Josephson junction resistance tuning

Abstract

The disclosure is related to the field of superconducting integrated circuits, in particular to methods for fabricating and tuning superconducting integrated circuit elements such as Josephson junctions. The disclosure includes exposing a Josephson junction (210) to an electron beam, ion beam, or electromagnetic radiation (205) in order to modify the resistance of the Josephson junction.

Description

[0001] JOSEPHSON JUNCTION RESISTANCE TUNING

[0002] Technical Field

[0003] The invention is in the field of superconducting integrated circuits, in particular methods for fabricating and tuning superconducting integrated circuit elements such as Josephson junctions.

[0004] Background

[0005] Superconducting qubits contain one or multiple Josephson junctions. A Josephson junction comprises two superconducting regions separated by a non-superconducting barrier, usually an electrical insulator.

[0006] The qubit frequency, i.e. the frequency difference between the ground state and first excited state, of a transmon qubit at cryogenic temperatures can be predicted from the normal state resistance of the Josephson junctions, i.e. their electrical resistance measured at room temperature. Consequently, the resistance of the Josephson junction(s) is carefully controlled in order to produce qubits with a desired qubit frequency.

[0007] Fine-grained control over qubit frequencies is required in order to scale up quantum computers, i.e. to increase the number of qubits in a single quantum processing unit. For example, frequency crowding makes it more difficult to address individual qubits as the number of qubits increases - a lack of precise control over qubit frequencies exacerbates the problem. However, even state of the art Josephson junction fabrication processes cannot produce Josephson junctions with the desired precision in their resistance values.

[0008] Prior art methods of modifying qubit frequencies include annealing Josephson junctions using a thermal source such as a laser to modify the resistance of the Josephson junction, for example as described in US 10,340,438 B2 and C. Granata et al, 2008 J. Phys.: Conf. Ser. 97 012110.

[0009] Summary of the Invention

[0010] A first aspect of the invention is a method for modifying the normal state resistance of a Josephson junction, said Josephson junction being provided on a substrate. The method comprises at least partially exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction to an electron beam, ion beam, or electromagnetic radiation. The method may comprise decreasing the normal state resistance of the Josephson junction relative to an initial resistance value by at least partially exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction with the electron beam, ion beam, or electromagnetic radiation.

[0011] The decrease in the normal state resistance of the Josephson junction relative to the initial resistance value may be greater than 5%, in particular greater than 10% or greater than 15%.

[0012] The method may further comprise increasing the normal state resistance of the Josephson junction relative to the initial resistance value by continuing to at least partially expose the Josephson junction and / or the region of the substrate close to the Josephson junction to the electron beam, ion beam, or electromagnetic radiation after decreasing the normal state resistance.

[0013] The increase in the normal state resistance of the Josephson junction relative to the initial resistance value may be greater than 5% of the initial resistance value, greater than 10% of the initial resistance value, or greater than 30% of the initial resistance value.

[0014] The parameters of the electron beam, ion beam, or electromagnetic radiation, and / or the parameters of the electron beam exposure, ion beam exposure, or electromagnetic radiation exposure, may be selected in order to control the resistance change of the Josephson junction.

[0015] The parameters of the electron beam exposure, ion beam exposure, or electromagnetic radiation exposure may include the exposure time during which the Josephson junction is exposed to the electron beam, ion beam, or electromagnetic radiation, and wherein the exposure time is selected to control the magnitude and direction of the resistance change of the Josephson junction.

[0016] The method may further comprise measuring the normal state resistance of the Josephson junction before at least partially exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction to the electron beam, ion beam, or electromagnetic radiation.

[0017] At least partially exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction to the electron beam, ion beam, or electromagnetic radiation may comprise scanning the Josephson junction with the electron beam, ion beam, or electromagnetic radiation. The scanning pattern and / or scanning speed may be selected in order to control the magnitude and direction of the resistance change of the Josephson junction. Alternatively, exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction to the electron beam, ion beam, or electromagnetic radiation may comprise exposing a single area or a number of discrete areas to a stationary electron beam, ion beam, or electromagnetic radiation, i.e. without scanning.

[0018] The Josephson junction may be part of a superconducting qubit, such that decreasing the normal state resistance of the Josephson junction increases the frequency of the qubit and increasing the normal state resistance of the Josephson junction decreases the frequency of the qubit.

