Helical resonator and ion trap system

Abstract

A helical resonator (140) for amplifying an AC voltage for driving an ion trap comprises: a coil (144) having, as a base material, a non-superconducting metal (for example, Cu) that does not exhibit superconductivity at a predetermined cryogenic temperature; and a shield (146) surrounding the coil (144) and having, as a base material, a non-superconducting metal that is the same as or different from that of the coil (144). A plating layer is provided on at least one of the surface of the base material of the coil (144) and the surface of the base material of the shield (146), the plating layer being plated with a superconductor (for example, Pb) exhibiting superconductivity at a predetermined cryogenic temperature. The base materials of the coil (144) and the shield (146) have a higher thermal conductivity than the superconductor.

Description

Helical resonator and ion trap system

[0001] The present invention relates to a helical resonator and an ion trap system including the helical resonator.

[0002] In recent years, there has been active research into quantum computers that use ions suspended in a vacuum, known as ion traps. Ion traps are characterized by their high quantum manipulation precision compared to other types of quantum computers. Recently, cryo-ion traps, which trap ions in an extremely low-temperature refrigerator, have become mainstream in order to reduce the electric field noise from the surface of the ion-trapping electrodes and the influence of background gases.

[0003] To perform large-scale quantum computation using ion trapping technology, it is necessary to significantly increase the number of traps and the number of ions that serve as quantum bits. To achieve stable and sufficiently deep ion confinement, a large AC voltage of several hundred volts must be applied to the trapping electrode. However, directly connecting a radio frequency (RF) signal source to the trapping electrode requires a very large power (e.g., on the order of 1 kW). Therefore, a technique has been widely used in which a helical resonator is provided between the RF signal source and the trapping electrode, and the voltage is amplified by the helical resonator (see, for example, Non-Patent Document 1).

[0004] JD Siverns, LR Simkins, S. Weidt, WK Hensinger, “On the application of radio frequency voltages to ion traps via helical resonators,” arXiv:1106.5013v3 [physics.atom-ph] (2011).

[0005] However, conventional ion trapping technology requires large power on the order of several hundred mW to several W, even when using a helical resonator. In particular, considering the increasing scale of ion traps and the development of cryo-ion traps, applying large power poses a major challenge because it generates heat.

[0006] The present invention has been made in view of the above problems, and has as its object to realize an ion trap with low power using a helical resonator.

[0007] The helical resonator according to the present invention amplifies an AC voltage for driving an ion trap, and includes a coil whose base material is a non-superconducting metal that does not exhibit superconductivity at a predetermined cryogenic temperature, and a shield whose base material is the same or a different non-superconducting metal as the coil and surrounds the coil. At least one of the surfaces of the base material of the coil and the shield is provided with a plating layer plated with a superconductor that exhibits superconductivity at the predetermined cryogenic temperature, and the base materials of the coil and the shield have a higher thermal conductivity than the superconductor.

[0008] The ion trap system according to the present invention comprises a radio frequency signal source that generates a radio frequency signal, the above-described helical resonator that generates an amplified AC voltage based on the radio frequency signal, and a trap electrode to which the amplified AC voltage is applied.

[0009] According to the present invention, at a predetermined cryogenic temperature, current flows only through the superconductor plating layer, eliminating the influence of the electrical resistance of the non-superconducting metal base material. Furthermore, the plating layer is cooled by the non-superconducting metal with high thermal conductivity, so the superconducting state can be maintained. Therefore, ion trapping can be realized with low power.

[0010] 1 is a schematic diagram of an ion trap system according to an embodiment of the present invention; 2 is a cross-sectional view of a coil of a helical resonator; 3 is a circuit diagram of an RLC series resonant circuit; 4 is a graph showing the results of a reflection measurement of a helical resonator; and 5 is a fluorescence image of ions trapped using the ion trap system according to the present embodiment.

[0011] Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. The same or similar components are designated by the same reference numerals throughout the drawings. The drawings are schematic, and the relationship between planar dimensions and thickness, and the thickness ratio of each component, differ from the actual ones. Furthermore, it goes without saying that the dimensional relationships and ratios of each component differ between the drawings.

[0012] This embodiment is directed to a helical resonator and ion trap system used in a cryo-ion trap. Figure 1 shows the configuration of an ion trap system 100 according to this embodiment. The ion trap system 100 includes an RF signal source 120 that generates a radio frequency (RF) signal, a helical resonator 140 that amplifies an AC voltage for driving the ion trap based on the RF signal generated by the RF signal source 120, and a trap electrode 160 to which the amplified AC voltage from the helical resonator 140 is applied.

