Ion trap systems built upon microfabricated chips have emerged as a promising platform for quantum computing to achieve reproducible and scalable structures. However, photoinduced charging of materials in such chips can generate undesired stray electric fields that disrupt the quantum state of the ion, limiting high-fidelity quantum control essential for practical quantum computing. While crude understanding of the phenomena has been gained heuristically over the past years, explanations for the microscopic mechanism of photogenerated charge carrier dynamics remains largely elusive. Here we present a photoinduced charging model for semiconductors, whose verification is enabled by a systematic interaction between trapped ions and photoinduced stray fields from exposed silicon surfaces in our chip. We use motion-sensitive qubit transitions to directly characterize the stray field and analyze its effect on the quantum dynamics of the trapped ion. In contrast to incoherent errors arising from the thermal motion of the ion, coherent errors are induced by the stray field, whose effect is significantly imprinted during the quantum control of the ion. These errors are investigated in depth, and methods to mitigate them are discussed. Finally, we extend the implications of our study to other photoinduced charging mechanisms prevalent in ion traps and discuss considerations in fabrication to reduce semiconductor charging.
Ion trap systems built upon microfabricated chips have emerged as a promising platform for quantum computing to achieve reproducible and scalable structures. However, photoinduced charging of materials in such chips can generate undesired stray electric fields that disrupt the quantum state of the ion, limiting high-fidelity quantum control essential for practical quantum computing. While crude understanding of the phenomena has been gained heuristically over the past years, explanations for the microscopic mechanism of photogenerated charge carrier dynamics remains largely elusive. Here we present a photoinduced charging model for semiconductors, whose verification is enabled by a systematic interaction between trapped ions and photoinduced stray fields from exposed silicon surfaces in our chip. We use motion-sensitive qubit transitions to directly characterize the stray field and analyze its effect on the quantum dynamics of the trapped ion. In contrast to incoherent errors arising from the thermal motion of the ion, coherent errors are induced by the stray field, whose effect is significantly imprinted during the quantum control of the ion. These errors are investigated in depth, and methods to mitigate them are discussed. Finally, we extend the implications of our study to other photoinduced charging mechanisms prevalent in ion traps and discuss considerations in fabrication to reduce semiconductor charging.
Abstract Silicon-based ion trap chips can benefit from existing advanced fabrication technologies, such as multi-metal layer techniques for two-dimensional architectures and silicon photonics for the integration of on-chip optical components. However, the scalability of these technologies may be compromised by semiconductor charging, where photogenerated charge carriers produce electric potentials that disrupt ion motion. Inspired by recent studies on charge distribution mechanisms in semiconductors, we developed a silicon-based chip with gold coated on all exposed silicon surfaces. This modification significantly stabilized ion motion compared to a chip without such metallic shielding, a result that underscores the detrimental effects of exposed silicon. With the mitigation of background silicon-induced fields to negligible levels, quantum operations such as sideband cooling and two-ion entangling gates, which were previously infeasible with the unshielded chip, can now be implemented.
Electrical charging of metal surfaces due to photoelectric generation of carriers is of concern in trapped ion quantum computation systems, due to the high sensitivity of the ions’ motional quantum states to deformation of the trapping potential. The charging induced by typical laser frequencies involved in Doppler cooling and quantum control is studied here, with microfabricated surface-electrode traps made of aluminum, copper, and gold, operated at 6 K with a single Sr+ ion trapped 100 μm above the trap surface. The lasers used are at 370, 405, 460, and 674 nm, and the typical photon flux at the trap is 1014 photons/cm2/sec. Charging is detected by monitoring the ion’s micromotion signal, which is related to the number of charges created on the trap. A wavelength and material dependence of the charging behavior is observed: Lasers at lower wavelengths cause more charging, and aluminum exhibits more charging than copper or gold. We describe the charging dynamic based on a rate-equation approach.
The stray charges induced on ion trap surfaces can push trapped ions away from a pseudopotential null point. This can cause the micromotion of the trapped ions that in turn results in ion heating. One of the causes of stray charges is laser scattering on dielectric surfaces. The amount of laser-induced stray charges can be related to the area of the dielectric surface exposed to the ions. In this paper, stray charges are intentionally created at various dielectric surfaces on a microfabricated ion trap chip using a laser, and the resulting ion motion is measured. The ion movements are estimated by comparing ion positions before and after laser irradiation. When the laser irradiates a dielectric surface with an area of approximately 1,750 μm2exposed to the ion, the ion moves approximately −4.8 μm in the axial direction. When the laser irradiates another dielectric surface with an area of approximately 140 μm2exposed to the ion, the ion moves approximately −0.6 μm. The ratio between the ion movements corresponds to the ratio between the areas of each dielectric surface directly exposed to the ion normalized to the distance between the ion and the laser-induced stray charges. The results indicate the importance of reducing the area of dielectric surfaces that are directly in the line of laser paths.
