional methods may reduce the cost of UHV vapour cells (the glass cells may be produced for < £1 each) and may increase production rates and supply of UHV vapour cells. The 3D printed UHV vapour cells may therefore enable cheaper and more widespread use of atomic devices.

[0015] 3D printing may also provide for improved repeatability / consistency in forming glass cells. The 3D printing process may be highly standardised and computer controlled to micron-level precision, thereby reducing variations that may be introduced into the vapour cells via conventional methods. The 3D printing may be performed using a commercial, off-the-shelf printer, thereby reducing the need for specialised fabrication equipment and operators.

[0016] Depositing the atomic vapour and generating a UHV within the glass cell may be performed using the same apparatuses and techniques that are used for producing conventional glass cells. The atomic vapour may be an alkali vapour (e.g., rubidium or lithium). Alkali metals are commonly used in vapour cells due to their relatively simple atomic structure, low melting point and ease of being addressed using common laser frequencies. The UHV may be generated by fluidly coupling the glass cell to a vacuum pump. UHV may be regarded as pressures of less than 1 x 108mbar. A UHV of 2 x 109mbar or less may be generated and retained within the glass cell.

[0017] Forming the glass cell via 3D printing may comprise: providing a photocurable resin; and using ultraviolet (UV) stereolithography to produce a preliminary printed part from the photocurable resin. UV stereolithography may be a preferable 3D printing / additive manufacturing process for forming the glass cell. The UV stereolithography may generally comprise using a UV light source (e.g., a laser) to selectively cure portions of a bath of photocurable resin, the cured portions forming the structure of the preliminary printed part. UV stereolithography may provide the ability to print smaller / larger parts, faster fabrication speed and / or greater scale-up capabilities for large scale production, when compared to other manufacturing techniques (or at least a suitable compromise of these different factors). In particular, the 3D printing may comprise inverse stereolithography, wherein the part is printed from the bottom up. Other known 3D printing / additive manufacturing techniques that are suitable for producing glass parts may be used instead. For example, the 3D printing may comprise any of (but is not limited to): fused deposition modelling, selective laser melting, directed energy deposition, direct ink writing, stereolithography, digital light projection, fused deposition modelling, micro-computer axial tomography, micro-CAL lithography, two- photon polymerisation, fused filament fabrication of molten glass, selective laser melting of silica or additive manufacturing using silica sol -gel ink. All or some of the subsequent manufacturing steps described herein may be used with these other techniques also; a preliminary printed part not formed via UV stereolithography may still be debound, sintered and / or polished as required.

[0018] Forming the glass cell via 3D printing may further comprise: washing the preliminary printed part so as to remove photocurable resin that has not been cured by the ultraviolet stereolithography; and debinding the preliminary printed part at a temperature of up to 600°C, thereby forming a debound part. Debinding may take place at between 550 and 650°C, 500°C and 700°C, or 400 and 800°C, for example. The washing and the debinding may improve the quality of the final 3D printed vapour cell; unwanted material may be removed and the polymeric matrix of the cured resin may be debound via thermal decomposition.

[0019] Forming the glass cell via 3D printing may further comprise sintering the debound part at a temperature of 1100°C or 1150°C. The sintering may take place at between 1050°C and 1150°C, between 1000°C and 1200°C, or 900°C and 1300°C, for example. Sintering may reduce the porosity of the printed glass shell by producing a glass y, amorphous polymerised structure. The sintering may increase the strength and / or the optical quality of the glass cell. Sintering may result in the glass cell shrinking (e.g., by approximately 20 or 25%) and the 3D printing of the preliminary 3D printed part may be configured to account for this shrinkage.

[0020] The photocurable resin may comprise: a monomer mixture; and an ultraviolet absorber at 0.035 wt% with respect to the monomer mixture. The monomer mixture may comprise 2-hydroxyethyl methacrylate (HEMA), tetra(ethylene glycol) diacrylate (TEGDA) and phenoxyethanol (POE). The monomer mixture may comprise 60 vol% HEMA, 10 vol% TEGDA and 30 vol% POE. These proportions may provide for sufficient polymerisation and viscosity for the UV stereolithography. The relative proportions may be varied depending on application. E.g., each proportion may be varied by ± 10% or ± 20%. The POE may be omitted entirely.

[0021] The UV absorber may absorb at least part of the light of the UV stereolithography, thereby altering the penetration depth of the light. The concentration of absorber may therefore vary the curing depth of the printing. The UV absorber may be Sudan Orange. 0.035 wt% Sudan Orange may provide an optimal printing speed, while maintaining formulation polymerisation / printability and optical quality. UV absorber at 0.035 wt% may be preferable to greater concentrations (e.g., 0.05 wt%). The concentration of absorber may be between 0.0325 and 0.0375 wt%, or 0.03 and 0.04 wt%, for example.

[0022] The photocurable resin may further comprise 40 vol% silica nanopowder with respect to the monomer mixture, the silica nanopowder having an average particle size of 40 nanometres. The silica nanopowder may be melted during the 3D printing process, e.g., during sintering, thereby forming a glassy, amorphous material that is capable of retaining UHV. The silica nanopowder may be Aerosil 0X50 silica nanopowder. A concentration of 40 vol% and an average particle size of 40 nanometres may provide for preferable physical and optical properties of the vapour cell. The concentration may be between 35 vol% and 45 vol%, or between 37.5 vol% and 42.5 vol%. The average particle size may be between 35 nanometers and 45 nanometres or between 30 nanometers and 50 nanometres, with a standard deviation of less than 5, less than 10, less than 15 or less than 20 nanometres. The average particle size may be determined using a sequence of graded sieves, or by dynamic light scattering (DLS) .

[0023] The method may comprise, prior to using ultraviolet stereolithography, doping the photocurable resin with a dopant configured to modify an optical property of the glass cell. The optical property may be transmissivity or polarisation. The doping may increase the optical absorption of the glass cell at a particular wavelength or range of wavelengths. The dopant may be configured so that the absorbed wavelength(s) do not interfere with wavelengths of the atomic vapour (e.g., spectroscopy lines for the atomic vapour material). The doping may enable the glass cell to filter certain wavelengths. A light source of a suitable wavelength (e.g., a laser) may be applied to the doped glass cell, thereby causing optical absorption -induced heating due to plasma surface resonance of the dopant. This may be used to increase the vapour pressure of the cell, without the need to provide electrical heating (e.g., using a coil wound around the cell). This may be particularly useful in magnetometer applications, as external magnetic fields associated with the heating can be reduced.

