Session H25: Trapped Ions and Polaritons

Electric-field noise from thermally-activated fluctuators in a surface ion trap

Electric-field noise originating from surfaces is a major source of motional decoherence in ion trap chips. A better understanding of this noise is important for the fabrication of low-noise quantum devices. We probe electric-field noise near the aluminum-copper surface of an ion trap chip in a previously unexplored high-temperature regime. A saturation of the noise amplitude occurs around 500 K, which, together with a small change in the frequency scaling, points to thermally activated two-state fluctuators as the origin of the noise. We find intriguing similarities in the distribution of activation energies extracted from our data and corresponding data from resistance fluctuation measurements for polycrystalline aluminum films. These similarities suggest atomic motion as a relevant microscopic mechanism, likely taking place at the metal trap surface.
  

Zero-temperature properties of the long-range transverse-field Ising model on the triangular lattice

We investigate the ground state properties of the transverse-field Ising model with long-range, antiferromagnetic interactions on the triangular lattice. Our study is motivated by experiments on trapped beryllium ions, where the ions form a 2D Wigner crystal with tunable 1/r^α Ising interactions in the range 0 ≤ α ≤ 3, where r is the distance between two spins. Our results are obtained using a projector variant of the stochastic series expansion (SSE) Monte Carlo method, which allows direct access to ground state properties throughout the phase diagram.   

Surface ion trap with dc-tunable height

We describe the design and operation of a novel surface-electrode ion trap that produces a radio-frequency-null along the axis perpendicular to the trap surface. This arrangement enables control of the vertical trapping potential and consequentially the ion height via dc-electrodes only. This architecture provides a platform for simplified vertical shuttling and trapping vertical ion strings without excess micromotion. As a first application, we show ion height-dependent measurements of electric-field noise, which may provide insight into understanding anomalous heating.   

A Cost Function Approach for Comparison of Harmonically Trapped Particle Transport Protocols

Transporting a particle in a harmonic potential between two locations arises naturally in array-based proposals for a scalable trapped ion quantum computer. Transport protocols parameterize the potential minimum location over time, and a protocol satisfying certain boundary conditions can ensure no excitation or local phase are incurred, which we call ideal transport. Ideal transport has an infinite solution space, but many solutions to ideal transport possess unrealizable changes in control field magnitudes or large excursions of the potential minimum beyond the interval of transport. We quantify deviations from a linear trajectory both in position and velocity, compute the overlap of the transported state with its translated initial state, and linearly combine these terms to construct a cost functional of the trajectory to compare the behavior of distinct quantum harmonic transport protocols without a full quantum simulation of particle displacement dynamics. We also show that not all terms can be simultaneously optimized in general, but rather that polynomial- and Fourier series-based trajectories exhibit different regions of superiority as quantified by the cost function.   

Quantum Control of a Trapped Ion using a Stimulated Brillouin Scattering Laser

Trapped ions are natural qubit candidates for quantum information processors and sensors, but high-fidelity quantum operations require high quality laser oscillators. In typical trapped-ion experiments, high finesse, free-space optical cavities made of ultralow expansion (ULE) glass are used to reduce the emission linewidth of commercial lasers to the level of the ion’s natural linewidth. Integration of these narrow laser systems into compact or scalable platforms presents a challenge, since it is not currently possible to fabricate integrated photonic resonators with quality factors as high as those in ULE cavities. In this work, we demonstrate the application of the stimulated Brillouin scattering (SBS) nonlinearity in a fiber-based cavity with a quality factor similar to those of integrated photonic resonators to achieve lasing at a linewidth comparable to that of a ULE cavity-stabilized laser. Additionally, we apply a novel technique to suppress temperature fluctuation of the central frequency of the fiber resonator to enable precise control of ions. The development of an ultranarrow laser compatible with integrated photonic resonators is a key part of the design of future portable quantum technologies based on trapped ions.   

High-field photoionization of Beryllium using a low-cost solid-state laser

The generation of a pure sample of ions is a prerequisite for a variety of experiments in atomic and molecular physics, in particular using ion traps. Resonant laser photoionization has the advantage of being element and isotope selective, therefore permitting precise control of the composition of the generated ionic species. In this talk we describe experiments employing a novel system for the photoionization of Beryllium at a 2T magnetic field in a Penning trap. Our system is based on injection-locking of a low-cost, high-power multimode diode at 470nm which can then be frequency doubled to the relevant wavelength near 235nm. The optical setup and its application will be presented.   

Characterization of a High-Brightness, Laser-Cooled Li+ Ion Source

Ion sources based on laser cooling have recently provided new pathways to high-resolution microscopy, ion milling, and ion implantation. Here, we present the design and detailed characterization of ⁷Li magneto-optical trap ion source (MOTIS) with a peak brightness of 1.3 x 10⁵ A m^-2 sr^-1 eV^-1 and a maximum continuous current of 1.16 ± 0.06 nA. These values significantly surpass previous Li MOTIS performance benchmarks. Using simple models, we discuss how the performance of this system relates to fundamental operating limits. This source will support a range of projects using lithium ion beams for surface microscopy and nanostructure characterization, including Li⁺ implantation for studies of ionic transport in energy storage materials.
  

