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Mode-locked laser driven gates for trapped ion quantum information processing

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

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

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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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.
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.
Highly-localized light-matter interactions are necessary for scaling trapped-ion architectures. In hyperfine qubits, counterpropagating beams generate entangling gates by coupling with motion, but this effect is undesirable during single-qubit operations. For that reason, single-qubit gates are traditionally implemented with copropagating beams, and the coexistence of two beam geometries adds hardware and computational overhead. In an effort towards collective performance improvement with minimal overhead, we design and implement pulse-amplitude and dephasing robust dynamically corrected gates using Space Curve Quantum Control (SCQC) and compare them against the constant-amplitude gate implementation. We perform gate set tomography on a four-qubit trapped-ion register, and we discover more than 50% error reduction when robust pulses are used. We find that counterpropagating robust gates often outperform their copropagating counterparts and reach error rates as low as (3.59±1.25)⋅10−3(3.59 \pm 1.25)\cdot 10^{-3}(3.59±1.25)⋅10−3, using diamond distance as a metric. This value establishes a laser-driven-gate error reference and is merely an order of magnitude higher than the best reported microwave\textit{microwave}microwave gate on a single\textit{single}single ion. Additional experiments reveal that robust pulses can effectively suppress non-Markovian errors that grow during runtime. Our work challenges the widely accepted belief that copropagating gates should be preferred for their weak motional coupling and invites the adoption of high-performance robust pulses that suppress multiple noise sources of the trapped-ion error budget.
Hardware-Economic Manipulation of Dual-Type 171{}^{171}171Yb+^++ Qubits
The dual-type qubit scheme is an emerging method to suppress crosstalk errors in scalable trapped-ion quantum computation and quantum network. Here we report a hardware-economic way to control dual-type 171Yb+^{171}\mathrm{Yb}^+171Yb+ qubits using a single 355 355\,355nm mode-locked pulsed laser. Utilizing its broad frequency comb structure, we drive the Raman transitions of both qubit types encoded in the S1/2S_{1/2}S1/2​ and the F7/2F_{7/2}F7/2​ hyperfine levels, and probe their carrier transitions and the motional sidebands. We further demonstrate a direct entangling gate between the two qubit types. Our work can simplify the manipulation of the 171Yb+^{171}\mathrm{Yb}^+171Yb+ qubits both at the hardware and the software level.
Pulsed lasers offer significant advantages over continuous wave (CW) lasers in the coherent control of qubits. Here we review the theoretical and experimental aspects of controlling the internal and external states of individual trapped atoms with pulse trains. Two distinct regimes of laser intensity are identified. When the pulses are sufficiently weak that the Rabi frequency Ω is much smaller than the trap frequency ω trap, sideband transitions can be addressed and atom-atom entanglement can be accomplished in much the same way as with CW lasers. By contrast, if the pulses are very strong Ω ≫ω trap, impulsive spin-dependent kicks can be combined to create entangling gates which are much faster than a trap period. These fast entangling gates should work outside of the Lamb-Dicke regime and be insensitive to thermal atomic motion. © 2013 Springer-Verlag Berlin Heidelberg.