[0019] One or both of the exposure area and acceleration voltage of the electron beam or ion beam may be selected in order to control the volume of the Josephson junction and / or a region of the substrate exposed to the electron beam or ion beam.

[0020] The method may further comprise imaging the Josephson junction, in particular using an electron beam or ion beam, prior to at least partially exposing the Josephson junction to the electron beam, ion beam or electromagnetic radiation.

[0021] The exposure area of a Josephson junction and / or a region of the substrate to be exposed may be determined based on the imaging of the Josephson junction.

[0022] The beam used for imaging the Josephson junction may be provided by the same source as the electron beam or ion beam used for modifying the normal state resistance of the Josephson junction.

[0023] The electron beam may be a focused electron beam.

[0024] The ion beam may be a focused ion beam.

[0025] The ion beam may be a helium ion beam, neon ion beam, or other noble gas ion beam.

[0026] A second aspect of the invention is a method of manufacturing a qubit. The method comprises manufacturing a qubit, wherein manufacturing the qubit includes providing at least one Josephson junction and modifying the qubit frequency by modifying the normal state resistance of the at least one or more Josephson junction according to the method of any preceding claim.

[0027] A third aspect of the invention is an apparatus for fabricating one or more Josephson junctions on a substrate comprising an electron beam source, in particular a focused electron beam source, an ion beam source, in particular a focused ion beam source, or an electromagnetic radiation source, in particular a laser, and configured to expose at least one Josephson junction and / or a region of the substrate close to the at least one Josephson junction to an electron beam, ion beam, or electromagnetic radiation to modify its resistance according to the method of any of claims 1 to 17.

[0028] Brief Description of the Drawings

[0029] Figure 1 is a flow chart depicting a method for modifying the normal state resistance of a Josephson junction.

[0030] Figure 2 depicts a first method of exposing a Josephson junction and / or a region of a substrate to an electron beam, ion beam, or electromagnetic radiation.

[0031] Figure 3 depicts a second method of exposing a Josephson junction and / or a region of a substrate to an electron beam, ion beam, or electromagnetic radiation.

[0032] Figure 4 depicts a third method of exposing a Josephson junction and / or a region of a substrate to an electron beam, ion beam, or electromagnetic radiation.

[0033] Detailed Description

[0034] Figure 1 is a flow chart depicting a method 100 for modifying the normal state resistance of a Josephson junction. In this context, “normal state resistance” means the resistance of the Josephson junction at a non-superconducting temperature.

[0035] At step 101 , the Josephson junction is provided on a substrate. The Josephson junction may be manufactured according to any suitable method, for example the “Manhattan” method described in Potts, A., Routley, P.R., Parker, G.J. et al. Novel fabrication methods for submicrometer Josephson junction qubits. Journal of Materials Science: Materials in Electronics 12, 289-293 (2001 )., or the Dolan method described in G. J. Dolan, Offset masks for lift-off photoprocessing, Appl. Phys. Lett. 31 , 337-339 (1977). The Josephson junction in general has a superconductor-insulator-superconductor structure. Some exemplary commonly used superconductors are aluminium, niobium, tantalum or titanium nitride. The insulator layer is typically a very thin layer of oxide, e.g., aluminium oxide. The substrate may be silicon, sapphire, ceramic or any other material suitable for use as a substrate for superconducting integrated circuits.

[0036] The Josephson junction may be formed as a component of a superconducting qubit, for example as a transmon qubit as described in Koch, J. et al. Charge-insensitive qubit design derived from the Cooper pair box. Phys. Rev. A, 76(4), 042319 (2007), or a unimon qubit as described in Hyyppa, E., Kundu, S., Chan, C.F. et al. Unimon qubit. Nat Commun 13, 6895 (2022).

[0037] When the Josephson junction is formed as part of a qubit, the Josephson junction’s normal state resistance accounts for almost the entire normal state resistance of the qubit. The qubit frequency, i.e. the transition frequency corresponding to the transition from the qubit ground state to the first excited state, which are typically used to represent computational basis states, varies based on the normal state resistance of the qubit. Therefore, by modifying the normal state resistance of the Josephson junction, the normal state resistance of the qubit can be modified thereby modifying the qubit frequency, allowing fine tuning of the qubit frequency.