[0024] The dopant may be dispersed throughout the photocurable resin prior to 3D printing. Performing the doping prior to 3D printing may result in a more homogenous modification of the glass cell optical property as compared to modifying a printed component. The absorption range of the dopant may be narrower than if the printed cell is doped or coated. The dopant may be dispersed non -homogenously in the photocurable resin, thereby creating regions with higher / lower concentrations of dopant and therefore a vapour cell with different glass cell properties (e.g., colour) at different areas.

[0025] The dopant may comprise a material that is reactive to the ultraviolet stereolithography so as to produce nanoparticles. Nanoparticles may provide a preferable dopant due to their absorption characteristics. By doping prior to printing, and then using the printing process itself to produce the nanoparticles, a narrower size distribution of nanoparticles may be generated. Nanoparticles produced the 3D printing may therefore provide a narrow absorption peak. Alternatively or additionally, the dopant may comprise quantum dots.

[0026] The dopant may comprise a salt and the nanoparticles may be metallic nanoparticles provided via reduction of a metal ions comprising the salt. The salt may be easily dispersed in the photocurable resin, and the nanoparticles may be naturally produced at substantially the same size by the UV of the 3D printing. The salt may be AuCD and the nanoparticles may be gold nanoparticles, for example. Rather than the dopant undergoing a reduction during the printing process, the final dopant may be produced by thermal decomposition or an optically induced chemical reaction, for example.

[0027] The method may further comprise depositing one or more layers of conductive or semiconductive material onto a surface of the glass cell. Multiple layers may be deposited on top of one another to produce a sufficiently thick / robust deposition. The deposited material may have a ‘finger-like’ structure, such that a relatively large area of the glass cell (e.g., > 10% of a side wall) is covered by the material, while still providing area for a laser beam to pass through. Multiple regions may be deposited, and these may be interdigitated. One or more different materials may be deposited on the same glass cell. For example, a vapour cell may comprise interdigitated silver and graphene depositions.

[0028] Conductive material (e.g., gold, silver or graphene) may be deposited onto the surface of the glass walls as ‘tracks’; electrode like structures to which apparatus may be electronically coupled. Sensing devices (e.g., a multimeter) may be coupled to the tracks such that voltage and / or current measurements may be taken. These measurements may provide information regarding the operation of the vapour cell, for example the temperature. Alternatively / additionally, a power supply may be coupled to the conductive tracks. The current may be controlled so as to provide resistive heating or active magnetic field correction. The tracks may provide electromagnetic shielding , which may be particularly important for quantum magnetometry. Silver tracks may be provided for taking measurements and graphene tracks may be provided for magnetic shielding, the two being interdigitated, for example.

[0029] Semiconductive material (e.g., silicon or perovskite) may be deposited onto the surface of the glass walls. The semiconductive material may be optically sensitive such that the deposited layers may be utilised as a photon sensor. A voltage sensor may be coupled to the semiconductor material, thereby allowing optical interactions (e.g., spectroscopy interrogation) to be measured. This may eliminate the need to provide separate photodetectors, reducing the apparatus’ footprint and cost. The deposited semiconductor materials may also allow sensors to be positioned closer to the atomic vapour (i.e., on the cell walls), which may reduce degradation of signals.

[0030] Depositing the one or more layers may comprise printing an ink onto the surface of the glass cell, the ink comprising the conductive or semiconductive material. Inkjet printing may be used, whereby the ink comprises the material that is to be deposited (e.g., silver nanoparticles). Inkjet printing may provide a cost effective and simple method for depositing the material. The adhesion of the material to the glass walls via inkjet printing may be stronger than other methods. Any other suitable deposition technique may be used to deposit the material onto the glass cell. Material may be deposited using a 3D printing method, chemical vapour deposition or physical vapour deposition , for example. Conductor / semiconductor components may be attached to the vapour cell using an adhesive.

[0031] Generating an ultra-high vacuum within the glass cell may comprises: coupling the glass cell to a tube; and coupling the tube to a vacuum system. The glass cell may be coupled to the tube using an adhesive. The adhesive may be a UV curable glue, the glue providing a substantially leak-proof and robust adhesion. The tube may be a copper tube and may provide a convenient means for coupling the glass cell to existing vacuum systems, regardless of the geometry of the glass cell.

[0032] The vacuum system may be configured to be fluidly decouplable from the glass cell such that the ultra-high vacuum is maintained within the glass cell. For example, the vacuum system may comprise a pump which is coupled to the glass cell via a valve. The valve may be operated such that, following generation of the UHV within the glass cell via the pump, the pump is then fluidly decoupled from the glass cell. This may provide a convenient means for loading the glass cell with atomic vapour and / or for conducting preliminary analysis. For example, leak testing or spectroscopy measurements may be performed on the vapour cell while it is still physically coupled to the vacuum system (but fluidly disconnected). The method may further comprise pinching off the tube such that the ultra -high vacuum is maintained within the glass cell, thereby forming a standalone ultra -high vacuum vapour cell. Following the generation of the UHV within the vapour cell and loading of the cell with atomic vapour, the tube may be pinched (e.g., crimped) so as to seal the UHV vapour cell. The pinched-off portion of the tube may be retained as a cap or seal. Once decoupled from the vacuum system, the UHV vapour cell may be used in any application which requires a conventionally manufactured reference cell.

[0033] The glass cell may comprise at least two chambers, the chambers being fluidly coupled to one another. The glass cell may comprise two or more cube -shaped (or cuboidal) chambers, coupled together via a connecting portion (which may be narrower than each of the two or more chambers). A multi-chambered glass cell may allow different investigations to be conducted simultaneously using the same atomic vapour. Furthermore, such a group of chambers could be filled via a conventional process, but with a similar degree of difficulty as filling a single chambered cell. A string of cells could be 3D printed (perhaps initially only 3 to 5 of them, but with the potential to develop to larger numbers) with a connecting tube and a single open tube for vapour loading. The cell (comprising multiple chambers) could be shaped so as to fit the chambers neatly into required locations in a device such as a magnetic gradiometer or an MEG helmet. There are a range of applications (gradiometers, MEG etc.) that require multiple sensors (and hence multiple chambers or cells) relatively close to each other in very specific relative positions and orientations. Multiple, conjoined chambers can be printed in an appropriate configuration for use in such applications. There is significant potential to decrease filling costs per cell with such a multi-chambered 3D printed cell approach.

