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Caltech's strontium atom chain reproduces energy ratios conformal field theory predicted 40 years ago
Caltech physicists measured energy ladders two conformal field theories have predicted for about 40 years, using a line of laser-trapped strontium atoms. It is the known-answer test a simulator must pass before it is trusted on unsolved systems.
The Scientist · Science desk
What happened
- The two theories tested were the Ising and tricritical Ising conformal field theories, and Caltech says this is the first direct measurement of their energy levels.
- Lasers pushed the atoms into Rydberg states, where neighbors interact strongly enough that the chain acts as one system, then were tuned until it reached a critical point.
- To read out the energy rungs, the team developed many body modulation spectroscopy, a technique that gently disturbs the entire atomic chain.
- The Nature study joined Manuel Endres's experimental group and Jason Alicea's theory group at Caltech with theorists at Université Paris-Saclay and the Technical University of Munich.
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Why it matters
- capability Caltech names quantum systems with no known answer as the next target, and a readout that reproduced two known ladders gives that work a calibrated starting point.
- precedent Measuring a known conformal field theory ladder before an unknown one sets the order of proof that later simulator claims about unsolved critical points will be expected to follow.
- constraint The atoms sat in a single line, so the result applies to one-dimensional chains; the release does not show the readout working in other array geometries.
A quantum simulator should first be tested on a problem whose answer is already known. Conformal field theories have told physicists for about 40 years how far apart the rungs of these energy ladders should sit, in precise ratios [3]. A measured ladder can be checked against that prediction rung by rung, the way a positive control is checked in a wet lab.
One atom chain can test a general theory because of universality. "Physicists call this trait universality -- the messy, microscopic details wash out and only a few essential features survive," Alicea said [12]. The Ising and tricritical Ising theories describe that behavior at a critical point between two states, one more ordered than the other [17].
Here the critical point is quantum. Temperature does not drive the transition. It comes from quantum effects close to absolute zero [7]. At that point, lasers can excite the chain into a sequence of specific energy states, which the researchers compare to the rungs of a ladder [8]. "The energy levels predicted by these theories are important because they encode profound information about the theories themselves," Alicea said [13].
Caltech's summary says the team watched the atoms fall into "the exact energy ratios predicted by theory" [10]. The release does not give the number of atoms in the chain, the measured ratios or their uncertainties. Those figures decide how much the word "exact" covers. One number in the release comes from a different experiment: the 6,100 atoms were trapped in a single array on a related neutral-atom platform in the same lab [16]. They were not this chain.
The release says the technique could now be used on quantum systems where scientists do not already know the answer [11]. I think that is the right next use, and reproducing two known ladders is how a method qualifies for it. The thing this doesn't tell you is how the readout behaves when there is no predicted ladder to compare against. The gap between measured and predicted rungs in this experiment is the best available estimate of the error a measurement on an unsolved system would carry.
These simulators are simpler than general-purpose quantum computers and are designed to reproduce particular quantum behaviors [15]. The hardware comes from computing work: the Endres lab uses the same optical tweezer technology to develop quantum computers [5]. "Our new tools borrow from quantum computing platforms," Sun said. "Over the past 10 years, people have been learning to control these systems, and now we are at the point where we can use them to do fundamental physics research." [14]
What to watch
- The Nature paper's numbers: chain length, measured rung ratios and their uncertainties against the conformal field theory predictions.
- A first use of many body modulation spectroscopy on a critical point where no theory has supplied the energy ratios in advance.
- Whether the readout carries over to the lab's larger neutral-atom arrays, such as the 6,100-atom platform.