粒子物理学家在大型强子对撞机中发现“神奇”现象
内容总结：
【本报综合报道】欧洲核子研究中心大型强子对撞机（LHC）近期取得突破性进展：科学家首次在顶夸克粒子系统中观测到量子“魔法态”。这一发现标志着量子计算理论与粒子物理学的跨界融合正开辟前沿科研新方向。
对撞机变身量子处理器
作为自然界最重的基本粒子，顶夸克与反顶夸克在LHC中每年产生约9000万对。因其衰变速度极快（仅存万亿分之万亿分之一秒），它们能保持量子纠缠状态——即一对粒子的自旋方向必然相反。牛津大学物理学家艾伦·巴尔指出：“对撞过程本身可视为量子计算过程，这为研究量子信息流动提供了全新平台。”
“魔法态”揭示量子优势本质
今年春季，CMS实验团队通过分析海量对撞数据，首次测得顶夸克对的“魔法态”。该概念源于量子计算领域，特指那些难以被经典计算机模拟的纠缠态，被视作实现量子计算加速的关键“燃料”。阿德莱德大学马丁·怀特教授解释：“自然界中能天然产生魔法态的系统极为罕见，顶夸克的发现提供了理想研究样本。”
意外收获推动粒子物理研究
在探测魔法态过程中，研究团队意外捕捉到顶夸克与反顶夸克强耦合形成的“顶偶素”。该粒子由理论预测至今已三十余年，此前普遍认为其信号过于微弱难以观测。ATLAS实验组夸克研究负责人马塞尔·沃斯表示：“这是量子信息研究方法带来的首个实质性衍生成果。”
跨界探索引发学科革新
这项研究正在催生更多前沿课题：从纠缠态在粒子衰变过程中的传递机制，到量子-经典转换的本质探索，甚至有望通过粒子系统验证“时间源于量子纠缠”的理论假说。罗切斯特大学物理学家雷吉娜·德米娜展望道：“我们或能借助LHC验证Page-Wootters机制，揭示时间并非自然界基本属性的可能性。”
尽管有学者对实验方法的理论基础提出质疑，但科学界普遍认为，这种跨界融合为运行十七年的LHC注入了新活力。正如沃斯所言：“当我们开始抽丝剥茧，永远不知道下一个发现会带来什么。”
中文翻译：
粒子物理学家在大型强子对撞机中发现“魔法”
引言
在大型强子对撞机（LHC）中，质子每年发生9000万次碰撞，其碎片中会产生已知最重的基本粒子——顶夸克和反顶夸克。在衰变成更轻粒子前的万亿分之万亿秒内，这两个粒子会相互飞离。但它们始终保持着量子力学纠缠，即每个粒子的状态都依赖于对方。若测得顶夸克朝某个方向自旋，反顶夸克必然朝相反方向自旋。
顶夸克具有特殊性。其他类型的夸克会在LHC探测器记录其状态前迅速结合形成复合粒子（如中子），但顶夸克在与其他夸克结合前就已衰变。其衰变产物承载着自旋信息——成为可观测的量子纠缠指纹。
2023年，LHC的ATLAS实验首次测量到顶夸克与反顶夸克之间的关联性，随后涌现出一系列纠缠态测量研究。这对LHC而言是全新探索。在这台对撞机运行十七年后，物理学家逐渐意识到可以利用它探索信息在量子系统中的传递规律——这正是量子计算领域的核心问题。夸克两种可能的自旋状态对应着量子比特的0和1状态。牛津大学物理学家艾伦·巴尔解释道：“这相当于将对撞产生新粒子的过程视作量子处理器，能探究对撞机原本并非为此设计、却完全有能力解决的全新问题。”
罗切斯特大学物理学家蕾吉娜·德米娜指出，量子信息理论与粒子物理的融合“确实是个新兴领域，眼下正掀起淘金热”。今春CMS实验测得顶夸克对的“魔法性”引发热议。在量子信息理论中，“魔法”是纠缠量子比特的特性，使得其状态难以用经典计算机模拟。量子计算机要实现超越经典算法的运算速度，必须持续获取魔法态作为“燃料”。
阿德莱德大学物理学家马丁·怀特表示：“学界正在寻找自然界中存在的魔法量子系统以便研究其特性，我们的发现为此增添了新案例。”去年他与孪生兄弟克里斯·怀特共同提出了顶夸克魔法性测量方案。
魔法微光
量子计算机能指数级加速特定算法，这种优势部分源于纠缠态将不同量子比特的0和1状态联结成概率网络，使量子计算机能同步处理所有可能状态。
伦敦玛丽女王大学的克里斯·怀特指出，过去人们认为高纠缠度是量子计算优势的保证，但“事实证明这种认知完全错误”。上世纪90年代戈特斯曼-克尼尔定理证明，某些高度纠缠的量子态（稳定子态）在经典计算机上的模拟效率与量子计算机相当。这意味着构建这类量子态无法获得任何加速优势。
