Carbon nanotubes play an important role. Image by Christoph Hohmann, MCQST.
How a Quantum Internet Would Differ From Today’s Internet
Imagine a quantum internet. Unlike today’s internet, where information is transmitted as light pulses made up of billions of photons, the quantum internet could encode information in the quantum states of single photons.
This difference matters because single photons can distribute quantum information and fragile quantum features such as entanglement. Entanglement is a phenomenon where two particles become linked in such a way that the state of one instantly influences the other, even across long distances.
Controlling photons one by one makes it possible to build ultra-secure communication systems, since any attempt to intercept the photons would immediately be detected. It also enables quantum teleportation, where the state of a particle can be transferred from one location to another without moving the particle itself. These processes are not possible with normal light sources, which is why single-photon technologies are an essential part of the coming quantum revolution.
The Challenge of Making Single Photons
So, to make a quantum internet, you need a light source that produces single photons (one and only one at a time), and a way to make those photons indistinguishable (identical in every way so they can exhibit quantum interference).
Until now, that combination has been hard to achieve outside of complex, cryogenically cooled laboratory setups. But new results from LMU Munich and collaborators (Husel, Trapp et al., Nat. Commun. 2024) show a practical path forward.
The researchers have created single photons that are partially indistinguishable, at room temperature, and at telecom wavelengths – the light frequencies used in today’s global fiber-optic networks.
Tiny Cavities Clean Up Nanotube Photons
The researchers used tiny, engineered defects in carbon nanotubes to create single-photon emitters at telecom wavelengths.
Left alone, these emitters have a problem: at room temperature, they dephase very quickly – meaning that vibrations and noise from their environment randomly scramble the “rhythm” of the photons’ waves, making each photon slightly different. As a result, each photon comes out slightly different in color or timing, making them distinguishable and therefore unsuitable for quantum interference.
The clever twist was to place the emitters inside a fiber-based microcavity – two highly reflective mirrors facing each other, one built into an optical fiber. In this setup, the cavity itself acts as a filter and timekeeper. Even though the nanotube defects are noisy at room temperature, any photon that escapes through the mirrors inherits the cavity’s clean, narrow properties.
Figure 1: Schematic of the experiment: defect-functionalized carbon nanotubes were coupled to an open fiber cavity. The emitted single photons interfere on a beam splitter, revealing an increased degree of indistinguishability.
This operating mode, called the incoherent good-cavity regime, essentially forces messy photons to align – improving their coherence and enhancing their degree of indistinguishability, while also boosting the number of photons collected.
How the Experiment Worked
- The emitters: chemically modified carbon nanotubes, glowing at ≈1465 nm in the telecom S-band.
- The cavity: The Qlibri microresonator platform offers a miniaturized turn-key Fabry–Pérot resonator. This device consists of two mirrors facing each other that trap light between them. In free space, the nanotube emitters would dephase at room temperature. Inside the resonator, however, the cavity’s narrow, stable properties “force” the emitted photons to line up and behave identically. This overcomes the dephasing problem and makes the photons indistinguishable.
- The laser: a fast pulsed supercontinuum source (the SuperK Extreme; the predecessor to the SuperK FIANIUM), tuned to excite the nanotubes without adding extra timing “jitters.”
- The payoff: photon indistinguishability improved by an estimated more than 100× compared to free-space emission, and with higher count rates than alternative filtering techniques could deliver.
To demonstrate that the photons were truly indistinguishable, the researchers tested for a distinctive quantum signature known as the Hong–Ou–Mandel effect. In this test, two photons are sent into a beam splitter.
If they are identical in every way, they will always leave together rather than separately, an effect also called quantum interference. This collective behavior cannot be observed with ordinary light – it is unique to indistinguishable single photons, and the team observed its signatures in their experiments.
Why It Matters
In quantum communication, photons are the couriers of information. To build quantum repeaters, perform teleportation, or run photonic quantum computers, those couriers need to be:
- Single: only one photon per pulse.
- Indistinguishable: identical, so they can interfere.
- Telecom-ready: able to travel through existing optical fibers with minimal loss.
So far, near-perfect indistinguishability has only been possible with sources that must be cooled to near absolute zero, which is quite impractical for everyday use. Showing that you can achieve enhanced indistinguishability at room temperature and in the telecom band is a major step towards removing two of the biggest obstacles to real-world quantum networks.
Looking Ahead Towards a Quantum Internet
The beauty of this approach is that it is not limited to carbon nanotubes. The same cavity-based approach could work with many types of quantum emitters, as long as they have the right dynamics.
This opens the door to scalable, fiber-compatible, room-temperature quantum light sources – devices that could one day sit alongside standard telecom equipment and form the backbone of a future quantum internet.
As the LMU team’s results show, careful engineering of light–matter interaction has the potential to make even noisy, imperfect emitters perform reliably. And that’s exactly the kind of practical progress quantum technologies need if they are ever to leave the lab and enter the real world.
In short: This research shows how to make photons that are not only single and indistinguishable, but also at the right wavelengths to travel our existing fiber-optic highways – all at room temperature. It’s a step toward putting quantum communication at ambient conditions into the same fiber optic cables that already carry our everyday internet.
Get all the details in the white paper Cavity-enhanced photon indistinguishability at room temperature and telecom wavelengths.
