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arXiv:1204.5004 · Review of Scientific Instruments

Compact radio-frequency resonator for cryogenic ion traps

Davide Gandolfi, M. Niedermayr, M. Kumph, Mike Brownnutt, R. Blatt

Published Aug 1, 2012• 20 citations
arXivHTMLPDF

Abstract

We report on the investigation and implementation of a lumped-component, radio-frequency resonator used in a cryogenic vacuum environment to drive an ion trap. The resonator was required to achieve the voltages necessary to trap (~100 V), while dissipating very little power. Ultimately, for an input voltage of 1.35 V, a voltage gain of 100 was measured at 5.7 K, using a design which dissipated only 18 mW. The resonator operated at a frequency of 7.64 MHz and had a Q of 700. Single (40)Ca(+) ions were confined in a trap driven by this device, providing proof of successful resonator operation at low temperature.

DOI
10.1063/1.4737889

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We report on the investigation and implementation of a lumped-component, radio-frequency resonator used in a cryogenic vacuum environment to drive an ion trap. The resonator was required to achieve the voltages necessary to trap (~100 V), while dissipating very little power. Ultimately, for an input voltage of 1.35 V, a voltage gain of 100 was measured at 5.7 K, using a design which dissipated only 18 mW. The resonator operated at a frequency of 7.64 MHz and had a Q of 700. Single (40)Ca(+) ions were confined in a trap driven by this device, providing proof of successful resonator operation at low temperature.
Trapping ions in Paul traps require high radio frequency voltages, which are generated using resonators. When operating traps in a cryogenic environment, an in-vacuum resonator showing low loss is crucial to limit the thermal load to the cryostat. In this study, we present a guide for the design and production of compact, shielded cryogenic resonators. We produced and characterized three different types of resonators and furthermore demonstrate efficient impedance matching of these resonators at cryogenic temperatures.
A combination of direct current (DC) fields and high amplitude radio frequency (RF) fields is necessary to trap ions in a Paul trap. Such high electric RF fields are usually reached with the help of a resonator in close proximity to the ion trap. Ion trap based quantum computers profit from good vacuum conditions and low heating rates that cryogenic environments provide. However, an impedance matching network between the resonator and its RF source is necessary, as an unmatched resonator would require higher input power due to power reflection. The reflected power would not contribute to the RF trapping potential, and the losses in the cable induce additional heat into the system. The electrical properties of the matching network components change during cooling, and a cryogenic setup usually prohibits physical access to integrated components while the experiment is running. This circumstance leads to either several cooling cycles to improve the matching at cryogenic temperatures or the operation of poorly matched resonators. In this work, we demonstrate an RF resonator that is actively matched to the wave impedance of coaxial cables and the signal source. The active part of the matching circuit consists of a varactor diode array. Its capacitance depends on the DC voltage applied from outside the cryostat. We present measurements of the power reflection, the Q-factor, and higher harmonic signals resulting from the nonlinearity of the varactor diodes. The RF resonator is tested in a cryostat at room temperature and cryogenic temperatures, down to 4.3 K. A superior impedance matching for different ion traps can be achieved with this type of resonator.
Trapped-ion qubits are one approach among many for achieving scalable quantum computers. An ion trap has to be operated with several dc and radio frequency (rf) signals to trap and control its qubits. Many ion trap setups are operated at cryogenic temperatures to reduce thermal influences, to reach very high vacuum, and to achieve high fidelity for quantum operations. A resonance circuit, consisting of a coil and the capacitance of the trap electrodes, is used to step-up a low power rf signal to high amplitudes in close proximity to the ion trap. These ac fields are used to confine ions70 μm70 \,{\rm \mu }\mathrm{m}70μmabove a surface ion trap chip. An increased quality factor (Q-factor) of the resonance circuit leads to a higher voltage gain, but the experiment also benefits from a better attenuation of parasitic frequency components in the confining electric field. The Q-factor is inversely proportional to the trap capacitance. Since ion traps are growing in size due to a larger number of qubits, the capacitance is increasing. Therefore, the development of a coil with low losses becomes even more important. In this work, we present the setup of a superconducting coil for a high-Q resonator, measurements of the Q-factor, and its temperature dependency. The coil is made of a niobium-titanium (NbTi) wire wound on a threaded bobbin made of ceramics and equipped with further thermalization structures. The superconducting resonator is a very promising approach to satisfy the needs for future trapped-ion quantum computing setups.
We report a simple, efficient, high voltage radio frequency (RF) generator powered by a single voltage source (1.5-7 V) to resonantly drive ion traps or other capacitive loads. Our circuit is able to deliver RF voltages > 500 V(p-p) at frequencies ranging from 0.1 to 10 MHz. This RF oscillator uses low-cost, commercially available components, and can be easily assembled onto a circuit board of a few cm(2). Because of its simplicity and good efficiency, this circuit is useful in applications requiring small size and low power consumption such as portable ion trap systems where the duration of operation under battery power is of concern.
Paul traps are widely used to confine electrically charged particles like atomic and molecular ions by using an intense radiofrequency (RF) field, typically obtained by a voltage drop on capacitative electrodes placed in vacuum. We present a RF drive realized on a compact printed circuit board and providing a high-voltage RF signal to a quadrupole Paul trap. The circuit is formed by using four interdependent resonant circuits - each of which is connected to an electrode of a Paul trap - fed by low-noise amplifiers, leading to an output voltage of peak-to-peak amplitude up to 200 V at 3.23 MHz. The presence of a single resonant circuit for each electrode ensures a strong control on the voltage drop on each electrode, e.g., by applying a DC field through a bias tee. Additionally, the moderate quality factor Q = 67 of the resonant circuits ensures a fast operation of the drive, which can be turned on and off in less than 10 μs. Finally, the RF lines are equipped with pickups that sample the RF in phase and amplitude, thus providing a signal that can be used to actively control the voltage drop at the trap's electrodes.
We present a novel ultrastable superconducting radio-frequency (RF) ion trap realized as a combination of an RF cavity and a linear Paul trap. Its RF quadrupole mode at 34.52 MHz reaches a quality factor of Q ≈ 2.3 × 105 at a temperature of 4.1 K and is used to radially confine ions in an ultralow-noise pseudopotential. This concept is expected to strongly suppress motional heating rates and related frequency shifts that limit the ultimate accuracy achieved in advanced ion traps for frequency metrology. Running with its low-vibration cryogenic cooling system, electron-beam ion trap, and deceleration beamline supplying highly charged ions (HCIs), the superconducting trap offers ideal conditions for optical frequency metrology with ionic species. We report its proof-of-principle operation as a quadrupole-mass filter with HCIs and trapping of Doppler-cooled 9Be+ Coulomb crystals.