Quantum Erasure Using Entangled Surface Acoustic Phonons

Abstract

Using the deterministic, on-demand generation of two entangled phonons, we demonstrate a quantum eraser protocol in a phononic interferometer where the which-path information can be heralded during the interference process. Omitting the heralding step yields a clear interference pattern in the interfering half-quanta pathways; including the heralding step suppresses this pattern. If we erase the heralded information after the interference has been measured, the interference pattern is recovered, thereby implementing a delayed-choice quantum erasure. The test is implemented using a closed surface acoustic wave communication channel into which one superconducting qubit can emit itinerant phonons that the same or a second qubit can later recapture. If the first qubit releases only half of a phonon, the system follows a superposition of paths during the phonon propagation: either an itinerant phonon is in the channel or the first qubit remains in its excited state. These two paths are made to constructively or destructively interfere by changing the relative phase of the two intermediate states, resulting in a phase-dependent modulation of the first qubit’s final state, following interaction with the half-phonon. A heralding mechanism is added to this construct, entangling a heralding phonon with the signaling phonon. The first qubit emits a phonon herald conditioned on the qubit being in its excited state, with no signaling phonon, and the second qubit catches this heralding phonon, storing which-path information which can either be read out, destroying the signaling phonon’s self-interference, or erased.

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Plain Language Summary

In an interferometer, any information about which path a particle takes destroys the interference pattern, a fundamental result in quantum mechanics. A quantum eraser experiment tests whether the interference pattern can be recovered by erasing this information. Quantum erasers have been demonstrated using photonic interferometers. Here, we realize a quantum eraser using another medium: phonons, the quanta of mechanical vibrations.

Our experiment generates, for the first time, a two-phonon entanglement. We use a surface acoustic wave communication channel into which two qubits can emit and recapture itinerant phonons. The first qubit is controlled to release “half” a phonon, a superposition of states where either a “signaling phonon” is in the channel or the first qubit remains in its excited state. We interfere these paths by changing the relative phase of the two intermediate states, leading to constructive or destructive modulation of the qubit’s final state.

To determine whether the qubit remains in its excited state or a phonon is emitted, we realize another first: the qubit emits a “heralding phonon” in the absence of the signaling phonon. The heralding phonon, which carries the path information, is later captured by the second qubit, thus extinguishing the interference pattern. However, we show that the pattern can be recovered by erasing the information gained by the second qubit.

This experiment implements one of the milestone experiments of quantum optics, here using phonons, opening the door for realizing other phonon analogs of fundamental quantum-optics experiments. Perhaps in the future we will be able to listen to quantum music.