[0038] In the method of the present invention, the normal state resistance of the Josephson junction can be either decreased or increased by exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction to an electron beam, ion beam, or to electromagnetic radiation. The present invention therefore provides more flexible control for fine-tuning Josephson junction resistances and consequently for fine-tuning qubit frequencies. In this context “exposing” the Josephson junction and / or region of the substrate to an electron beam, ion beam or electromagnetic radiation means that the electron beam, ion beam or electromagnetic radiation is incident on the Josephson junction and / or region of the substrate. The term “exposing” may include irradiating the Josephson junction and / or region of the substrate with the electron beam, ion beam or electromagnetic radiation.

[0039] Step 101 of providing the Josephson junction may also include placing the entire quantum processing unit inside an annealer in order to cause global thermal annealing, which artificially expedites the oxide aging process. The purpose of this step is to make the oxide become more stable so that the qubit resistance shows almost no further change for an extended period even when stored in ambient environment.

[0040] After the Josephson junction has been provided, the normal state resistance of the Josephson junction or the normal state resistance of the qubit that the Josephson junction is part of may be measured at step 102. After measuring the normal state resistance of the Josephson junction or qubit, the desired change in the Josephson junction resistance, qubit resistance of qubit frequency can, at step 103, be used to select, i.e. determine, the parameters of the electron beam, ion beam, or electromagnetic radiation, e.g. from a laser, and the parameters of the electron beam exposure, ion beam exposure, or electromagnetic radiation exposure.

[0041] Exposure of the Josephson junction and / or a region of the substrate adjacent to the Josephson junction by the electron beam, ion beam, or electromagnetic radiation leads to an increase in normal state resistance of the Josephson junction under some conditions and a decrease in the normal state resistance of the Josephson junction under other conditions. Exposure of the Josephson junction and / or a region of the substrate adjacent to the Josephson junction by the electron beam, ion beam, or electromagnetic radiation may lead to a change in the normal state resistance of the Josephson junction greater than 5%.

[0042] In particular, exposure of the Josephson junction and / or a region of the substrate to the electron beam, ion beam or electromagnetic radiation may lead to an increase in the normal state resistance of the Josephson junction depending on the exposure parameters. The increase in the normal state resistance of the Josephson junction may be greater than 5%, in particular greater than 10%, more in particular of the order of 30% or more.

[0043] Exposure of the Josephson junction to the electron beam, ion beam or electromagnetic radiation may also lead to decreased normal state resistance under certain conditions. The decrease in the normal state resistance of the Josephson junction may be greater than 5%, in particular greater than 10% or greater than 15%.

[0044] Where an electron beam is used, by controlling the parameters of the electron beam, such as acceleration voltage and electron beam current, and the parameter of the electron beam exposure, such as the area of the Josephson junction and / or the region of the substrate that is exposed to the electron beam, and the length of time for which the Josephson junction and / or the region of the substrate are is exposed to the electron beam, the change in the normal state resistance of the Josephson junction can be controlled.

[0045] The parameters of the electron beam and electron beam exposure that may be controlled include the exposure area (or scanning area, if scanning is used), the acceleration voltage, exposure time (or scanning speed, if scanning is used), and electron beam current (or electron beam source aperture size). The exposure (or scan) area and the acceleration voltage determine the volume of the Josephson junction to be treated, while the beam current (or aperture size) and the exposure time (or scan speed) determine the energy transferred to the Josephson junction by the electron beam. The relationship between the parameters mentioned above and the change in the resistance of the Josephson junction vary based on the materials used to manufacture the Josephson junction, the method of manufacturing the Josephson junction, and the size of the Josephson junction, amongst other factors. The relationship between these parameters and the change in the resistance value for a given set of these factors can be readily determined by experiment, by simply measuring the change in resistance produced by different combinations of parameters. This list of parameters should not be seen to be limiting. Where an ion beam is used, by controlling the parameters of the ion beam, such as acceleration voltage, focus, and beam current, the area of the Josephson junction that is exposed to the ion beam, and the length of time for which the Josephson junction is exposed to the ion beam, the change in the normal state resistance of the Josephson junction can be controlled.

[0046] The parameters of the ion beam and ion beam exposure that may be controlled include the exposure area (or scanning area, if scanning is used), the acceleration voltage, exposure time (or scanning speed, if scanning is used), and ion beam current (or ion beam source aperture size). The exposure (or scan) area and the acceleration voltage determine the volume of the Josephson junction to be treated, while the beam current (or aperture size) and the exposure time (or scan speed) determine the energy transferred to the Josephson junction by the ion beam. The relationship between the parameters mentioned above and the change in the resistance of the Josephson junction vary based on the materials used to manufacture the Josephson junction, the method of manufacturing the Josephson junction, and the size of the Josephson junction, amongst other factors. The relationship between these parameters and the change in the resistance value for a given set of these factors can be readily determined by experiment, by simply measuring the change in resistance produced by different combinations of parameters. This list of parameters should not be seen to be limiting.

