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arXiv:1801.04043 · Physical Review Letters

18-Qubit Entanglement with Six Photons’ Three Degrees of Freedom

Xi‐Lin Wang, Yi-Han Luo, He-Liang Huang, Ming-Cheng Chen, Zu-En Su, Chang Liu, Chao Chen, Wei Li, Yuqiang Fang, Xiao Qiang Jiang, Jun Zhang, Li Li, Nai-Le Liu, Chao‐Yang Lu, Jian-Wei Pan

Published Jun 28, 2018• 452 citations
arXivHTMLPDF

Abstract

Full control of multiple degrees of freedom of multiple particles represents a fundamental ability for quantum information processing. We experimentally demonstrate an 18-qubit Greenberger-Horne-Zeilinger entanglement by simultaneous exploiting three different degrees of freedom of six photons, including their paths, polarization, and orbital angular momentum. We develop high-stability interferometers for reversible quantum logic operations between the photons' different degrees of freedom with precision and efficiencies close to unity, enabling simultaneous readout of 2^{18}=262 144 outcome combinations of the 18-qubit state. A state fidelity of 0.708±0.016 is measured, confirming the genuine entanglement of all 18 qubits.

DOI
10.1103/physrevlett.120.260502

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Full control of multiple degrees of freedom of multiple particles represents a fundamental ability for quantum information processing. We experimentally demonstrate an 18-qubit Greenberger-Horne-Zeilinger entanglement by simultaneous exploiting three different degrees of freedom of six photons, including their paths, polarization, and orbital angular momentum. We develop high-stability interferometers for reversible quantum logic operations between the photons' different degrees of freedom with precision and efficiencies close to unity, enabling simultaneous readout of 2^{18}=262 144 outcome combinations of the 18-qubit state. A state fidelity of 0.708±0.016 is measured, confirming the genuine entanglement of all 18 qubits.
For quantum computation using photons, performing deterministic quantum gate operations is a challenge due to the probabilistic nature of the photon-photon interaction. Encoding qubits in multiple degrees-of-freedom of photons and controlling operations between them is one of the promising ways to navigate the probabilistic behavior. Using single-photon discrete-time quantum walk in combination with polarization and path degrees-of-freedom, we experimentally demonstrate the realization of a universal set of quantum gates with high fidelity at room temperature. The deterministic realization of quantum gates through photonic quantum walk are characterized via quantum state tomography. For a three-qubit system using a single photon, the first qubit is encoded using polarization information, and the other two qubits are encoded using path information, closely resembling a Galton-board setup. To generate a six-qubit Greenberger-Horne-Zeilinger state, entangled photon pairs are used to entangle the two three-qubit modules on which gate operations are performed. We also provide insights into the mapping of photonic quantum walk operations to quantum circuits and propose methods to resourcefully scale. This demonstration marks a significant progress towards using quantum walks for quantum computing and provides a framework for using fewer photons in combination with different degrees-of-freedom of photon to scale the number of qubits.
Abstract Quantum technology, leveraging non-classical characteristics, enables breakthroughs in computation, communication, and metrology. Entanglement generation is fundamental. However, efficiently creating large-scale entanglement remains challenging. Using 2 n physical modes within a single particle has been exploited for encoding n -qubit entanglement, yet this approach is believed to be unscalable. Here, using sequentially quantum correlations on a single particle, we present a Ququart Encoding scheme to encode and process multi-qubit entanglement within ququart. We implement a programmable integrated-photonic entanglement generator on a silicon chip, encoding up to 5-qubit entanglement and quantum circuits within 2 ququarts. Within a ququart, we generate high-fidelity 3-qubit W and Greenberger-Horne-Zeilinger states and verify quantum teleportation and phase estimation. Encoding a pair of entangled photons as ququarts yields a 5-qubit Greenberger-Horne-Zeilinger state at 688 Hz, with fidelity 0.956 ± 0.053, confirming genuine 5-qubit entanglement. While the 2 n physical-mode overhead persists for general purposes, our scheme reduces it from exponential to linear under certain circumstances, potentially accelerating large-scale quantum systems.
