This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The vast majority of modern quantum technologies—from quantum cryptography to the quantum internet to the quantum computer—rely on one essential 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 100%: Using conventional methods, a success rate of at most 54% 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.
The findings are published in the journal Physical Review Letters. "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 specific moment—the sound persists for a while. At the moment of the hammer blow, the sound wave is at its strongest, and afterward 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 afterward 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 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 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.
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