Ion surface trap

Abstract

The invention relates to an ion surface trap (10) having an electrode pair (12) which comprises a first trap electrode (14.1) and a second trap electrode (14.2) and is designed to form a trap volume for at least one ion when an electrical AC voltage is applied, and having a sensor element (18) for detecting photons (20) which are emitted from the at least one ion, wherein the sensor element (18) comprises a superconductor layer, forms the second trap electrode (14.2), and does not have a superconductor layer - separating layer -superconductor layer structure.

Description

[0001] ion surface trap

[0002] The invention relates to an ion surface trap comprising (a) an electrode pair comprising a first trap electrode and a second trap electrode, and configured to form a trap volume for at least one ion upon application of an alternating electrical voltage, and (b) a sensor element for detecting photons emitted by the at least one ion. According to a second aspect, the invention relates to a method for operating such an ion surface trap. Ion surface traps are used to hold one ion, two ions, or several (i.e., 3, 4, 5,..., N) ions in a predetermined spatial region. For example, an ion surface trap is used in an atomic clock or a quantum computer. An atomic clock and / or a quantum computer comprising an ion surface trap according to the invention are also subject matter of this invention.

[0003] To determine the calculation result, the quantum state of the ion must be read out. To do this, the fluorescence of individual ions must be detected. The larger the photodetector used to detect the fluorescence, the less space is available for the trap electrodes, through which, for example, an alternating electric field is applied to build up the trapping potential for the ion. Ion surface traps are often implemented on a chip. To detect light emitted by the ion, special sensors are arranged on the chip, as described, for example, in the article “State Readout of a Trapped Ion Qubit Using a Trap-Integrated Superconducting Photon Detector” by Todaro et al., Phys. Rev. Lett. (2021). From the article “State readout of a trapped ion qubit using a trap-integrated superconducting photon detector” by Todaro et al. in Physical Review Letters, Vol.126 2021 No. 1 describes a generic ion surface trap capable of reading qubits. There are no optical elements between the ion and the detector, so ion fluorescence can be used to measure the quantum efficiency of the detector and its dependence on the angle of incidence and the incidence polarization.

[0004] The article “UV-sensitive superconducting nanowire single photon detectors for integration in an ion trap” by Slichter et al, in Optics Express, Vol. 25 2017 No. 8 pp. 8705-8720 describes a single-photon detector used in a Paul trap with planar radiofrequency electrodes.

[0005] The paper “Fluorescence detection of a trapped ion with a monolithically integrated single-photon-counting avalanche diode” by Setzer et al, in: arXiv: 2105.01235v2 [quant-ph], 07.07.2021 (pp. 1-6) DOI: 10.48550 / arXiv.2105.01235 shows a generic ion surface trap for detecting fluorescent light from a trapped ion using single-photon avalanche photodiodes integrated together with a surface ion trap on a chip.

[0006] DE 102019 114 842 A1 describes an ion trap for holding at least one ion in a spatial region defined by a ponderomotive potential. Electrodes of an electrode structure arranged around the spatial region are controlled such that ions in the ion trap are captured by a laser beam.

[0007] DE 10 2018 121 942 B3 discloses an ion trap comprising two electrodes configured to generate an electric field comprising an attractive ponderomotive potential in at least one region. An electric resonator supplies the electrodes of the ion trap with alternating voltage and has an oscillating circuit arranged within the vacuum chamber. This makes it easier to calibrate the alternating voltage field of the ion trap.

[0008] US Pat. No. 6,710,334 B1 describes a quadrupole ion trap mass spectrometer for large molecules using cryogenic particle detectors as molecular detectors. Cryogenic particle detectors have mass-independent detection efficiency and show no decrease in detection efficiency with increasing molecular mass compared to ionizing detectors used in conventional quadrupole ion trap mass spectrometers.

[0009] US Pat. No. 5,994,694 A describes an ultra-high mass time-of-flight mass spectrometer that uses a cryogenic particle detector as an ion detector. The cryodetector improves performance and sensitivity. It uses a cryogenically cooled Nb-Al2O3-Nb superconductor-insulator-superconductor (SIS) tunnel junction detector operating at 1.3 K. The STJ detector has charge discrimination capability. Because the cryogenic STJ detector responds to ion energy and does not rely on secondary electron generation, it can detect large molecular ions with a velocity-independent efficiency of nearly 100%.

