Mixed-Species Ion Chains for Quantum Networks
Dissertation Committee Chair: Prof. Christopher Monroe
Prof. Christopher Jarzynski, Dean’s Representative
Prof. Gretchen Campbell
Prof. James Williams
Prof. Norbert Linke
Quantum computing promises solutions to some of the world's most important problems that classical computers have failed to address. The trapped-ion-based quantum computing platform has a lot of advantages for doing so: ions are perfectly identical and near-perfectly isolated, feature long coherent times, and allow high-fidelity individual laser-controlled operations. One of the greatest remaining obstacles in trapped-ion-based quantum computing is the issue of scalability. The approach that we take to address this issue is a modular architecture: separate ion traps, each with a manageable number of ions, are interconnected via photonic links. To avoid photon-generated crosstalk between qubits and utilize advantages of different kinds of ions for each role, we use two distinct species -- 171Yb+ as memory qubits and 138Ba+ as communication qubits. The qubits based on 171Yb+ are defined within the hyperfine “clock” states characterized by a very long coherence time while 138Ba+ ions feature visible-range wavelength emission lines. Current optical and fiber technologies are more efficient in this range than at shorter wavelengths.
We present a theoretical description and experimental demonstration of the key elements of a quantum network based on the mixed-species paradigm. The first one is entanglement between an atomic qubit and the polarization degree of freedom of a pure single photon. To verify the purity of single photons, we measure the second-order correlation function and find g(2)(0)=(8.1±2.3)×10-5without background subtraction, which is consistent with the lowest reported value in any system. Next, we show mixed-species entangling gates with two ions using the Mølmer-Sørensen and Cirac-Zoller protocols. Finally, we theoretically generalize mixed-species entangling gates to long ion chains and characterize the roles of normal modes there. In addition, we explore sympathetic cooling efficiency in such mixed-species crystals. Besides these developments, we demonstrate new techniques for manipulating states within the D3/2-manifold of zero-nuclear-spin ions -- a part of a protected qubit scheme promising seconds-long coherence times proposed by Aharon et al. in 2013.