Trapped-Ion Qubits

Technical Overview

Trapped atomic ions are a leading candidate technology for large-scale quantum information processing because of their demonstrated long coherence times and low-error operations. In our lab, strontium (Sr) ions are laser cooled and electromagnetically trapped in an ultrahigh vacuum (UHV), providing a high degree of isolation from the environment. Both internal and external quantum states of individual trapped ions can be utilized for quantum processing tasks. These states are manipulated by a combination of lasers and other electromagnetic fields, and quantum information is read out via ion fluorescence.

Program Goals

  • Develop a flexible platform for the investigation and advancement of trapped-ion quantum state manipulation.
  • Integrate optics and electronics technologies on chip with ion trap arrays to enable large-scale quantum processing with trapped ions.
  • Increase all aspects of processing speed and fidelity, from ion trap loading through quantum-logic gates to state readout, to explore the possibilities of useful quantum simulation, computation, and sensing.

Research Highlights

At Lincoln Laboratory, we trap ions in surface electrode traps. We use cryocooled vacuum systems to achieve UHV rapidly and to cool the ion trap structures. We currently have three main areas of research:

  • Ion quantum-logic processing speed is fundamentally limited by trap frequencies; in order to increase the speed, these trap frequencies must be increased, requiring trap size reduction. However, it is now well known that as ions get closer to the trap electrodes, motional heating of the ions increases rapidly, limiting the fidelity of two-qubit gates. This electric-field-noise-induced motional heating is termed "anomalous" in the scientific community because, while the noise appears to come from the ion trap electrode surfaces, its microscopic origin is currently unknown. We perform experiments aimed at studying the dependence of the noise on trap-electrode material, temperature, fabrication method, and surface preparation in an effort to understand the origin of, and mitigate or eliminate, anomalous heating. We systematically study this noise using an ion trap apparatus (Figure 1) capable of accessing the whole trap-electrode temperature range from ambient to 4 K and by employing the extensive capabilities of Lincoln Laboratory's trap fabrication facilities.
  • To date, demonstrations of quantum processing with ions have been limited to systems of small size (~1–10 ions). Quickly and reliably preparing many ions in a useful array configuration is a significant scalability challenge. Additionally, as the system size grows, a technique for fast and reliable replacement of ions lost from the trap will become increasingly necessary. We are currently building a new ion trap apparatus dedicated to exploring a method of defect-free loading of two-dimensional (2D) ion arrays and for site-selectively replacement of ions lost from the array.
    Photograph of the trapped ion experimental apparatus highlighting the magneto-optical trap to rapidly and efficiently load the trap Figure 1. Photograph of the trapped-ion experimental apparatus highlighting the magneto-optical trap to rapidly and efficiently load the trap. (Click on image to view larger version.)

     

    The strength of this technique relies on our previously demonstrated method of high-rate loading of ion traps from a laser-cooled source of neutral atoms (Figure 1).  We have designed and fabricated 2D arrays (Figure 2) of surface-electrode ion traps using a custom multilayer niobium process developed at Lincoln Laboratory.

     

    Ion trapsFigure 2. (left) Photograph of a 1D ion trap loaded with 16 ions. (right) Photograph of a 2D ion trap with the center trap loaded with 4 ions. (Click on image to view larger version.)
  • In trapped-ion experiments, cooling and control laser light sent to the ions is typically routed through windows in the UHV chamber, and fluorescence light for readout from the ions is collected and directed onto large imaging systems or photon counters using bulk optics. Additionally, the electronics required to operate the traps and run the experiments are located outside the vacuum system, necessitating a large number of wires and connections to the trap inside. While this method has proved to be enormously fruitful in performing experiments for systems of small numbers of ions, it is not scalable to larger-scale systems. We are developing several techniques that will lead to scalable ion trap realization (Figure 3). In collaboration with the Semiconductor Laser Group led by Prof. Rajeev Ram at MIT, we are exploring ways to integrate optics and electronics into the ion trap itself to develop a path toward scalable control and readout of ions. We recently demonstrated stable ion trapping in a trap fabricated in a CMOS foundry and are currently designing traps with the integration of visible-light waveguides for cooling- and control-light delivery, and avalanche photodiodes and electronics for ion-state readout via photon counting.
Illustration of the on-chip distribution of the classical control signal and qubit readout for scalable trap arraysFigure 3. Illustration of the on-chip distribution of the classical control signal and qubit readout for scalable trap arrays. (Click on image to view larger version.)

 

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