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arXiv:1704.02141 · Physical Review Letters

High-Fidelity Preservation of Quantum Information During Trapped-Ion Transport

Peter Kaufmann, Timm F. Gloger, Delia Kaufmann, M. Johanning, Christof Wunderlich

Published Jan 2, 2018• 37 citations
arXivHTMLPDF

Abstract

A promising scheme for building scalable quantum simulators and computers is the synthesis of a scalable system using interconnected subsystems. A prerequisite for this approach is the ability to faithfully transfer quantum information between subsystems. With trapped atomic ions, this can be realized by transporting ions with quantum information encoded into their internal states. Here, we measure with high precision the fidelity of quantum information encoded into hyperfine states of a ^{171}Yb^{+} ion during ion transport in a microstructured Paul trap. Ramsey spectroscopy of the ion's internal state is interleaved with up to 4000 transport operations over a distance of 280 μm each taking 12.8 μs. We obtain a state fidelity of 99.9994( _{-7}^{+6})% per ion transport.

DOI
10.1103/physrevlett.120.010501

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A promising scheme for building scalable quantum simulators and computers is the synthesis of a scalable system using interconnected subsystems. A prerequisite for this approach is the ability to faithfully transfer quantum information between subsystems. With trapped atomic ions, this can be realized by transporting ions with quantum information encoded into their internal states. Here, we measure with high precision the fidelity of quantum information encoded into hyperfine states of a ^{171}Yb^{+} ion during ion transport in a microstructured Paul trap. Ramsey spectroscopy of the ion's internal state is interleaved with up to 4000 transport operations over a distance of 280 μm each taking 12.8 μs. We obtain a state fidelity of 99.9994( _{-7}^{+6})% per ion transport.
We realize fast transport of ions in a segmented microstructured Paul trap. The ion is shuttled over a distance of more than 10(4) times its ground state wave function size during only five motional cycles of the trap (280 μm in 3.6 μs). Starting from a ground-state-cooled ion, we find an optimized transport such that the energy increase is as low as 0.10±0.01 motional quanta. In addition, we demonstrate that quantum information stored in a spin-motion entangled state is preserved throughout the transport. Shuttling operations are concatenated, as a proof-of-principle for the shuttling-based architecture to scalable ion trap quantum computing.
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.
This dissertation describes a prototype experiment aiming at the realization of scalable quantum information. The essential feature is the usage of a novel microstructured ion trap derived from the Paul trap. It allows for storing and manipulating a large number of ions, as compared to conventional linear Paul traps. This thesis describes how the way is paved towards the realization of quantum information experiments in this ion trap. An analysis of the electrostatic properties of the ion trap is presented, which is laying the foundation for understanding the limits of confinement stability and effects beyond standard Paul trap behavior. The focus of this work lies on the realization and characterization of single and dual qubit operations, which are achieved by means of (semiclassical) atom-light interaction. In our experiment, the qubit is implemented in the Zeeman sublevels of the ion’s ground state, i.e. in the spin of the bright electron of a 40Ca+ ion. The main body of this thesis then describes the realization of the necessary steps of preparation, manipulation and readout of this qubit. The preparation includes optical pumping and cooling close to the motional quantum ground state by means of sideband cooling. Several possible techniques for these steps are tested and analyzed. Coherent manipulations are carried out by means of stimulated Raman transitions. Here, a strong emphasis is put on the characterization of the various decoherence mechanisms, which are dominated by the motional excitation of the ion due to thermalization of the ion with the trap electrodes, and by imperfections in the ion-laser interactions. As by-product of the latter investigation, a new measurement scheme for the experimental determination of atomic dipole matrix elements is presented.
Trapped ion systems are the leading candidate for quantum information processing because many of the critical components have already been demonstrated. Scaling trapped ion systems to large numbers of ions is currently believed possible, but much work remains to prove it. Microfabricated surface ion traps are increasing in popularity for their ease of mass production and their ability to manipulate individual ions and interact arbitrary pairs of ions. Even with the advent of scalable ion traps, detection of an individual ion trapped in a high vacuum poses a challenge. The internal state of the ion chosen for a quantum bit can be measured via exposure to a probe beam that causes one state to scatter light (a "bright" state), but not the other state (a "dark" state). In free space, a single ion acts like a point source that emits in all directions; a standard two inch lens system can only collect about 2% of the light emitted by the ion. Poor light collection results in a high error rate and slow determination of the internal state of the ion. Fast, high fidelity state detection is necessary for quantum error correction and loophole-free Bell experiments at short (less than 100\\,km) distances, and high efficiency collection is necessary to rapidly interconnect separate quantum computers. We demonstrate state detection fidelities of 99%, 99.856(8)% and 99.915(7) % which correspond to detection times of 10.5, 28.1 and 99.8 us, respectively.
We report reliable transport of (9)Be(+) ions through an "X junction" in a 2D trap array that includes a separate loading and reservoir zone. During transport the ion's kinetic energy in its local well increases by only a few motional quanta and internal-state coherences are preserved. We also examine two sources of energy gain during transport: a particular radio-frequency noise heating mechanism and digital sampling noise. Such studies are important to achieve scaling in a trapped-ion quantum information processor.