[0034] Partitioning the volume of the vapour cell into multiple chambers may result in atomic polarisation created by laser pumping being maintained throughout a greater fraction of the atomic vapour when an external magnetic field is applied. In current cuboidal cells , the polarisation decays rapidly with distance from the cell wall even in magnetic fields as low as 15 nT, which limits the dynamic range to fields of this order. By contrast, partitioning the atomic vapour into a series of thin chambers (through which a single laser beam is applied) may reduce the loss of polarisation, thereby increasing the dynamic range of operation and enabling the sensor to operate in, and sense, larger external magnetic fields. The glass cell may have a volume no greater than 1 cm3. Ordinarily, the glass cell may be cuboidal in shape (e.g., a cube with dimensions of approximately 1 cm). Such a cuboidal shape may be simple to print. However, more complex geometries may be printed depending upon the application. The 3D printing-based manufacturing process may be able to produce smaller vapour cells (e.g., < 1 mm3) and / or cells with more complex geometries (e.g., substantially flat / thin cells, or cells of intricate design), which may enable their use in a wider range of applications.

[0035] According to another aspect of the present invention, there is provided a method of using a 3D printed ultra-high vacuum vapour cell in an optically pumped magnetometer (OPM), the method comprising: providing a 3D printed ultra-high vacuum vapour cell containing an atomic vapour; illuminating the atomic vapour with a light source, thereby transferring a plurality of atoms of the atomic vapour to an optically pumped state; and measuring, using a detector, a transmission of light from the light source through the ultra-high vacuum vapour cell, the transmission corresponding to the proportion of the atoms of the atomic vapour that are in the optically pumped state.

[0036] Providing a 3D printed ultra-high vacuum vapour cell containing an atomic vapour may comprise manufacturing the 3D printed UHV vapour cell according to a method of the present invention.

[0037] The operation of OPMs is well understood. For example, Tierney2, describes the fundamental physics associated with optical pumping, and the application of OPMs to magnetoencephalography (MEG). The skilled person will understand that the 3D printed UHV vapour cells described herein may replace conventional reference cells in any existing OPM method / system.

[0038] The use of a 3D printed UHV vapour cell in OPM may be particularly advantageous.

[0039] The 3D printed vapour cells may reduce the size of the OPM such that it can be used in devices worn on the human body. An OPM -based device, for example a device for

[0040] 2T. M. Tierney et al., “Optically pumped magnetometers: From quantum origins to multichannel magnetoencephalography”, NeuroImage, vol. 199, 2019, pgs. 598-608 conducting MEG, may contain a plurality of OPMs (perhaps up to 50 separate OPMs or more). Therefore, the aforementioned cost savings and ease of manufacturing may be particularly beneficial in such devices.

[0041] The light source may be a laser that is tuned to an atomic transition frequency of the atomic vapour. For example, the atomic vapour may comprise87Rb and the laser be tuned to a 795 nm wavelength, such that only the DI transitions of the87Rb are interrogated. A 3D printed UHV vapour cell may be used to lock the laser to the correct transition, the hyperfine absorption spectrum of the atomic vapour providing a sharp reference feature.

[0042] The light used to illuminate the atomic vapour may be circularly polarised such that atoms are pumped to a steady state. As explained in Tierney, circularly polarised light can pump the atomic vapour into a single magnetic quantum state (resulting from Zeeman shifting of the hyperfine structure due to an applied magnetic field), the optical pumping arising from selection rules of which magnetic quantum state the atoms can be pumped to by the circularly polarised light.

[0043] The 3D printed ultra-high vacuum vapour cell may comprise an optically absorptive dopant, wherein the method may comprise illuminating the optically absorptive dopant so as to induce heating. For example, the 3D printing resin may be doped using a gold salt, thereby resulting in gold nanoparticles being dispersed in the glass of the vapour cell, as discussed above. The gold nanoparticles may absorb a particular range of wavelengths (~ 550 nm). A light source, e.g., a laser, may be provided to illuminate these nanoparticles, thereby resulting in heating of the vapour cell via surface plasma resonance.

[0044] Heating the vapour cell will increase the pressure of the atomic vapour, which in turn increases the spin exchange rate. This may enable more accurate magnetic field measurements, due to the increased collision rate of the atoms in the vapour cell. The use of optically-induced heating may be beneficial, as the heating can be generated without the need to provide electrical currents to the vapour (e.g., by wrapping a coil of current carrying wire around the cell), which would produce its own magnetic field. The optically pumped magnetometer may be operated in a spin exchange relaxation free (SERF) mode such that the measured transmission of light through the ultra -high vacuum vapour cell is dependent upon an external magnetic field. Again, the SERF mode is described in Tierney, and comprises the pumping the atomic vapour into a steady state and observing the relaxation from this state, which may be highly sensitive to external fields.

[0045] The optically pumped magnetometer may be used to measure a biomagnetic field. The OPM may be used as part of an MEG apparatus to measure magnetic fields associated with a subject’s brain, for example. Alternatively, the OPM may be used to measure magnetic fields associated with cardiac activity. The use of OPMs in biomedical applications is beneficial, as OPMs do not require cryogenic cooling . The use of an OPM comprising a 3D printed vapour cell may be further beneficial; the vapour cells may be made smaller, more lightweight or comprising more complex geometries, such that they can be more easily positioned around / adjacent to the body (e.g., disposed on a hat / helmet which is worn by a subject when conducting MEG).

[0046] According to a further aspect of the present invention, there is provided an ultra-high vacuum vapour cell, the ultra-high vacuum vapour cell comprising: a 3D printed glass cell, the glass cell retaining an ultra-high vacuum; and an atomic vapour contained within the glass cell.

[0047] The UHV vapour cell may be manufactured according to a method of the present invention.

