Improvements in or relating to quantum computing

Abstract

According to the invention there is provided a quantum processor comprising a substrate having a first surface, a first module having a top surface and a lower surface and having a plurality of electrodes formed on the top surface, each electrode being coupled to a signal generator configured to output a radio frequency signal to generate an ion trap above the first module, a second module having a top surface and a lower surface having a plurality of electrodes formed on the top surface, each electrode being coupled to a signal generator configured to output a radio frequency signal to generate an ion trap above the second module, the second module being a first distance in a first direction from the first module and a controller. The substrate comprises a plurality of substrate electrodes, formed on its first surface and located at least between the first module and the second module and the electrodes are electrically coupled to a substrate signal generator configured to output a radio frequency signal to generate an ion trap above the space between the first and second modules and wherein the controller is configured to control the signal generators.

Description

[0001] IMPROVEMENTS IN OR RELATING TO QUANTUM COMPUTING

[0002] The present invention relates to providing a modular system for an ion trap quantum computer.

[0003] Quantum computing in general, unlike so-called “classical computing”, relies on the quantum mechanical properties of particles or matter to produce or alter data. The data may be represented by quantum bits or “qubits”, which is a two state quantum mechanical system. Unlike classical computing, the qubit may be in superposition of quantum states. Another feature of quantum computing is the entanglement between qubits in which the state of one particle or atom is influenced by another particle or atom.

[0004] Quantum mechanical qubits are able to encode information as combinations of zeros and ones simultaneously. Such properties open numerous complex numerical applications that are traditionally difficult for classical computers. Examples include artificial Intelligence, image processing and recognition, cryptography, or secure communications and so on.

[0005] Within an ion hyperfine electronic states (Zeeman split states) can be revealed by the use of a magnetic field and the different electron levels used as the different qubit states and electrons moved between the levels using microwave radiation or lasers.

[0006] On a single chip, or module, there are a plurality of quantum gates. For example, on a module of size 300x300mm there may be 4096 gate zones. However, there is a limit to the size of individual modules which can be easily manufactured. One solution is therefore to create an array of modules, as depicted in Figure 1 , with a few micrometers of spacing between each module. Ions, or qubits, can be transferred between the different modules as necessary and therefore significantly larger computers can be developed. For example, a 10x10 array of modules, may have a 409,600 different gate zones.

[0007] However, one disadvantage of the use of a plurality of modules is that ions must be shuttled between the modules and this can be problematic, particularly where there is misalignment between modules. It can be difficult to arrange the modules sufficiently close to each other that there is no energy barrier, over which an ion must overcome, between the modules. If there is an energy barrier it can be difficult to move the ion between modules. Furthermore, if the ion overcomes the energy barrier and moves between modules additional thermal noise will be generated.

[0008] It is therefore desirable to generate an arrangement in which ions can be shuttled between modules.

[0009] It is an aim of the invention to provide a method of manufacturing a scalable system in which transfer of ions between modules is seamless.

[0010] According to the invention there is therefore provided a substrate having a first surface, a first module affixed to the substrate and having a top surface, a lower surface and having a surface linear Paul trap comprising a plurality of electrodes formed on the top surface, each RF electrode being coupled to a signal generator configured to output a radio frequency signal to generate an ion trap above the first module, a second module affixed to the substrate and spaced apart from the first module and having a top surface, a lower surface and having a surface linear Paul trap comprising a plurality of electrodes formed on the top surface, each RF electrode being coupled to a signal generator configured to output a radio frequency signal to generate an ion trap above the second module, the second module being spaced apart by a first distance in a first direction from the first module and a controller and wherein the substrate comprises a surface linear Paul trap comprising a plurality of substrate electrodes, formed on its first surface and located at least in the space between the first module and the second module, the RF electrodes being electrically coupled to a substrate signal generator configured to output a radio frequency signal to generate an ion trap above the space between the first and second modules and wherein the controller is configured to control the signal generators.

[0011] Thus, the substrate electrodes generate a radio frequency field above the substrate so that an ion can be shuttled along a minimum energy path. The field generated by the radio frequency electrodes confine ions in a second direction, perpendicular to the first direction, and a third direction, parallel to the first and second directions. The ion trap above the substrate therefore forms an ion trap continuous path between the ion trap above the first module and the ion trap above the second module.

[0012] The first and second modules have a space therebetween and the substrate ion trap is formed above the substrate in the space between the first and second modules. The first surface of the substrate is at a different height from the top surface of both the first and second modules.

