An international team of researchers achieved a historic breakthrough in quantum physics: make a levitated nanoparticle undergo such a large “delocalization” that it behaves like an extended wave.
The result, published in Physical Review Lettersopens new possibilities for exploring the boundaries between classical matter and quantum behavior.
What was previously considered an exclusive domain of electrons, atoms or molecules, begins to tread mesoscopic terrain thanks to precision optical techniques.
From particle to wave: what did scientists do?
To understand the experiment we must go back to two central concepts: the wave-particle duality in quantum physics and the technique of optical levitation.
In it quantum worldthe particles can present a wave behavior or that “probability wave” that describes your position. However, in larger systems, that wave often collapses due to decoherence.
The team used a silica nanoparticle (the size of a virus) and confined it within an optical trap in an ultra-stable vacuum chamber.
Diagram of the experiment showing how the levitated nanoparticle is cooled, expanded and measured in an optical trap. (Photo: Physical Review Letters).They then applied a three-phase protocol:
- Quantum cooling: They reduced the energy of the nanoparticle until it approached the lowest state allowed by quantum physics.
- Controlled relocation: They abruptly decreased the laser intensity to “loosen” the confinement, allowing the nanoparticle to spread spatially and its waveform (the probability) to broaden.
- careful measurement: They evaluated the new spatial distribution and verified that quantum coherence persisted.
Thanks to this protocol, they managed to increase the quantum coherence length from about 21 picometers to more than 70 picometers in the best case.
That is to say: the nanoparticle, being partially released from confinement, “stretches” like a wave.
It shows the temporal evolution of the position variation, which shows quantum expansion. (Photo: Physical Review Letters).What is striking is that this expansion of the wave function did not imply a loss of quantum purity: the state maintained its coherencedespite the quantum recoil caused by photon scattering, one of the most important sources of decoherence in this type of experiments.
Thus, what happened was that the nanoparticle managed “behave like an extended wave” in a broader region of space. This is a phenomenon that until now was believed to be reserved for much smaller scales.
Implications and future challenges
The experiment opens several interesting lines for the future of applied quantum physics:
- Quantum sensors: A nanoparticle with extended coherence could function as an ultrasensitive detector of electric and gravitational forces or fields, surpassing current technologies.
- Connection between quantum mechanics and gravity: This technique could allow us to experience gravitational entanglements between quantumly delocalized masses.
- System scaling: Although double slit-type quantum interference has not yet been achieved with the nanoparticle, the authors believe that with repeated pulses and lower decoherence the delocalization could grow exponentially.
However, challenges remain: maintaining quantum coherence against external noise sources, control photon scattering and extend those effects to even larger objects without the wave collapsing prematurely.
Key technical aspects of quantum levitation
Here we explain, with items, some critical elements of the experiment:
- Modulated optical trap: The trap created by the laser was not static, but modulated to control when the nanoparticle is released and when it is contained.
- Quantum coherence length: is the measure that indicates how far the wave function can extend while maintaining coherence. The increase achieved was from 9 pm to more than 70 pm.
- Quantum delocalization: the more freedom of position the particle has, the more “wavey” it becomes. But that expansion is limited by decoherence and zero-point motion.
- Photon decoherence and recoil: Coherence dissipation is exacerbated by photon scattering from the confinement beam, an effect that the researchers had to minimize to preserve quantum integrity.
