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Coherent Ion Transport in a Multi-electrode Trap Array

R. Bowler

Published Jan 1, 2015• 6 citations
OpenAlex

Abstract

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.

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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.
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.
Scaling quantum information processors is a challenging task, requiring manipulation of a large number of qubits with high fidelity and a high degree of connectivity. For trapped ions, this can be realized in a 2D array of interconnected traps in which ions are separated, transported, and recombined to carry out quantum operations on small subsets of ions. Here, functionality of a junction connecting orthogonal linear segments in a 2D trap array to reorder a two-ion crystal is demonstrated. The secular motion of the ions experiences low energy gain and the internal qubit levels maintain coherence during the reordering process, therefore demonstrating a promising method for providing all-to-all connectivity in a large-scale, 2D or 3D trapped-ion quantum information processor.
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.
Quantum information processing combines information theory with laws of quantum mechanics to provide an interesting new study that promises significant technological advances in the field of computation. The quest for a physical quantum information processor not only tests the limits of quantum mechanics but also motivates the development of new control techniques for quantum systems. This thesis documents the implementation of the necessary components of a quantum computer in a new atomic ion species, while demonstrating an entangling procedure that is uniquely insensitive to certain types of phase noise. Quantum bits are stored in the ground state hyperfine levels of individual trapped cadmium ions, and the collective vibration of the ions in the trap potential form a quantum databus through which information can be transferred. Quantum state measurements and initialization processes are accomplished through optical pumping, and quantum logic operations are performed through interactions with applied electromagnetic fields. The spin-dependent force, which is the underlying principle of many entanglement schemes for trapped ions, is investigated in detail in a series of Schrodinger cat experiments that generates entangled wavepackets well separated in the momentum-position phase space (alpha = 6). Phase control of the interaction in the gate scheme proposed by Molmer and Sorensen results in phase coherence between single-qubit rotations and a robust two-qubit entangling logic gate that can operate on magnetic-field insensitive clock qubits at finite temperature. The coherence time of the clock qubit (∼ 1s) is long compared to the gate time (∼ 100mus). Finally, quantum state tomography is performed on two ions, featuring a set of universal quantum logic gates that is sufficient for any quantum computation, with an entangling gate fidelity of 0.83. The combination of the tools developed here is sufficient to perform universal quantum computation. With the advent of scalable, multi-zone ion trap structures, the concept of a quantum computation device may become reality in the not so distant future.
Large-scale quantum information processors must be able to transport and maintain quantum information and repeatedly perform logical operations. Here, we show a combination of all of the fundamental elements required to perform scalable quantum computing through the use of qubits stored in the internal states of trapped atomic ions. We quantified the repeatability of a multiple-qubit operation and observed no loss of performance despite qubit transport over macroscopic distances. Key to these results is the use of different pairs of 9Be+ hyperfine states for robust qubit storage, readout, and gates, and simultaneous trapping of 24Mg+ "re-cooling" ions along with the qubit ions.
Abstract The trapped-ion quantum charge-coupled device (QCCD) architecture is a leading candidate for advanced quantum information processing. In current QCCD implementations, imperfect ion transport and anomalous heating can excite ion motion during a calculation. To counteract this, intermediate cooling is necessary to maintain high-fidelity gate performance. Cooling the computational ions sympathetically with ions of another species, a commonly employed strategy, creates a significant runtime bottleneck. Here, we demonstrate a different approach we call exchange cooling. Unlike sympathetic cooling, exchange cooling does not require trapping two different atomic species. The protocol introduces a bank of “coolant" ions which are repeatedly laser cooled. A computational ion can then be cooled by transporting a coolant ion into its proximity. We test this concept experimentally with two 40Ca+ ions, executing the necessary transport in 107 μs, an order of magnitude faster than typical sympathetic cooling durations. We remove over 96%, and as many as 102(5) quanta, of axial motional energy from the computational ion. We verify that re-cooling the coolant ion does not decohere the computational ion. This approach validates the feasibility of a single-species QCCD processor, capable of fast quantum simulation and computation.