Short answer: A quantum internet is a proposed network that would enable quantum communication between any two points on Earth and connect quantum processors. A Science review by Wehner, Elkouss and Hanson proposes stages of development, each distinguished by new functionality instead of just more qubits. The authors say a full-blown quantum internet is still some way away, though the first long-range quantum networks are already being planned.
The vision
The review begins from the classical internet, which it describes as a vast network enabling simultaneous long-range classical communication that has had a revolutionary impact. The vision of a quantum internet, in the authors' words, is to fundamentally enhance internet technology by enabling quantum communication between any two points on Earth. They say such a network may operate in parallel to the internet we have today and connect quantum processors, in order to achieve capabilities that are provably impossible using only classical means.
The Science summary adds a security motivation. It says that if the internet attained the capability of transmitting quantum information, many security concerns about interception and theft of information would be addressed. That is the review's framing of the motivation, and this article does not evaluate it.
Stages defined by functionality
The review defines stages of development toward what it calls a full-blown quantum internet. Each stage is distinguished by an increasing amount of functionality, at the expense of increasing experimental difficulty. The authors say an experimental implementation has reached a certain stage only if the functionality of that stage and all previous stages is available to all the end nodes using the network.
A key point in the paper is that the subsequent stage must offer fundamentally new functionality not available in the previous one. It is not enough to improve parameters or offer more of the same by increasing the number of qubits. The authors note that the stages and tests are written for systems that prepare and transmit qubits for clarity, but can also be phrased in terms of qudits or continuous variables.
Why a staged framework
The authors say they expect the classification to be instrumental in guiding and assessing experimental progress, and in stimulating the development of new applications by providing a common language and reference frame for the different scientific and engineering disciplines involved. For each stage, they describe application protocols that are already known and that can be realized with the functionality available at that stage.
An opening for software
The review points out that a simpler protocol, or a better theoretical analysis, may be found in the future that solves the same task but is less demanding in terms of functionality. In parallel to what they call the daunting experimental challenges of making a quantum internet a reality, they say there is an opportunity for quantum software developers to design protocols that can realize a task with fewer requirements.
What the Science summary says about status
The Science summary says a full-blown quantum internet, with functional quantum computers as nodes connected through quantum communication channels, is still some ways away, but that the first long-range quantum networks are already being planned. It does not describe any deployed network, and this article makes no claim about one.
What this does not tell you
This article summarises the abstract and introduction of one review and its Science summary. It does not list the individual stages or the applications assigned to each, and readers who need them should take them from the review. It says nothing about which projects are underway.
FAQ
What is a quantum internet?
According to the review, a network that enables quantum communication between any two points on Earth and connects quantum processors, possibly operating in parallel to the internet we have today.
How are the stages defined?
By functionality. Each stage offers fundamentally new functionality that the previous one lacks, and a stage counts as reached only if its functionality and that of all earlier stages is available to all end nodes.
Is it about adding more qubits?
No. The authors say a later stage must be more than "more of the same" from increasing the number of qubits.
Does a quantum internet exist today?
The Science summary says a full-blown one is still some ways away, while the first long-range quantum networks are already being planned.
Where do software developers fit in?
The authors see an opportunity to design protocols that realize a task with less demanding functionality.
All field notes