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Quantum Electronics and Laser Science Conference

First steps toward a multiplexed trapped ion quantum processor

Brian DeMarco, Ben-Kish, J. Britton, Hughes, Jelenkovic, Langer, Leibfried, Meyer, Rosenband, Rowe, Itano, Wineland

Published Jan 1, 2002• 1 citation
Publisher

Abstract

Summary form only given. We discuss experiments with trapped /sup 9/Be/sup +/ ions in a micro-machined linear Paul trap that has two trapping regions separated by 1.2 mm. This trap geometry permits experiments that are the first steps toward a multiplexed, scalable quantum computer. We demonstrate coherent manipulations on a single ion that is transported between trapping regions, separation of two ions, and a quantum logic gate on a single ion that is made possible by the significantly reduced heating rate in this trap. We demonstrate adiabatic transport of a single ion between the trapping regions using time varying electrode potentials. A single ion is transported adiabatically over 1.2 mm between the trapping regions in 500 /spl mu/s with a measured increase of 0.1 /spl plusmn/ 0.1 motional quanta. A single ion has been transported between traps non-adiabatically in times as short as 50 /spl mu/s. Through manipulation of the electrode potentials we also show controlled separation of two ions initially held in one region to two regions that are separated by 1.2 mm.

DOI
10.1109/qels.2002.1031401

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Summary form only given. We discuss experiments with trapped /sup 9/Be/sup +/ ions in a micro-machined linear Paul trap that has two trapping regions separated by 1.2 mm. This trap geometry permits experiments that are the first steps toward a multiplexed, scalable quantum computer. We demonstrate coherent manipulations on a single ion that is transported between trapping regions, separation of two ions, and a quantum logic gate on a single ion that is made possible by the significantly reduced heating rate in this trap. We demonstrate adiabatic transport of a single ion between the trapping regions using time varying electrode potentials. A single ion is transported adiabatically over 1.2 mm between the trapping regions in 500 /spl mu/s with a measured increase of 0.1 /spl plusmn/ 0.1 motional quanta. A single ion has been transported between traps non-adiabatically in times as short as 50 /spl mu/s. Through manipulation of the electrode potentials we also show controlled separation of two ions initially held in one region to two regions that are separated by 1.2 mm.
We investigate the dynamics of single and multiple ions during transport between and separation into spatially distinct locations in a multizone linear Paul trap. A single 9Be+^{9}\mathrm{Be}^{+}9Be+ ion in a $\ensuremath{\sim}2\text{ }\text{ }\mathrm{MHz}$ harmonic well was transported $370\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ in $8\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$, corresponding to 16 periods of oscillation, with a gain of 0.1 motional quanta. Similar results were achieved for the transport of two ions. We also separated chains of up to 9 ions from one potential well to two distinct potential wells. With two ions this was accomplished in $55\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$, with excitations of approximately two quanta for each ion. Fast transport and separation can significantly reduce the time overhead in certain architectures for scalable quantum information processing with trapped ions.
Quantum information processors are predicted to enable a significant speedup in solving certain classes of problems compared to their classical counterparts. For one scheme of a processor implementation, required elements include qubits with long coherence times, laser-induced universal gates, information transport, and motional-state initialization using a second coolant ion species. Techniques described in this thesis include the use of a multiple electrode segmented trap, wherein information transport is achieved through the transport of qubit ions between spatially distinct locations. While experiment timescales had previously been dominated by ion transport and re-cooling of motional states, I have achieved transport and separation of qubit ions on timescales comparable to quantum logic gates. This work has been extended to achieve fast transport of qubit and coolant two-species ion chains. The developed techniques for ion transport result in low motional excitations, reducing the need for re-initialization of ion motional states. This can enable significant improvement in the operation time of an ion-based quantum information processor.
