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Credit: Pixabay/CC0 Public Domain
Credit: Pixabay/CC0 Public Domain
Researchers around the world are working on a network that could connect quantum computers to each other over long distances. Andreas Reiserer, Professor of Quantum Networks at the Technical University of Munich (TUM), explains the challenges that must be overcome and how atoms captured in crystals can help.
Professor Reiserer, what is the quantum internet and how does it differ from the classical internet as we know it?
The idea is the same: we use today’s Internet to connect computers to each other, while the quantum Internet allows quantum computers to communicate with each other. But in technical terms, the quantum internet is much more complex. This is why only smaller networks have been implemented so far.
Why do we need a quantum network?
There are two main applications: Firstly, networking quantum computers allows them to increase their computational power; second, a quantum network will make absolutely interception-proof communication encryption possible. But there are also other applications, for example the networking of telescopes to achieve a previously impossible resolution in order to observe the depths of the universe, or the possibility of synchronizing atomic clocks around the world with extreme precision, making it possible to investigate physics completely new. questions.
How do quantum computers exchange information?
For the most part, exactly the same way as the classic internet: using photons. These photons are transmitted through optical cables. In the classic internet, very strong signals are used, pulses of light made up of billions of photons. Here the information is transmitted using a binary code: Light on or off, similar to Morse code.
The quantum internet is different: it still uses a binary code, but information is not carried by multi-photon light pulses, but rather by individual photons. This makes it possible to transmit quantum mechanical states that contain extremely large amounts of information.
Why is it so much harder to build a quantum Internet?
The photons are lost on their way through the optical cable. In a normal network, signals can be easily amplified using repeaters that add more photons to the light pulses. But in the quantum Internet, if a single photon is lost, all transmitted information will be irretrievably destroyed. This type of loss is the biggest problem in building a functional network. It could be solved using quantum repeaters, which my group is currently working on.
What challenges do you face?
Transmission over short distances already works very well. However, the loss grows exponentially as distances increase. To build quantum repeaters, we divide the overall distance into many small subsegments. Buffers, actually small quantum computers, store the quantum state after each subsegment until a photon is transmitted to the next subsegment.
Then, what is known as quantum teleportation can be used to subsequently “forward” the information to the transmitted photon. Doing this requires small, efficient quantum computers, which we are in the process of developing.
What do these little quantum computers look like?
The best systems investigated so far use individual atoms that are captured in a vacuum with laser light and cooled to very low temperatures. However, this approach requires an entire laboratory full of optical components, which makes implementing this approach on a small scale difficult.
Instead, we use silicon crystals in which individual atoms have been embedded and you could say are trapped in the crystal. The erbium atoms we use maintain their quantum mechanical properties under these conditions. This structure also requires low temperatures, but is technically much, much simpler.
We were able to show that this system works in principle and that erbium atoms, when excited, generate photons suitable for carrying quantum information. A big advantage here is that we can build thousands or even millions of these structures on a single silicon chip.
Why is it important?
The need for buffering in repeaters would mean that it would take a long time to transport information from one place to another. To achieve a faster rate, we use what is called multiplexing. This means that the process is carried out as many times as possible in parallel. Our technology makes this feasible and we are already working on realizing it.
Will we all be using the quantum internet in the future?
The situation could end up being similar to that of the classic Internet: in the beginning, hardly anyone could have imagined that today everyone would be walking around with Internet access in their pocket, using satellites to determine our location and surfing the Internet. We are still at a very early stage when it comes to the quantum internet.
Our current research still focuses on the fundamentals, looking at questions like: Can we connect these systems? Can we succeed in spreading quantum states across the world? The potential of this type of system as we know it today would already be revolutionary for some fields, and I’m sure there will be many applications that no one is thinking about today.
Andreas Gritsch et al, Narrow Optical Transitions in Erbium-Implanted Silicon Waveguides, Physical Review (2022). DOI: 10.1103/PhysRevX.12.041009
Andreas Gritsch et al, Purcell enhancement of single-photon emitters in silicon, Optics (2023). DOI: 10.1364/OPTICA.486167