Surface science for improved ion traps
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Introduction Trapped atomic ions are used to explore the feasibility of quantum information processing (QIP) algorithms,1–4 mainly aided by their inherent well-determined energy-level structure and relative isolation from the surrounding world. They have demonstrated all of the basic DiVincenzo criteria:5 ability to initialize states of well-defined quantum bits (qubits), implementation of universal gate sets, long coherence times relative to gate durations, qubit specific readout, and scalability. Therefore, they are promising candidates for physical qubits to be used in QIP. However, as the scalability requirement motivates greater miniaturization with ions confined closer to the trap-electrode surfaces (Figure 1), the importance of interactions between ions and the trap electrodes grows. Ideally, trapped ions only feel the force associated with an applied harmonic potential well and their mutual Coulomb repulsion. The applied potentials produce three-dimensional confinement, where the three motional modes for a single trapped ion typically have frequencies ranging from 0.1 MHz to 10 MHz. Multiple ions in the trap strongly repel each other, leading to a collective mode structure with 3N modes for N trapped ions. These modes are ideally laser-cooled to the ground state of motion, preparing a well-defined initial state
for quantum logic operations that rely on Coulomb coupling between nearby qubit ions. In practice, however, the motion of the ions may be perturbed by stray electric fields from the environment, causing their quantum states to decohere. Even one quantum of motion absorbed from the environment during a two-qubit logic gate will ruin the fidelity of this operation. In the context of quantum logic operations with ions, decoherence of the ions’ motion refers mainly to the process whereby the initial and final motional-state wave functions in a two-qubit gate do not overlap. Many sources of motional decoherence have been identified. These include trap frequency instability, radiative heating such as that caused by Johnson noise (thermal electronic noise) from resistive elements, external electronic noise, field emission, and collisions with background gases.6 In this article, we review efforts to understand and eliminate another noise source that has been a particular nuisance for trapped-ion QIP; that is, heating of the motional modes of ions (effectively, excitation of phonons in a collection of ions) from electric-field fluctuations originating from the surface of the trap electrodes. This is commonly referred to as anomalous heating, because the origin cannot be easily explained by any of the more obvious sources listed previously.
D.A. Hite, National Institute of Standards and Technology, Colorado; [email protected] Y. Colombe, National Institute of Standards and Technology, Colorado; [email protected] A.C. Wilson, National Institute of Standards and Technology, Colorado; [email protected] D.T.C. Allcock, National Institute of Standards and Technology, Colorado; [email protected] D. Leibfried,
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