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| Fig. 1: Rydberg atoms (blue) are trapped individually inside tweezer arrays (red) and coupled by Rydberg lasers (orange). (Source: M. Wu, after Bernien et al. [2]) |
Nature isn't classical, as argued by Richard Feynman, to study Physics we need a quantum computer. [1] Though the realization of quantum computers has progressed rapidly recently, a universal quantum computer that can emulate any given Hamiltonian is yet in the absence. Physicists have been working on engineering quantum materials in order to build highly tunable, coherent quantum simulators that serve as a tool to solve problems in many-body physics that require heavy numerical calculation and also a candidate to the realization of quantum computers. In this paper, I will review the realization of Rydberg-atom simulator and how it leads us to quantum many-body scars.
Using neutral atoms as a platform has the advantages that (1) such systems have coherent properties, (2) it is easy to create a large number of atoms, and (3) atoms can be strongly coupled to light. However, the disadvantages include the weak interaction between atoms and the challenge to control neutral atoms individually.
Rydberg atoms are atoms being in the excited states with a large quantum number. They thus have a long lifetime and strong interaction. The Lukin group (Bernien et al.) from Harvard University realized a 51-qubits quantum simulator by trapping Rydberg atoms inside tightly focused laser beams in order to individually control them (as shown in Fig. 1) that circumvents the disadvantages described above. [2] With clever trapping skills, they are able to create a quantum material system that simulates the Ising-type quantum spin model.
They observe different phases of ordered states that break various discrete symmetries by performing adiabatic sweeping. Moreover, by suddenly changing the laser detuning, they observe persistent oscillations of the system between the ground state and the excited states. This result remains a puzzle because the dynamic seems to be non-ergodic while the system doesn't seem to be an integrable system.
Thermalization in classical mechanics is based on the ergodic hypothesis. If, after a long period of time, all microstates of the system are accessed with equal probability, the system reaches thermal equilibrium. In quantum mechanics, however, this definition cannot be directly translated since the probability of finding the system in a given state is based on the choice of the initial state, making one unable to track a trajectory in the phase space. Therefore, the eigenstate thermalization hypothesis (ETH), describing thermalization in isolated quantum systems using properties of eigenstates, is proposed. Isolated quantum systems that present ergodic dynamics are systems that obey ETH and are regarded as being able to reach thermal equilibrium.
The highly non-equilibrium quantum matter has been realized in a variety of platforms. Those systems such as integrable systems and many-body localized systems strongly violate ETH. This motivates the question that whether systems that only weakly violate ETH exist?
Such systems are later proved to exist and the distinct behavior of this system is called "quantum many-body scarring."
A state that is likely to exist in unstable classical periodic orbits in classically chaotic systems is called a scar. The name "scar" comes from the fact that the system seems to carry an imprint of the past that continuously draws them back to their original configuration. This resembles the oscillation observed in the experiment, which has invoked many discussions.
Shortly after the publication of the experimental result, Turner et al. published a theoretical paper using the same system as the experimental work done by the Lukin group but using L = 32 as the system size instead. [3] They further interpret the experimental observation as a result of weak ergodicity breaking due to the special eigenstates in the spectrum. This resembles quantum scars in chaotic non-interacting systems.
© Michelle Wu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. P. Feynman, "Simulating Physics With Computers," Int. J. Theor. Phys. 21, 467 (1982).
[2] H. Bernien et al., "Probing Many-Body Dynamics on a 51-Atom Quantum Simulator," Nature 551, 579 (2017)
[3] C. J. Turner et al., "Weak Ergodicity Breaking From Quantum Many-Body Scars," Nat. Phys. 14, 745 (2018).