[0048] According to a further aspect of the present invention, there is provided an optically pumped magnetometer comprising: an ultra-high vacuum vapour cell comprising: a 3D printed glass cell, the glass cell retaining an ultra-high vacuum; and an atomic vapour contained within the glass cell; a light source configured to illuminate the atomic vapour; and a detector configured to measure transmission of light from the light source through the ultra-high vacuum vapour cell. The UHV vapour cell may be manufactured according to a method of the present invention. The UHV vapour cell-based 0PM may be used according to a method of the present invention.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Embodiments of the invention will be described, purely by way of example, with reference to the accompanying drawings, in which:

[0051] Figure 1 shows a schematic diagram of an example 3D printed vapour cell;

[0052] Figure 2 shows pictures of some example 3D printed vapour cells;

[0053] Figure 3 shows a schematic diagram of a UV stereolithography 3D printing apparatus used to manufacture atomic vapour cells;

[0054] Figure 4 shows pictures of a 3D printed vapour cell undergoing debinding and sintering steps;

[0055] Figure 5 shows the effect of doping on the absorption properties of 3D printed glass;

[0056] Figure 6 shows experimental results for a functionalised graphene strip on a 3D printed glass slab;

[0057] Figure 7 shows a schematic diagram for generating UHV within, loading atomic vapour into, and analysing an atomic vapour cell;

[0058] Figure 8 shows the rubidium hyperfine structure obtained via Doppler -free spectroscopy using a 3D printing vapour cell;

[0059] Figure 9 shows experimental results for laser locking using a 3D printed vapour cell;

[0060] Figure 10 shows experimental results for the beam profile of a laser passing through a 3D printed reference cell; and Figure 11 shows experimental results illustrating the transmission and polarisation properties of a 3D printed vapour cell.

[0061] DETAILED DESCRIPTION

[0062] Referring to Figure 1 , and example of an ultra-high vacuum (UHV) vapour cell 100 is shown. The UHV vapour cell 100 generally comprises walls 102 that enclose a chamber 110. Depending upon the application and / or geometry of the vapour cell 100, the walls 102 may comprise a neck portion 104. The neck 104 may assist in coupling the vapour cell 100 to vacuum system, compared to a flush opening in the walls 102. The walls 102 of the vapour cell 100 are manufactured using a 3D printing process (discussed further below) and comprise a glass material.

[0063] The vapour cell 100 comprises a seal or cap 106 that is used to close the chamber 110. During manufacture of the vapour cell 100, the neck 104 may be coupled (e.g., glued) to a tube, the tube being fluidly coupled to a vacuum system which may be used to produce an UHV within the chamber 110 and to deposit an atomic vapour 112 (e.g., an alkali vapour such as Rb) within the chamber 110. After the UHV is formed and the atomic vapour 112 is deposited, the tube may be pinched off, thereby forming the cap 106 and sealing the vapour cell 100. The vapour cell 100 may then be removed from any manufacturing apparatus, thus forming a standalone cell that can be used in the same way as a conventionally manufactured atomic vapour reference cell.

[0064] The walls 102 of the vapour cell 100 may be functionalised so as to provide desirable optical properties and / or additional functionality. The vapour cell 100 may be functionalised by doping a printing resin that is used during the 3D printing process to form the walls 102. Such doping may modify the optical transmission properties of the walls 102. For example, the printing resin may be doped such that the walls 102 contain nanoparticles (e.g., gold), which may result in certain wavelengths of light being preferentially absorbed by the walls 102 of the vapour cell 100.

[0065] Alternatively / additionally to doping materials that comprise the vapour cell 100, materials may be deposited onto surfaces of the vapour cell 100, e.g., onto an exterior surface of one or more of the walls 102. Tracks 120 may be deposited onto the surface of the cell 100 using inkjet printing, for example. The inkjet printing may be used to deposit a layer of conductive material (e.g., silver, gold or graphene) so as to produce electrically conductive electrodes on / about the vapour cell 100. An electrical current may be passed through the tracks 120, which may provide heating or magnetic field shielding during use of the vapour cell 100. More complex materials and / or structures may be deposited so as to provide additional functionality. For example, semiconductor materials (e.g., perovskite nanocrystals) may be inkjet printed onto the surface of the vapour cell 100 so to provide photosensitive tracks / features 120. In such an example, the tracks 120 may act as an optical sensor, enabling optical information (e.g., alkali absorption spectra) to be obtained from the vapour cell without the need to provide additional photodetectors.

[0066] Referring to Figure 2, photographs are shown depicting examples of vapour cells 100 according to the present invention. Image a) shows a 3D printed UHV vapour cell 100, similar to that described above with regard to Figure 1. In this image, the vapour cell is being addressed by a pair of perpendicularly propagating laser beams. A green laser beam 312b passes through the vapour cell 100 from the left-hand side of the image, while a red laser beam 312a passes through the vapour cell from the right-hand side of the image. The scale bar in image a) is 1 cm.

[0067] Image b) shows a 3D printed UHV vapour cell 100a that comprises two distinct chambers 110a, 110b. The vapour cell 100a may be produced using the same 3D printing process of the present invention as used for a single -chambered vapour cell (like that of Figure 1) and may comprise all of the same features / functionality. However, a first chamber 110a and a separate second chamber 1 10b are formed. The two chambers 110a,b are fluidly coupled to one another by a connecting portion 111. The connecting portion 111 , e.g., a tube that connects a side of the first chamber 110a to a side of the second chamber 110b, means that the atomic vapour may be dispersed throughout both of the chambers 110a,b. The scale bar in image b) is 5 mm.

[0068] Image c) shows a vapour cell 100 in which the surface of the cell has been functionalised by deposition of conductive tracks 120. The tracks have been deposited using inkjet printing. In this instance, the tracks comprise a pair of interdigitated tracks ; a first track 120a comprising silver is interleaved with a second track 120b comprising graphene. Each track 120a, b has a width of 300 microns. A sheet resistance of 0.53 / sq and 207 Q / sq, was achieved for the silver nanoparticle track 120a and the graphene track 120b respectively, which is comparable to the values achieved on other substrates . The tracks 120a, b may be used for heating or magnetic field shielding, for example. Tracks comprising perovskite nanocrystals may be deposited, thereby allowing the tracks to be used as a photon sensor.

[0069] The vapour cells shown in Figure 2 are cuboidal in shape (the two -chambered vapour cell of image b) comprising two connected cuboidal chambers). A cuboidal shape may generally be desirable. The geometry provides five clear surfaces, four of which may allow for two perpendicular beams to pass through the cell with minimal distortion (as per image a)). The cuboidal shape may also be easy to produce and / or may provide flat surfaces for functionalisation. However, it will be understood that the 3D printing process may allow for essentially any geometry to be produced. For example, the performance of the vapour cell in quantum / optical applications may be improved by increasing the optical pathlength, which can be enabled by increasing one or more dimensions of the vapour cell.

