Quantum dot emitter delivers near-identical telecom photons at 40 million per second

Quantum technologies, devices that perform specific functions leveraging quantum mechanical effects, could soon outperform their classical counterparts on some tasks. Quantum emitters, devices that release individual particles of light (i.e., photons), are central components of many of these technologies, including quantum communication systems and quantum computers.

To enable the reliable operation of quantum technologies, emitters should emit photons with high consistency and coherence. In other words, they should ensure that the quantum properties of emitted photons remain stable and predictable.

Researchers at University of Copenhagen's Niels Bohr Institute, Ruhr-University Bochum, University of Basel and Sparrow Quantum ApS recently developed a new photon emitter based on quantum dots, tiny structures that can trap electrons in confined regions and enable the controlled emission of individual photons.

The device they developed, introduced in a paper published in Nature Nanotechnology, operates in the telecommunication O-band, thus, it is directly compatible with existing communication systems. This means that it could potentially be used to develop large-scale quantum communication systems, and potentially even a quantum internet.

"The motivation was to connect a premier quantum light source with the optical technology we already know how to scale," Marcus Albrechtsen, first author of the paper, told Tech Xplore.

"Specifically, quantum dots are excellent emitters, but the best ones historically operated at wavelengths that are incompatible with telecommunications and silicon photonics. We wanted quantum-coherent quantum dots directly in the original telecom band (the so-called O-band around 1,300 nm)."

Coherent quantum dot-based telecom emitters

This recent study emerged from a long-term collaboration between scientists at the Niels Bohr Institute, Ruhr University Bochum and the University of Basel. Its primary goal was to enable the emission of photons at telecom wavelengths, but without introducing noise (i.e., unwanted environmental disturbances) that typically destroy coherence and disrupt the quantum states of emitted photons.

"The material breakthrough at Bochum was to grow a strain-reduction layer on top of the quantum dots without introducing material defects that would later degrade the photons," said Albrechtsen.

"Using advanced nanofabrication at the Niels Bohr Institute cleanroom, these samples were machined into quantum photonic circuits with electrical control, without compromising the inherent high quality of the crystals. We then measured the samples in optical laboratories in an ultra-cold -269°C environment, or 4 Kelvin, only 4 degrees above absolute zero, using a special cryo-station in Copenhagen."

Quantum dots are essentially tiny artificial atoms confined in a chip made of semiconducting materials. Just like real atoms, these tiny structures have their own discrete energy states and can be excited or relaxed.

"We fabricate devices around these dots and direct a laser onto them, which causes the dots to become excited," explained Albrechtsen.

"After a short while they decay, emitting exactly one single photon into the surrounding nanophotonic waveguide structure. The electrical control of the devices allows stabilizing the slow noise, such that back-to-back photons become nearly identical."

In the team's device, individual quantum dots sit in a so-called p-i-n diode. This is a device with three layers (i.e., a p-type semiconductor, an intrinsic, undoped semiconductor region, and an n-type semiconductor), which stabilizes nearby electrical charges that would otherwise shift the energy of photons from one emission event to the next.

"We engineer a photonic crystal waveguide around the dots," said Albrechtsen. "This enhances single photon emission (a process known as the Purcell effect) into the waveguide, reducing the time window where residual noise can disturb the process and allowing routing and manipulating the single photon for quantum applications."

The new photon emitter created by Albrechtsen and his colleagues enables the emission of bright photons, all while retaining quantum coherence and operating directly at telecom wavelengths. The combination of these three properties proved difficult to achieve so far and could be highly advantageous for the development of large quantum communication systems.

"We demonstrate quantum dots in the O-band with emission lines only about 8% broader than the fundamental lifetime limit," said Albrechtsen.

"This means that across several seconds, with over 40 million single photons per second sent into the waveguide, at least 92% of them are the same. This is quantum coherence, and it is vital for practical applications of such photon-matter interfaces."

Contributing to the development of a quantum internet

This recent study could open new exciting possibilities for the development of quantum communication systems and potentially even for the realization of a quantum internet.

"Until now, emitted photons would be filtered to only use the photons that are identical, however, this came with steep efficiency prices that limited the scope and usability," explained Leonardo Midolo, the principal investigator of the study.

"Now the full power of quantum dots is unleashed, making them compatible with telecommunications infrastructure and capabilities for scalable applications. This bridges quantum-coherent emitters with low-loss fibers and silicon photonics, without frequency conversion."

The device developed by the researchers could be used to create scalable quantum-photonic systems, including large secure communication systems and highly accurate sensing technologies. In addition, it could be integrated with other components to create fault-tolerant quantum computers (i.e., quantum computing systems that are robust against noise and make minimal errors).

"The next step is to move from one excellent emitter to more complex photonic integrated circuits," said Midolo.

"We want multiple telecom quantum dots on the same chip, tuned into resonance and connected by low-loss optical circuits. Here, a key advantage of telecom-band operation is the direct compatibility with silicon-on-insulator photonic integrated circuits, which have been largely developed over the past two decades."

As part of their next studies, Midolo and his colleagues plan to scale up their design and integrate multiple quantum dot-based photon emitters. This could enable the introduction of new entanglement-based quantum communication protocols, which might be promising for the future realization of a quantum internet.

"In the paper we tune the emission over a wide range, which is important for matching several quantum dots despite natural fabrication differences," said Midolo.

"Future directions will include heterogeneous integration with silicon photonics, which offers ultra-low loss photonic chips to route and process the quantum information. For example, these technologies can be combined using micro-transfer printing or wafer bonding.

"In fact, the native host material of the quantum dots, gallium arsenide, is itself an excellent material for photonic integrated circuits and, at these telecommunication wavelengths, its loss is far lower compared to the near-infrared wavelengths where the best quantum dots so far operated."