[0013] The signal generator coupled to the module electrodes and the signal generator coupled to the substrate electrodes are preferably configured to output the same radio frequency so that the fields generated by the electrodes are all similar. There may be a single signal generator, or alternatively there may be one or more signal generators for the module electrodes and one or more signal generators for the substrate electrodes. However, they are configured to output the same radio frequency.

[0014] The plurality of electrodes on each of the first and second modules, adjacent the edge of the module and in a second direction perpendicular to the first direction and within the plane of the module, have a first length and wherein the plurality of electrodes on the substrate have a second length in the second direction and wherein the second length is at least 10% greater than the first length. It may be at least 50% greater. The substrate electrodes having a different width than the module electrodes means that they can generate a confining potential at a different position. As the top of the substrate is at a lower height than the top of the modules the substrate electrodes need to generate a confining field at a higher position to create an ion path at a similar height to the ion path above the modules.

[0015] The substrate RF electrodes extend between the first and second module. In particular, the substrates occupy at least 80%, and preferably at least 90%, of the first distance between the first and second modules. By having the electrodes extend almost all the distance between the modules a continuous ion path can be generated. Preferably, the electrodes extend the entire distance between the modules.

[0016] The controller may be configured to control the signal generator coupled to the module electrodes to generate a module radio frequency field which generates an ion trap of having a time average energy depth of x and to control the signal generator coupled to the substrate electrodes to generate a substrate radio frequency field, between ion trap above the first module and the ion trap above the second module, where the maximum energy barrier on the ion trap continuous path between the first and second modules, is less than x. Thus, there is a continuous ion path from above the first module to above the second module.

[0017] Preferably, the maximum energy barrier is less than x / 10, or more preferably less than x / 100. In absolute terms the energy barrier is preferably less than 10meV and more preferably less than 1 meV.

[0018] The radio frequency potential applied to the substrate electrodes preferably has a greater amplitude than the radio frequency potential applied to module electrodes.

[0019] By providing a path between modules greater distances between modules may be achievable. In particular a distance of at least 5pm may be achievable. The distance between the first and second modules may be less than 1 mm.

[0020] The first module and the second module may further comprise a second set of electrodes, each coupled to signal generators configured to output a DC signal. These second set of electrodes allow a potential well, in which an ion may be confined, to be created. These electrodes confine the ion in the first direction. Thus, the first set of electrodes (which generate an RF field) confine an ion in the second and third directions and the second set of electrodes confine an ion in a first direction.

[0021] The substrate may also further comprise a second set of electrodes, each coupled to a signal generators configured to output a DC signal.

[0022] The first module may be affixed to the first surface of the substrate by its lower surface and the second module is affixed to the first surface of the substrate by its lower surface. Thus, the modules are on top of the substrate.

[0023] One or more of the signal generators may comprise a DAC.

[0024] According to the invention there is provided a method of trapping an ion on a device comprising a first module and a second module affixed to a substrate, the second module being spaced apart by a first distance in a first direction from the first module, each of the first and second modules having a surface linear Paul trap comprising a first set of module electrodes, each coupled to a signal generator configured to output a radio frequency signal and having a second set of module electrodes, each coupled to a signal generator configured to output a DC signal, the substrate comprising a surface linear Paul trap located at least in the space between the first module and the second module and comprising plurality of substrate electrodes, each substrate electrode being coupled to a signal generator configured to output a radio frequency signal. The method comprises generating, using the first set of module electrodes of the first and second modules, a radio frequency signal to generate an ion trap above the first module and an ion trap above the second module, the ion trap having a time average energy depth of x, and generating, using the substrate electrodes, a radio frequency signal of the same frequency, to generate a continuous ion trap path above the substrate between the ion trap above the first and the ion trap above the second module, wherein the maximum energy on the continuous ion trap path is less than x and trapping an ion in at least one of the ion traps.

[0025] According to the invention there is further provided a method of manipulating an ion comprising the method of trapping an ion as described above and further comprisinggenerating a potential well above the first module using the electrodes coupled to the signal generators configured to generate a DC signal, trapping an ion at the intersection of the potential well and the ion trap generated by the radio frequency signals, and generating a sequence of signal generator signals for the electrodes on the first module and the second to move the ion from the ion trap above the first module to the continuous ion trap path above the substrate and onto the ion trap above the second module.

