Fiber coupled single photon source meets the requirements of quantum computing

Due to the ability of quantum computers to crack many encryption methods used in current communication systems, the security of our current communication systems is facing threats. To address this crisis, scientists are developing quantum communication systems that utilize quantum mechanics to provide stronger security. A key component of these systems is the single photon source. In order for quantum communication systems to function properly, single photons must be injected into optical fibers with extremely low loss.

In conventional systems, single-photon emitters, such as quantum dots and rare-earth 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.

A research team led by associate professor Kaoru Sanaka from the department of physics at Tokyo University of Science has found a solution to this issue. The team members 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 rare-earth ion confined in a tapered optical fiber is selectively excited by a laser to generate single photons,” Sanaka said. “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 first prepared a silica fiber doped with neodymium ions (Nd3+). Nd3+ 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 approach was significantly more efficient in 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,” Sanaka said.

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,” said Sanaka.

Further studies should focus on improving the wavelength of single photons to realize in practical settings of spectroscopy and imaging analysis, the researchers said. Overall, this fiber-coupled single-photon source represents a major step for practical quantum technologies, paving the way for secure, unhackable communication networks.

Source: photonics