[0010] The article “Quasiparticle trapping and the quasiparticle multiplier” by NE Boot, in: Applied Physics Letters, Vol. 50 1987 No. 5, pp. 293-295, ISSN 0003-6951 (P) describes the detection of phonons, electromagnetic radiation and nuclear particles using superconducting tunnel junctions.

[0011] WO 2015 / 128438 A1 describes a system comprising a cryostat and a surface electrode trap on a silicon substrate. Planar electrodes configured to generate a trapping potential are located on the front side of the substrate. A first radio-frequency electrode extends parallel to the front side of the substrate, and a direct current electrode extends parallel to the front side, adjacent to and electrically insulated from the first radio-frequency electrode. The surface electrode trap is arranged in the cryostat, which cools the surface electrode trap to a maximum of 150 K.

[0012] DE 10 2021 124 396 A1 describes an ion surface trap with a pair of electrodes comprising a first trap electrode and a second trap electrode, forming a trap volume for an ion. The ion surface trap also has two DC electrodes for sealing the trap volume and an energy-sensitive superconductor sensor for detecting photons emitted by the ion, which sensor has a superconductor layer-separation layer-superconductor layer structure. The first superconductor layer forms the first trap electrode.

[0013] It has proven to be very difficult to detect radiation, such as fluorescence radiation, emitted by the ion.

[0014] The invention is based on the object of improving the detection of photons emitted by the trapped ion.

[0015] The invention solves the problem by means of a generic ion surface trap in which the sensor element comprises a superconductor layer and forms a trap electrode. In particular, the superconductor layer forms the trap electrode. Preferably, the sensor element does not have a superconductor layer-separation layer-superconductor layer structure.

[0016] The invention further solves the problem by a method comprising the steps of (i) introducing an ion into an ion surface trap according to the invention, (ii) applying a trapping voltage to the first trapping electrode and the sensor element so that the ion remains trapped in the ion surface trap, and (iii) detecting at least one photon emitted by the ion by means of the sensor element.

[0017] An advantage of the invention is that the radiation emitted by the ion only has to travel a short distance to be detected by the sensor and—as provided in a preferred embodiment—does not pass through any optical elements such as optical windows, lenses, glass fibers, etc. The probability of being absorbed on the way from the ion to the sensor is therefore low.

[0018] Due to the sensor's proximity to the trapped ion, the sensor can also detect photons over a comparatively wide solid angle range. The inventive design also allows the sensor to have a comparatively large spatial extent and thus cover an even larger solid angle range than if it were limited to the space between the electrodes. Another advantage is that the invention generally allows for higher integration density. Alternatively or additionally, this results in simplified manufacturing, particularly in series production.

[0019] The invention is based in particular on the idea of ​​eliminating the separation of sensor and trap electrode known from the prior art. In other words, at least one trap electrode forms part of the structure used to detect individual photons.

[0020] An advantage of the invention is that the function of the trap electrode and that of the sensor are combined. This allows for a particularly simple structure. The superconductor layer is preferably designed such that upon absorption of a photon with a predetermined minimum energy, the superconductivity initially collapses locally, resulting in a local increase in electrical resistance. This in turn leads to the critical current density being exceeded in the vicinity of the photon's impact point, causing superconductivity to collapse. In this way, the zone in which superconductivity collapses continues to expand. This causes a measurable increase in the electrical resistance of the superconductor layer. This increase occurs from a very good approximation of 0 ohms to a significantly higher value, usually more than 1 kiloohm.

[0021] In the context of the present description, the sensor element is understood to mean, in particular, any structure of the ion surface trap by means of which a photon having a predetermined minimum energy can be detected, provided the photon impinges on the sensor element. The minimum energy is preferably at least 0.5 eV, in particular at least 0.8 eV.

[0022] A superconductor layer is a layer of material that becomes superconducting at a temperature below a transition temperature. The superconductor layer is preferably made of a high-temperature superconductor. It is advantageous if the high-temperature superconductor has a transition temperature above 77 Kelvin.

[0023] However, it is also possible that the superconductor material is not a high-temperature superconductor. For example, the superconductor material is NbN, Nb, NbTiN, MoSi, or WSi. It is advantageous if the superconductor layer thickness is between 5 nm and 500 nm.

[0024] The electrode pair is understood to be the combination of the first trap electrode and the second trap electrode. It is possible, but not necessary, for at least one of the trap electrodes to be composed of two, three, or more sub-electrodes. It is also possible, but not necessary, for individual sub-electrodes to be electrically insulated from each other. Alternatively, the individual sub-electrodes are electrically connected to each other so that they are at the same electrical potential.

