On Oct. 29, 1969, a set of electrical signals emerged from the University of California, Los Angeles and traveled to one at the Stanford Research Institute in Palo Alto. This marked the inception of a new era for humanity, as this was the first data ever transmitted over Arpanet, the precursor of the internet. Unquestionably, the internet has had a revolutionary impact on our society.
On similar lines, a quantum internet's vision is to provide fundamentally new technologies by enabling quantum communication between any two points on Earth. Such a quantum internet will work in synergy with the "classical" internet that we have today to connect quantum information processors to achieve unparalleled capabilities that are impossible using only classical information.
As with any emerging technology, it is hard to predict all the quantum internet capabilities. The best-known application of a quantum internet today is quantum key distribution (QKD), enabling two remote network nodes to establish an encryption key. However, a quantum internet has many other applications with advantages that are unattainable with the present internet. More applications include secure access to remote quantum computers and more accurate clock synchronization. Moving ahead, more applications are likely to be discovered in the next decade.
A quantum internet requires spearheading three quantum hardware elements: quantum channel, quantum repeaters and quantum processors. Quantum channel supports the transmission of qubits similar to standard telecom fibers. Since they are inherently lossy, we require quantum repeaters to reach longer distances, thus scaling both entanglement and key distribution capabilities. The final element is the end nodes, i.e., the quantum processors connected to the quantum internet.
The stages of development toward a full-blown quantum internet can be identified as:
Trusted repeater network: A network with at least two end nodes using QKD to exchange encryption keys.
Prepare and measure: Enabling end-to-end QKD without the need to trust intermediary repeater.
Entanglement generation: Creation of quantum entanglement with a deterministic nature along with local measurements.
Quantum memory: This stage involves having a quantum memory with local control at end nodes.
Qubit with fault tolerance: The ability to perform local operations fault tolerance.
Quantum Computing: The final stage consists of quantum computers that can arbitrarily exchange quantum communication.
Based upon this vision for the quantum network, the current experimental status of long-distance quantum networks is at the lowest stage, i.e., trusted repeater networks. Building and scaling quantum networks is a challenging endeavor, requiring sustained and concerted efforts in physics, computer science, and engineering to succeed. Although it is hard to predict the exact components of a future quantum internet, we will likely see the first multimode quantum networks' birth in the next few years. This development would bring the exciting opportunity to test the ideas and functionalities that so far only exist on paper and are potential components of a future large-scale quantum internet.
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