为寻找量子优势，物理学家开始探寻与稳定子态差异最大的纠缠态，这些态被命名为魔法态（马丁·怀特坦言“这个命名很糟糕，但沿用20年已难以更改”）。2014年，物理学家发现赋予魔法态量子优势的关键在于“情境性”——量子力学中较冷门的特性。该特性指量子测量结果会受同步测量的其他属性影响，被测量属性并非固定待发现，而是随情境变化。稳定子态是特例——可被视为非情境性系统，假想其始终具备完整确定属性。但魔法态无法规避情境性，因而难以用经典方法模拟。
当量子信息研究者开始探索在量子系统中产生增强魔法性的方法时，包括怀特兄弟在内的粒子物理学家开始思考：基础粒子系统中会如何呈现魔法性？克里斯·怀特提出：“LHC本身就是量子系统，顶夸克也是量子系统，我们能否直接观测这个系统是否具有魔法性？”2024年底他们合作提出测量方案，这对长期希望合作的孪生兄弟终于实现首次学术协作。
德米娜在会议上受怀特兄弟启发，将提案带给CMS团队。她感叹道：“这对分别在英美和澳洲工作的双胞胎，虽远隔重洋却仍保持着纠缠状态。”为提取顶夸克的魔法性，CMS团队通过分析海量对撞数据，统计不同方向飞散的顶夸克对的自旋状态，构建出完整的自旋关联矩阵，进而计算出魔法值。结果证实纠缠夸克对确实具有魔法性，标志着这个曾属小众的量子计算概念正式进入粒子物理领域。
研究魔法性的主要意义在于提升量子计算机性能，而非揭示基本粒子新特性。但为实现精确测量开发的灵敏方法却带来意外发现：物理学家观察到顶夸克与反顶夸克有时会形成超强纠缠，结合成名为顶偶素的单一粒子。ATLAS顶夸克研究组负责人马塞尔·沃斯指出，顶偶素虽在1990年就被预言，但一直被认为“效应过于微弱而无法被LHC观测”。今年3月和7月，CMS与ATLAS相继发布了顶偶素测量结果，沃斯称此为“最具实质性的衍生成果”。
待解之谜
粒子物理与量子信息理论的交叉点令人振奋，科学家得以利用LHC探究纠缠态的深层奥秘。沃斯提出：“顶夸克衰变后纠缠系统将如何演化？其衰变产物是否仍与反顶夸克保持纠缠？量子场论认为应当如此，但从未有人验证过。”
这些实验还可能为量子-经典转变提供新见解——即量子物体如何从不确定态转变为确定态。当顶夸克衰变时，似乎会“自主测量”其自旋方向，根据选择生成特定运动方向的粒子。巴尔指出：“数学上这与测量过程完全等效。”这为理解量子-经典转变提供了新视角。
德米娜则希望探索时间本质：“有理论认为时间并非自然界基本属性，而是涌现现象。”她梦想在基本粒子系统中验证1983年提出的佩奇-沃特斯机制——该理论认为整体宇宙是永恒不变的，但内部观察者因与周期性物体（如钟表指针）的纠缠而感知到时间流逝。2013年该效应已在光子实验中得到验证。
不过波恩大学物理学家赫伯特·德赖纳在两篇预印本中提出质疑：测量纠缠需将衰变产物的角运动与夸克自旋关联，但“这种转换必须依赖某种理论，若使用量子力学理论，就无法再验证量子力学”。这场争论仍在持续。在马丁·怀特看来，这系列实验正预示着LHC运行十七年后需要新研究方向。沃斯则比喻道：“就像扯线头，你永远不知道会带出什么。”
英文来源：
Particle Physicists Detect ‘Magic’ at the Large Hadron Collider
Introduction
Ninety million times a year, when protons crash together at the Large Hadron Collider (LHC), they produce, in their wreckage, a top quark and an anti-top quark, the heaviest known elementary particles. In the trillionth of a trillionth of a second before the particles decay into lighter pieces, they fly apart. But they remain quantum mechanically entangled, meaning each particle’s state depends on the other’s. If the top quark is measured to spin in one direction, the anti-top quark must spin the opposite way.
