MSGID: 1:317/3 63d9eaf5   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Researchers take a step toward novel quantum simulators    
      
     Date:   
         January 31, 2023   
     Source:   
         DOE/SLAC National Accelerator Laboratory   
     Summary:   
         If scaled up successfully, the team's new system could help answer   
         questions about certain kinds of superconductors and other unusual   
         states of matter.   
      
      
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   FULL STORY   
   ==========================================================================   
   Some of the most exciting topics in modern physics, such as   
   high-temperature superconductors and some proposals for quantum computers,   
   come down to the exotic things that happen when these systems hover   
   between two quantum states.   
      
      
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   Unfortunately, understanding what's happening at those points, known as   
   quantum critical points, has proved challenging. The math is frequently   
   too hard to solve, and today's computers are not always up to the task   
   of simulating what happens, especially in systems with any appreciable   
   number of atoms involved.   
      
   Now, researchers at Stanford University and the Department of Energy's   
   SLAC National Accelerator Laboratory and their colleagues have taken   
   a step toward building an alternative approach, known as a quantum   
   simulator. Although the new device, for now, only simulates the   
   interactions between two quantum objects, the researchers argue in a   
   paper published January 30 in Nature Physics that it could be scaled   
   up relatively easily. If so, researchers could use it to simulate more   
   complicated systems and begin answering some of the most tantalizing   
   questions in physics.   
      
   "We're always making mathematical models that we hope will capture the   
   essence of phenomena we're interested in, but even if we believe they're   
   correct, they're often not solvable in a reasonable amount of time" with   
   current methods, said David Goldhaber-Gordon, a professor of physics   
   at Stanford and a researcher with the Stanford Institute for Materials   
   and Energy Sciences (SIMES). With a path toward a quantum simulator,   
   he said, "we have these knobs to turn that no one's ever had before."   
   Islands in a sea of electrons The essential idea of a quantum simulator,   
   Goldhaber-Gordon said, is sort of similar to a mechanical model of the   
   solar system, where someone turns a crank, and interlocking gears rotate   
   to represent the motion of the moon and planets.   
      
   Such an "orrery" discovered in a shipwreck dating back more than 2000   
   years is thought to have produced quantitative predictions of eclipse   
   timings and planetary locations in the sky, and analogous machines were   
   used even into the late 20th century for mathematical calculations that   
   were too hard for the most advanced digital computers at the time.   
      
   Like the designers of a mechanical model of a solar system, researchers   
   building quantum simulators need to make sure that their simulators line   
   up reasonably well with the mathematical models they're meant to simulate.   
      
   For Goldhaber-Gordon and his colleagues, many of the systems they are   
   interested in -- systems with quantum critical points such as certain   
   superconductors -- can be imagined as atoms of one element arranged in   
   a periodic lattice embedded within a reservoir of mobile electrons. The   
   lattice atoms in such a material are all identical, and they all interact   
   with each other and with the sea of electrons surrounding them.   
      
   To model materials like that with a quantum simulator, the simulator   
   needs to have stand-ins for the lattice atoms that are nearly identical   
   to each other, and these need to interact strongly with each other and   
   with a surrounding reservoir of electrons. The system also needs to be   
   tunable in some way, so that experimenters can vary different parameters   
   of the experiment to gain insight into the simulation.   
      
   Most quantum simulation proposals don't meet all of those requirements   
   at once, said Winston Pouse, a graduate student in Goldhaber-Gordon's   
   lab and first author of the Nature Physicspaper. "At a high level,   
   there are ultracold atoms, where the atoms are exactly identical, but   
   implementing a strong coupling to a reservoir is difficult. Then there   
   are quantum dot-based simulators, where we can achieve a strong coupling,   
   but the sites are not identical," Pouse said.   
      
   Goldhaber-Gordon said a possible solution arose in the work of French   
   physicist Fre'de'ric Pierre, who was studying nanoscale devices in which   
   an island of metal was situated between specially designed pools of   
   electrons known as two- dimensional electron gases. Voltage-controlled   
   gates regulated the flow of electrons between the pools and the metal   
   island.   
      