[0026] The signal generator coupled to the module electrodes preferably generates a module radio frequency field which generates an ion trap having a time average energy depth of x and the signal generator coupled to the substrate electrodes preferably generates a substrate radio frequency field, between ion trap above the first module and the ion trap above the second module, where the maximum energy barrier on an ion trap continuous path between the first and second modules, is less than the time average energy depth x.

[0027] Figure 1 depicts an array of modules in a quantum computer;

[0028] Figure 2 depicts a portion of a module; and

[0029] Figure 3 depicts a substrate according to the invention. Referring to Figure 1 , there is an example arrangement of a plurality of modules 10 within a quantum computer. An example of part of a module is depicted in Figure 2 which depicts a surface linear Paul trap. The module comprises a plurality of electrodes to which voltages can be applied to form a potential well to trap an ion. Some electrodes 12 are RF electrodes, controlled by one or more DACs 17, which generate an RF field. There may be a separate DAC 17 for each electrode or alternatively one DAC 17 may control several electrodes 12. Although coupled to the electrodes the DACs may be remote. The RF field generated by these electrodes creates an ion trap in which the ion is positioned at a minimum energy and restricted by an energy barrier in the y and z direction. The generated RF field therefore restricts the ion in the y and z directions depicted in the figure. The RF field generated by the plurality of RF electrodes therefore creates a path of minimum energy in an x direction along which an ion can travel.

[0030] The RF electrodes 12 and associated DACs 17 generate a radio frequency field with a time average energy depth. As an example, for RF DACs generating 200V amplitude at 20MHz at a height of 125um there would be a time average energy depth of 500meV for a Yb171 ion.

[0031] Additionally there are DC electrodes 11 , controlled by a DAC 16 configured to generate a DC field. The DACs 16 are coupled to the electrodes 11 and as depicted in figure 2 the DACs are often located remotely from the DC electrodes simply due to space constraints. There may be a different DAC for each electrode or a DAC may control a plurality of the electrodes. Different DC voltages can be applied to different electrodes to generate a potential well in which the ion is located. As the DC voltages of different electrodes are changed the ion is moved, along the path of minimum energy, in an x direction. Thus the combination of the RF electrodes 12 and the DC electrodes 11 control the position of the ion. The ion may therefore be moved around above the module as desired.

[0032] As can be seen from Figure 2 the both the RF electrodes and the DC electrodes are adjacent to the edge of the module in a first direction. In particular they abut the edge of the module.

[0033] According to the invention modules are placed on a substrate 20, as depicted in Figure 3. Between the modules there are a plurality of RF electrodes 22 each coupled to a DAC 27 configured to output a radio frequency. As on the modules, the RF electrodes form a linear surface Paul trap. The DAC 27 preferably outputs the same radio frequency as the DAC 17 controlling the RF electrodes on the substrate. The RF electrodes on the substrate generate an RF field above the substrate which has a minimum energy path surrounded by an energy barrier. If the minimum energy path is less than the trap depth (time average energy depth) above the module then the ion will be able to pass above the substrate between the modules.

[0034] The minimum energy path above the substrate must be less than the time average energy depth above the module. However, the minimum energy path should preferably have as low energy as possible to reduce the heating. Preferably, the maximum energy level on the minimum energy path should be less than 1 meV. This would be similar to the energy of a junction on a module. Additionally, the potential energy gradient of the minimum energy path should be minimised to reduce heating.

[0035] The DACs may all be controlled by a controller 50 which controls the voltages generated by each DAC.

[0036] As the RF electrodes 22 on the substrate are at a different height from the RF electrodes 12 on the module a different strength of field may be applied so that the minimum energy path above the substrate is at a similar height to the ion path above the substrates. The different strength of field may be achieved by applying a larger voltage (at the same RF frequency) and / or using electrodes which are wider in the y direction, namely in a direction within the plane of the substrate but perpendicular to the minimum distance between the modules. By having the minimum energy path at a similar height a continuous path is created between the ion trap above adjacent modules.

[0037] Calculating the height of an ion trap above electrodes is well known and is given by:

[0038] Where zo is the height of the electric field minimum a is the width of the central DC electrodes and b and c are the widths of the rf electrodes. Calculation of the ion height is described in detail in “Analytic model for electrostatic fields in surface-electrode ion traps” M. G. House Physical Review A 78 033402 2 September 2008, the contents of which is incorporated herein. The position of the minimum energy above the substrate can therefore be controlled by the width of the electrodes. The position of the minimum energy above the substrate can therefore be controlled to be a similar (absolute) height to the minimum energy of the ion trap above the modules.