[0025] When the trapping voltage is applied to the electrode pair, a trap volume forms. The trap volume is the area of ​​space that the ion cannot leave.

[0026] According to a preferred embodiment, the ion surface trap has a current source which is designed to automatically apply a measuring current to the superconductor layer, wherein the measuring current is selected to be large enough that an impact of a photon with a predetermined minimum energy at an impact location on the superconductor layer causes normal conduction in an environment of the impact location, which spreads over the entire cross section of the superconductor layer, in particular over the entire cross section of the superconductor layer at the impact location.

[0027] In particular, the current source is connected to a trap electrode in such a way that the electrical ground potential of the current source corresponds to the potential of the trap electrode. In particular, the ground potential of the current source does not correspond to the ground potential of the ion trap.

[0028] The ion surface trap preferably has a trap voltage source connected to the first trap electrode and the sensor element for applying a trap voltage. The trap voltage thus exists between the ground potential of the current source on the one hand and the first trap electrode on the other. The trap voltage source can be designed to output a DC voltage, an AC voltage, or a DC and AC voltage. The trap voltage is preferably at least 10 V and / or at most 300 V.

[0029] The potential of one of the trap electrodes can be grounded. However, both trap electrodes are preferably at the same potential. The sum of the two (signed) potentials is then preferably 0 V.

[0030] The ion surface trap preferably has at least two DC electrodes arranged to generate an electric field by means of which the position of the ion relative to the ion surface trap can be changed. Preferably, one of the DC electrodes is at the same potential as one of the two trap electrodes.

[0031] DC electrodes are understood to be, in particular, electrodes that can generate a static electric confinement field. The confinement field is preferably designed such that the trap volume is closed in all spatial directions.

[0032] The ion surface trap preferably has a resistivity meter for measuring a change in the electrical resistance of the superconducting layer. If the electrical resistance of the superconducting layer changes, it can be concluded that a photon with at least the specified minimum energy has struck. The current source can be part of the resistivity meter. The resistance change is typically from a good approximation of 0 Ω (if the superconducting layer is superconducting) to 1 kΩ or more (if the superconducting layer is normally conducting).

[0033] Preferably, the superconductor layer is designed such that it has a resistance of at least 50 ohms in the normally conducting state at 1 Kelvin below the transition temperature of the superconductor material from which the superconductor layer is constructed.

[0034] The ion surface trap preferably has a substrate onto which the trap electrodes and, optionally, the equalization electrodes are applied. The substrate is preferably non-conductive. The substrate is preferably a semiconductor, quartz glass, or corundum. Alternatively, the substrate can be silicon coated with a non-conductor, for example. Other substrate materials are possible.

[0035] It should be noted that a structure of the ion surface trap, such as the first trap electrode, can be applied directly to the substrate. However, this is not necessary. Rather, it is also possible for one of the aforementioned structures to be applied to another of the aforementioned structures, which in turn is directly or indirectly connected to the substrate. Preferably, the first trap electrode, the second trap electrode, and the superconductor layer form an integral unit with the substrate.

[0036] Preferably, the superconductor layer is electrically insulated and potential-separated from the first trap electrode.

[0037] According to a preferred embodiment, the ion surface trap has an ion introduction device for introducing an ion into a trap region of the ion surface trap. It is also advantageous if the ion surface trap has an evaluation unit configured to automatically carry out a method comprising the steps of (i) controlling the ion introduction device so that an ion is introduced into the ion surface trap, (ii) controlling the trap voltage source to apply the trap voltage between the first trap electrode and the sensor element so that the ion remains trapped in the ion surface trap, and (iii) detecting at least one photon emitted by the ion by means of the sensor element based on a change in the resistance of the superconductor layer.

[0038] The ion introduction device preferably has an evaporator for generating a gas from particles of a pure chemical substance, for example a metal, in particular an alkali metal, and a photoionizer for ionizing metal atoms, in particular alkali metal atoms, so that ions are formed therefrom. The photoionizer preferably has an electrode arrangement and a control unit connected to the electrode arrangement so that the generated ions can be introduced individually into the ion surface trap. It is advantageous if the ion surface trap has an evaluation unit connected to the resistance meter. The evaluation unit is preferably designed to carry out a method according to the invention.

