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arXiv:2006.01162 · Physical Review A

Entangling nuclear spins by dissipation in a solid-state system

Xin Wang, Huili Zhang, Wengang Zhang, Xiao-Long Ouyang, Xianzhi Huang, Yefei Yu, Yanqing Liu, Xiuying Chang, Dong-Ling Deng, Luming Duan

Published Sep 11, 2020• 3 citations
arXivHTMLPDF

Abstract

Entanglement is a fascinating feature of quantum mechanics and a key ingredient in most quantum information processing tasks. Yet the generation of entanglement is usually hampered by undesired dissipation owing to the inevitable coupling of a system with its environment. Here, we report an experiment on how to entangle two $^{13}\mathrm{C}$ nuclear spins via engineered dissipation in a nitrogen-vacancy system. We utilize the electron spin as an ancilla and combine unitary processes together with optical pumping of the ancilla to implement the engineered dissipation and deterministically produce an entangled state of the two nuclear spins, independent of their initial states. Our experiment demonstrates the power of engineered dissipation as a tool for generation of multiqubit entanglement in solid-state systems.

DOI
10.1103/physreva.102.032615

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Entanglement is a fascinating feature of quantum mechanics and a key ingredient in most quantum information processing tasks. Yet the generation of entanglement is usually hampered by undesired dissipation owing to the inevitable coupling of a system with its environment. Here, we report an experiment on how to entangle two 13C^{13}\mathrm{C}13C nuclear spins via engineered dissipation in a nitrogen-vacancy system. We utilize the electron spin as an ancilla and combine unitary processes together with optical pumping of the ancilla to implement the engineered dissipation and deterministically produce an entangled state of the two nuclear spins, independent of their initial states. Our experiment demonstrates the power of engineered dissipation as a tool for generation of multiqubit entanglement in solid-state systems.
We propose a practical implementation of a two-qubit entanglement engine which denotes a scheme to generate quantum correlations through purely dissipative processes. On a diamond platform, the electron spin transitions of two nitrogen-vacancy (NV) centers play the role of artificial atoms (qubits), interacting through a dipole-dipole Hamiltonian. The surrounding carbon-13 nuclear spins act as spin baths playing the role of thermal reservoirs at well-defined temperatures and exchanging heat through the NV center qubits. In our scheme, a key challenge is therefore to create a temperature gradient between two spin baths, for which we propose to exploit the recent progress in dynamical nuclear polarization, combined with microscopy superresolution methods. We discuss how these techniques should allow us to initialize such a long lasting out-of-equilibrium polarization situation between them, effectively leading to suitable conditions to run the entanglement engine successfully. Within a quantum master equation approach, we make theoretical predictions using state-of-the-art values for experimental parameters. We obtain promising values for the concurrence, reaching theoretical maxima.
Generating robust entanglement among solid-state spins is key for applications in quantum information processing and precision sensing. Here we show a dissipative approach to generate such entanglement among the hyperfine coupled electron nuclear spins using the rapid optical decay of electronic excited states. The combined dark state interference effects of the optical and microwave driving fields in the presence of spontaneous emission from the short-lived excited state leads to a dissipative formation of an entangled steady state. We show that the dissipative entanglement is generated for any initial state conditions of the spins and is resilient to external field fluctuations. We analyze the scheme for both continuous and pulsed driving fields in the presence of realistic noise sources.
Spins in a solid-state environment and photons are both promising physical systems regarding the implementation of qubits. Whereas electron and nuclear spins act very well as localized qubits that are able to store and process quantum information, photons are the primary candidates to transmit quantum information between distant locations. Entanglement, on the other hand, is a fundamental resource of quantum information processing enabling, e.g., secure communication over long distances. The generation and manipulation of entanglement are thus fundamental steps in order to leverage the benefits of quantum information processing. The aim of this dissertation is therefore to theoretically investigate entangling mechanisms for various physical systems. A functioning source of entangled photons is a basic hardware component required for quantum communication. To this end, we study a system composed of semiconductor quantum wells inside a microcavity, known as intersubband cavity system, in which the interaction of the electronic system