[0039] If the heights of the minimum energy positions above the substrate and the modules are similar then minimum energy paths will connect and this well known concept is used in in “T-junction ion trap array for two-dimensional ion shuttling, storage and manipulation” by W. K. Hensinger et al Applied Physics Letters 88, 034101 , 17 January 2006 and “High-Fidelity Transport of Trapped-Ion Qubits through an X-Junction Trap Array” by Blakestad et al, Physical Review letters 16 April 2009, the contents both of which are incorporated herein. Thus, by generating an ion trap above the substrate the minimum energy path between the two modules connects to provide a continuous path between the modules along which an ion can be moved.

[0040] The RF electrodes on the substrate preferably occupy the distance between the first and second modules. In particular they preferably occupy at least 80%, or preferably at least 90% of the distance in a first direction between the modules. Preferably they occupy the entire distance in a first direction between the modules.

[0041] In particular, the height of the ion path between the modules should not fall below the level of the top surface of the modules. This is controlled by the controller controlling the RF DACs 27 to generate a field such that the minimum energy path above the substrate is at a height (from the surface of the substrate) greater than the height of the modules. As such the height of the minimum energy path above the substrate is greater than the height of the modules. Preferably, the height of the minimum energy path above the substrate is substantially equal to the height of the modules plus the height of the minimum energy path above the modules.

[0042] As can be seen from Figure 3 the width of the substrate RF electrodes 22 in a y direction, namely in within the plane of the substrate but perpendicular to the shortest distance between the modules, is greater than the width of the module RF electrodes. In particular, they are preferably at least 10% greater. For example, they may be twice as wide. The greater width of the electrodes increases the height of the minimum energy path above the substrate. As discussed above the RF fields above the modules generates ion trap having a time average energy depth of x. A lowest energy path is generated above the substrate by the substrate RF electrodes and, according to the invention the maximum barrier on the ion trap path above the substrate and between the modules is less than x. Thus, if changes in the potential by the DC electrodes move a trapped ion above the substrate the energy barrier is less than the ion trap depth above the modules. The maximum energy barrier in the continuous bath should preferably be as low as possible and preferably less than x / 10 and more preferably less than x / 100. As an absolute value the maximum energy barrier should be less than 10meV. At less than 1 meV the energy barrier would be less than an ion experiences going through a junction.

[0043] The ion trap generated above the substrate will also have a time average energy depth. In order for the time average energy depth above the substrate to be similar to the time average energy depth above to the modules the radio frequency potential applied to the substrate electrodes has a greater amplitude than the radio frequency potential applied to the module electrodes. Preferably the time average energy depth above the substrate is within 20% of the time average energy depth above the modules.

[0044] Just as the RF and DC electrodes on the modules abut the edge of the modules so the RF and DC electrodes on the substrate are also adjacent to the edge of the modules. This helps to achieve a continuous path between the modules with the lowest possible maximum energy barrier.

[0045] The apparatus described above is used to generate, using the RF module electrodes of the first and second modules, a radio frequency signal to generate an ion trap above the first and second modules, the ion trap having a time average energy depth of x. A radio frequency signal of the same frequency, is generated by the substrate RF electrodes to generate a continuous ion trap path above the substrate between the ion trap above the first and the ion trap above the second module. The maximum energy on the continuous ion trap path is less than the time average energy depth above the first or second module. Using the DC electrodes a potential well is generated above the first module using the electrodes coupled to the DACs configured to generate a DC signal and this traps an ion at the intersection of the potential well and the ion trap generated by the radio frequency signals. A sequence of DAC signals for the DC electrodes on the first module, the substrate and the second module are generated to move the ion from the ion trap above the first module along the continuous ion trap path above the substrate and onto the ion trap above the second module.

[0046] Although the arrangements above are described using DACs any form of signal generator may be used.

[0047] Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure.

[0048] “and / or” where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example “A and / or B” is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein.

[0049] Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described.