[0070] Referring to Figure 3, a 3D printing apparatus 200 is shown. The apparatus 200 may be used during a method of manufacturing a UHV vapour cell (such as those described above). The apparatus comprises a build platform 202, upon which the walls 102 of the UHV vapour cell(s) are built via stereolithographic 3D printing.

[0071] The apparatus comprises a resin bath 204 containing a photocurable resin 206, from which the wall 102 of the UHV vapour cells are formed. The apparatus 200 may be large enough that a plurality of cells can be produced simultaneously, as shown in Figure 3. Although not shown, additional support structures (that do not form part of the final UHV vapour cell) may be 3D printed, thereby providing sufficient support for the walls 102. This may be required for certain vapour cell geometries, e.g., cells that are relatively tall or cells that have overhanging parts. In the case of inverted stereolithographic printing, as shown in Figure 3, light 208 is projected from the underside of the resin bath 204 and directed / controlled as necessary to produce the stereographically printed walls 102. The light 208 may be generated by a UV laser.

[0072] The 3D printing may be performed using a commercially available ultraviolet (UV) stereolithography printer. Apparatus 200 may comprise a Cellink Lumen X stereolithography printer with a 50 -micron hatching distance, for example. In one example, the printer was configured with 6.5 second exposure time for each layer and a UV laser power of 40 mW / cm2at a wavelength of 405 nm. Simulations performed by the inventors showed that these parameters, in conjunction 0.035 wt% absorber content in the resin, provided optimal polymerisation and curing depth. The exact parameters used for the 3D printing may vary dependent on the application (e.g., cell geometry or resin material).

[0073] The photocurable material may comprise a monomer mixture. The monomer mixture may comprise 60 vol% 2-hydroxyethyl methacrylate (HEMA), 10 vol% tetra (ethylene glycol) diacrylate (TEGDA) and 30 vol% phenoxyethanol (POE). The photocurable resin may be degassed (e.g., at 200 mBar vacuum for three minutes), so as to reduce the amount of air trapped in the resin.

[0074] Additives may be added to the monomer mixture, which may improve the 3D printing process and / or the quality of the final UHV vapour cell. The additives may comprise at least one of a photoiniator, an inhibitor and / or a UV absorber. For example, the monomer mixture may be blended with a 0.2 wt% of bis (2,4,6 trimethylbenzoyl) - phenylphosphineoxide as a photoinitiator, 0.1 wt% of hydroquinone monomethyl ether as an inhibitor and 0.035 wt% of Sudan Orange UV absorber (with respect to the amount of monomer mixture).

[0075] A silica nanopowder may be added to the monomer mixture. The silica nanopowder may have an average particle size of 40 nanometers and may be added at 40 vol% of the monomer mixture. The silica nanopowder may be Aerosil 0X50, for example. The silica nanopowder may be gradually added to the monomer mixture under bath sonification, so as to reduce agglomeration.

[0076] The HEMA may form a solvation layer on the nanoparticles, thereby allowing photocurable resin with high silica loading, which may improve the structural and optical properties of the UHV vapour cell. The TEGDA may be used to improve the crosslinking of the polymer, which may produce a stronger mechanical structure. The relative amount of POE may be adjusted to alter the viscosity of the resin, such that it is suitable for printing. 30 vol% POE was found to result in a suitably optimised viscosity of 14.3 mPa- s. Referring to Figure 4, manufacturing steps for producing a UHV vapour cell are illustrated. The 3D printing process described above with regard to Figure 3 may produce a preliminary printed part (also known as a green part). Following the 3D printing, the green part may be washed so as to remove non-polymerised resin. This may comprise immersing the green part in propylene glycol methyl ether acetate (PGMEA) and washing using a roller at 80 rpm for 5 minutes, and repeating the washing three times. The green part may be post cured by being exposed to light, which may increase the mechanical strength of the part. Post curing may comprise exposing the green part to a UV flood light for 10 minutes, for example.

[0077] A debinding step may then be performed, thereby producing a debound part (also referred to as a brown part). The debinding may comprise placing the green part in a furnace at a temperature of up to 600°C. The debinding step may debind (i.e. remove) the polymeric matrix of the vapour cell via thermal decomposition and may help to reduce internal stress. In one example, the debinding step was performed using a Carbolite ash furnace. The temperature of the furnace was increased at a rate of 0.35°C / min up to a temperature of 600°C, with dwellings at 130°C for 2 hours, 320°C for 4 hours and 600°C for 2 hours.

[0078] Despite heat treating the green part in the debinding step, the brown part may have an undesirably high porosity, which would make it unsuitable for use as a UHV vapour cell. It was found that although the brown part had the same geometry as the green part (i.e., no discernible volume shrinkage), there was a 51 % weight loss resulting in a structure with a porous morphology. A sintering step may subsequently be performed to reduce the porosity, whereby the silica nanoparticles within the part merge and form amorphous glass. The sintering step may comprise sintering the brown part in a furnace at a temperature of up to 1150°C. The sintering may be performed using an inert atmosphere, which may help avoid reaction / contamination of the vapour cell. In one example, the sintering step was performed using a Carbolite tube furnace. The brown part was sintered at temperature of 1150°C for 12 hours (or 1100 °C), with 3°C / min heating and cooling rates and a 1 bar argon flow.

[0079] Following the sintering, a finished part is formed. The finished part may be further cleaned or polished in order to improve the surface of the vapour cell. During the sintering process, the cell may experience shrinkage. In one example, the shrinkage was found to be approximately 30% in horizontal direction and vertical direction. The preliminary printed part (green part in Figure 4) comprised a 1 cm3cuboid-shaped chamber with a wall thickness of 1.5 mm. Following sintering, the finished part had dimensions of 7 mm x 7 mm x 7 mm, with a wall thickness of 1.4 mm. The manufacturing method may comprise calculating an expected shrinkage and accounting for this when determining the dimensions of the preliminary printed part, such that the final dimensions of the finished part / UHV vapour cell are as required. The expected shrinkage may be based upon known properties of the photocurable resin, e.g., the solid weight concentration of silica.