[0047] Where electromagnetic radiation is used, by controlling the parameters of the electromagnetic beam (such as wavelength, power, and beam width), the area of the Josephson junction that is exposed to the electromagnetic radiation, and the length of time for which the Josephson junction is exposed to the electromagnetic radiation, the change in the normal state resistance of the Josephson junction can be controlled. An example of an electromagnetic radiation can be a laser, but other focused electromagnetic radiation can also be used.

[0048] The parameters of the electromagnetic radiation and electromagnetic radiation exposure that may be controlled include the exposure area or beam width (or scanning area, if scanning is used), the power or flux of electromagnetic radiation incidence on the Josephson junction, exposure time (or scanning speed, if scanning is used), and wavelength. The wavelength and exposure (or scan) area determine the volume of the Josephson junction to be treated, while the power or flux and the exposure time (or scan speed) determine the energy transferred to the Josephson junction by the electromagnetic radiation. The relationship between the parameters mentioned above and the change in the resistance of the Josephson junction vary based on the materials used to manufacture the Josephson junction, the method of manufacturing the Josephson junction, and the size of the Josephson junction, amongst other factors. The relationship between these parameters and the change in the resistance value for a given set of these factors can be readily determined by experiment, by simply measuring the change in resistance produced by different combinations of parameters. This list of parameters should not be seen to be limiting.

[0049] The table below shows the initial resistances of two Josephson junctions and their resistances measured after 5, 10 and 15 minutes, where one Josephson junction was exposed to an electron beam, and another was not exposed to the electron beam, thus being used as a test Josephson junction. The measurement of the normal resistance of the not exposed Josephson junction demonstrates that the change in resistance of the Josephson junction is indeed caused by exposure to the electron beam rather than environmental factors, with a small and negligible variation in resistance values.

[0050] The electron beam parameters for exposure and modification of the normal resistance of the Josephson junction can be as follows, namely electron beam current of the order of 25 nA, acceleration voltage of the order of 20 kV, magnification of the order of x18,000, field of view (i.e. total exposure area) of the order of 2.75pm2and working distance of the order of 4.7mm.

[0051] Om 5m 10m 15m

[0052] Exposed to electron beam 7340 6999 6148 9491

[0053] Not exposed (control) 7325 7358 7350 7358

[0054] Table A

[0055] This table further demonstrates that exposure of a Josephson junction to an electron beam first leads to an initial decrease in resistance after 5 minutes of exposure and a further decrease after 10 minutes of exposure, followed by a large increase in resistance after 15 minutes of exposure. The initial decrease in the normal state resistance observed is greater than 10%, in contrast to prior art methods for adjusting Josephson junction resistance using thermal annealing, which only leads to an increase in the Josephson junction resistance or negligible decrease in resistance of around 1%. The subsequent increase in the normal state resistance observed is greater than 10%, in particular of the order of 30% for a 15 minute exposure time, as can be seen in Table A.

[0056] Thus, in the method of the present invention, the normal resistance of the Josephson junction can be either decreased or increased by selecting the appropriate parameters of the electron beam, ion beam, or electromagnetic radiation and / or appropriate parameters of the electron beam exposure, ion beam exposure, or electromagnetic radiation exposure. The present invention therefore provides more flexible control for fine-tuning Josephson junction resistances and qubit frequencies.

[0057] The effect of the electron beam, ion beam or electromagnetic radiation exposure on the resistance of the Josephson junction persists after the electron beam, ion beam or electromagnetic radiation is removed.

[0058] By performing steps 102 and 103, the method 101 can advantageously be adapted to each Josephson junction on a given QPU chip, taking into account small variations in the manufacturing process in order to precisely fine tune the Josephson junction resistance / qubit frequency. However, these steps may not necessarily be performed every time the method 100 is performed. For example, if Josephson junctions are repeatedly produced using the same method each time, the properties of the Josephson junctions may be similar enough that using the same parameters for each Josephson junction provides a sufficiently accurate change in the Josephson junction resistance.