[0050] It will further be appreciated by those skilled in the art that although the invention has been described by way of example with reference to several embodiments. It is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1 . A quantum processor comprising: a substrate having a first surface; a first module affixed to the substrate and having a top surface, a lower surface and having a surface linear Paul trap comprising a plurality of electrodes formed on the top surface, each electrode being coupled to a signal generator configured to output a radio frequency signal to generate an ion trap above the first module; a second module affixed to the substrate and spaced apart from the first module and having a top surface, lower surface and having a surface linear Paul trap comprising plurality of electrodes formed on the top surface, each electrode being coupled to a signal generator configured to output a radio frequency signal to generate an ion trap above the second module, the second module being spaced apart by a first distance in a first direction from the first module; a controller; wherein the substrate comprises a surface linear Paul trap comprising a plurality of substrate electrodes, formed on its first surface and located at least in the space between the first module and the second module, the electrodes being electrically coupled to a substrate signal generator configured to output a radio frequency signal to generate an ion trap above the space between the first and second modules and wherein the controller is configured to control the signal generator.

2. A processor according to any one of the preceding claims wherein the signal generator coupled to the module electrodes and the signal generator coupled to the substrate electrodes are configured to output the same radio frequency.

3. A processor according to any one of the preceding claims wherein the ion trap above the substrate forms an ion trap continuous path between the ion trap above the first module and the ion trap above the second module.

4. A processor according to any one of the preceding claims wherein the controller is configured to control the signal generator coupled to the substrate electrodes and the signal generator coupled to the module electrodes such that the continuous path does not fall below the level of the top surface of the modules.

5. A processor according to any one of the preceding claims wherein the plurality of electrodes on each of the first and second modules, adjacent the edge of the module and in a second direction perpendicular to the first direction and within the plane of the module, have a first length and wherein the plurality of electrodes on the substrate have a second length in the second direction and wherein the second length is at least 10% greater than the first length.

6. A processor according to any one of the preceding claims wherein the controller is configured to control the signal generator coupled to the module electrodes to generate a module radio frequency field which generates an ion trap of having a time average energy depth of x and to control the signal generator coupled to the substrate electrodes to generate a substrate radio frequency field, between ion trap above the first module and the ion trap above the second module, where the maximum energy barrier on an ion trap continuous path between the first and second modules, is less than the time average energy depth x.

7. A processor according to either claim 6 or claim 7 wherein the maximum energy barrier is less than 10meV.

8. A processor according to any one of the preceding claims wherein the radio frequency potential applied to the substrate electrodes has a greater amplitude than the radio frequency potential applied to module electrodes.

9. A processor according to any one of the preceding claims wherein the module electrodes are coupled to a module signal generator and the substrate electrodes are coupled to a substrate signal generator.

10. A processor according to any one of the preceding claims wherein the first module and the second module further comprise a second set of electrodes, each coupled to signal generators configured to output a DC signal.

11. A processor according to any one of the preceding claims wherein the substrate further comprises a second set of substrate electrodes, each coupled to signal generators configured to output a DC signal.

12. A processor according to any one of the preceding claims wherein the first module is affixed to the top surface of the substrate by its lower surface and the second module is affixed to the top surface of the substrate by its lower surface.

13. A processor according to any one of the preceding claims wherein the substrate electrodes extend the full distance between the first and second module.

14. A method of trapping an ion on a device comprising a first module and a second module affixed to a substrate, the second module being spaced apart by a first distance in a first direction from the first module, each of the first and second modules having a surface linear Paul trap comprising a first set of module electrodes, each coupled to a signal generator configured to output a radio frequency signal and having a second set of module electrodes, each coupled to a signal generator configured to output a DC signal, the substrate comprising a surface linear Paul trap located at least in the space between the first module and the second module and comprising a plurality of substrate electrodes, each substrate electrode being coupled to a signalgenerator configured to output a radio frequency signal, the method comprising: generating, using the first set of module electrodes of the first and second modules, a radio frequency signal to generate an ion trap above the first module and an ion trap above the second module, the ion trap having a time average energy depth of x; and generating, using the substrate electrodes, a radio frequency signal of the same frequency, to generate a continuous ion trap path above the substrate between the ion trap above the first and the ion trap above the second module, wherein the maximum energy on the continuous ion trap path is less than x; trapping an ion in at least one of the ion traps.

15. A method of manipulating an ion comprising a method of trapping an ion according to claim 1 and further comprising: generating a potential well above the first module using the electrodes coupled to the signal generator configured to generate a DC signal, wherein trapping an ion comprises trapping at the intersection of the potential well and the ion trap generated by the radio frequency signals; and generating a sequence of signal generator signals for the electrodes on the first module and the second to move the ion from the ion trap above the first module to the continuous ion trap path above the substrate and onto the ion trap above the second module.

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