Top quarks are special. Other types of quarks quickly group together to form composite particles (such as neutrons) before the LHC’s detectors can record their states. But top quarks decay before combining with other quarks. The particles they decay into contain a record of their spins — an observable fingerprint of their entanglement.
The ATLAS experiment at the LHC measured the correlations between top and anti-top quarks for the first time in 2023. A cascade of further entanglement measurements have followed.
Such efforts are new inside the walls of the LHC. Seventeen years after the machine switched on, particle physicists are realizing that they can use the collider to explore how information flows through quantum systems — a question at the foundations of quantum computing. The two possible spins of the quarks correspond to the 0 and 1 states of a qubit, a unit of quantum information. “It is treating the process of colliding things together and forming new particles as a quantum processor,” said Alan Barr, a physicist at the University of Oxford who works on the ATLAS experiment. “You can investigate a whole different set of questions that colliders were not really designed to do in the first place but are very capable of addressing.”
This convergence of quantum information theory and particle physics “really is an emergent field,” said Regina Demina, a physicist at the University of Rochester who works on the CMS experiment at the LHC. “It’s like a gold rush right now.
One buzzy result came this spring, when the CMS experiment measured the “magic” of a pair of top quarks. In quantum information theory, magic is a property of entangled qubits that makes their state difficult to simulate on a classical computer. For quantum computers to run algorithms faster than classical computers can, they must be fed a supply of magic states as a kind of fuel.
“People seem to be saying, ‘We just want to find any quantum system where magic is there in nature, so that we can study the properties of magic,’” said Martin White, a physicist at the University of Adelaide who proposed the magic top quark measurement along with Chris White, his identical twin, last year. “This is adding to that list.”
A Little Bit of Magic
Quantum computers can run certain algorithms exponentially faster than regular computers. This speedup is possible in part because of entanglement, which links the 0 and 1 states of different qubits, creating a network of contingent possibilities. The quantum computer can manipulate all the possible states at once, rather than one after another.
High levels of entanglement between qubits used to be thought of as a surefire way to give quantum computers a performance advantage. “Intuition would make you think, the more entanglement we have, the better our quantum computer is,” said Chris White, of Queen Mary University of London. However, he said, “that actually turns out to be completely false.”
In the 1990s, a quantum information breakthrough came in the proof of the Gottesman-Knill theorem. The theorem revealed that certain highly entangled quantum states — called stabilizer states — can be simulated just as efficiently on a classical computer as it can on a quantum computer. Create these states out of qubits, and you won’t find any speedup at all.
In search of quantum advantage — the ability of a quantum computer to outperform classical computers on certain tasks — physicists began to look for entangled states that differed as much as possible from stabilizer states. These states earned the name magic states. (“It’s an appalling word,” said Martin White, but after 20 years, there’s probably no changing it now.)
In 2014, physicists found the missing piece that gives magic states their quantum boost. The key is contextuality — a lesser-known feature of quantum mechanics. Contextuality says that the outcome of a quantum measurement will depend on the other properties that are being measured at the same time. The measured properties aren’t fixed and waiting to be discovered; they’re contextual. Stabilizer states are an exception to the rule — it’s possible to treat them as noncontextual and imagine that they have a full set of definite properties at any given time. But for magic states, there’s no getting around their contextuality, making them hard to simulate classically.