[0059] At step 104 of the method 100, the Josephson junction may be imaged using an electron beam, e.g. in an electron microscope, in order to determine the precise location for electron beam, ion beam or electromagnetic radiation exposure in step 105. This may be particularly advantageous where the subsequent step of at least partially exposing the Josephson junction to effect a change in the resistance is performed using the same electron beam, for example of the same electron microscope.

[0060] Alternatively, at step 104 of the method 100, the Josephson junction may be imaged using an ion beam, e.g. in a focused ion beam microscope, in order to determine the precise location for electron beam, ion beam or electromagnetic radiation exposure in step 105. This may be particularly advantageous where the subsequent step of at least partially exposing the Josephson junction to effect a change in the resistance is performed using the same ion beam source, for example of the same focused ion beam microscope.

[0061] However, other imaging techniques, such as atomic force microscopy or focused ion beam may be used instead of electron beam microscopy or ion beam microscopy.

[0062] Steps 102, 103, 104 and 104 may be performed in any suitable order, for example steps 104 and 105 may be performed in parallel with steps 102 and 103, before steps 102 and 103, or after steps 102 and 103.

[0063] Steps 104 and 105 may not necessarily be performed every time the method 100 is performed. Imaging the Josephson junctions with the electron beam or ion beam, does not have a significant effect on the resistance of the Josephson junctions given the relatively short exposure time and lower beam energy or beam focus used for electron beam and focused ion beam imaging than that used to modify the resistance of the Josephson junction.

[0064] As already mentioned above, it is not essential that the same electron microscope or focused ion beam microscope be used for step 106 of at least partially exposing the Josephson junction to the electron beam or ion beam in order to change the resistance of the Josephson junction. An electron microscope, ion beam microscope (or other imaging system such as an atomic force microscope) may be used to determine the target location for the exposure to the electron beam, ion beam or electromagnetic radiation in step 106, e.g. relative to the chip or wafer on which the Josephson junction is located, and a separate electron beam source, ion beam source or electromagnetic radiation source may be used to change the resistance of the Josephson junction at step 106.

[0065] Under imaging conditions, the change in resistance during imaging is relatively low Nevertheless, while the change in the resistance of the Josephson junction during imaging is relatively low, it may be taken into account in the overall process of modifying the Josephson junction resistance, for example by performing steps 102 and 103 after imaging has been performed in step 104, or repeating steps 102 and 103 after imaging in step 104. Basically, after imaging, the resistance will be measured again and will be considered as the initial resistance for determining the parameters to obtain the desired resistance.

[0066] Step 106 of the method 100 includes at least partially exposing the Josephson junction and / or a region of the substrate on which the Josephson junction is located to the electron beam, ion beam, or electromagnetic radiation. Exposure of the Josephson junction and / or a region of the substrate to the electron beam, ion beam or electromagnetic radiation may include exposing: part of the Josephson junction, the whole Josephson junction, part of the substrate, part of the Josephson junction and part of the substrate, or the whole Josephson junction and part of the substrate. Where part of the substrate is exposed to the electron beam, ion beam or electromagnetic radiation, the exposed part of the substrate is preferably a region adjacent to the Josephson junction, such that thermal energy deposited in the part of the substrate by exposure of the substrate is transferred to the Josephson junction.

[0067] In one embodiment, the electron beam is generated by an electron beam source, in particular an electron microscope. However, the method of the present invention is not limited to use with electron beams generated by electron microscopes. It will be understood that except where electron microscopy is explicitly used, other electron beam sources may be used to generate the electron beam to which the Josephson junction and / or a region of the substrate is exposed. The electron beam can be a focused electron beam or a nonfocused electron beam.

[0068] In one embodiment, the ion beam is generated by an ion beam source, in particular a focused ion beam microscope. However, the method of the present invention is not limited to use with ion beams generated by focused ion beam microscopes. It will be understood that except where focused ion beam microscopy is explicitly used, other ion beam sources may be used to generate the ion beam used to modify the normal state resistance of the Josephson junction. The ion beam can be a focused ion beam or a non-focused ion beam.

[0069] In one embodiment of the present invention, the method for modifying the normal state resistance of a Josephson junction may include only step 106. This may be useful in cases whether the needed parameters for the electron beam, ion beam or electromagnetic radiation exposure are already known, for example in repeated production runs, or when the parameters are determined separately.

[0070] Figure 2 depicts a first method of exposing a Josephson junction 210 and / or a region of the substrate according to the invention to an electron beam, ion beam, or electromagnetic radiation.

