The speck that moves both ways at once

Original: "Mesoscopic mechanical superpositions by gluing individual quantum systems"

arXiv:2607.20195 · 2026-07-22 · CC BY 4.0 · 1 min

A single electron that hasn't yet decided when to arrive makes a levitating nanoparticle oscillate left and right at the same time.

If a runner could grab the relay baton in two places at once, he'd sprint off in two directions simultaneously. Something similar happens to a particle hovering in a laser beam. A single electron, sitting in a superposition of two arrival times, collides with it and hands over its uncertainty. Now the particle moves left and right at the same time.

The electron is prepared so that it seems to arrive both earlier and later. At the moment of collision it sticks to the particle, and a short electric pulse gives the particle a kick. If the electron arrived earlier, the kick goes one way; if later, the opposite way.

Everything happens extremely fast: the electric field switches in 100 nanoseconds, and the particle doesn't have time to shift during that window. So its state stays quantum.

Half a period of the particle's oscillation later, the particle is measured, and by repeating the experiment many times, an interference pattern emerges: a set of fringes that proves the particle really was in two states at once.

Previously, to get such a state, you had to stretch the particle out in space, but that quickly destroyed its quantum properties. The new approach is simpler: the electron is already in a superposition, and it passes that superposition to the particle on impact. If this experiment succeeds, it will let us test quantum laws on objects five times heavier than today's record holders.

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A particle with a radius of 50 nanometers contains about $10^{10}$ atoms, and its mass exceeds the electron mass by a factor of $10^{12}$. Yet a single electron can put it into a quantum superposition of two different momenta. For comparison: it is as if a collision with a speck of dust made a soccer ball fly in two opposite directions at the same time.

The idea of a macroscopic quantum superposition echoes Schrödinger's thought experiment about a cat that is simultaneously alive and dead. In Ted Chiang's story "Story of Your Life" (and the film "Arrival"), the aliens perceive time nonlinearly, which resembles the time-bin state, where the electron is in a superposition of two arrival times.

|\psi_{e^-}\rangle = \frac{|t_e\rangle + e^{i\phi}|t_\ell\rangle}{\sqrt{2}}

The electron is in a superposition of early and late arrival with a relative phase φ.

|\psi_0\rangle = \mathcal{N}\left(e^{i\delta p \hat{x}/\hbar} + e^{i\phi}e^{-i\delta p \hat{x}/\hbar}\right)|0\rangle

A superposition of two coherent states with opposite momenta ±δp.

P_{\Delta x}(\phi, x) = \mathcal{N}^2 \left(A + B\cos\phi + C\sin\phi\right)

The probability of finding the particle in an interval of width Δx depends on the phase φ through the coefficients A, B, C, which account for decoherence.

Scientists

David Deutsch Erwin Schrödinger John Stewart Bell Louis de Broglie Lov Grover Paul Benioff

Concepts

schrodingers cat schrodinger cat state superposition macroscopic superposition Schrödinger equation semiconductor qubits braiding mach zehnder interferometer optical microscopy magnetic pattern heisenberg model magnetic optical microscopy quantum optomechanics feldman cousins construction synthetic images schwarzschild metric digital twin training geometric confinement quantum clock interference semiclassical

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The speck that moves both ways at once