Â鶹ÒùÔº

With the rise of quantum computers, the security of our existing communication systems is at risk. Quantum computers will be able to break many of the encryption methods used in current communication systems. To counter this, scientists are developing quantum communication systems, which utilize quantum mechanics to offer stronger security. A crucial building block of these systems is a single-photon source: a device that generates only one light particle at a time.

These photons, carrying quantum information, are then sent through optical fibers. For quantum communication systems to work, it is essential that single photons are injected into optical fibers with extremely low loss.

In conventional systems, single-photon emitters, like quantum dots and rare-earth (RE) element ions, are placed outside the fiber. These photons then must be guided to enter the fiber. However, not all photons make it into the fibers, causing high transmission loss. For practical quantum communication systems, it is necessary to achieve a high-coupling and channeling efficiency between the optical fiber and the emitter.

In a new study, a research team led by Associate Professor Kaoru Sanaka from the Department of Â鶹ÒùÔºics, Tokyo University of Science (TUS), Japan, have found a solution to this issue. They have developed a highly efficient fiber-coupled single-photon source, where single photons are generated directly inside an optical fiber. Unlike previous approaches, a single atom was selectively excited in this method.

"In our approach, a single isolated RE ion confined in a tapered optical fiber is selectively excited by a laser to generate single photons," explains Dr. Sanaka.

"Unlike conventional approaches, where single-photon generation and transmission are separate steps, here single photons can be generated and efficiently guided directly within the fiber with significantly reduced loss."

The team also included third-year Ph.D. candidate Mr. Kaito Shimizu and Assistant Professor Tomo Osada, also from TUS. Additionally, Associate Professor Mark Sadgrove cooperated with the research team to provide fiber devices. Their study is published in .

The team first prepared a silica fiber doped with neodymium ions (Nd³⁺). Nd³⁺ ions were selected because they can emit photons across a wide range of wavelengths, including telecom standard, making them versatile for different quantum applications.

The doped silica fibers were then tapered using a heat-and-pull process, wherein a section of the fiber is heated and pulled to gradually reduce its thickness. This process allowed them to access spatially separated individual Nd³⁺ within the tapered section.

This resulted in a novel approach where a single Nd³⁺ was selectively excited using a pump laser at room temperature, generating single photons directly into the fiber's guided mode. For testing, the emitted photons were then collected from one end of the fiber.

Using an analytical approach called autocorrelation, where a photon signal is compared with its delayed version, the researchers experimentally validated that only one photon was being emitted at a time and that they can be efficiently guided within the fiber. The team also confirmed that the tapering of the fiber does not alter the natural optical properties of the ion.

Notably, the results showed that this new approach was significantly more efficient at collecting photons than their previous non-selective excitation method, where multiple Nd³⁺ were excited together. This collection efficiency can be enhanced even further if photons are collected from both sides of the fiber.

"Our approach allows highly efficient transmission of single photons from source to end," notes Dr. Sanaka.

Since this method uses commercially available optical fibers, it is cost-effective, wavelength selectable, and straightforward to integrate into a fiber-based communication network. Moreover, unlike most current quantum technologies that require expensive cryogenic systems, this system operates at room temperature. These features can make this system a strong candidate for next-generation all-fiber-integrated quantum communication networks.

Beyond quantum communications, this approach could also power future quantum computing technologies.

"By individually operating multiple isolated ions within the same fiber, it is possible to develop a multi-qubit processing unit. It may also enable qubit encoding protocols," adds Dr. Sanaka.

A qubit or quantum bit is the basic unit of quantum information. Further studies should focus on improving the wavelength of single photons in practical settings of spectroscopy and imaging analysis.

Overall, this fiber-coupled single-photon source represents a major step for practical quantum technologies, paving the way for secure, unhackable communication networks.