2022年ノーベル物理学賞：「量子もつれ」

2022年ノーベル物理学賞：「量子もつれ」の理解

「量子もつれ」という言葉を聞くと、それを説明できる人は世界中で一握りではないかと思われます。2022年のノーベル物理学賞は、フランスのサクレー大学のアラン・アスペ博士、米国のクラウザー研究所のジョン・クラウザー博士、オーストリア・ウィーン大学のアントン・ツァイリンガー博士に「量子もつれ」の研究に対して授与されました。

物理学の専門知識がない人でも理解できるように、今回は「量子もつれ」が何であり、なぜこれが注目されたのかを解説することを目的に、アイストマガジン編集部の見解をもとにコラムとしてご紹介します。本記事はアイストマガジン編集部の理解に基づくものであり、学術的に完璧でないかもしれませんが、その点をご理解いただいた上でお読みいただければ幸いです。

量子もつれ：量子粒子の強い相関

量子力学の世界と「量子もつれ」が何を意味するのかを理解するために、新原理コンピューティング研究センターの松崎雄一郎氏の協力を得ました。彼は物理学の最前線で研究を行っている専門家で、「物理学科出身ですか？ そうではない。なるほど」と、慎重に言葉を選びながら説明を始めました。

1. 量子の本質： 量子は粒子と波動の性質を併せ持つものです。この微細な世界では、日常的な物理現象とは異なる現象が起こり、「量子力学」として整理されています。この量子を観測すると、一つの状態だけでなく、複数の状態を取ることがあります。これが「量子重ね合わせ」と呼ばれる状態です。「シュレーディンガーの猫」として知られる思考実験では、箱の中で猫が同時に死んでいる状態と生きている状態が共存しているという奇妙な話を通じて、これが量子重ね合わせの概念です。

2. 量子もつれ： 量子のもう一つの特異な性質は、量子同士が相互作用すると非常に強い相関を示すことです。これにより、一方が1ならばもう一方も1、逆に0ならば0の状態を示すということができます。初期の状態がわかっていれば、一方を測定すればもう一方の状態が確実にわかります。これが「量子もつれ」と呼ばれるものです。松崎氏は「例えば、AさんとBさんがじゃんけんを100回して100回連続あいこになった場合、強い相関があるといえます。これは現実には確率としてほぼありえないことですが、強い相関を示す一つの例になっています。もちろん現実の世界でも、AさんとBさんが前もって同じ順番で出す約束をしていたり、次に出すものを会話したりするなど、連絡を随時とりあいながらじゃんけんをすると相関を強めることはできます。しかし、現実世界で考えうる影響を取り除いた状況で、非常に強い相関関係を示すのが量子の世界なのです」といった具体例を挙げながら説明しました。

3. ノーベル賞受賞者の業績： 2022年のノーベル物理学賞は、光子を使った「量子もつれ」の実験でベルの不等式を破り、量子情報分野を開拓した業績に贈られました。彼らが示した「ベルの不等式」は、CERN（欧州原子核研究機構）のジョン・スチュワート・ベル博士が提唱した、2つの粒子間の相関を記述する不等式です。2つの相関した量子において、個々の量子の状態が古典的には既に生まれた時に0または1に決まっているとすれば、ベルの不等式が成り立ちます。しかし、「量子もつれ」と呼

Title: Nobel Prize in Physics 2022: Understanding "Quantum Entanglement"

When you hear the term "quantum entanglement," explaining it might not be a common skill around the world. The 2022 Nobel Prize in Physics was awarded to three researchers, Dr. Alain Aspect from the University of Paris-Saclay, Dr. John Clauser from the Clauser Research Institute in the U.S., and Dr. Anton Zeilinger from the University of Vienna in Austria, for their research on "quantum entanglement."

In an attempt to provide insights that even those without a background in physics can grasp, we present a column outlining what "quantum entanglement" is and why their work earned them this prestigious recognition. Please note that this article represents the understanding of the AIST Magazine editorial team, and while it may not be academically exhaustive, we hope it offers valuable information.

Quantum Entanglement: Strong Correlation Among Quantum Particles

To delve into the world of quantum physics and what "quantum entanglement" entails, we sought assistance from Yuichiro Matsuzaki of the New Principles Computing Research Center, an expert at the forefront of research on new technologies based on quantum mechanics. Matsuzaki carefully explained, "Are you all physics graduates? No? I see," as he began the discussion.

