A drop of ink falls into a glass of water, and within a minute you can't tell where it landed. In the quantum world, it's even more dramatic: information about a local event spreads across the entire system so quickly that any point-like sensor soon stops feeling anything. This is called delocalization, and for quantum sensors, it sounds like a death sentence.
But physicists have found a loophole. In the Kitaev chain, a theoretical model of superconducting nanowires, special Majorana modes can emerge. These are excitations that behave like halves of an electron: each has two 'personalities' (quadratures), and in the topological phase they are physically pulled apart to opposite ends of the chain. One half lives on the left, the other on the right. It is this spatial separation, the authors showed, that pins the quantum Fisher information (QFI)—a measure of sensitivity — to a nonzero value. Even after a time exponentially large compared to the system size, a sensor at one end continues to sense what happened at the other.
This is not just resilience to noise. An asymmetry of the encoding axis emerges: the signal at the left edge carries information about a phase that the right edge does not respond to. Such a 'lopsided' memory cannot be confused with an ordinary localized signal inside the gap. Numerical simulations confirmed: even moderate disorder at the sites (as if a few atoms in the chain were slightly shifted) does not erase the asymmetry. And with interactions that conserve parity of particle number, the plateau at the boundary continues to shine in real time.
The protocol is ridiculously simple: prepare the chain in a product state, let it evolve under the Hamiltonian, and read the signal from a single site. No complex entangled states, no tomography. Topology itself protects information, just as the shape of a vessel determines where liquid flows.
So the ink drop stops spreading. The topological phase locks it in two fixed points, and even after an eternity, you can tell where it fell.
Majorana modes are named after the Italian physicist Ettore Majorana, who mysteriously disappeared in 1938. His particles are the only ones that are their own antiparticles.
In the series 'The Expanse', the alien protomolecule uses topological defects for instantaneous communication across any distance — much like our separated quadratures.