[0039] The ion surface trap preferably comprises a relief resistance element connected in parallel to the superconductor layer, the ohmic relief resistance of which is smaller than the ohmic superconductor layer resistance when the superconductor layer is normally conductive. The ohmic relief resistance is preferably smaller than half, in particular smaller than one-fifth, and particularly preferably smaller than one-tenth, of the ohmic superconductor layer resistance. When the superconductor layer is in the normally conductive state, it heats up due to the electrical measuring current flowing through it. The relief resistance element causes such a large proportion of the measuring current to flow through the relief resistance that sufficiently little heat is dissipated by the remaining current in the superconductor layer, which has become completely or partially normally conductive, so that it becomes superconducting again after a certain relaxation time.Preferably, the relaxation time is no more than 100 nanoseconds.

[0040] It is advantageous if the ion surface trap is implemented on a chip. In particular, the electrode pair, the DC electrodes, and the sensor element are integral components of the chip. These structures are produced, in particular, by successively depositing different layers on top of one another and / or etching out parts of the layer structure.

[0041] If multiple ions are to be trapped in the ion surface trap, which represents a preferred embodiment, it is advantageous if it can be determined with at least a sufficiently high probability which ion a photon detected by the sensor element originates from. It is therefore advantageous if at least the superconducting layer has at least two spatially separated and electrically insulated detection sections.

[0042] The invention also relates to an ion surface trap system comprising (a) an ion surface trap according to the invention and (b) a cooling device, in particular a cryostat, for cooling the ion surface trap. The method preferably comprises the steps of (i) evaporating a pure substance so that it is in gaseous form, (ii) photoionizing the gaseous pure substance, and (iii) moving at least one ion of the gaseous pure substance into a trap volume of the ion surface trap.

[0043] The invention is explained in more detail below with reference to the accompanying drawings.

[0044] Figure 1 a is a schematic view of an ion surface trap according to a first embodiment and

[0045] Figure 1 b is a schematic view of an ion surface trap according to a second embodiment.

[0046] Figure 1 schematically shows an ion surface trap 10 according to the invention, which has an electrode pair 12. The electrode pair 12 has a first trap electrode 14.1 and a second trap electrode 14.2.

[0047] The first trap electrode 14.1 is formed, for example, by a metallization, which can consist in particular of gold. The second trap electrode 14.2 is constructed of a superconductor, for example, niobium, tantalum, or a high-temperature superconductor, for example, yttrium barium copper oxide.

[0048] The ion surface trap 10 also has a first DC voltage electrode 16.1 and a second DC voltage electrode 16.2, which are connected to a positioning voltage source 17 which outputs a positioning voltage llpos.

[0049] The second trap electrode 14.2 forms a sensor element 18, which has an ohmic superconductor layer resistance Ris in the normal conducting state. Using the sensor element 18, a schematically drawn photon 20 emitted by an ion 22 (also schematically drawn) can be detected. The ion 22 is positioned at a predetermined position P22 by the trap voltage lltrap. It is possible for the trap electrode 14.2 to have additional, non-superconducting sections. In this case, only the superconducting part of the trap electrode 14.2 is the sensor element 18.

[0050] The ion surface trap 10 has a current source 24 connected to opposite ends of the second trap electrode 14.2, which generates a measuring current Imess through the second trap electrode 14.2. A corresponding measuring voltage Umess is approximately zero when the second trap electrode is superconducting and jumps to a higher value when the superconductivity breaks down.

[0051] By means of a resistance meter 26, which in the present case can be designed as a voltmeter, a resistance change of the electrical resistance of the second trap electrode 14.2 is measured.

[0052] The current source 24 is connected to a first pole 25.1 of a trap voltage source 27, so that the reference potential of the current source 24 corresponds to the potential of this pole 25.1. For example, the potential is ground.

[0053] The two trap electrodes 14.1, 14.2 are arranged on a substrate 28, for example made of corundum.

[0054] Figure 1b shows a second, simplified schematic view of an ion surface trap 10 according to the invention according to a second embodiment. A relief resistance element 30 can be seen.

[0055] Schematically shown is a cooling device 32 in the form of a cryostat, which brings at least the electrode pair 12 and the sensor element 18 to an operating temperature Tb which is below a transition temperature Tsprung of the superconductor material.

