Chuck DarwinSeventeen years after the Large Hadron Collider 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
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
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
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