A fault-tolerant quantum computer is expected to require thousands of qubits. Trapped ion architectures provide a modular approach where the quantum register is divided into multiple subregisters connected by physically moving the corresponding ions. Transporting ions at scale comes with several challenges such as the need to connect thousands of control lines to an ion trap chip. Multiplexing the required control voltages from few input signals to multiple electrodes offers a solution to this wiring challenge. Here we demonstrate a quantum processing unit that combines a surface ion trap with a time multiplexer via a sample-and-hold technique that initially charges electrodes to fixed voltages and disconnects them during qubit operations. We characterize the unit's performance by measuring motional heating rates below one phonon per second in both open and closed switch configurations. We further characterize the sample and hold process and find that sampling intervals below 50 ms are sufficient to keep expected gate errors from decaying charges during the hold phase below 10−410^{-4}10−4. Our results indicate that the multiplexing scheme is compatible with high-fidelity operations.
Abstract It was recently proposed to use small groups of trapped ions as qubit carriers in miniaturized electrode arrays that comprise a large number of individual trapping zones, between which ions could be moved [1,2] This approach might be scalable for quantum information processing with a large numbers of qubits. Processing of quantum information is achieved by transporting ions to and from separate memory and qubit manipulation zones in between quantum logic operations. The transport of ion groups in this scheme plays a major role and requires precise experimental control and fast transport times. In this paper we introduce a theoretical framework to study ion transport in external potentials that might be created by typical miniaturized Paul trap electrode arrays. In particular we discuss the relationship between classical and quantum descriptions of the transport and study the energy transfer to the oscillatory motion during near‐adiabatic transport. Based on our findings we suggest a numerical method to find electrode potentials as a function of time to optimize the local potential an ion experiences during transport. We demonstrate this method for one specific electrode geometry that should closely represent the situation encountered in realistic trap arrays.
The major challenges in trapped-ion quantum computation are to scale up few-ion experiments to many qubits and to improve control techniques so that quantum logic gates can be carried out with higher fidelities. This thesis reports experimental progress in both of these areas. In the early part of the thesis we describe the fabrication of a surface-electrode ion trap, the development of the apparatus and techniques required to operate it and the successful trapping of 40Ca+ ions. Notably we developed methods to control the orientation of the principal axes and to minimise ion micromotion. We propose a repumping scheme that simplifies heating rate measurements for ions with low-lying D levels, and use it to characterise the electric field noise in the trap. Surface-electrode traps are important because they offer a route to dense integration of electronic and optical control elements using existing microfabrication technology. We explore this scaling route by testing a series of three traps that were microfabricated at Sandia National Laboratories. Investigations of micromotion and charging of the surface by laser beams were carried out and improvements to future traps are suggested. Using one of these traps we also investigated anomalous electrical noise from the electrode surfaces and discovered that it can be reduced by cleaning with a pulsed laser. A factor of two decrease was observed; this represents the first in situ removal of this noise source, an important step towards higher gate fidelities. In the second half of the thesis we describe the design and construction of an experiment for the purpose of replacing laser-driven multi-qubit quantum logic gates with microwave-driven ones. We investigate magnetic-field-independent hyperfine qubits in 40Ca+ as suitable qubits for this scheme. We make a design study of how best to integrate an ion trap with the microwave conductors required to implement the gate and propose a novel integrated resonant structure. The trap was fabricated and ions were successfully loaded. Single-qubit experiments show that the microwave fields above the trap are in excellent agreement with software simulations. There are good prospects for demonstrating a multi-qubit gate in the near future. We conclude by discussing the possibilities for larger-scale quantum computation by combining microfabricated traps and microwave control.
We demonstrate single-qubit operations by transporting a beryllium ion with a controlled velocity through a stationary laser beam. We use these to perform coherent sequences of quantum operations, and to perform parallel quantum logic gates on two ions in different processing zones of a multiplexed ion trap chip using a single recycled laser beam. For the latter, we demonstrate individually addressed single-qubit gates by local control of the speed of each ion. The fidelities we observe are consistent with operations performed using standard methods involving static ions and pulsed laser fields. This work therefore provides a path to scalable ion trap quantum computing with reduced requirements on the optical control complexity.