METHOD FOR COOLING AT LEAST ONE ION AND SYSTEM

Abstract

A method for cooling at least one ion (8) within a capture potential of an ion trap (2), comprising: - capturing the at least one ion (8) in the capture potential, wherein the at least one ion (8) oscillates about a potential minimum of the capture potential between a first inflection point region (13) and a second inflection point region (14), - illuminating the at least one ion (8) with a laser beam in the first inflection point region (13) and / or the second inflection point region (14) for cooling the at least one ion, wherein the laser beam has a beam center characteristic of a peak intensity and the beam center is spaced from the potential minimum, and - decreasing a distance of the beam center from the potential minimum until the beam center overlaps with the potential minimum, for further cooling of the at least one ion. Furthermore, a system (1) is specified.

Description

The present disclosure relates to a method for cooling at least one ion and a system. Publication WO 2025 / 008434A1 describes an ion trap, an ion trap system, and a quantum computer arrangement. Publication US 2004 / 0238754A1 describes a method and a device for cooling and focusing ions. Publication CN118410882A describes an ion cooling method and device, and a quantum computer. Typically, during ion capture, hot ions oscillate in the capture potential with relatively large amplitudes. The lasers used for cooling typically have a comparatively small cooling cross-section with the hot ions. The task is to provide a method for efficiently cooling ions. Furthermore, a cooling system should be provided. These objectives are achieved through the subject matter of the independent claims. Advantageous embodiments, implementations, and further developments are the subject matter of the respective dependent claims. A method for cooling at least one ion within the capture potential of an ion trap is described. In particular, the ion trap is configured to capture at least one ion within the capture potential. The at least one ion is, in particular, a quantum bit (qubit), which is a fundamental unit of information in a quantum computing process. For example, the ion trap is a processing unit of a quantum computing device. For example, the ion trap comprises a set of electrodes configured to provide the capture potential. The set of electrodes is configured, for example, to confine and / or manipulate the ion within a processing region of the ion trap. For instance, a high-frequency (HF) voltage is applied to at least some of the electrodes in the set, providing a time-varying electric field in the processing region that constitutes at least part of the capture potential. Furthermore, a direct current (DC) voltage is applied to at least some other electrodes in the set, providing a static electric field in the processing region that constitutes at least another part of the capture potential.The capture potential is formed, in particular, by a superposition of the time-varying electric field and the static electric field, configured to confine and / or manipulate the ion. For example, the ion intersects a capture axis and / or oscillates around and along a capture axis within the processing area. In particular, the ion oscillates radially around and axially along the capture axis within the processing area. The capture potential, for example, has a potential minimum. This potential minimum can be a predefined potential minimum range that extends along the capture axis, particularly within the processing area. The ion experiences different oscillation frequencies due to the capture potential, namely a first oscillation frequency along a slow axis and a second oscillation frequency along a fast axis. The first oscillation frequency is, for example, lower than the second oscillation frequency, in particular by at least one order of magnitude. The slow axis is, for example, aligned axially with the capture axis. An axial limit with respect to the capture axis is achieved primarily by at least some of the other electrodes in the electrode array. The fast axis is, for example, aligned radially with respect to the capture axis. A radial limit with respect to the capture axis is achieved primarily by at least some of the electrodes in the electrode array. The processing area is configured to include at least one ion, or, for example, a plurality of ions. For instance, a maximum of 100 ions or a maximum of 60 ions are provided in the processing area. The enclosed ions, for example, form a corresponding quantum register. The processing area can, for example, be an initialization area, with the auxiliary area configured for charging the at least one ion, in particular for capturing the at least one ion, and / or also for cooling the at least one ion. For example, cooling is carried out using electromagnetic radiation from a cooling laser. The at least one ion is trapped in the capture potential, oscillating around a potential minimum of the capture potential between a first inflection point region and a second inflection point region. In particular, the ion is not cooled to a predetermined temperature during capture. This means that the captured ion is a hot ion. A hot ion is characteristic of an ion with a comparatively high kinetic energy. This means that the hot ion has a greater kinetic energy than the ion would have if cooled to the predetermined temperature. The at least one ion oscillates, for example, around the potential minimum of the capture potential between the first and second inflection points along the slow axis. Each inflection point is particularly characteristic of a region where the ion's kinetic energy decreases to zero. For example, the first and second inflection points of the ion refer to the region where the velocity reaches zero before the direction of the oscillatory motion is reversed. This occurs, for example, at the ends of the ion's trajectory, where all of the ion's kinetic energy is converted into potential energy within the capture