[0013] The helical resonator 140 includes a coupling antenna 142, a coil 144, and a cylindrical shield 146 that surrounds the coupling antenna 142 and the coil 144. The shield 146 is grounded and has holes 148a and 148b on the RF signal source 120 side and the trapping electrode 160 side, respectively.

[0014] One end of the coupling antenna 142 is connected to the RF signal source 120 through a hole 148a in the shield 146, and the other end is connected to the grounded shield 146. The coupling antenna 142 is a smaller coil (e.g., two turns) than the main coil 144, and couples the RF signal into the helical resonator 140 by magnetic field coupling.

[0015] The coil 144 is made of a non-superconducting metal that does not exhibit superconductivity at a predetermined cryogenic temperature (e.g., approximately 4 K). The non-superconducting metal of the coil 144 is a metal with a relatively high thermal conductivity. Examples of such non-superconducting metals include, but are not limited to, copper (Cu) and aluminum (Al). One end of the coil 144 is connected to a grounded shield 146, and the other end is connected to the trapping electrode 160 via a hole 148b in the shield 146. The shield 146 is made of a non-superconducting metal that may be the same as or different from the non-superconducting metal of the coil 144.

[0016] At least one of the surfaces of the base material of the coil 144 and the shield 146 is provided with a plating layer formed by plating a superconductor that exhibits superconductivity at a predetermined cryogenic temperature (e.g., approximately 4 K). The superconductor used for the plating layer is a material whose superconducting transition temperature is higher than the base temperature of the refrigerator used in the cryo-ion trap. Therefore, the electrical resistance of the plating layer becomes zero below the superconducting transition temperature. Examples of such superconductors include, but are not limited to, Pb, Nb, NbTi, NbN, and NbTiN. For example, the superconducting transition temperature of Pb is 7.2 K. The superconductor plating layer is formed, for example, by electrolytic plating.

[0017] In order to maintain the superconducting state of the helical resonator 140 and increase the Q value (Quality Factor) described below, it is preferable to provide a plating layer on both the surface of the base material of the coil 144 and the surface of the base material of the shield 146.

[0018] The diameter Dc and length lc of the coil 144 and the diameter Ds of the shield 146 are determined depending on the resonant frequency of the helical resonator 140, with Dc ranging from 20 mm to 50 mm, lc ranging from 30 mm to 80 mm, and Ds ranging from 30 mm to 100 mm.

[0019] 2 shows a cross-sectional view of the coil 144. The diameter d of the non-superconducting rod 202 made of the base material (e.g., Cu) of the coil 144 ranges from 1 mm to 3 mm, for example. The thickness t of the plating layer 204 of the coil 144 and / or shield 146 is sufficiently thicker than the penetration depth of the superconductor (e.g., Pb), but is preferably thinner in terms of thermal conductivity, and ranges from 1 μm to 10 μm, for example. In FIG. 2, the wire diameter of the coil 144 is represented as φ.

[0020] For example, the helical resonator 140 of this embodiment is designed with Cu as the base material of the coil 144 and the shield 146, Pb as the superconductor constituting the plating layer 204, Ds = 42 mm, Ic = 40.5 mm (3 mm pitch, 13.5 turns), Dc = 24 mm, φ = 2 mm, and t ≈ 6 μm. Pb has a relatively large critical current and can maintain a superconducting state to some extent even when a large current is passed through it.

[0021] In this embodiment, the extremely low temperature environment of the cryo-ion trap is actively utilized to plate the surface of the base material (non-superconducting metal) of the coil 144 and / or the shield 146 with a superconductor. In the superconducting state, RF current flows on the surface of the material. Therefore, in the superconducting state, RF current flows only through the plating layer 204, which has zero electrical resistance, and does not flow through the non-superconducting metal base material. This eliminates the influence of the electrical resistance of the non-superconducting metal.

[0022] One problem with superconductors is their extremely low thermal conductivity. Although the electrical resistance of superconductors becomes zero at extremely low temperatures, application of an RF signal from RF signal source 120 causes Joule loss throughout the entire circuit of helical resonator 140, heating coil 144 and shield 146. However, because the superconductor plating layer 204 can be cooled by the base material (non-superconducting metal) with high thermal conductivity, the inflow of heat due to application of the RF signal can be immediately suppressed.

[0023] In this way, by taking advantage of the low electrical resistance of the superconductor and the high thermal conductivity of the non-superconducting metal, it is possible to realize a low-loss helical resonator 140 that can withstand high voltages.

