Observation of Entangled States of a Fully Controlled 20-Qubit System (1711.11092v3)

Abstract: We generate and characterise entangled states of a register of 20 individually controlled qubits, where each qubit is encoded into the electronic state of a trapped atomic ion. Entanglement is generated amongst the qubits during the out-of-equilibrium dynamics of an Ising-type Hamiltonian, engineered via laser fields. Since the qubit-qubit interactions decay with distance, entanglement is generated at early times predominantly between neighbouring groups of qubits. We characterise entanglement between these groups by designing and applying witnesses for genuine multipartite entanglement. Our results show that, during the dynamical evolution, all neighbouring qubit pairs, triplets, most quadruplets, and some quintuplets simultaneously develop genuine multipartite entanglement. Witnessing genuine multipartite entanglement in larger groups of qubits in our system remains an open challenge.

• The paper demonstrates genuine multipartite entanglement in a fully controlled 20-qubit system.
• It employs laser-induced XY interactions on trapped 40Ca⁺ ions to generate and probe entanglement dynamics.
• Novel entanglement witnesses are introduced to detect complex qubit correlations, advancing quantum simulation methods.

Observation of Entangled States of a Fully Controlled 20-Qubit System

The paper presents a significant advance in the generation and characterization of entangled states in a quantum system with 20 qubits. This investigation focuses on generating entanglement among qubits encoded into the electronic states of trapped ions, utilizing an Ising-type Hamiltonian to achieve this through laser-induced interactions.

The experimental framework employs a linear chain of 20 $^{40}$Ca$^+$ ions confined in a Paul trap. Each qubit is defined by the distinct electronic states of these ions, linked by a quadrupole transition. The qubit system's dynamics are driven by an "XY" Hamiltonian, effectively a model capturing interactions that can be modulated through external laser fields. This system configuration is instrumental in studying out-of-equilibrium dynamics and the subsequent entanglement in a controlled quantum register.

The authors demonstrate genuine multipartite entanglement (GME) in this 20-qubit system. They achieve this by initially preparing the qubits in a Néel-ordered state and evolving them dynamically under the Hamiltonian. The research focuses on how entanglement percolates through neighboring qubit pairs and triplets during these dynamics, detecting strong manifestation of GME even among quadruplets and quintuplets of qubits.

One of the paper's significant contributions is its advancement in methods for characterizing multipartite entanglement without reliance on exhaustive quantum state tomography. The authors develop new entanglement witnesses tailored to the system, allowing them to ascertain GME through measurement settings feasible under their experimental constraints. The witnesses are constructed via semantic analysis and numerical optimizations, making them robust and well-suited to detect GME even in larger neighboring qubit groups.

Analyzing the properties and dynamics of $\langle Z_i \rangle$ — indicators of localized qubit excitation — and the GME, they confirm that entanglement emerges swiftly across qubit pairs and triples, peaking at specific interaction times. The witnessed GME demonstrates the potential complexity of entangled states generated by the system's dynamics.

This research has profound implications for quantum computation, as the capability to reliably generate and manage entangled states is central to advancing quantum information processing. The experiment showcases the potential of trapped ion systems to serve as universal quantum simulators. Practically, this could herald increased capability in executing quantum calculations exceeding classical systems' power.

Looking forward, the demonstrated ability to generate complex entangled states sets the stage for deeper exploration into quantum simulations involving even larger systems or multiple particle interactions with tailored Hamiltonians. Indeed, the future orientation could explore more optimized interactions, potentially enabling more efficient and scalable quantum computation frameworks. The capacity of these systems to generate substantial GME attests to their potential role in developing the next generation of quantum technologies.