with the cavity field can reach the ultrastrong coupling regime. The ground state contains a finite number of photons, and we find that these photons are entangled. The amount of entanglement is quantified analytically, and maximal entanglement is found to be possible. The technique of entanglement purification allows to restore maximal entanglement that has decreased, e.g. due to decoherence. Purification requires quantum memory, a role for which electron spins in electrically-defined quantum dots are well suited. However, existing purification protocols are rather unpractical in this case. Here, the concept of asymmetric bilateral two-qubit operations is introduced to purify spin entanglement by harnessing the typical interaction between electrons in neighboring quantum dots. As it turns out, this concept can be applied to a variety of qubit systems, e.g. superconducting qubits or spins in nitrogen-vacancy centers in diamond. The latter example offers several possibilities to implement a qubit, of which the intrinsic nitro- gen nuclear spin has proven its viability in many ways. We develop a scheme to deterministically couple two nuclear spin qubits, in which the interaction is mediated by an optical cavity. It is found that an entangling two-qubit gate, also required for universal quantum computation, can be implemented with operation times below 100 nanoseconds, i.e. several orders of magnitude faster compared to the decoherence time of the nuclear spin. The verification of entanglement, which is required e.g. in entanglement-based quantum com- munication to detect an eavesdropping attack, typically involves a measurement of qubit states. Using the input-output formalism, we derive a fully quantum-mechanical model of an optical readout scheme to measure the spin state of an electron in a self-assembled quantum dot.
Engineering the properties of a reservoir and its coupling to a quantum system is a powerful tool for simulating quantum thermodynamic processes and for generating otherwise inaccessible steady states. Yet tailoring both the reservoir and its coupling within a single platform remains challenging. Here we introduce a platform, in which an ancilla qubit mediates the coupling of a target system to a reservoir, providing independent control over the interaction form, coupling strength, and effective reservoir temperature. Our implementation uses the electron spin of a single nitrogen-vacancy center in diamond as the ancilla and a proximal 13^{13}13C nuclear spin as the target. By alternating engineered unitary interactions with dissipative ancilla resets, we realize dynamics naturally described by a collision model, enabling straight-forward tracking of the work, heat, coherence, and entropy generated at every collision. We experimentally demonstrate conventional thermalization and also realize anti-thermalization: the stabilization of the target system in a temperature opposite to that of its reservoir. Finally, harnessing this steady-state engineering, we utilize the nuclear spin as a quantum battery, achieving a steady-state ergotropy exceeding 70%70\%70% of the theoretical maximum.
Nitrogen-vacancy (NV) defect centers in diamond are strong candidates to generate entangled states in solid-state environments even at room temperature. Quantum correlations in spatially separated NV systems, for distances between NVs ranging from a few nanometers to a few kilometers, have been recently reported. In the present paper, we consider the entanglement transfer from two-mode microwave-squeezed (entangled) photons, which are in resonance with the two lowest NV electron spin states, to initially unentangled NV centers. We first demonstrate that the entanglement transfer process from quantum microwaves to isolated NV electron spins is feasible. We then proceed to extend the previous results to more realistic scenarios where 13C^{13}\mathrm{C}13C nuclear spin baths surrounding each NV are included, quantifying the entanglement transfer efficiency and robustness under the effects of dephasing/dissipation noisy nuclear baths. Finally, we address the issue of assessing the possibility of entanglement transfer from the squeezed microwave light to two remote nuclear spins closely linked to different NV centers.
Entanglement is a striking feature of quantum mechanics and an essential ingredient in most applications in quantum information. Typically, coupling of a system to an environment inhibits entanglement, particularly in macroscopic systems. Here we report on an experiment, where dissipation continuously generates entanglement between two macroscopic objects. This is achieved by engineering the dissipation using laser- and magnetic fields, and leads to robust event-ready entanglement maintained for 0.04s at room temperature. Our system consists of two ensembles containing about 10^{12} atoms and separated by 0.5m coupled to the environment composed of the vacuum modes of the electromagnetic field. By combining the dissipative mechanism with a continuous measurement, steady state entanglement is continuously generated and observed for up to an hour.