Session M09: Advancing Techologies and Techniques for Trapped Ions

Multimode Motional Quantum State Tomography in a Trapped Ion System

We present a new method to determine an arbitrary motional state of multiple normal modes in a trapped ion chain. We use individual gate operations and state detection on multiple ions to quickly reconstruct the density matrix and Wigner function of a high-dimensional motional state. We apply this method in a room temperature trapped ion system with ¹⁷¹Yb⁺ ions and reconstruct a motional Bell state. The method offers a new approach to study complex motional states.   

Combined ion trap and surface-science system for investigating electric-field noise from surfaces

Electric-field noise from surfaces can pose problems in experiments in the quantum regime.  In quantum information processing with trapped ions, this noise causes heating of the ions’ motion, which complicates large-scale quantum computing by limiting the fidelity of multi-qubit operations.  Noise levels far above intrinsic Johnson noise have been observed for the past few decades, yet a complete description of the mechanisms responsible for this noise remains to be developed.  There is strong evidence that it originates from processes on electrode surfaces.  We report on progress towards investigating electric field noise from surfaces using a “stylus” ion trap integrated with a surface-science cluster of instruments.  The combined system allows for in situ treatment and characterization of surfaces and UHV transfer into the proximity of a trapped ion.  Ion heating-rate measurements are performed to quantify the distance- and frequency-dependence of noise originating from a sample surface.  This combined setup may allow for high-throughput correlation of heating with surface features such as morphology and adsorbate coverage.  This may aid a more complete understanding of electric-field noise from surfaces where highly specific data can inform theoretical models.   

Spin-motion coupling using the transverse field gradient of a laser beam

As we continue to push toward an ion-based gyroscope interferometer [1], we observe spin-motion coupling of an ion using a single laser beam that drives a stimulaed Raman transition. Transferring momentum to an ion during a laser-driven transition requires a spatial gradient in the Rabi frequency which is typically achieved through the interference of two non-copropagating laser beams. Here [2], we demonstrate momentum transfer perpendicular to the laser beam's wavevector using the spatial gradient of the transverse beam profile. This effect may already be playing an unappreciated role in trapped-ion qubit gates.

[1] W. C. Campbell and P. Hamilton, J. Phys. B. 50, 064002 (2017)

[2] Adam D West et al., Quantum Sci. Technol. 6 024003. (2021)

    

Single Photon Generation with 138Ba+ Ions Excited by Pulsed Light for Quantum Networking

Trapped ions are among the most promising platforms for quantum computing and quantum communication due to their long coherence and excellent controllability. Individual traps with ion chains can be combined into a larger quantum system offering scalability for universal quantum computing. ¹³⁸Ba⁺ ions are especially attractive for quantum networking because they emit light at 493 nm, allowing us to benefit from fiber technologies for remote entanglement over long distances. We aim to demonstrate S_1/2→P_1/2 excitation with ultra-short pulsed light by a frequency-doubled Ti:Sapphire laser and the generation of single photons by spontaneous emission.   

Detecting and minimizing RF breakdown on microfabricated surface ion traps

RF breakdown is a major limiting factor in the maximum RF voltage microfabricated surface ion traps can sustain.  The complicated physics involved in breakdown makes it difficult to predict a priori how susceptible new trap designs will be to this destructive process.  We have developed two techniques for detecting RF breakdown events in situ, one using free-space RF field detectors, and the other monitoring the back-reflected RF signal from the trap itself. Here we describe these techniques and share the results of an extended study of RF breakdown on many different traps.  Our results highlight the danger of ramping up the RF voltage too quickly for the initial use of a new trap.  We present a procedure for safely turning on new traps, by increasing the voltage slowly and monitoring for breakdown.  Also, we briefly describe our most recent fabrication efforts to mitigate breakdown in future traps.

    

Coherently coupled mechanical oscillators in the quantum regime

We couple spectrally separated pairs of quantum harmonic oscillator modes of motion in linear multi-ion crystals via parametric modulation of the confining potential. The choice of the modulation determines which pair of modes are coupled together.  We realize the coherent exchange of single motional quanta between the coupled modes while controlling the timing, strength, and phase of the coupling. We demonstrate high fidelity quantum state transfer between modes and use the coupling to entangle motional modes and observe Hong-Ou-Mandel type interference. Controllable coupling of this type between harmonic oscillators can aid high-fidelity operations for quantum information processing with discrete and continuous variables, quantum simulations, and precision measurements. It can also enable cooling and quantum logic spectroscopy involving motional modes that do not couple directly to laser light.