   In studying Pierre and his lab's work, Pouse, Goldhaber-Gordon and   
   their colleagues realized these devices could meet their criteria. The   
   islands - - stand-ins for the lattice atoms -- interacted strongly   
   with the electron gases around them, and if Pierre's single island   
   were expanded to a cluster of two or more islands they would interact   
   strongly with each other as well. The metal islands also have a vastly   
   larger number of electronic states compared with other materials, which   
   has the effect of averaging out any significant differences between   
   two different invisibly tiny blocks of the same metal - - making them   
   effectively identical. Finally, the system was tunable through electric   
   leads that controlled voltages.   
      
   A simple simulator The team also realized that by pairing up Pierre's   
   metal islands, they could create a simple system that ought to display   
   something like the quantum critical phenomenon they were interested in.   
      
   One of the hard parts, it turned out, was actually building   
   the devices. First, the basic outlines of the circuit have to be   
   nanoscopically etched into semiconductors. Then, someone has to deposit   
   and melt a tiny blob of metal onto the underlying structure to create   
   each metal island.   
      
   "They're very difficult to make," Pouse said of the devices. "It's   
   not a super clean process, and it's important to make a good contact"   
   between the metal and the underlying semiconductor.   
      
   Despite those difficulties, the team, whose work is part of broader   
   quantum science efforts at Stanford and SLAC, was able to build a device   
   with two metal islands and examine how electrons moved through it under   
   a variety of conditions. Their results matched up with calculations which   
   took weeks on a supercomputer -- hinting that they may have found a way to   
   investigate quantum critical phenomena much more efficiently than before.   
      
   "While we have not yet built an all-purpose programmable quantum computer   
   with sufficient power to solve all of the open problems in physics," said   
   Andrew Mitchell, a theoretical physicist at University College Dublin's   
   Centre for Quantum Engineering, Science, and Technology (C-QuEST) and a   
   co-author on the paper, "we can now build bespoke analogue devices with   
   quantum components that can solve specific quantum physics problems."   
   Eventually, Goldhaber-Gordon said, the team hopes to build devices   
   with more and more islands, so that they can simulate larger and larger   
   lattices of atoms, capturing essential behaviors of real materials.   
      
   First, however, they are hoping to improve the design of their two-island   
   device. One aim is to decrease the size of the metal islands, which could   
   make them operate better at accessible temperatures: cutting-edge ultralow   
   temperature "refrigerators" can reach temperatures down to a fiftieth   
   of a degree above absolute zero, but that was barely cold enough for   
   the experiment the researchers just finished. Another is to develop a   
   more reliable process for creating the islands than essentially dripping   
   molten bits of metal onto a semiconductor.   
      
   But once kinks like those are worked out, the researchers believe, their   
   work could lay the foundation for significant advances in physicists'   
   understanding of certain kinds of superconductors and perhaps even more   
   exotic physics, such as hypothetical quantum states that mimic particles   
   with only a fraction of the charge of an electron.   
      
   "One thing David and I share is an appreciation for the fact that   
   performing such an experiment was even possible," Pouse said, and for the   
   future, "I am certainly excited."  The research was funded primarily by   
   the DOE Office of Science, with the early stages supported by the Gordon   
   and Betty Moore Foundation.   
      
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   ==========================================================================   
   Story Source: Materials provided by   
   DOE/SLAC_National_Accelerator_Laboratory. Original written by Nathan   
   Collins. Note: Content may be edited for style and length.   
      
      
   ==========================================================================   
   Related Multimedia:   
       * The_experimental_setup   
   ==========================================================================   
   Journal Reference:   
      1. Winston Pouse, Lucas Peeters, Connie L. Hsueh, Ulf Gennser,   
      Antonella   
         Cavanna, Marc A. Kastner, Andrew K. Mitchell, David   
         Goldhaber-Gordon.   
      
         Quantum simulation of an exotic quantum critical point in a two-site   
         charge Kondo circuit. Nature Physics, 2023; DOI: 10.1038/s41567-022-   
         01905-4   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/01/230131160535.htm   
      
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