[0071] The Josephson junction 210 depicted in Figure 2 is a generic Josephson junction formed by a process such as that as described in Potts, A., Routley, P.R., Parker, G.J. etal.: “Novel fabrication methods for submicrometer Josephson junction qubits”, Journal of Materials Science: Materials in Electronics 12, 289-293 (2001 ). However, the present invention is not limited to use with Josephson junctions of a particular form or method of manufacture provided that the Josephson junction 210 and / or adjacent substrate can be at least partially exposed to an electron beam, ion beam, or electromagnetic radiation. The Josephson junction 210 is formed on a substrate 201 and is made up of a base electrode 202, dielectric layer 203, and counter electrode 204. However, further layers can also be part of the Josephson junction. For example, each layer mentioned above can be made of a combination of layers.

[0072] The electrical properties of the Josephson junction 210, such as its normal state resistance, are determined by the properties of the overlapping regions of base electrode 202, dielectric layer 203 and counter electrode 204. In some contexts, the term “Josephson junction” may be understood to mean only the overlapping regions of the base electrode 202, dielectric layer 203 and counter electrode 204, but in the present context the term “Josephson junction” should be understood to mean the overlapping regions of the base electrode 202, dielectric layer 203, and counter electrode 204 as well as adjacent parts of the base electrode 203 and counter electrode 204. In this context, “adjacent” may mean the parts of the base electrode 203, and counter electrode 204 close enough to overlapping regions of the base electrode 203, dielectric layer 203, and counter electrode 204 such that when they are exposed to the electron beam, ion beam or electromagnetic radiation in accordance with the present method a change in the normal state resistance of the Josephson junction 210 is observed.

[0073] In the example of Figure 2, an electron beam, ion beam or electromagnetic radiation 205 is incident on a region 206 on the upper surface of the counter electrode 204. In this context, “upper” means distal to the substrate 201. In Figure 2, the exposed region 206 is significantly smaller than the surface area of the Josephson junction 210, thus the electron beam, ion beam or electromagnetic radiation 205 may be scanned across the Josephson junction 210 and / or substrate in order to expose more of the Josephson junction 210. An example scanning pattern 207 is shown in Figure 2, but this specific scanning pattern should not be seen as limiting on the invention. Other scanning patterns may also be used. Furthermore, while the scanning pattern 207 is shown entirely within the overlapping regions of the base electrode 202, dielectric layer 203, and counter electrode 204, other scanning patterns may be used that extend partially or even wholly outside of the overlapping region.

[0074] Figure 3 shows an alternative embodiment to that of Figure 2 in which the electron beam, ion beam or electromagnetic radiation 305 is incident on the Josephson junction 210 without scanning, i.e. without moving the electron beam, ion beam or electromagnetic radiation during exposure. A stationary electron beam, ion beam or electromagnetic radiation as depicted in Figure 3 may be used, for example, where a larger region 306 on the upper surface of the counter electrode 204 is exposed. While Figure 3 shows only the upper surface of the counter electrode 204 being exposed to the electron beam, ion beam or electromagnetic radiation 306, other components of the Josephson junction 210, such as the base electrode 202, may also be directly exposed to a stationary electron beam, ion beam or electromagnetic radiation 306, either alternatively or in addition to the counter electrode 204. Furthermore, the substrate may be exposed to a stationary electron beam, ion beam or electromagnetic radiation in regions adjacent to the Josephson junction 210, either alternatively or in addition to the counter electrode 204 and any other components of the Josephson junction 210.

[0075] Figure 4 shows a further alternative embodiment in which a region 406 of the substrate 201 adjacent to the Josephson junction 210 is exposed to the electron beam, ion beam or electromagnetic radiation 405. The electron beam, ion beam or electromagnetic radiation 405 may be stationary, as depicted in Figure 3, or it may move with a scanning pattern as depicted in Figure 2, for example. The electron beam, ion beam or electromagnetic radiation may alternatively be incident on both the Josephson junction 210 and the substrate 201 simultaneously.

[0076] The method for modifying the normal state resistance of a Josephson junction described above may be part of a larger method of manufacturing a qubit. As explained above, increasing the normal state resistance of the Josephson junction decreases the frequency of the qubit including the Josephson junction. Conversely, decreasing the normal state resistance of the Josephson junction increases the frequency of the qubit including the Josephson junction. As such, a method of manufacturing a qubit, may include first manufacturing the qubit, which includes the Josephson junctions and subsequently modifying the qubit frequency by modifying the normal state resistance of the at least one Josephson junction using the method described above.