1. Quantum Nature: Quantum entities possess both particle and wave characteristics. In this microscopic realm, distinct from our familiar everyday world, unique physical phenomena occur, organized systematically under "quantum mechanics." When observing the quantum, a particle may exist not in a single state but in multiple states simultaneously—a phenomenon known as "quantum superposition." You might have heard of the "Schrodinger's cat" thought experiment, where a cat in a box is both alive and dead simultaneously, illustrating the concept of quantum superposition.

2. Quantum Entanglement: Another peculiar property of quantum entities is their strong correlation when they interact. This means that if one quantum particle is in a state of 1, the other particle entangled with it will also be in the state of 1, and vice versa. If the initial states are known, measuring one particle instantly determines the state of the other—a feature referred to as "quantum entanglement." Matsuzaki used the analogy of a real-world scenario: "For instance, if persons A and B play rock-paper-scissors 100 times and tie consecutively, this indicates a strong correlation. While such a scenario is improbable in reality, it serves as an extreme example of a strong correlation. In the quantum world, achieving such correlations, seemingly impossible in classical mechanics, defines 'quantum entanglement.'"

3. Contributions of the Nobel Laureates: The 2022 Nobel Prize in Physics recognized the laureates for their experiments using entangled photons to violate Bell's inequalities, pioneering achievements in quantum information. The "Bell inequalities," formulated by Dr. John Stewart Bell at CERN, describe the correlation between two particles. If the states of individual particles are predetermined, following classical rules, Bell's inequalities hold. However, quantum entanglement, as proved by the laureates' experiments, defies this description. Their work showcased the "violation of Bell's inequalities," proving the existence of quantum entanglement.

In the 1970s, Professor Clauser's group utilized entangled photons, measuring the polarization of photons sent in opposite directions to reveal a "violation" that could not be explained by Bell's inequalities. However, this initial experiment had limitations, as the settings of the filters were fixed, and external influences on entangled photons were not entirely eliminated.

Dr. Aspect corrected these limitations by randomizing filter settings, confirming the strong correlation of entangled photons and proving the "violation of Bell's inequalities." Yet, his experiment also faced challenges as neighboring filters could influence each other.

Professor Zeilinger addressed this issue by employing signals from different galaxies to alter filter settings and minimize potential influences. Even with randomized detector settings, he demonstrated that entangled photons maintained a strong correlation.

Through these approaches, the presence of entangled quantum particles with robust correlation was established.

Technological Challenges in Utilizing Quantum Behavior

1. Quantum Computing: With the confirmation of quantum entanglement, technologies utilizing quantum properties have garnered attention, especially quantum computing. According to Matsuzaki, quantum computing requires enhanced "error correction techniques" due to the susceptibility of quantum states to noise. He explains, "In applications using quantum, quantum computing has gained attention. The utilization of quantum behaviors such as quantum superposition and quantum entanglement enables faster computational processing compared to current digital computers. However, since these phenomena occur in the extremely small quantum world, they are highly susceptible to noise. Noise, which was not a significant issue in classical digital computers, becomes a significant obstacle when performing large calculations with quantum computers. Therefore, implementing quantum error correction techniques to rectify errors in quantum bits (qubits) has become essential."

2. Quantum Entanglement in Secure Communication: Quantum entanglement finds application in secure communication, particularly in quantum cryptography. The unique property of entangled photons allows the instantaneous determination of the state of one photon by measuring the other, regardless of the distance between them. This characteristic is leveraged to achieve "quantum teleportation" of quantum states in communication, creating a secure channel free from eavesdropping concerns. Matsuzaki notes, "To realize quantum communication and quantum networks, further technological development is required. Achieving quantum teleportation involves precise measurement of one quantum and the implementation of detectors capable of distinguishing whether the detected information is part of communication or external noise. Additionally, maintaining the coherence time (the time quantum superposition remains undisturbed by external factors) of quantum entanglement, and developing techniques for creating strong interactions, are crucial challenges. Furthermore, the creation of high-precision lenses for collecting photons, which travel in three dimensions, poses an important task."

In conclusion, Matsuzaki emphasizes the difficulties researchers face in overcoming these technological barriers: "Utilizing technologies based on quantum behavior, including quantum networks, has been theoretically and experimentally understood. However, overcoming this formidable barrier and making active use of the strange phenomena of quantum mechanics that cannot be explained by intuition is a challenging task. Yet, as a researcher, I want to overcome this difficult barrier."