[0056] The two poles of current source 24 are each connected to an inductor 34.1, 34.2, which acts as a low-pass filter. This prevents the high-frequency component of the trap voltage Utrap from reducing the stability of the measurement voltage Umess. Two capacitors 36.1, 36.2 separate the potential of the trap electrodes 14.1, 14.2 from the potential of the trap voltage source 27.

[0057] List of reference symbols

[0058] 10 ion surface traps

[0059] 12 pairs of electrodes

[0060] 14.1 first trap electrode

[0061] 14.2 second trap electrode

[0062] 16.1 first DC electrode

[0063] 16.2 second DC electrode

[0064] 17 Positioning voltage source

[0065] 18 Sensor element

[0066] 20 photons

[0067] 22 Ion

[0068] 24 Power source

[0069] 25 pole

[0070] 26 resistance meters

[0071] 27 Trap voltage source

[0072] 28 Substrat

[0073] 30 Relief resistance element

[0074] 32 Cooling device

[0075] 34 Inductance

[0076] 36 Capacitor

[0077] Ri8 superconducting film resistor

[0078] Rso ohmic relief resistance

[0079] U pos positioning voltage

[0080] U mess measuring voltage

[0081] Imess measuring current

Claims

Patent claims 1 . ion surface trap (10) with (a) a pair of electrodes (12) comprising a first trap electrode (14.1) and a second trap electrode (14.2) and designed to form a trap volume for at least one ion upon application of an alternating electrical voltage, and (b) a sensor element (18) for detecting photons (20) emitted by the at least one ion, characterized in that (c) the sensor element (18) comprises a superconductor layer and forms the second trap electrode (14.2) and does not have a superconductor layer-separation layer-superconductor layer structure.

2. Ion surface trap (10) according to claim 1, characterized by a power source (24) which is designed for automatic (i) applying a measuring current (Imess) to the superconductor layer which is chosen to be large enough that the impact of a photon (20) with a predetermined minimum energy at an impact location on the superconductor layer causes a normal conduction in the vicinity of the impact location, which spreads over the entire cross section of the superconductor layer and (ii) Detecting a change in a quantity characterising the measuring current (Imess). The ion surface trap (10) according to one of the preceding claims, characterized by a trap voltage source connected to the first trap electrode and the sensor element (18) for applying a trap voltage (11trap). The ion surface trap (10) according to one of the preceding claims, characterized by at least two DC voltage electrodes arranged to generate an electric field by means of which an ion position of the ion relative to the ion surface trap (10) can be changed. The ion surface trap (10) according to one of the preceding claims, characterized by a resistance meter (26) for measuring a change in resistance of the superconductor layer. The ion surface trap (10) according to one of the preceding claims, characterized by (a) a substrate (28) on which the first trap electrode (14.1) and the superconductor layer are applied, (b) wherein the superconductor layer is electrically insulated and potential-separated from the first trap electrode. Ion surface trap (10) according to one of the preceding claims, characterized by (a) an ion introduction device for introducing an ion into a trap region of the ion surface trap (10) and (b) an evaluation unit which is designed to automatically carry out a method comprising the steps (i) controlling the ion introduction device so that an ion is introduced into the ion surface trap (10), (ii) controlling the trap voltage source to apply the trap voltage between the first trap electrode and the sensor element (18) so that the ion remains trapped in the ion surface trap (10), and (iii) detecting at least one photon (20) emitted by the ion by means of the sensor element (18). Ion surface trap (10) according to one of the preceding claims, characterized by a relief resistance element (30) connected in parallel to the superconductor layer (18) and whose ohmic relief resistance (R30) is smaller than the ohmic superconductor layer resistance (Ri s) of the superconductor layer (18) when the superconductor layer (18) is normally conductive. Ion surface trap (10) according to one of the preceding claims, characterized in that the current source (24) (a) is connected to the second trap electrode (14.2) via an inductance (34) and / or (b) is connected to the trap voltage source (27) via at least one capacitor (36). A method for operating an ion surface trap (10) according to one of the preceding claims, characterized by the steps: (i) introducing an ion into the ion surface trap (10), (ii) applying a trap voltage to the first trap electrode (14.1) and the sensor element (18) so that the ion remains trapped in the ion surface trap (10), and (iii) detecting at least one photon (20) emitted by the ion by means of the sensor element (18). A method according to claim 10, characterized by the steps of (i) evaporating a pure substance so that it is in gaseous form, (ii) photoionization of the gaseous pure substance and (iii) moving at least one ion of the gaseous pure substance into a trap volume of the ion surface trap (10).

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