Even a Perfect Qubit Loses Nearly Half the Photons It Is Sent. TU Wien Physicists Say the Fix Is to Flip the Photon's Shape Backward in Time, and a Specially Built Waveguide Can Do It.
A photon emitted by a qubit is loud at first and fades like a struck bell, which is exactly the wrong shape for the next qubit to absorb. Engineering the waveguide's dispersion reverses the pulse and pushes the absorption probability to 100% in theory.
Nearly every quantum technology on the drawing board, from quantum cryptography to a quantum internet to networked quantum computers, depends on one basic handoff: one qubit emits a photon, and another qubit absorbs it. That handoff fails far more often than most people realize. With conventional methods, the best achievable absorption probability is about 54%, meaning the photon is simply lost in almost half of all attempts. A team at TU Wien in Vienna now says the loss can be engineered away by reversing the shape of the photon, and that a fairly simple trick in the waveguide can do it. The proposal appears in Physical Review Letters.
"When a qubit emits a photon, you must not picture the photon as a tiny particle that is simply shot out," said Dr. Zeyu Kuang of TU Wien's Institute of Theoretical Physics. "The photon is a wave, and a wave has a certain shape and a certain extension." That shape resembles a struck bell: the wave is strongest the instant it is emitted and then decays exponentially, a "sawtooth" profile that is loud at the front and quiet at the back.
The trouble is that the receiving qubit prefers the opposite. "This follows from time-reversal symmetry in quantum mechanics," said Oliver Diekmann, a co-author. "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." A pulse that builds gradually and peaks at the end would be absorbed with near-certainty. The sawtooth is a poor match, and that mismatch is where the missing 46% goes.
The question was how to turn the pulse around. "In a vacuum, light always travels at exactly the same speed, namely the speed of light," said Professor 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." The team proposes placing both qubits inside a waveguide whose dispersion is deliberately shaped so that the photon's fast and slow frequency components reorder themselves in flight, arriving at the second qubit with the sawtooth exactly reversed. In their calculations, that yields a theoretical absorption probability of 100%.
"We calculated how this goal can be achieved and simulated the process on a computer," said Professor 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." The word passive matters: the scheme needs no active pulse shaping, no timed control fields and no feedback, only a waveguide built with the right dispersion.
The paper, "Passive Quantum State Transfer in a Dispersion-Engineered Waveguide," is a theoretical proposal, and the next step is for an experimental group to fabricate a waveguide with the prescribed dispersion and test it with real emitters. If it works, the same idea could apply to any platform where qubits talk through guided photons, including superconducting circuits, trapped atoms and solid-state emitters.
Originally reported by Phys.org.