We demonstrate a SWAP gate between laser-cooled ions in a segmented microtrap via fast physical swapping of the ion positions. This operation is used in conjunction with qubit initialization, manipulation and readout, and with other types of shuttling operations such as linear transport and crystal separation and merging. Combining these operations, we perform quantum process tomography of the SWAP gate, obtaining a mean process fidelity of 99.5(5)%. The swap operation is demonstrated with motional excitations below 0.05(1) quanta for all six collective modes of a two-ion crystal, for a process duration of 42 µs. Extending these techniques to three ions, we reverse the order of a three-ion crystal and reconstruct the truth table for this operation, resulting in a mean process fidelity of 99.96(13)% in the logical basis.The last decade has seen substantial progress towards scalable quantum computing with trapped ions. Gate fidelities reach fault-tolerance thresholds [1], and first steps towards realizing decoherence-free qubits have been demonstrated [2]. Moreover, microfabricated, segmented ion traps continue to mature as an experimental low-noise environment [3,4] hosting multi-qubit systems [5,6]. In the seminal proposal from Kielpinsky, Monroe and Wineland [7] for such quantum CCD chip, scalability is reached through ion shuttling operations, where trapped-ion qubits are moved between different trap sites through application of suitable voltage waveforms to the trap electrodes. Since the first demonstration of ion shuttling in segmented traps [8], the development of trap control hardware has progressed [9,10]. This has recently led to demonstrations of fast ion shuttling at low final motional excitation [11,12]. It is currently an open question if a trapped-ion quantum computer should be based on large processing units hosting thousands of qubits [13,14] or on a modular architecture of medium-sized nodes with photonic interconnectivity [15]. With current technology, the possibilities for high-fidelity coherent control and readout of ion strings consisting of more than a few ions are limited, such that ion shuttling is required in either case. For universal quantum computation, two-qubit gates need to be performed between arbitrary pairs of ions, such that reordering ion strings becomes a necessary. Furthermore, if multiple ion species [16] are employed for sympathetic cooling [17] or ancilla-based syndrome readout via inter-species entangling gates [18,19], deterministic ion reconfiguration is ultimately required.To that end, segmented ion traps bearing junctions with T[20], X [21,22] or Y[23] geometry have been developed and tested. Junctions increase the design complexity of the traps and allow only for sequential ion transport. Shuttling through junctions may yield large motional excitations, which precludes the execution of two-qubit gates. In this work, we perform ion reorder- ing via on-site swapping of ions through application of suitable electric potentials. , where an external magnetic field lifts t...

We demonstrate sensing of inhomogeneous dc magnetic fields by employing entangled trapped ions, which are shuttled in a segmented Paul trap. As sensor states, we use Bell states of the type |↑↓ + e iϕ |↓↑ encoded in two 40 Ca + ions stored at different locations. Due to the linear Zeeman effect, the relative phase ϕ serves to measure the magnetic field difference between the constituent locations, while common-mode fluctuations are rejected. Consecutive measurements on sensor states encoded in the S 1/2 ground state and in the D 5/2 metastable state are used to separate an ac Zeeman shift from the linear dc Zeeman effect. We measure magnetic field differences over distances of up to 6.2 mm, with accuracies of around 300 fT, sensitivities down to 12 pT/ √ Hz, and spatial resolutions down to 10 nm. For optimizing the information gain while maintaining a high dynamic range, we implement an algorithm for Bayesian frequency estimation.

We demonstrate the deterministic generation of multipartite entanglement based on scalable methods. Four qubits are encoded in 40 Ca + , stored in a micro-structured segmented Paul trap. These qubits are sequentially entangled by laser-driven pairwise gate operations. Between these, the qubit register is dynamically reconfigured via ion shuttling operations, where ion crystals are separated and merged, and ions are moved in and out of a fixed laser interaction zone. A sequence consisting of three pairwise entangling gates yields a four-ion GHZ state |ψ = 1 √ 2 (|0000 + |1111 ), and full quantum state tomography reveals a Bell state fidelity of 94.4(3)%. We analyze the decoherence of this state and employ dynamic decoupling on the spatially distributed constituents to maintain 69(5)% coherence at a storage time of 1.1 seconds.The key challenge for the realization of quantum information processing devices which actually outperform classical information technology lies in the scaling to a sufficient complexity, while maintaining high operational fidelities. With trapped ions and superconducting circuits being the leading candidates for scalable highfidelity quantum computing (QC) platforms, few-qubit architectures have been realized [1,2], and elementary quantum algorithms [3,4] as well as fundamental building blocks for quantum error correction [5,6] have been demonstrated. For trapped ions, a possible route to scalability was opened up with the seminal proposal of the quantum CCD [7], where ions are stored in segmented, micro-chip-based radiofrequency traps [8,9] and shuttled between distinct trap sites in order to realize quantum logic operations on selected subgroups of qubits [10][11][12][13][14]. Based on these methods, a complete methods set for QC [15] and a fully programmable two-qubit quantum processor [16] have been shown. It is rather likely that any trapped-ion based large-scale QC architecture [17][18][19] will involve ion shuttling operations. As a benchmark for quantum information processing capabilities, the generation and properties of multipartite entangled states have been studied intensively. On the one hand, generating and maintaining such states lies at the heart of quantum computing, on the other hand large multipartite entangled states represent a resource for the measurement-based approach to QC [20,21]. The first generation of a four-particle Greenberger-Horne-Zeilinger (GHZ) states has been accomplished at a state fidelity of 57% by the NIST group [22] [26] has also been demonstrated, QC generally requires deterministic entanglement generation with capabilities for storage and individual manipulation and readout of the qubits. In this work, we demonstrate the scalable generation of GHZ states of up to four trapped ions. Our method is based on single-qubit rotations, pairwise two-qubit entangling gates and shuttling operations. In analogy to arithmetic-logic-units (ALU) in the von-Neumann computer architecture [19], the computational gate operations are driven by laser beams which are di...

We present a multi-purpose toolkit for digital processing, acquisition and feedback control designed for physics labs. The kit provides in a compact device the functionalities of several instruments: function generator, oscilloscope, lock-in amplifier, proportional-integral-derivative filters, Ramp scan generator and a Lock-control. The design combines Field-Programmable-Gate-Array processing and microprocessor programing to get precision, ease of use and versatility. It can be remotely operated through the network with different levels of control: from simple out-of-the-shelve Web GUI to remote script control or in-device programmed operation. Three example applications are presented in this work on laser spectroscopy and laser locking experiments. The examples includes side-fringe locking, peak locking through lock-in demodulation, complete in-device Pound-Drever-Hall modulation and demodulation at 31.25 MHz and advanced acquisition examples like real-time data streaming for remote storage. Hardware / device FIG. 1: Layer structured design for the client-server architecture of the toolkit. The elements are grouped horizontally by site and type. In columns, the user possible strategies are shown. The first column follows the Red Pitaya STEMLab original design philosophy. The dashed line separates the client part (end-user device) and the server part (STEMLab device). The arrows show communication direction between elements.

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