Four years ago, theoretical physicists proposed a new quantum-communication scheme with a striking feature: it did not require the transmission of any physical particles. The research raised eyebrows, but now a team of physicists in China claims it has demonstrated that the "counterfactual" scheme works. The group built an optical apparatus that it says can transfer a simple image while sending (almost) no photons in the process.
The theoretical proposal was put forward by scientists at Texas A&M University (TAMU) in the US and the King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia. It is based on the phenomenon of wave–particle duality. Specifically, it uses the fact that the presence of an object blocking an arm of an interferometer can be inferred by virtue of its collapsing the wavefunction of an interrogating photon – even though it has no physical contact with the photon. The work also relies on what is known as the quantum Zeno effect, which stipulates that an ongoing series of weak measurements will stifle the quantum-mechanical evolution of a particle and almost certainly cause it to remain in its initial state.
The communication protocol is defined in terms of two characters Alice and Bob – and it is Bob who sends the message. Alice fires single photons at a chain of interferometers, created by a series of beam splitters and mirrors. At the output of the final interferometer, photons end up in one of two detectors monitored by Alice. Bob, meanwhile, can choose whether or not to switch on a measuring device in the right-hand arm of each interferometer.
Left or right
If Bob switches on his devices, he forces the photon injected by Alice to behave as a particle and therefore follow a definite path – going either left or right – through each interferometer. But since the beam splitters are highly reflective, and photons are always reflected to the left, Bob – employing the quantum Zeno effect – causes the photon to remain in the left-hand channel as it travels through the apparatus and as such triggers Alice's right-hand detector. But if Bob instead switches his devices off, the photon's wavefunction is allowed to evolve and the photon instead ends up in the left-hand detector.
Intriguingly, therefore, Alice learns of Bob's decision – whether or not to turn on the devices – even though no photon passes between them. In neither case does Bob's equipment interact with a photon. As such, Bob can send Alice a message by using the states "on" and "off" to represent the ones and zeros of a binary code, even though he sends no physical particle to Alice.
The counterfactual protocol put forward by the TAMU-KACST group, which is led by TAMU's Suhail Zubairy, was actually slightly more complicated. It involved the addition of an extra chain of interferometers in the right-hand arm of each existing interferometer. This was done to make sure that any photons that enter the communication channel between Alice and Bob are lost.
Eve the eavesdropper
That fix clearly didn't satisfy everyone. After Zubairy and colleagues had published their research in Physical Review Letters, Lev Vaidman of Tel-Aviv University in Israel sent a comment to the journal arguing that photons would not pass between Alice and Bob only when Bob switches his devices on. With the devices off, reckoned Vaidman, a weak measurement would in fact reveal photons to be present in the channel. Saying that Zubairy's group has a "naive classical approach to the past of the photons", Vaidman adds that the misconception could allow an eavesdropper (Eve) to uncover part of the message being transmitted.
Notwithstanding the debate that ensued, Jian-Wei Pan of the University of Science and Technology of China in Hefei and team set about building an experiment to put the protocol to the test. As they point out in a paper describing the work in the Proceedings of the National Academy of Sciences, a completely counterfactual scheme would require an infinite number of interferometers, which is clearly not practical. So instead they used a simplified design – employing just two interferometers (one each for the external and internal chains) and sending each photon back and forth multiple times, thanks to the use of nanosecond timing and phase stabilization.
Pan and colleagues transmitted a 100 × 100 monochrome bit map of a Chinese knot. After five hours of painstakingly transmitting each of the 10,000 bits multiple times (to overcome channel loss), the researchers were able to clearly reproduce the image, successfully transmitting the correct bit value – black or white – 87% of the time. Comparing that figure with the rate at which photons erroneously leaked through the communication channel – just 1.4% – they conclude that they had indeed sent the information counterfactually. In other words, the vast majority of the transmitted bits, they say, were not associated with the passage of any physical particle.
Imaging delicate art
Despite their positive results, the Chinese researchers say that further experiments are necessary. Among the possible tests that could be carried out, they say, are weak measurements at the output of each inner interferometer to establish whether photons are in fact leaking through the communication channel. The researchers do not explicitly discuss the possibility of developing a practical ultra-secure communication scheme on the back of their work, but they do raise the possibility of "counterfactual imaging". Involving an array of optical switches that are used to send data counterfactually to a camera, the technique, they suggest, could prove handy in imaging delicate pieces of ancient art that cannot be exposed to direct light.
As to exactly what is physically transmitting information from Bob to Alice, if not particles, that remains an open question. Hatim Salih, who was lead author on the theory paper and is now at the UK's University of York, is convinced that the culprit must be the photon's wavefunction. As such, he argues, the research would help settle a decades-old debate among physicists about the reality of the wavefunction. It must be real, says Salih, who is also co-founder of QuBet, a UK-based quantum technology company.
About the author
Edwin Cartlidge is a science writer based in Rome