The photon changes shape during propagation.
Image Credit: © Oliver Diekmann / TU Wien
Scientific Frontline: Extended "At a Glance" Summary: Shaping Photons for Reliable Quantum Communication
The Core Concept: Researchers have developed a theoretical method to increase the probability of a photon being successfully transmitted and absorbed between two qubits to nearly 100 percent by time-reversing its temporal waveform.
Key Distinction/Mechanism: Conventionally, a photon emitted by a qubit takes on a "sawtooth" waveform (strong at first, decaying exponentially), which is poorly suited for absorption by a receiving qubit, resulting in a maximum success rate of about 54 percent. The new method proposes engineering the dispersion relation of an optical waveguide linking the qubits, which passively reverses the photon's wave shape into a gradually rising pulse that reaches its maximum at the end, perfectly matching the ideal absorption waveform.
Major Frameworks/Components:
- Time-Reversal Symmetry: A principle in quantum mechanics stating that if a process (emission) occurs perfectly with a specific waveform, the reversed process (absorption) dictates the ideal waveform required for perfect execution.
- Wave-Particle Duality: The understanding that an emitted photon acts as a wave with specific temporal extension and shape, not merely a discrete particle.
- Dispersion Relation: A mathematical description utilized in the waveguide design where different frequency components of the photon's wave travel at different speeds, effectively altering the pulse shape during propagation.
Branch of Science: Quantum Physics, Theoretical Physics, Quantum Optics, and Quantum Information Science.
Future Application: The technique offers a passive, technically feasible approach to vastly improve the efficiency and reliability of quantum data transmission, which is foundational for quantum computing, quantum cryptography, and the development of a secure quantum internet.
Why It Matters: Transmission loss is a major hurdle in quantum technology; solving this inefficiency without active, complex interventions clears a significant bottleneck in scaling quantum communication networks.
 |
| Stefan Rotter, Zeyu Kuang, Oliver Diekmann and Carlos Gonzalez-Ballestero (from left to right) Photo Credit: TU Wien |
Photons can carry quantum information from one qubit to another—but the transfer does not always succeed. Researchers at TU Wien have now shown how tailoring a photon’s pulse shape can dramatically increase the success rate.
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 100 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.
Quantum Particles and the Ringing of Bells
"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 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 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.
Time-Reversing the Photon Pulse
"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 so as to make the waveform of an emitted photon pulse reverse itself exactly, thereby enabling a theoretical absorption probability of 100 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."
Published in journal: Physical Review Letters
Title: Passive Quantum State Transfer in a Dispersion-Engineered Waveguide
Authors: Zeyu Kuang, Oliver Diekmann, Lorenz Fischer, Stefan Rotter, and Carlos Gonzalez-Ballestero
Source/Credit: Technische Universität Wien
Edited by: Scientific Frontline
Reference Number: qs091426_02