Entanglement is one of the strangest phenomena predicted by quantum mechanics, the theory that underlies most of modern physics: It says that two particles can be so inextricably connected that the state of one particle can instantly influence the state of the other—no matter how far apart they are.
A century ago, entanglement was at the center of intense theoretical debate, leaving scientists like Albert Einstein baffled. Today, entanglement is accepted as a fact of nature and is actively being explored as a resource for future technologies including quantum computers, quantum communication networks and high-precision quantum sensors.
Ref: Quantum entanglement at ambient conditions in a macroscopic solid-state spin ensemble. Science Advances (20 November 2015) | DOI: 10.1126/sciadv.1501015
Entanglement is a key resource for quantum computers, quantum-communication networks, and high-precision sensors. Macroscopic spin ensembles have been historically important in the development of quantum algorithms for these prospective technologies and remain strong candidates for implementing them today. This strength derives from their long-lived quantum coherence, strong signal, and ability to couple collectively to external degrees of freedom. Nonetheless, preparing ensembles of genuinely entangled spin states has required high magnetic fields and cryogenic temperatures or photochemical reactions. We demonstrate that entanglement can be realized in solid-state spin ensembles at ambient conditions. We use hybrid registers comprising of electron-nuclear spin pairs that are localized at color-center defects in a commercial SiC wafer. We optically initialize 10^3 identical registers in a 40-μm^3 volume (with 0.95^+0.05 / -0.07 fidelity) and deterministically prepare them into the maximally entangled Bell states (with 0.88 ± 0.07 fidelity). To verify entanglement, we develop a register-specific quantum-state tomography protocol. The entanglement of a macroscopic solid-state spin ensemble at ambient conditions represents an important step toward practical quantum technology.