Quantum information researchers began looking for ways to generate and enhance magic in quantum systems. This caught the attention of a few particle physicists — including Martin and Chris White — who wondered how magic appears in systems of elementary particles. “We thought, the LHC is a quantum system. Top quarks are a quantum system. Can we look at that system and just see if it’s magic or not?” Chris White said.
They proposed a way to do so in late 2024. The paper is their first collaboration. “I found it really quite emotional when it was released. We wanted to work together for many years,” Martin White said.
When Demina met the brothers at a conference, they inspired her to bring the proposal to her group at CMS. “They are identical twins, and one works in the U.K., and the other one works in Australia. They were moved very far apart, but are still in an entangled state,” she mused.
To glean the magic of top quarks, CMS analyzed a huge bank of collision data, tallying the spins of top quark pairs that flew off in all different directions. Doing this allowed the team to fill out a so-called spin correlation matrix, a complete description of the correlations between the particles’ spins in the x, y, and z directions. From this matrix, physicists calculate magic.
The entangled quark pairs did indeed have magic. CMS’s measurement marked the entry of the once-niche quantum computing concept into the realm of particle physics.
The main point of studying magic is to potentially improve quantum computers rather than reveal new insights about elementary particles. But the sensitive methods developed for doing such a detailed measurement led to something unexpected: The physicists observed that the top quark and anti-top quark were sometimes extra-entangled. In these cases, the quarks were binding strongly to form a single particle, an elusive state called toponium. Toponium was predicted in 1990 but “was thought to be a too-subtle effect” for a collider such as the LHC to see, said Marcel Vos, a leader of the top quark research group at ATLAS.
CMS and ATLAS posted their measurements of toponium in March and July, respectively. “That’s our first tangible spin-off from all this,” Vos said.
Threads To Pull
What some physicists find exciting about the new overlap between particle physics and quantum information theory is the chance to use the LHC to probe subtle questions about entanglement.
For instance: “What happens to your entangled system after the top quark decays? Will the daughters of the top quark still be entangled with the anti-top quark?” Vos asked. “Quantum field theory says they should be, but no one’s ever tested it.”
The experiments might also offer new insights about the quantum-to-classical transition — how a quantum object goes from an uncertain state to a single definite state. This famously happens when a quantum object is measured, but in this case, the mystery crops up when the top quark decays into lighter particles. Initially, the quark is in an uncertain state of both possible spin directions at once. When it decays, the quark appears to choose one spin direction, and the particles it generates travel in certain directions based on that choice of spin. It’s as if the top quark is forced to “measure” its own spin during its decay. “Mathematically, it’s an equivalent process to making a measurement,” Barr said. That gives physicists a fresh angle on the quantum-to-classical transition.
Demina hopes to probe questions about time. “There is a certain theory that suggests that time is not a fundamental property of nature, but it is an emergent property,” she said. One famous mechanism for how this can work was described by Don Page and William Wootters in 1983. They argued that the universe as a whole may be timeless and unchanging, while observers inside the universe can perceive temporal evolution. This perception arises because various possible spatial configurations are entangled with the spatial configurations of an object with some periodic pattern, like the hands of a clock. The effect was demonstrated with photons in 2013. “My dream is to perform this experiment in a system of elementary particles, to demonstrate the Page-Wootters mechanism,” Demina said.
Others have raised concerns that these top quark experiments cannot reliably test quantum mechanics at all. Herbert Dreiner, a physicist from the University of Bonn in Germany, argued in two recent preprints that the approach is circular: To measure entanglement, you need to relate the angular motion of the outgoing decay products to the top and anti-top quarks’ spins. But “in order to translate one into the other, you have to use some theory,” Dreiner said. “And if you’re using quantum mechanics, you can’t test for quantum mechanics.”
That debate is ongoing. To some, this whole line of experimentation is a sign that, after 17 years of collision experiments at the LHC, new goals are needed. “There is a sense that you’re always looking for new things to do,” Martin White said.
“There is a lot of skepticism,” Vos said. Still, “you start pulling on the thread, and you don’t know what you’re going to come up with.”