potential. Specifically, the ion oscillates between the first and second inflection points at the first oscillation frequency. The first inflection point and / or the second inflection point is located, for example, on the capture axis. For instance, the first inflection point and the second inflection point are spaced apart axially from each other relative to the capture axis. For example, the ion, and in particular each ion, oscillates with an unknown amplitude, so that the respective first and second inflection points are not precisely known. Since one state of motion of the ion, and in particular each ion, is thermal, a probability distribution of the first and second inflection points results. Specifically, the first inflection point region includes all locations of the first inflection points according to the probability distribution, and the second inflection point region includes all locations of the second inflection points according to the probability distribution. For example, the method is configured to cool a large number of ions within the capture potential. Each of the ions has a first inflection point and a second inflection point. The effective energy of each ion can differ, resulting in different first and second inflection point locations for different ions. Specifically, the first inflection point region includes all locations of the first inflection points of different ions, and the second inflection point region includes all locations of the second inflection points of different ions. For example, the first inflection point region and the second inflection point region each intersect with the capture axis, with the potential minimum being located between the first inflection point region and the second inflection point region. For example, the first inflection point area and the second inflection point area are each stretched along a principal extension direction of the capture axis. The at least one ion is illuminated with a laser beam in the first inflection point region and / or the second inflection point region for cooling purposes. The laser beam has a beam center characteristic of a peak intensity, and the beam center is spaced from the potential minimum. In particular, the beam center is spaced from the potential minimum such that the laser beam interacts with the ion within the first inflection point region and / or the second inflection point region. For example, the beam center is spaced from the potential minimum along the capture axis. For example, the beam center is spaced from the potential minimum by at least 20 µm, at least 50 µm, or at least 100 µm. For example, a cross-section perpendicular to a principal propagation direction of the laser beam is characteristic of a beam profile. The beam profile is, for example, characteristic of a spatial distribution of the laser beam intensities across the cross-section. The highest intensity of the laser beam within the cross-section is characteristic of the peak intensity. A location of the peak intensity within the cross-section is, for example, characteristic of the beam center. The cross-section is defined here and in the following, for example, such that it intersects the capture axis. The beam profile can be a Gaussian profile, a top-hat or flat-top profile, a multimode profile, or a Bessel and Airy profile. The first frequency of the laser beam used to cool the ion is, for example, set such that absorption and spontaneous emission of the laser beam by the ion result in a net reduction of the ion's kinetic energy. While the center of the laser beam is spaced away from the potential minimum, the laser beam is operated, for example, to have the specified first frequency. For example, if at least one ion in the area of ​​the first inflection point region and / or the second inflection point region is illuminated with the laser beam to cool the ion, the distance of the respective inflection point of the ion to the potential minimum decreases during cooling. The distance between the beam center and the potential minimum is reduced until the beam center overlaps with the potential minimum, further cooling the at least one ion. This distance can be reduced, for example, at least partially or completely discretely, or at least partially or completely continuously. In a discrete reduction, the distance is decreased in clearly defined, separate steps, e.g., in at least two steps. In a continuous reduction, the distance decreases uniformly over time. The distance is reduced, in particular, along the capture axis. When the distance is reduced, the laser beam is operated, for example, in such a way that it has the specified first frequency, so that absorption and spontaneous emission of the laser beam by the ion leads to a further net reduction in the kinetic energy of the ion. If the beam center overlaps with the potential minimum, the ion is cooled, for example, so that the ion forms the qubit that is particularly suitable for carrying out the quantum computing process. Advantageously, the laser beam, and in particular its center, is moved to the position or positions where the hottest ion is optimally cooled. This is characteristic of regions where the oscillating ions are furthest from the potential minimum, e.g., characteristic of a trap center. These regions are the first and second inflection point regions where the ion has zero velocity before returning to the potential minimum. In this way, the ion is advantageously optimally cooled at these points, particularly without the need to adjust the first frequency of the laser beam. This allows the captured hot ion to cool down more quickly, and no hot ions remain compared to a static laser beam position. Advantageously, more deterministic loading is possible, and loading times for a larger number of ions are faster compared to a static laser beam position. According to at least one embodiment of the method, the laser beam has a beam waist that is characteristic of a region around the beam center in which the intensity of the laser beam is at least 1 / e² of the peak