Non-demolition readout of a mechanical oscillator state in a symmetric linear ion crystal

Continuous-variable (CV) systems are an alternative to approaches using two-level systems for quantum information processing. An infinite-dimensional (or CV) Hilbert space, such as that of a harmonic oscillator (HO), can be used to encode "logical" qubit states (mutually orthogonal superpositions of energy eigenstates) for which quantum error correction (QEC) is possible through coupling to an auxiliary quantum system. Trapped-ion systems are a candidate for CV QEC by encoding information in motional degrees of freedom, which can be treated as HOs, while reading out error syndromes via internal electronic states. However, atomic motion is affected by photon recoil such that any information stored in motional modes is typically destroyed during fluorescence readout. Readout schemes that do not affect the motional state are thus an important prerequisite for motional-state verification in trapped-ion CV QEC. We demonstrate that the axial out-of-phase (OOPH) mode in a Be+-Mg+-Be+ crystal is nearly unaffected after scattering thousands of photons off the Mg+ ion. We also demonstrate a projective measurement that leaves the OOPH mode in number state |0> or |1> with high fidelity. It may be possible to modify this measurement to accommodate error syndrome readout for bosonic codes.   

Miniaturize 3D ion traps using novel 3D printing technology

Scalable architectures for quantum information processing with trapped ions will necessarily comprise thousands of computation sites. 3D traps are cumbersome and scaling to thousands of trapping sites appears to be quite challenging. Current approaches to scalable trapped ion quantum information use planar traps with limited trap depth, inefficient confinement. Although more challenging to make, miniature 3D traps are a better candidate which allows for higher secular frequencies thereby speeding up quantum operations and increasing resilience to electric-field noise. Here we use a novel 3D printing technology: two-photon polymerization direct laser writing, to fabricate miniature 3D printed traps with ion-electrode distance of 100 µm, and successfully trapped in such traps. The high flexibility of printing 3D structures using this method paves the way for creating a scalable QCCD quantum computer.   

Electrically adjustable inductance for cryogenic RF applications M. Schubert1*, M. Neumann1, M. Schilling1, B. Hampel1 1Institut für Elektrische Messtechnik und Grundlagen der Elektrotechnik, TU Braunschweig, Hans-Sommer-Str. 66, Braunschweig 38106, Germany *marjan.schubert@tu-bs.de

Trapped-ion qubits are one of many approaches to realize 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. The properties of materials and semiconductor components change significantly at low temperatures, which makes the impedance matching and frequency tuning of resonance circuits very difficult. Electrically adjustable components for RF circuits enable the fine-tuning during an experiment without the need of time-consuming cooldown cycles.

In this work, we integrate an electrically tunable cryogenic resonance circuit with a voltage-controlled inductor to provide RF fields for trapping ions. The structure consists of two ideally coupled coils. The current and thus the magnetic flux from one coil can be adjusted with a field-effect transistor used as a voltage-controlled resistor. The relationship between the inductance of the coil setup and the magnetic field generated by the current can be derived using the law of induction. Simulations show that with a sufficient coupling factor, significant changes in inductance can be achieved. We will present cryogenic measurements of an electrically tunable resonance circuit, which can be used to fine-tune the impedance matching and the resonant frequency of circuits.   

Site-resolved ion imaging and precision beam positioning for Penning-trap quantum simulation and sensing

Laser-cooled ions in a Penning trap are a promising candidate for near-term quantum simulation of exotic forms of quantum magnetism as well as the search for dark matter using quantum sensing [1,2]. To this end we have recently implemented a high bandwidth time-correlated single-photon-counting camera which allows efficient single-site detection of individual ions in large 2D ion crystal, a prerequisite to investigate spatial correlations in many-body quantum systems. The large amount of image data is analysed by an object detection algorithm using an artificial neural network. Next, to overcome the issue of precise entangling laser beam alignment in the difficult to access Penning trap system, we have developed a laser beam delivery system based on compact piezo-actuated optical mirrors which allow an efficient beam position tuning. This system also enables us to maximize the ratio of spin-spin interaction strength to spontaneous emission, a critical ratio for experiments with Be-9 ions in a Penning trap. In this talk I will give an overview of these technical developments and present first results.

[1] H. Ball et. al., Rev. Sci. Instrum. 90, 053103 (2019)

[2] K. A. Gilmore et. al., Science 373, 673 (2021)