[0080] Referring to Figure 5, the effects of doping-based functionalisation of 3D printed glass components and vapour cells are shown. As discussed above, the photocurable resin may be doped prior to 3D printing so as to modify an optical property of the vapour cell. In one example, AuCh was dispersed in the photocurable resin, the concentration of which can be controlled in order to vary the transparency of the resultant 3D printed glass. During UV stereolithography, the Au+within the salt is reduced to form gold nanoparticles. This process may result in nanoparticles with a narrow size distribution, thereby providing sharper absorption peaks.

[0081] Image a) of Figure 5 shows four glass slabs produced using the described 3D printing method, wherein the concentration of Au+is increased from 0 to 2 mM, thereby resulting in a progressively darker cranberry colouring (in order of top left, top right, bottom left, bottom right). The absorption spectroscopy graphs of image b) show that doping the photocurable resin with AuCF in this way results in an absorption peak at approximately 550 nm, as expected for gold nanoparticles. By contrast, providing the doping after the 3D printing (e.g., by immersing the brown part in AuCF isopropanol solution), has been shown to result in a broader absorption peak across 500 nm to 700 nm. Doping of the vapour cell glass may be used to provide integrated optical filters. Additionally, a surface plasma resonance (SPR) of the gold nanoparticles may be used to provide a local, optically-activated heat source (e.g., by shining a corresponding 550 nm laser onto the doped glass).

[0082] It may also be possible for different portions of the vapour cell to be provided with different concentrations of dopant, by selectively providing different dopant concentrations to regions of the photocurable resin during the 3D printing stage. For example, some side walls of the vapour cell may have zero doping, while some walls may be doped. Image c) of Figure 5 shows a printed design comprising doped glass pillars (cranberry colour) of different heights on a non -doped, transparent glass. This may allow for multiple reflections, which may enable longer optical length in small volumes.

[0083] Referring to Figure 6, the effects of deposition-based functionalisation on 3D printed glass components and vapour cells are also shown. Deposition-based functionalisation may be used in addition to, or instead of, doping -based functionalisation. Shown inset of image a), a 1cm by 2mm graphene strip is shown inkjet-printed onto a gold nanoparticle doped glass slab. The graph of image a) depicts the I-V curve of the graphene strip.

[0084] In one example, tracks were inkjet -printed using a Dimatix DMP-2831 inkjet printer and Samba cartridge of 2.4 pL drop volume. For silver tracks (like the 300 micron interdigitated tracks of Figure 2, image c) three layers of silver nanoparticle ink (XTPL IJ36) were deposited using 20 micron drop spacing. For graphene tracks, ten layers of graphene ink (Merck 793663) were printed using 10 micron drop spacing.

[0085] In addition to providing, for example, heating / magnetic shielding, the printed tracks may be used as detectors. The graph of image b) shows the measured change in resistance of the graphene strip caused by optically -induced heating of the gold nanoparticle doped glass slab. The graphene strip was printed onto one side of glass slab, and a green laser light used to illuminate the opposing side. A resistance drop in the graphene of 2% was observed, indicating a temperature change of approximately 30°C. Thus, the SPR-based optical heating of doped 3D printed glass may be used to provide substantial heating for a vapour cell. This may be particularly desirable in certain applications, e.g., for use in optically pumped magnetometers for brain imaging, where heating via current carrying wires or the environment is not desirable.

[0086] Referring to Figure 7, an apparatus 300 is shown for manufacturing and / or analysing UHV vapour cells 100. The apparatus 300 may be used to generate the UHV within the 3D printed vapour cell 100 and to load it with atomic vapour. The apparatus 300 may also be used to conduct spectroscopy analysis of the vapour cell 100, for example. After the vapour cell 100 has been 3D printed (using any of the printing, debinding, sintering and / or functionalisation methods described above), it is coupled to a tube 106. The tube 106 may be an annealed copper tube, though any suitable connection means could be used depending upon the wider apparatus 300. The vapour cell 100 may be coupled to the tube 106 using an adhesive, such as a UV curing glue. The tube 106 is used to fluidly couple the vapour cell to a vacuum system of the apparatus 300; this may be any suitable laboratory / commercial apparatus that is used for manufacturing vapour cells.

[0087] Generally, the tube 106 is coupled to a UHV-flange 302, said UHV-flange 302 being configured to retain the vapour cell 100 in a fixed position and to allow further apparatus to be fluidly coupled to the vapour cell 100. The apparatus 300 further comprises a vacuum / turbo pump 306, the pump 306 being coupled to the flange 302 and the vapour cell 100 so that a UHV can be pulled therein. The pump 306 may be coupled to the flange 302 via a valve 308, e.g., an angle valve. The valve 308 allows the pump 306 to be fluidly disconnected from the vapour cell 100, once the UHV is generated therein. The vapour cell 100 / apparatus 300 may be baked as part of the UHV generation step, so as to enable surface degassing. In one example, the baking process lasted for 24 hours and pressures of 2 x 109mbar may be obtained. Following decoupling of the pump 306 via the valve 308, leak testing may be performed to ensure that the vacuum is retained.

[0088] The apparatus 300 further comprises additional insertion ports / dispensers 304 that are coupled to the UHV flange 302. Following the generation of the UHV, the dispensers 304 are used to insert the atomic vapour into the apparatus 300, so as to load the vapour cell 100. Any suitable material may be loaded into the vapour cell 100 depending on the application. Rubidium and lithium are commonly used reference cells. The atomic vapour may comprise85Rb and87Rb, or a purified version of just one of these isotopes.

[0089] Following the UHV generation and atomic vapour loading, the vapour cell 100 may be analysed while still physically coupled to the vacuum system. The apparatus 300 may comprise absorption spectroscopy apparatus for investigating the atomic absorption lines of the atomic vapour.

[0090] In the example of Figure 7, the apparatus 300 is configured to enable longitudinal spectroscopy and transversal / pump-probe spectroscopy. The apparatus 300 comprises a first detector 310a that is configured to receive a longitudinal spectroscopy beam 312a, the longitudinal beam 312a being provided coaxially within the UHV flange 302 and the tube 106 so as to pass through the vapour cell 100 from the underside (as viewed in Figure 7). The apparatus 300 further comprises a second detector 310b and a pump-probe spectroscopy beam 312b. The pump-probe spectroscopy beam 312b counterpropagates through the vapour cell 100, thereby enabling Doppler-free spectroscopy.