[0077] The method for modifying the normal state resistance may be performed using a specific apparatus or device for modifying the normal state resistance of a Josephson junction. Such an apparatus may include a stage for retaining a substrate on which the Josephson junction is disposed and an electron beam source, ion beam source or electromagnetic radiation source configured to provide the electron beam, ion beam or electromagnetic radiation to which the Josephson junction and / or a region of the substrate adjacent to the Josephson junction is exposed. In particular, the electron beam source can be a focused electron beam source, the ion beam source can be a focused ion beam source, or the electromagnetic radiation source can be a laser.

[0078] The apparatus may also include a drive system configured to move the stage and / or the electron beam, ion beam or electromagnetic radiation such that the electron beam, the ion beam or electromagnetic radiation can be scanned across the Josephson junction and / or a region of the substrate close to the Josephson junction. The apparatus may be a modified electron microscope, modified focused ion beam microscope or a modified atomic force microscopy or a different, dedicated device.

[0079] The following is a list of embodiments of the invention that may be claimed later:

[0080] 1. A method for modifying the normal state resistance of a Josephson junction, said Josephson junction being provided on a substrate, the method comprising irradiating at least partially the Josephson junction and / or a region of the substrate adjacent to the Josephson junction with an electron beam.

[0081] 2. The method of embodiment 1 , wherein the method further comprises imaging the Josephson junction using the electron beam prior to irradiating at least partially the Josephson junction and / or a region of the substrate adjacent to the Josephson junction with the electron beam.

[0082] 3. The method of embodiment 2, wherein the irradiated area of a Josephson junction and / or a region of the substrate is determined based on imaging the Josephson junction using the electron beam.

[0083] 4. The method of any preceding embodiment, wherein one or both of the irradiation area and acceleration voltage of the electron beam are selected in order to control the volume of the Josephson junction and / or a region of the substrate exposed to the electron beam.

[0084] 5. The method of any preceding embodiment, wherein the parameters of the electron beam irradiation are selected in order to control the resistance change of the Josephson junction.

[0085] 6. The method of any preceding embodiment, wherein irradiating the Josephson junction and / or a region of the substrate adjacent to the Josephson junction leads to a change in the normal state resistance of the Josephson junction greater than 5%.

[0086] 7. The method of any preceding embodiment, wherein the change in the normal state resistance of the Josephson junction is an increase in the normal state resistance of the Josephson junction.

[0087] 8. The method of embodiment 7, wherein the increase in the normal state resistance of the Josephson junction is greater than 5%, in particular greater than 10%, more in particular of the order of 30% or more.

[0088] 9. The method of any of embodiments 0 to 6, wherein the change in the normal state resistance of the Josephson junction is a decrease in the normal state resistance of the Josephson junction.

[0089] 10. The method of embodiment 8, wherein the decrease in the normal state resistance of the Josephson junction is greater than 5%, in particular greater than 10%. The method of any of any preceding embodiment, wherein the method further comprises, before irradiating at least part of the Josephson junction and / or a region of the substrate adjacent to the Josephson junction: measuring the normal state resistance of the Josephson junction; and selecting the parameters of the electron beam in order to change the normal state resistance of the Josephson junction to a desired value. The method of any preceding embodiment, wherein irradiating at least partially the Josephson junction and / or a region of the substrate adjacent to the Josephson junction comprises scanning the Josephson junction and / or region of the substrate adjacent to the Josephson junction with the electron beam. The method of any preceding embodiment, wherein the Josephson junction is part of a superconducting qubit, and wherein modifying the normal state resistance of the Josephson junction modifies the frequency of the qubit. A method of manufacturing a qubit, the method comprising manufacturing a qubit, wherein manufacturing the qubit includes providing at least one Josephson junction on a substrate; and modifying the qubit frequency by modifying the normal state resistance of the at least one or more Josephson junction according to the method of any preceding claim. The method of embodiment 14, wherein modifying the qubit frequency comprises increasing the normal state resistance of the Josephson junction thereby decreasing the qubit frequency. The method of embodiment 14, wherein modifying the qubit frequency comprises decreasing the normal state resistance of the Josephson junction thereby increasing the qubit frequency. An apparatus for fabricating one or more Josephson junction on a substrate comprising an electron beam source, in particular a focused electron beam source, configured to irradiate at least one Josephson junction to modify its resistance according to the method of any of embodiment 1 to 13.

