Scalable Creation of Long-Lived Multipartite Entanglement

Abstract

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...

Precise detection of multipartite entanglement in four-qubit Greenberger–Horne–Zeilinger diagonal states

“…When p 1 > p 16 , the criterion gives a lower bound of 1/q corresponding to the straight-line boundary AB in Figs.1 and 2, with q = 9 and q = 22.5, respectively. When p 1 < p 16 , the criterion gives an upper bound of 1/q corresponding to the straight-line boundary FG in Fig.2, with q = 12.5. Criterion II can be written as cases.…”

“…Thus, − α−14v α−8+2v ≥ 5. The equation for line GH in Fig.1 16 ) (notice that p 16 = 0 in Fig.1). Therefore, v ≤ 2 13 .…”

Entanglement certification from theory to experiment

“…For more details on the setup see, e.g., [244]. Depending on the specific choice of atomic species and electronic energy levels, the qubit transitions can either be driven by spatially addressing the ions with laser light (e.g., the setups used in [245][246][247][248]), or (near-field [249,250], or far-field [251][252][253]) frequency addressed microwave signals. Although the generation of GME is not necessarily the raison d'être for many current multi-qubit devices, the controlled generation and detection of GME is often considered as a means to benchmark their functionality [254][255][256].…”

Trapping and sympathetic cooling of single thorium ions for spectroscopy

“…This establishes new options for singleion spectroscopy with exotic ions, eventually quantum logic spectroscopy using the Ca as a read-out of quan-tum states and transitions of Th ions which escape direct observation via detection of fluorescence or which cannot be interrogated using the electron-shelving method. In the future, we plan to employ the advanced quantumcomputing [29] and quantum-sensing [30] techniques with entangled Ca ion crystals for precise investigations on the unique system of 229 Th + and 229m Th + .…”