The eerie music of a theremin—the instrument that’s played without touching it—seemingly comes from thin air. Eerier still is a new result from Jérôme Faist (of ETH Zürich in Switzerland) and colleagues. By holding a metallic resonator (the gold square attached to the copper post in figure 1) above an electronic chip, the researchers enhance how the chip exhibits the quantum Hall effect. But whereas a theremin’s arms are active antennas that send and receive RF signals, neither the resonator nor the chip does the same. Instead, the effect stems entirely from the ground state of the electromagnetic vacuum.
Even the darkest, emptiest vacuum isn’t devoid of electromagnetic fields. For the same reason that a particle can’t perfectly settle at the bottom of a bowl-shaped potential—because to do so would violate Heisenberg’s uncertainty principle—the energy in any of a system’s electromagnetic modes can never be exactly zero. But the set of allowed modes can be restricted through the placement of conductive materials, which impose electromagnetic boundary conditions. One manifestation of that is the Casimir effect, a measurable attraction between two conductors that stems from the diminished electromagnetic vacuum energy in the space between them (see the article by Alex Stange, David Campbell, and David Bishop, Physics Today, January 2021, page 42).
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