intensity. In particular, the beam waist is characteristic of the region around the beam center in the cross-section in which the intensities are still sufficient to interact with the ion for cooling. The beam waist corresponds in particular to a region in the cross-section that has a shape dependent on the beam profile. The beam waist, especially the region at the capture axis, is, for example, elongated along the principal direction of extension of the capture axis. According to at least one embodiment of the method, the at least one ion in the first turning point region and / or in the second turning point region is illuminated with the laser beam, wherein the first turning point region and / or the second turning point region overlaps at least partially with the beam waist. For example, the first inflection point region and / or the second inflection point region do not completely overlap with the beam waist. Alternatively, the first inflection point region and / or the second inflection point region completely overlap with the beam waist. Advantageously, due to such a beam waist, the ion is cooled particularly effectively in the first inflection point region and / or in the second inflection point region. According to at least one embodiment of the method, the at least one ion in the first turning point region and / or in the second turning point region is illuminated with the laser beam, wherein the beam waist does not overlap with the potential minimum. According to at least one embodiment of the method, when the distance of the beam center to the potential minimum is reduced, the beam center is shifted from the first inflection point region and / or from the second inflection point region towards the potential minimum. In this embodiment, the beam center is actively moved towards the potential minimum. For example, the capture potential is not adjusted to reduce the distance. For example, the beam center is moved by adjusting optics that control the direction of the laser beam. According to at least one embodiment of the method, when the distance of the beam center to the potential minimum is reduced, the capture potential is adjusted such that the first inflection point region or the second inflection point region is shifted toward the potential minimum. In this embodiment, the capture potential is actively adjusted so that the potential minimum is shifted toward the beam center. For example, the beam center is not moved to reduce the distance. For example, the capture potential is adjusted by adjusting a current supplied to the set of electrodes. In particular, the potential minimum is shifted toward the beam center by adjusting the DC voltages applied to at least some of the other electrodes of the set of electrodes. It is also conceivable that when the distance of the beam center to the potential minimum is reduced, the beam center is moved from the first inflection point region and / or the second inflection point region towards the potential minimum, and the capture potential is adjusted to move the first inflection point region or the second inflection point region towards the potential minimum. According to at least one embodiment of the method, the beam center oscillates between the first inflection point region and the second inflection point region when the at least one ion in the first inflection point region and in the second inflection point region is illuminated by the laser beam. In particular, the beam center oscillates along the capture axis. Advantageously, the beam center, by oscillating between the first and second inflection point regions, sweeps across the capture axis and thus also overlaps with potentially cooler ions, which have inflection points closer to the potential minimum. This advantageously cools even the already cooler ions further, and the overall cooling time is significantly reduced. According to at least one embodiment of the method, the vibration amplitude is reduced when the distance of the beam center to the potential minimum is decreased. For example, the vibration amplitude is reduced at least partially or completely discretely, or at least partially or completely continuously. For example, if the beam center overlaps with the potential minimum, the vibration amplitude is reduced to zero. According to at least one embodiment of the method, the laser beam has approximately the same frequency when illuminating the at least one ion in the first inflection point region and / or in the second inflection point region and when decreasing the distance of the beam center to the potential minimum. In particular, the laser beam is operated such that it has the same initial frequency during cooling and during further cooling. This advantageously cools the ion optimally without the need to adjust the laser frequency. According to at least one embodiment of the method, the at least one ion is trapped within the capture potential along a slow axis and a fast axis. The slow axis extends, in particular, along the capture axis, and the fast axis extends, in particular, perpendicular to the capture axis. According to at least one embodiment of the method, the first inflection point region and / or the second inflection point region are located on the slow axis. In particular, the first inflection point region and the second inflection point region are each stretched along the slow axis, and a principal extension direction of the stretching extends along the slow axis. This means that the ion oscillates at the first oscillation frequency between the first inflection point region and the second inflection point region along the capture axis. According to at least one embodiment of the method, the ion trap comprises a set of electrodes configured to implement the slow axis and the fast axis. The slow axis and the fast axis are specifically dependent on the capture potential, which is provided, in particular, by the set of electrodes. According to at least one embodiment of the method, at least one ion is confined along the slow axis by at least two DC electrodes