[0091] The apparatus 300 further comprises additional optical apparatus (e.g., a beam splitter 314 and mirror 316) that is used to direct / combine the beams (e.g., laser beams). For simplicity, much of the optical apparatus used for spectroscopy (e.g., beam splitters, waveplates, mirrors, neutral-density filters) has been omitted from the diagram of Figure 7. However, the skilled person would understand that any suitable spectroscopy apparatus may be set up around the apparatus 300; although coupled to the vacuum system, the vapour cell 100 may be used in largely the same way as if it were on an optical bench. Likewise, the skilled person would understand that the detectors 310 (e.g., photodiode detectors) may be coupled to suitable analysis equipment (e.g., spectrum analysers) and that the beams 312 may be provided via one or more lasers (e.g., a laser provided with scan-sweep apparatus) such that the absorption spectra of the atomic vapour can be obtained.

[0092] The apparatus 300 may provide a convenient platform for preliminary analysis of the vapour cell 100 following UHV generation / atomic vapour loading. However, the vapour cell 100 may be decoupled from the apparatus 300, while still retaining vacuum, so that it can be used elsewhere as a standalone reference cell and / or incorporated into other apparatuses, for example. The tube 106 may be ‘pinched off’ so as to form a cap on the vapour cell, as shown in Figure 1. The pinching off may comprise compressing (e.g., crimping) the copper tube so that an airtight seal is formed where once the interior of the tube 106 coupled the vapour cell 100 to the vacuum system. Once decoupled, the 3D printed vapour cell 100 be used in place of a conventional vapour cell in any suitable atomic system, e.g., in an optically pumped magnetometer.

[0093] Referring to Figure 8, analysis using apparatus like that of Figure 7 may be used to obtain Doppler-free spectra from the 3D printed vapour cell. In one example, the vapour cell was loaded with Rb and interrogated with light from a Toptica laser that was tuned to the rubidium absorption lines. Using a pump-probe spectroscopy setup (0.06 mW probe beam power and 0.4 mW pump beam power, with beam diameters of 2 mm), the hyperfine structure for87Rb F = 2 — > F' = 1, 2, 3 and85Rb F = 3 — > F' = 2, 3, 4 can be obtained.

[0094] Referring to Figure 9, the hyperfine structure may be used for laser locking; a common application for Rb vapour cells. By using the sharp hyperfine absorption peaks of the Rb as a reference, the laser may ‘locked’ to the region around to this frequency us ing a lock in amplifier. This may be used to prevent laser frequency drift, thereby improving frequency stability. During analysis of the 3D printed vapour cell, the laser was locked to the85Rb Cooler transition. The error signal from the lock in amplifier was measured over a period of twenty minutes whilst the laser was locked using the 3D printed vapour cell. A measurement with the laser unlocked was also taken, so that a comparison could be made with the inherent stability of the laser itself. The Allan deviation of all three measurements was calculated for a range of time scales ; the results for the 3D printed vapour cell and the unlocked laser are shown in image a) of Figure 9. The 3D printed vapour cell is significantly better than the unlocked laser for all time scales. The same measurement was also done for a Thorlabs 75mm standard vapour cell, with the comparison against the 3D printed vapour cell shown in image b) of Figure 9. The 3D printed cell is also shown to have a comparable or even better Allan deviation than the normal cell adjusted for the ratio of the corresponding optical path lengths (Doppler- free spectroscopy requires the beam to pass through the cell twice, so the Thorlabs vapour cell has a longer path length 15 times greater than the 1 cm33D printed vapour cell).

[0095] Referring to Figures 10 and 11 , the results of investigations of the optical properties a 3D printed vapour cell are shown. The optical properties of a vapour cell are of substantial importance for quantum technologies; any defects or aberrations may significantly impact the ability to measure quantum-level effects.

[0096] In one example, a 3D printed vapour cell with horizontally -built surfaces had some surface roughness and the undulations can be seen at the vertically -built walls, caused by the layer-by-layer printing process. Following an additional polishing step (using grit 1000 sanding paper) to minimise the outside wall undulations, the cell caused small distortions to the Gaussian beam; Figure 10 shows an image of the beam scattering and graphs of the measured beam profile in x and y directions. The intensity parallel to the building surface showed negligible distortions while the intensity distribution perpendicular to the building surface encountered more scatterings and exhibited widened Gaussian shape. However, the overall transparency is > 90% across the wavelength range from 400 nm to 1000 nm, as shown in Figure 5 (at least for clear glass, i.e., not doped).

[0097] Further investigations were conducted to investigate how polarisation of the light beam was affected by the 3D printed vapour cell. In one example, optical components were set up such that a laser passed through a polarising beamsplitter (PBS) and a halfwaveplate, before going through the 3D printed vapour cell (which was under internal vacuum so that no gases in the cell could alter the polarisation; it was the optical characteristics of the cell itself that were being tested). The laser then travelled through another PBS and onto a photodiode. The photodiode was placed to be almost touching the PBS, which in turn was placed very close to the vapour cell. This was to reduce the optical pathlength to be as small as possible after the cell, as the cell walls caused the laser to disperse and become divergent upon exit. The half-waveplate was then turned through 90°, and the voltage signal from the photodiode was recorded. Measurements were taken with and without the cell present, and can be seen in Figure 11 , image a). The signal from the photodiode was normalised in both cases, since the magnitude of the signal with the cell present was weaker than without the cell. The data shows a ‘sinusoidal-squared’ curve for both with and without the vapour cell, which is to be expected since intensity oc sin20 (or oc cos20 depending on p- or s- polarisation of interest) for the linear polarisation angle 0 made with the plane of incidence of an (ideal) PBS. The data shows that there is a slight decrease in maximal transition when the 3D printed vapour cell is present, indicating that the vapour cell adds some ellipticity to light.

[0098] The experimental setup was then adjusted to take a measurement of the polarising stability of the 3D printed vapour cell. The polarising beamsplitter -photodiode system was removed and replaced by a polarisation analyser, with the rest of the setup remaining. The half-waveplate was arbitrarily set to 0°. The polarisation analyser then recorded the polarisation of the light, characterised by the azimuth and ellipticity, ov er a given time, with and without the presence of the vapour cell in the beam path. The data was smoothed by taking a rolling average, with a window size 6% of the total data pool, to reduce the noise caused from the precision of the polarisation analyser (±0.1 °). The results are shown in Figure 1 1 , image b) and show that the polarisation is stable with the vapour cell present. The results indicate that while the linearity of the polarisation characterised by the azimuth remains unchanged, the cell causes a subtle increase in ellipticity (as also indicated by image a) albeit rather small ( ~ 0.3°). This is mostly caused by imperfect optical quality and dispersivity of the cell. Since this change is not substantial for most purposes of a vapour cell, the polarisation can be regarded as remaining stable and unchanged.