[0024] The ion trap system 100 can be regarded as a series RLC resonant circuit. Figure 3 shows a circuit diagram of an RLC series resonant circuit. If the impedance of the entire circuit is denoted as Z, the resistance as R, the capacitance as C, the inductance as L, and the angular frequency as ω, then the impedance Z is given by equation (1).

[0025] The resonant angular frequency ω of this resonant circuit ０ is the angular frequency at which Z is smallest, and is expressed as in equation (2).

[0026] The Q value is introduced as a parameter that represents the properties of a resonant circuit. The Q value is defined as in equation (3).

[0027] The Q value of a series resonant circuit is R, L, C, ω ０The Q value represents the sharpness (sharpness) of the frequency characteristics of the resonance, and the larger the Q value, the greater the sharpness in the vicinity of the resonance frequency.

[0028] The voltage across the inductor (coil) is V(t) = Lω ０ I ｍａｘ cosω ０ t, where I ｍａｘ is the maximum value of the current, and t is time. At the resonant frequency, the voltages across the capacitor and inductor are equal throughout the circuit, so the voltage applied to the capacitor of trapping electrode 160 is approximately equal to the voltage applied to coil 144, and is expressed as in equation (5).

[0029] At this time, the Joule loss P ｄ is expressed as in equation (6).

[0030] Equation (7) is obtained from equations (2) and (4) to (6).

[0031] From equations (4) and (7), it can be seen that by maintaining the superconducting state of the helical resonator 140 and reducing the resistance R of the circuit, a high Q value can be obtained and the voltage applied to the trapping electrode 160 can be maximized.

[0032] To evaluate the helical resonator 140, Figure 4 shows the results of reflection measurements of the helical resonator 140, in which the non-superconducting metal is Cu and the superconductor is Pb. Specifically, Figure 4 shows the reflection spectrum |S11| obtained at a temperature of 6 K and an RF input power of -20 dBm, and the results of fitting the reflection spectrum with a Lorentz function. Figure 4 shows that the reflection spectrum is nearly consistent with the Lorentz function. Furthermore, this reflection measurement shows that resonance occurs near 37.210 MHz. The Q value calculated by fitting the reflection spectrum shown in Figure 4 reaches approximately 10,000.

[0033] Figure 5 shows a fluorescence image obtained by trapping strontium (Sr) ions at an RF intensity of approximately 3 mW using the ion trap system 100 of this embodiment. It is clear from Figure 5 that Sr ions have been trapped. Thus, ion trapping was successfully achieved at an RF intensity 1 / 1000 of that of the conventional system. Therefore, the ion trap system 100 of this embodiment can realize an ion trap with low power consumption and low thermal load.

[0034] Note that the constant input power (P = V ２ To maximize the voltage at a voltage (V / R), the impedance of the helical resonator 140 can be increased by increasing the diameter Ds of the shield 146 and the number of turns and diameter Dc of the coil 144. In a normal conductor coil, increasing the number of turns increases the resistance, but in a superconductor coil, increasing the number of turns does not increase the resistance, and the inductance can be increased.

[0035] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the spirit of the present invention. Other embodiments and modifications made by those skilled in the art are also included in the present invention.

[0036] 100 Ion trap system 120 RF signal source 140 Helical resonator 142 Coupled antenna 144 Coil 146 Shield 160 Trap electrode 202 Non-superconducting rod 204 Plating layer

Claims

1. A helical resonator that amplifies an AC voltage for driving an ion trap, comprising: a coil whose base material is a non-superconducting metal that does not exhibit superconductivity at a predetermined cryogenic temperature; and a shield that surrounds the coil and whose base material is the same or a different non-superconducting metal as the coil, wherein a plating layer plated with a superconductor that exhibits superconductivity at the predetermined cryogenic temperature is provided on at least one of the surface of the base material of the coil and the surface of the base material of the shield, and the base materials of the coil and the shield have a higher thermal conductivity than the superconductor.

2. The helical resonator according to claim 1, wherein the plating layer is provided on both the surface of the base material of the coil and the surface of the base material of the shield.

3. The helical resonator according to claim 1, wherein the thickness of said plating layer is greater than the penetration depth of said superconductor.

4. The helical resonator according to claim 3, wherein the thickness of the plating layer is 1 μm to 10 μm.

5. A helical resonator according to claim 1, wherein the superconducting transition temperature of said superconductor is higher than the base temperature of a refrigerator used in said ion trap.

6. An ion trap system comprising: a radio frequency signal source that generates a radio frequency signal; a helical resonator according to any one of claims 1 to 5 that generates an amplified AC voltage based on the radio frequency signal; and a trap electrode to which the amplified AC voltage is applied.

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