of the electrode set. In particular, the at least two DC electrodes of the electrode set correspond to at least two of the at least some further electrodes of the electrode set. According to at least one embodiment of the method, the distance between the beam center and the potential minimum is reduced within a time interval. For example, the time interval is at most 10 s, at most 5 s, or at most 2 s, for example, about 1 s. This allows the method for cooling a trapped ion—especially a hot ion—that is fed into the ion trap to be cooled particularly quickly. According to at least one embodiment of the method, a plurality of ions are trapped in the capture potential, wherein the ions oscillate around the potential minimum of the capture potential between the first inflection point region and the second inflection point region. According to at least one embodiment of the method, the ion trap is the processing unit of a quantum computer. Furthermore, a system for cooling at least one ion within a trapping potential of an ion trap is provided, and the system is configured to carry out the procedure. This means that the features relating to the procedure also apply to the system and vice versa. According to at least one embodiment, the system comprises a laser device, which is specifically designed to generate the laser beam. According to at least one embodiment, the system comprises an ion trap, which is particularly designed to capture at least one ion within the capture potential. Furthermore, a computer program is specified which includes instructions which, when the computer program is executed by a computer, cause the computer program to perform the procedure described herein. In particular, the computer program includes instructions for trapping the at least one ion in the trapping potential by controlling the ion trap, for illuminating the at least one ion with the laser beam by controlling the laser device, for reducing the distance of the beam center to the potential minimum, and for controlling the laser device and / or the ion trap. Furthermore, a computer-readable storage medium is specified on which the computer program described here is stored. The method and the system are explained in more detail below with reference to exemplary embodiments and the accompanying figures. Fig. 1 shows a flowchart of the method according to one embodiment. Fig. 2 shows a schematic representation of the ion in the system according to one embodiment. Fig. 3 shows a quantum computer system according to one embodiment. Elements that are identical or similar, or have the same effect, are marked with the same reference symbols in the figures. The figures and the proportions of the elements depicted within them are not to be considered as being to scale. Rather, individual elements may be exaggerated for better representation and / or clarity. The process stage S1 according to the embodiment of Fig. 1 comprises the capture of the at least one ion 8 in the capture potential, wherein the at least one ion 8 oscillates around a potential minimum of the capture potential between a first inflection point region 13 and a second inflection point region 14. Subsequently or simultaneously with process stage S1, the at least one ion 8 is illuminated with a laser beam in process stage S2 in the first turning point region 13 and / or in the second turning point region 14 for cooling the at least one ion 8, wherein the laser beam has a beam center characteristic for a peak intensity and the beam center is spaced away from the potential minimum. After process stage S2, in process stage S3 the distance of the beam center to the potential minimum is reduced until the beam center overlaps with the potential minimum for further cooling of the at least one ion 8. Fig. 2 shows a temporal evolution of the ion 8 trapped in the capture potential, showing, for example, four time points that develop from the process stage S1 at the top to the process stage S3 at the bottom. At the first time point above, which corresponds specifically to process stage S1, ion 8 is a hot ion with a comparatively high kinetic energy. This means that between the first turning point region 13 and the second turning point region 14, ion 8 oscillates along a capture axis 11. The first turning point region 13 and the second turning point region 14 refer to the region in which the velocity of ion 8 reaches zero before it reverses direction in its oscillating motion, which is represented in Fig. 2 by a slow ion 10, indicated by a black circle. Between the first turning point region 13 and the second turning point region 14, e.g., in a potential minimum region, ion 8 has a comparatively high velocity, indicated by a fast ion 9, which is indicated in Fig. 2 by a black ellipse. Since ion 8 oscillates with an unknown amplitude, its positions are not precisely known. Because one state of motion of ion 8 is thermal, a probability distribution of its position is characteristic of an ion cloud profile 12, as shown in Fig. 2. Initially, as in the first time point above, the beam waist 15 of the laser beam is spaced away from the potential minimum, in particular overlapping with the first and the second inflection point region 14. For example, the laser beam can oscillate between the inflection point regions or the laser beam can be directed exclusively onto the first or the second inflection point region 14. Subsequently, once the hot ion has cooled down as shown above at the first time point, the beam waist 15 is moved towards the potential minimum, further cooling the ion 8. At the fourth time point, as shown below, the initially hot ion is finally cooled down further, making it particularly usable for quantum computing processes. System 1 according to Fig. 3 is, in particular, a quantum computer system 1 with an ion trap 2 as the processing unit of the quantum computer system 1 and a laser device 3, wherein at least the ion trap 2 is arranged within a chamber 4. The ion trap 2 and the laser device 3 are each connected via the chamber 4 to external components