2. 量子エンタングルメント 

古典力学では説明できない量子力学的な相関は量子エンタングルメント (量子もつれ) と呼ばれています。例えば、2 つのスピンを考えた場合、片方のスピンの状態が定まるともう片方のスピン状態もそれに応じて定まり、その距離には寄らないとされています。量子エンタングルメントに関する議論は Einstein-Podolsky-Rosen(1934) に端を発したものですが、その量子力学の本質をついた議論は、量子重力論への発展、近年注目されている量子コンピュータへの応用など最先端の研究へと繋がっています。中辻・酒井研究室では量子エンタングルメントの状態を有する “量子スピン液体” やエキゾティックな“トポロジカル相転移”を伴った新奇な二次元磁気秩序の存在を示唆する振る舞いの観測に多数成功してきました。 以下では、我々が世界で初めて観測した、様々な種類の量子スピン状態を紹介していきます。

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Highly entangled liquid state in frustrated magnets

From light years away

A whisper from an unknown

Altered density

Among the many mind-blowing facets of quantum mechanics, probably the most bizarre one is the non-local entanglement – particles that are light-years away can instantly communicate with one another. This seemingly paradoxical feature rules the microscopic world: two electron spins forming a singlet state (i.e., up-down configuration) are entangled. The wavefunction of the individual spin is no longer well defined. In the pioneer paper about quantum entanglement by Schrodinger, he described this term as “the best possible knowledge of a whole does not necessarily include the best possible knowledge of all its parts.”  Researchers have demonstrated that entanglement is far more than just a fascinating theoretical concept. Rather, it is an invaluable physical resource that may revolutionize our ways of collecting and processing information– the rapidly emerging field of quantum information and computing.

One fascinating creation by quantum entanglement is a quantum spin liquid (QSL) – an intrinsic topological order without spontaneous symmetry breaking. The QSL is like a “magical ballroom” of magnetic moments, where they dance in an entangled and dynamical way, with infinitely many partners simultaneously. The long-range entanglement makes QSL a fertile ground for exotic low-energy excitations with fractional quantum statistics; some of those bear striking similarities with fundamental particles that shy away from detection in high energy experiments. A natural route to a QSL is magnetic frustration; namely, the magnetic interaction energies cannot be simultaneously minimized, leading to robust quantum fluctuations that avoid a long-range magnetic order even at absolute zero. The experimental quest for a QSL has been the central goal of the frustrated magnetism community for decades; if proven to exist, QSLs would be an exciting step toward next-generation quantum information technologies. The publications below describe our contributions to this extremely challenging yet exciting exploration. Please feel free to check them out.

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