Mitigation of frequency noise due to mechanical vibration in a cryogenic trapped-ion quantum processor

Trapped-ion systems equipped with closed-cycle cryocoolers confer several cost-efficient advantages over traditional room-temperature designs. For example, operation of ion traps at cryogenic temperatures has been shown to lower rates of anomalous motional heating, a limit to multi-qubit logic fidelity, by up to two orders of magnitude. This environment also permits the use of superconducting materials to shield the ion qubits from magnetic field fluctuations. Additionally, cryopumping quickly produces ultrahigh vacuum conditions without the need for extended high-temperature chamber bakeouts, greatly reducing the cycle time of ion trap prototyping. However, mechanical vibrations caused by the cryocooler compressor can degrade quantum gate performance. Specifically, we have identified uncompensated Doppler shifts due to the relative motion of the ion with respect to the laser driving our gates as a leading source of decoherence. Here we describe the recent implementation of an interferometric stabilization scheme that actively modulates the qubit laser frequency to compensate for the motion induced by the cryocooler vibration. We observe a significant increase in the laser-ion coherence time and investigate the effect of this frequency stabilization on two-qubit gate performance.   

Observation of the Polariton Drag Effect

In this work, we report the observation of polariton drag effect in a semiconductor structure that has been designed to maximize the light-matter coupling, the namely strong coupling of exciton-polaritons in a solid-state microcavity. We show that collisions of polaritons with the free electrons results in a change in the angle of emission of the photons from the cavity structure. The effect is asymmetric, significantly slowing down the polaritons when they move oppositely to the electrons, while the polaritons are only slightly accelerated by electrons moving in the same direction. In conclusion, we have demonstrated proof of principle that a DC current can directly alter the momentum of photons moving in the cavity; this has the direct effect of changing the angle of emission. This polariton drag effect is beam steering using a DC current to tune the angle of a light beam since the experiment is a photon-in, photon-out system.   

Exciton-Polariton Condensate in a Ring Microcavity

Ring microchannels for exciton-polaritons were created using optical photolithography followed by an inductively coupled plasma etch to pattern the top mirror of the microcavity in a ring shape. Due to the variation in the thickness of the microcavity across the ring, there was a gradient of potential energy across each ring. Polariton condensates were created by non-resonantly pumping the ring at the highest energy region using short laser pulses. A polariton condensate was formed at the location of the pump spot which flowed towards the minimum energy region. We observed natural oscillations of the condensate about this energy minimum by time resolving the photoluminescence using a streak camera. The time period of the oscillations was in good agreement with the numerical simulation of the time evolution of a wave packet in a rigid pendulum potential. A time-reversal -symmetry-broken rotational mode was also observed in the ring. The temporal oscillations in energy and density of the polaritons at a given spatial point provide a direct measurement of the exciton-exciton interaction strength (~ 5 μeV-μm²) in a GaAs quantum well.   

Characterization of High-Q Microcavity Quantum Wells

The development of high-Q microcavity quantum well samples has led to long lifetime polaritons, thus allowing for long distance propagation of polaritons. However, long lifetime polaritons have very narrow line widths, as a result we can no longer resolve the polariton branches in the reflectivity spectrum of these samples. This makes accurately determining the detuning of polariton populations difficult. Accurate detuning information is essential for quantitative work involving absolute density of the polariton population. In this work, we will discuss our modern characterization techniques. We have two methods which we currently use. One method involves fitting large angle dispersion curves of the lower polariton with the theoretical predictions of a two state model. The other method involves measuring accurate dielectric functions for our materials, and then comparing experimental reflectivity data to the predictions of a transfer matrix model to determine the cavity mode. Both of these methods depend on the measurement of the rabbi splitting by photoluminescent emission scans. We found these two methods agree well with each other.   

Long-Range Exciton Transport in Microcavity Exciton-Polariton Systems

It has previously been assumed that the transport distance of excitons in microcavity exciton-polariton systems is limited by the bare exciton diffusion length (≤ 1 μm). In the case of non-resonant excitation, this implies that excitons created by the pump laser would be mostly confined to the pump location. While this is clearly generally true given the close similarities between the spatial pump and lower polariton potential profiles, we show that there is an important minority population of slightly polaritonic excitons that can move distances similar to those of the much more photonic polaritons (≈ 30 μm). This population, which resides in the bottleneck region of the lower polariton branch, is often not detected in a typical photoluminescence experiment due to limited numerical aperture. While small in number compared to the total population of excitons, they can substantially outnumber the polaritons below the bottleneck. This has significant implications for creating potential landscapes using a non-resonant pump, and for measurements of the interaction strength between polaritons.