Electrical charging of metal surfaces due to photoelectric generation of carriers is of concern in trapped ion quantum computation systems, due to the high sensitivity of the ions’ motional quantum states to deformation of the trapping potential. The charging induced by typical laser frequencies involved in Doppler cooling and quantum control is studied here, with microfabricated surface-electrode traps made of aluminum, copper, and gold, operated at 6 K with a single Sr[superscript +] ion trapped 100 μm above the trap surface. The lasers used are at 370, 405, 460, and 674 nm, and the typical photon flux at the trap is 10[superscript 14] photons/cm[superscript 2]/sec. Charging is detected by monitoring the ion’s micromotion signal, which is related to the number of charges created on the trap. A wavelength and material dependence of the charging behavior is observed: Lasers at lower wavelengths cause more charging, and aluminum exhibits more charging than copper or gold. We describe the charging dynamic based on a rate-equation approach.
An ion trap is a device to confine charged particles by utilizing electromagnetic fields.Since the trapped ions have a feasibility of the individual state manipulation, a long coherence time, and an ideal isolation from the surroundings, the ion-trap technology becomes one of the leading candidates for the physical implementation of quantum information processing.In order to build a large-scale integrated iontrap system for realizing complex quantum algorithms, microfabrication technologies have been applied to construct ion traps.The use of micro-electromechanical system (MEMS) ion-trap chip allows a scalable architecture of ion-trap arrays and integration of functional components to the trap chip, but a few side effects also arise.One of the most particular problems is stray fields generated by the charges accumulated on the sidewalls of thick dielectric pillars or the native oxide grown on metal surfaces, since the stray fields lead to ion micromotions which can cause heating and escape of the trapped ions.This dissertation presents a silicon surface ion-trap chip with dielectric sidewalls shielded by metal films.The oxide pillars supporting the top electrodes are fabricated to have overhang structures, and the upper and lower part of the pillars are coated by separate aluminum layers which are electrically isolated from each other.In order to evaluate the effects of the native ii metal oxide on the electrode surface to the charging phenomenon, a trap chip with an additional gold layer on the aluminum electrodes is also fabricated.An ultra-high vacuum (UHV) chamber, electrical connections, and an optical setup are prepared to trap 174 Yb + ions, and the ions are successfully trapped by using the experimental setup including the fabricated ion-trap chip.To evaluate the effectiveness of the proposed electrode structures, charging is intentionally induced on the chip surface by injecting a 355-nm pulse laser with 40-μW power perpendicularly to the chip surface.The intensity of the stray field is estimated by measuring the displacement of ion position after charging is induced.When the Al-based chips with exposed and Al-coated dielectric sidewalls are used, the standard deviation of the intensities of stray fields are 3.53, and 5.63 V/m, respectively.However, the standard deviation of the intensity of stray fields in the case of using Au-coated chip is 1.56 V/m, and any stray field is measured when the laser was injected on the surface or sidewalls of the gold-coated electrodes.These experimental results indicate that the gold coating on the surface of the aluminum electrodes and the sidewalls of dielectric pillars is effective for eliminating the generation of stray fields induced by static charges.
The purpose of this work is to investigate methods by which the quantum states of trapped ions may be better controlled and measured for the applications of quantum computing, quantum simulation, and the generation of atomic frequency standards. We report on two primary projects: first, we control the relative coupling strengths of carrier and first-order motional sideband interactions of a trapped ion by placing it in a resonant optical standing wave. Our configuration uses the surface of a microfabricated chip trap as a mirror, avoiding the technical challenges of in-vacuum optical cavities. Displacing the ion along the standing wave, we show a periodic suppression of the carrier and sideband transitions, with the cycles for the two cases 180◦ out of phase with each other. This technique allows for the suppression of off-resonant carrier excitations when addressing the motional sidebands, and has applications in quantum computation and quantum simulation. Second, we investigate methods proposed in Ref. [36] which may improve the performance of trapped ion (or atom) frequency standards when the interrogating local oscillator is of limited quality. These methods could be applied to future generations of small-scale frequency standards, which are in demand for applications such as local GPS networks and distributed sensors. We propose a scheme for implementing these methods in a 40Ca+ based frequency standard, and outline the technical requirements for doing so. Finally, we present preliminary results characterizing single-ion coherence times and ion “state destruction” events in our system, and discuss how they will affect the realization of the proposed scheme.