The vast majority of modern quantum technologies – from quantum cryptography to the quantum internet to the quantum computer – rely on one absolutely essential basic element: the transmission of photons. Two qubits (two atoms, for example) exchange information: one qubit emits a photon, and the other qubit absorbs it.
However, this process does not work perfectly. The probability that the photon is actually absorbed is considerably lower than one hundred percent: using conventional methods, a success rate of at most around 54 percent can be achieved — in almost half of all cases, the photon is lost. A team at TU Wien has now developed a proposal for how this problem can be solved: the shape of the photon has to be reversed — and this can be achieved with a fairly simple trick.
"When a qubit emits a photon, you must not picture the photon as a tiny particle that is simply shot out," says Dr. Zeyu Kuang from the Institute of Theoretical Physics at TU Wien. "The photon is a wave, and a wave has a certain shape and a certain extension."
The same is true of sound waves: when you strike a bell with a hammer, the sound wave is not produced only at that one very specific moment — the sound persists for a while. At the moment of the hammer blow the sound wave is at its strongest, and afterwards it gently fades away.
The wave produced when a photon is emitted from a qubit has a very similar shape: at the beginning it is very pronounced, and afterwards it decays exponentially. You could say that the photon’s wave has a "sawtooth" shape.
This "photon sawtooth" now travels away from the first qubit and can be guided in a targeted way through a waveguide to the second qubit, where it is meant to be absorbed. The absorption probability, however, depends on the waveform of the photon. And this very sawtooth shape — strong at first, then decaying — is in fact poorly matched to the waveform that the second qubit can absorb most efficiently. A time-reversed pulse would be much better: it rises gradually before reaching its maximum at the end.
“This follows from time-reversal-symmetry in quantum mechanics,” Oliver Diekmann explains. “Under ideal conditions, quantum dynamics are reversible. If a qubit perfectly emits a photon with a particular waveform, the time-reversed process tells us which waveform that qubit can absorb perfectly.”
The question for the TU Wien team was therefore: how can the photon’s temporal waveform be reversed? "In a vacuum, light always travels at exactly the same speed, namely the speed of light," says Prof. Stefan Rotter. "But in an optical waveguide this is not necessarily the case. Different frequency components of the wave travel faster than others – mathematically this is described by the so-called dispersion relation." Based on this idea, the researchers propose to place the two qubits in a waveguide and engineer its dispersion relation such as to make the waveform of an emitted photon pulse reverses itself exactly, thereby enabling a theoretical absorption probability of one hundred percent.
“We calculated how this goal can be achieved and simulated the process on a computer,” says Prof. Carlos Gonzalez-Ballestero. “Our results indicate that the required setup should be technically feasible. This passive approach could significantly improve the absorption of photons by qubits and thereby increase the efficiency of many quantum technologies.”
Physical Review Letters
Computational simulation/modeling
Not applicable
Passive Quantum State Transfer in a Dispersion-Engineered Waveguide
4-Sep-2026