of the quantum computer system 1 by a plurality of connections 5. The connections 5 connect, for example, the ion trap 2 and the laser device 3 to electronic devices 6 and a classical computer 7 – for controlling the devices. The electronic devices 6 include, for example, a laser controller, a wavemeter, an acousto-optic modulator, an electro-optic modulator, a detector, a signal generator, an amplifier, a power supply, a piezoelectric controller, a motor, analog-to-digital converters, and signal generators such as high-frequency generators, microwave signal generators, low-frequency signal generators, and / or DC signal generators. The electronic devices 6 may also be partially located within the chamber 4. The chamber 4 may be an ultra-high vacuum chamber, an extreme-high vacuum chamber, and / or a cryostat. For example, ion trap 2 is provided with a magnetic arrangement configured to generate a magnetic gradient to the trapped ions 8 of ion trap 2. The invention is not limited to the exemplary embodiments described. Rather, the invention encompasses every new feature as well as every combination of features, which in particular includes every combination of features in the claims, even if that feature or combination itself is not expressly specified in the claims or exemplary embodiments. Reference sign 1 System 2 Ion trap 3 Laser device 4 Chamber 5 Connections 6 Electronic devices 7 Classical computer 8 Ions 9 Fast ion 10 Slow ion 11 Capture axis 12 Ion cloud profile 13 First inflection point region 14 Second inflection point region 15 Beam waist S1...S3 Process stages

Claims

Method for cooling at least one ion (8) within a capture potential of an ion trap (2), comprising: - capturing the at least one ion (8) in the capture potential, wherein the at least one ion (8) oscillates about a potential minimum of the capture potential between a first inflection point region (13) and a second inflection point region (14), - illuminating the at least one ion (8) with a laser beam in the first inflection point region (13) and / or the second inflection point region (14) for cooling the at least one ion, wherein the laser beam has a beam center characteristic of a peak intensity and the beam center is spaced away from the potential minimum, and - reducing a distance of the beam center to the potential minimum until the beam center overlaps with the potential minimum, for further cooling of the at least one ion. The method according to claim 1, wherein the laser beam has a beam waist (15) that is characteristic for a region around the beam center in which the intensity of the laser beam is at least 1 / e2 of the peak intensity, and when the at least one ion (8) in the first turning point region (13) and / or in the second turning point region (14) is illuminated with the laser beam, the first turning point region (13) and / or the second turning point region (14) overlap at least partially with the beam waist (15). Method according to claim 2, wherein - when the at least one ion (8) in the first turning point region (13) and / or in the second turning point region (14) is illuminated with the laser beam, the beam waist (15) does not overlap with the potential minimum. Method according to one of claims 1 to 3, wherein when reducing the distance of the beam center to the potential minimum, - the beam center is moved from the first inflection point region (13) and / or the second inflection point region (14) in the direction of the potential minimum and / or - the capture potential is adjusted to move the first inflection point region (13) or the second inflection point region (14) in the direction of the potential minimum. Method according to one of claims 1 to 4, wherein - when the at least one ion (8) is illuminated with the laser beam in the first turning point region (13) and in the second turning point region (14), the beam center oscillates between the first turning point region (13) and the second turning point region (14), and - when the distance of the beam center to the potential minimum is reduced, the oscillation amplitude is reduced. Method according to one of claims 1 to 5, wherein the laser beam has approximately the same frequency when illuminating the at least one ion (8) in the first turning point region (13) and / or in the second turning point region (14) and when reducing the distance of the beam center to the potential minimum. Method according to any one of claims 1 to 6, wherein the at least one ion (8) is captured within the capture potential along a slow axis and a fast axis, and the first inflection point region (13) and / or the second inflection point region (14) are located on the slow axis. Method according to claim 7, wherein the ion trap (2) comprises a set of electrodes configured to implement the slow axis and the fast axis. Method according to claim 7 or 8, wherein the at least one ion (8) is trapped along the slow axis by at least two direct current electrodes of the set of electrodes. Method according to any one of claims 1 to 9, wherein the distance of the beam center to the potential minimum is reduced within a time interval. Method according to any one of claims 1 to 10, wherein initially a plurality of ions are captured in the capture potential, wherein the ions oscillate around the potential minimum of the capture potential between the first inflection point region (13) and the second inflection point region (14). Method according to any one of claims 1 to 11, wherein the ion trap (2) is a processing unit of a quantum computer. System (1) for cooling at least one ion (8) within a trapping potential of an ion trap (2), wherein the system (1) is configured to carry out the method according to one of the preceding claims. Computer program with instructions which, when the computer program is executed by a computer, cause the computer program to execute the method according to any one of claims 1 to 12. Computer-readable storage medium on which the computer program according to claim 14 is stored.

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