[0099] The experimental setup was further modified to include a linear polariser before the half-waveplate, to ensure that the light going into the waveplate was linearly polarised (though a small amount of ellipticity in the light is expected after the linear polariser due to imperfections) such that varying polarisations of light via the wavepl ate through the vapour cell could be analysed. The half-waveplate was then turned through 90°, with a measurement taken every 4° on the polarisation analyser, providing the azimuth and ellipticity. This was repeated with and without the vapour cell, and results are seen in Figure 11 , image c). The half-waveplate was then switched for a quarter-waveplate and the same measurement technique was repeated, this time going through 180°. The results can be seen in Figure 1 1 , image d). In both cases, minimal changes to the polarisation are seen. Small changes of ellipticity for given waveplate turns are observed, which matches the data discussed above. A slight change to the azimuth is also observed; adding the vapour cell has the same effect as turning the half-waveplate by ~ 4°.

[0100] Although specific examples have been described, the skilled person will appreciate that variations are possible, within the scope of the invention, which should be determined with reference to the accompanying claims.

Claims

CLAIMS1. A method of manufacturing an ultra-high vacuum vapour cell, the method comprising: forming a glass cell via 3D printing; generating an ultra-high vacuum within the glass cell; and depositing an atomic vapour within the glass cell.

2. The method of claim 1 , wherein forming the glass cell via 3D printing comprises : providing a photocurable resin; and using ultraviolet stereolithography to produce a preliminary printed part from the photocurable resin.

3. The method claim 2, wherein forming the glass cell via 3D printing further comprises: washing the preliminary printed part so as to remove photocurable resin that has not been cured by the ultraviolet stereolithography; and debinding the preliminary printed part at a temperature of between 500°C and 700°C, thereby forming a debound part.

4. The method of claim 3, wherein forming the glass cell via 3D printing further comprises: sintering the debound part at a temperature of between 1050° and 1150°C.

5. The method of any of claims 2 to 4, wherein the photocurable resin comprises: a monomer mixture; and an ultraviolet absorber at between 0.03 wt% and 0.04 wt% with respect to the monomer mixture.

6. The method of claim 5, wherein the photocurable resin further comprises: between 37.5 vol% and 42.5 vol% silica nanopowder with respect to the monomer mixture, the silica nanopowder having an average particle size of between 30 nanometres and 50 nanometers.

7. The method of any of claims 2 to 6, further comprising: prior to using ultraviolet stereolithography, doping the photocurable resin with a dopant configured to modify an optical property of the glass cell.

8. The method of claim 7, wherein the dopant comprises a material that is reactive to the ultraviolet stereolithography so as to produce nanoparticles.

9. The method of claim 8, wherein the dopant comprises a salt and the nanoparticles are metallic nanoparticles provided via reduction of metal ions comprising the salt.

10. The method of any preceding claim, further comprising depositing one or more layers of conductive or semiconductive material onto a surface of the glass cell.

11. The method of claim 10, wherein depositing the one or more layers comprises printing an ink onto the surface of the glass cell, the ink comprising the conductive or semiconductive material.

12. The method of any preceding claim, wherein generating an ultra-high vacuum within the glass cell comprises: coupling the glass cell to a tube; and coupling the tube to a vacuum system.

13. The method of claim 12, comprising using a valve to fluidly decouple the glass cell from the vacuum system such that the ultra-high vacuum is maintained within the glass cell.

14. The method of claim 12 or 13, further comprising pinching off the tube such that the ultra-high vacuum is maintained within the glass cell and the ultra-high vacuum vapour cell may be separated from the vacuum system.

15. The method of any preceding claim, wherein the glass cell comprises at least two chambers, the chambers being fluidly coupled to one another.

16. The method of any preceding claim, wherein the glass cell has a volume no greater than 1 cm3.

17. A method of using a 3D printed ultra-high vacuum vapour cell in an optically pumped magnetometer, the method comprising: providing a 3D printed ultra-high vacuum vapour cell containing an atomic vapour; illuminating the atomic vapour with a light source , thereby transferring a plurality of atoms of the atomic vapour to an optically pumped state; and measuring, using a detector, a transmission of light from the light source through the ultra-high vacuum vapour cell, the transmission corresponding to the proportion of the atoms of the atomic vapour that are in the optically pumped state.

18. The method of claim 17, wherein the light source is a laser that is tuned to an atomic transition frequency of the atomic vapour.

19. The method of claims 17 or 18, wherein the light used to illuminate the atomic vapour is circularly polarised such that atoms are pumped to a steady state.

20. The method of any of claims 17 to 19, wherein the 3D printed ultra-high vacuum vapour cell comprises an optically absorptive dopant, and wherein the method comprises illuminating the optically absorptive dopant so as to induce heating.

21. The method of any of claims 17 to 20, wherein the optically pumped magnetometer is operated in a spin exchange relaxation free, SERF, mode such that the measured transmission of light through the ultra-high vacuum vapour cell is dependent upon an external magnetic field.

22. The method of claim 21, wherein the optically pumped magnetometer is used to measure a biomagnetic field.

23. An ultra-high vacuum vapour cell, the ultra-high vacuum vapour cell comprising: a 3D printed glass cell, the glass cell retaining an ultra-high vacuum; andan atomic vapour contained within the glass cell.

24. An optically pumped magnetometer comprising: an ultra-high vacuum vapour cell comprising: a 3D printed glass cell, the glass cell retaining an ultra-high vacuum; and an atomic vapour contained within the glass cell; a light source configured to illuminate the atomic vapour; and a detector configured to measure transmission of light from the light source through the ultra-high vacuum vapour cell.

25. The method of any of claims 17 to 22, wherein providing a 3D printed ultra-high vacuum vapour cell comprises manufacturing the vapour cell according to any of claims 1 to 17; or the ultra-high vacuum vapour cell of claim 23 or the optically pumped magnetometer of claim 24, wherein the ultra-high vacuum vapour cell is manufactured according to any of claims 1 to 17.

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