A quantum memory for photonic entanglement is the basic component in quantum networks. Photons entangled in multiple degrees of freedom enable the mutual transformation of Hilbert spaces for the same particle and different particles, thereby connecting heterogeneous nodes in the quantum network. So far, the quantum storage for photonic entanglement is limited to two degrees of freedom. Here, we demonstrate the heralded storage of single-photon hybrid entanglement involving three degrees of freedom (polarization, orbital angular momentum, and path) using a solid-state medium. The fidelity of the output state with the ideal maximum entangled state is $94.0\ifmmode\pm\else\textpm\fi{}0.8%$. The quantum property of the retrieved state is verified by testing the hypothesis of noncontextual hidden-variable theories. These results demonstrate the entanglement interface between light and matter involving multiple degrees of freedom, paving the way for the construction of hybrid quantum networks.
Quantum information science has leaped forward with the exploration of high-dimensional quantum systems, offering greater potential than traditional qubits in quantum communication and quantum computing. To advance the field of high-dimensional quantum technology, a significant effort is underway to progressively enhance the entanglement dimension between two particles. An alternative effective strategy involves not only increasing the dimensionality but also expanding the number of particles that are entangled. We present an experimental study demonstrating multi-partite quantum non-locality beyond qubit constraints, thus moving into the realm of strongly entangled high-dimensional multi-particle quantum systems. In the experiment, quantum states were encoded in the path degree of freedom (DoF) and controlled via polarization, enabling efficient operations in a two-dimensional plane to prepare three- and four-particle Greenberger-Horne-Zeilinger (GHZ) states in three-level systems. Our experimental results reveal ways in which high-dimensional systems can surpass qubits in terms of violating local-hidden-variable theories. Our realization of multiple complex and high-quality entanglement technologies is an important primary step for more complex quantum computing and communication protocols. Experimental demonstrations of genuine high-dimensional multipartite quantum nonlocality have been lacking so far. Here, the authors fill this gap using entangled photons, surpassing qubit-based limits and paving the way for device-independent quantum information processing in more complex systems.
Recently, great progress has been made in the entanglement of multiple photons at various wavelengths and with different degrees of freedom for optical quantum information applied in diverse scenarios. However, multiphoton entanglement in the transmission window of green light under water has not yet been reported. Here, by combining femtosecond-laser-based multiphoton entanglement and entanglement-maintaining frequency up-conversion techniques, we successfully generate a green two-photon polarization-entangled Bell state and a green three-photon Greenberger-Horne-Zeilinger state, whose state fidelities are $0.893\ifmmode\pm\else\textpm\fi{}0.002$ and $0.595\ifmmode\pm\else\textpm\fi{}0.023$, respectively. Our result provides a scalable method to prepare green multiphoton entanglement, which may have wide applications in underwater quantum information.
Multipartite entanglement is one of the core concepts in quantum information science with broad applications that span from condensed matter physics to quantum physics foundation tests. Although its most studied and tested forms encompass two-dimensional systems, current quantum platforms technically allow the manipulation of additional quantum levels. We report the experimental demonstration and certification of a high-dimensional multipartite entangled state in a superconducting quantum processor. We generate the three-qutrit Greenberger-Horne-Zeilinger state by designing the necessary pulses to perform high-dimensional quantum operations. We obtain the fidelity of $76\mathrm{%}\ifmmode\pm\else\textpm\fi{}1\mathrm{%}$, proving the generation of a genuine three-partite and three-dimensional entangled state. To this date, only photonic devices have been able to create and certify the entanglement of these high-dimensional states. Our work demonstrates that another platform, superconducting systems, is ready to exploit genuine high-dimensional entanglement and that a programmable quantum device accessed on the cloud can be used to design and execute experiments beyond binary quantum computation.