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arXiv:1005.4144 · Physical Review Letters

Ultrafast Gates for Single Atomic Qubits

Wesley C. Campbell, Jonathan Mizrahi, Qudsia Sara Quraishi, Crystal Senko, David Hayes, David Hucul, Dzmitry Matsukevich, Peter Maunz, Christopher R. Monroe

Published Aug 26, 2010• 158 citations
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Abstract

We demonstrate single-qubit operations on a trapped atom hyperfine qubit using a single ultrafast pulse from a mode-locked laser. We shape the pulse from the laser and perform a π rotation of the qubit in less than 50 ps with a population transfer exceeding 99% and negligible effects from spontaneous emission or ac Stark shifts. The gate time is significantly shorter than the period of atomic motion in the trap (Ω(Rabi)/ν(trap)>10(4)), demonstrating that this interaction takes place deep within the strong excitation regime.

DOI
10.1103/physrevlett.105.090502

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Single- and multi-qubit operations for trapped ion hyperfine or Zeeman qubits are typically implemented by continuous-wave lasers driving stimulated Raman transitions. Here we report two distinct regimes for driving these transitions with mode-locked pulsed lasers instead of cw systems. In the resolved-sideband regime, multiple pulses from a mode-locked laser constructively accumulate transition amplitude to drive the gate. In the regime where the pulse train is shorter than the oscillation period of motional modes, the mode spectrum is unresolvable and all allowed sidebands are driven simultaneously. We demonstrate a single qubit gate in about 50 ps in this regime, and discuss extensions to multi-qubit gates.
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One of the major problems in building a quantum computer is the development of scalable and robust methods to entangle many qubits. Quantum computers based on trapped atomic ions are one of the most mature and promising platforms for quantum information processing, exhibiting excellent coherence properties, near-perfect qubit detection efficiency, and high-fidelity entangling gates. Entangling operations between multiple ions in a chain typically rely on qubit state-dependent forces that modulate their Coulomb-coupled normal modes of motion. However, scaling these operations to large qubit numbers in a single chain must account for the increasing complexity of the normal mode spectrum, and can result in a gate time slowdown or added complexity of the control forces. In this thesis, I present an alternative route to the scalability problem using optical interactions faster than any state evolution. The experiments shown here represent a proof of principle for quantum manipulation of atoms in the strong coupling regime. This work relies on spin dependent forces (SDK) with short laser pulses and use it as our fundamental building block for thermometry and non-trivial motional state preparation. Together with a robust stabilization of the ion trap and high light collection efficiency, we demonstrate two-ion entanglement with ten ultrafast pulses. Due to the nature of the interaction, the demonstrated entangling operation can be made arbitrarily fast only limited by laser engineering.
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