Claims

Claims1. A method for modifying the normal state resistance of a Josephson junction, said Josephson junction being provided on a substrate, the method comprising at least partially exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction to an electron beam, ion beam, or electromagnetic radiation.

2. The method of claim 1 , wherein the method comprises decreasing the normal state resistance of the Josephson junction relative to an initial resistance value by at least partially exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction with the electron beam, ion beam, or electromagnetic radiation.

3. The method of claim 2, wherein the decrease in the normal state resistance of the Josephson junction relative to the initial resistance value is greater than 5%, in particular greater than 10% or greater than 15%.

4. The method of any of claims 1 to 3, wherein the method further comprises increasing the normal state resistance of the Josephson junction relative to the initial resistance value by continuing to at least partially expose the Josephson junction and / or the region of the substrate close to the Josephson junction to the electron beam, ion beam, or electromagnetic radiation after decreasing the normal state resistance.

5. The method of claim 4, wherein the increase in the normal state resistance of the Josephson junction relative to the initial resistance value is greater than 5% of the initial resistance value, greater than 10% of the initial resistance value, or greater than 30% of the initial resistance value.

6. The method of any preceding claim, wherein the parameters of the electron beam, ion beam, or electromagnetic radiation, and / or the parameters of the electron beam exposure, ion beam exposure, or electromagnetic radiation exposure, are selected in order to control the resistance change of the Josephson junction.

7. The method of claim 6, wherein the parameters of the electron beam exposure, ion beam exposure, or electromagnetic radiation exposure include the exposure time during which the Josephson junction is exposed to the electron beam, ion beam, orelectromagnetic radiation, and wherein the exposure time is selected to control the magnitude and direction of the resistance change of the Josephson junction.

8. The method of any of any preceding claim, wherein the method further comprises measuring the normal state resistance of the Josephson junction before at least partially exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction to the electron beam, ion beam, or electromagnetic radiation.

9. The method of any preceding claim, wherein at least partially exposing the Josephson junction and / or a region of the substrate adjacent to the Josephson junction to the electron beam, ion beam, or electromagnetic radiation comprises scanning the Josephson junction with the electron beam, ion beam, or electromagnetic radiation.

10. The method of claim 9, wherein the scanning pattern and / or scanning speed are selected in order to control the magnitude and direction of the resistance change of the Josephson junction.

11. The method of any preceding claim, wherein the Josephson junction is part of a superconducting qubit, such that decreasing the normal state resistance of the Josephson junction increases the frequency of the qubit and increasing the normal state resistance of the Josephson junction decreases the frequency of the qubit.

12. The method of any preceding claim, wherein one or both of the exposure area and acceleration voltage of the electron beam or ion beam are selected in order to control the volume of the Josephson junction and / or a region of the substrate exposed to the electron beam or ion beam.

13. The method of claim 12, wherein the method further comprises imaging the Josephson junction, in particular using an electron beam or ion beam, prior to at least partially exposing the Josephson junction to the electron beam, ion beam or electromagnetic radiation.

14. The method of claim 13, wherein the exposure area of a Josephson junction and / or a region of the substrate to be exposed is determined based on the imaging of the Josephson junction.

15. The method of claim 13 or 14, wherein the beam used for imaging the Josephson junction is provided by the same source as the electron beam or ion beam used for modifying the normal state resistance of the Josephson junction.

16. The method of any preceding claim, wherein the electron beam is a focused electron beam.

17. The method of any of claims 1 to 15, wherein the ion beam is a focused ion beam.

18. The method of claim 17, wherein the ion beam is a helium ion beam, neon ion beam, or other noble gas ion beam.

19. A method of manufacturing a qubit, the method comprising manufacturing a qubit, wherein manufacturing the qubit includes providing at least one Josephson junction; and modifying the qubit frequency by modifying the normal state resistance of the at least one or more Josephson junction according to the method of any preceding claim.

20. An apparatus for fabricating one or more Josephson junctions on a substrate comprising an electron beam source, in particular a focused electron beam source, an ion beam source, in particular a focused ion beam source, or an electromagnetic radiation source, in particular a laser, and configured to expose at least one Josephson junction and / or a region of the substrate close to the at least one Josephson junction to an electron beam, ion beam, or electromagnetic radiation to modify its resistance according to the method of any of claims 1 to 18.

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