*There has been a lot of loose talk lately about a coming “Quantum Internet”. I was asked about it recently by a journalist and gave him this curmudgeonly answer, hoping to redirect some of the naive enthusiasm:*

…First let me remark that “quantum internet” has become a bit of a buzzword that can lead to an inaccurate idea of the likely role of quantum information in a future global information infrastructure. Although quantum concepts such as *qubit* and *entanglement* have revolutionized our understanding of the nature of information, I believe that the Internet will remain almost entirely classical, with quantum communication and computation being used for a few special purposes, where the unique capabilities of quantum information are needed. For other tasks, where coherent control of the quantum state is not needed, classical processing will suffice and will remain cheaper, faster, and more reliable for the foreseeable future. Of course there is a chance that quantum computers and communications links may some day become so easy to build that they are widely used for general purpose computing and communication, but I think it highly unlikely.

**Would the quantum internet replace the classical one, or would the two somehow coexist?**

As remarked above, I think the two would coexist, with the Internet remaining mostly classical. Quantum communication will be used for special purposes, like sharing cryptographic keys, and quantum computing will be used in those few situations where it gives a significant speed advantage (factoring large numbers, some kinds of search, and the simulation of quantum systems), or for the processing of inherently quantum signals (say from a physics experiment).

**Would quantum search engines require qubits transmitted between the user’s computer and the web searcher’s host? Or would they simply use a quantum computer performing the search on the host machine, which could then return its findings classically?**

It’s not clear that quantum techniques would help search engines, either in transmitting the data to the search engine, or in performing the search itself. Grover’s algorithm (where coherent quantum searching gives a quadratic speedup over classical searching) is less applicable to the large databases on which search engines operate, than to problems like the traveling salesman problem, where the search takes place not over a physical database, but over an exponentially large space of virtual possibilities determined by a small amount of physical data.

On the other hand, quantum techniques could play an important supporting role, not only for search engines but other Internet applications, by helping authenticate and encrypt classical communications, thereby making the Internet more secure. And as I said earlier, dedicated quantum computers could be used for certain classically-hard problems like factoring, searches over virtual spaces, simulating quantum systems, and processing quantum data.

**When we talk about quantum channels do we mean a quantum communication link down which qubits can be sent and which prevents them decohering, or are these channels always an entangled link? …**

A quantum channel of the sort you describe is needed, both to transmit quantum signals and to share entanglement. After entanglement has been shared, if one has a quantum memory, it can be stored and used later in combination with a classical channel to transmit qubits. This technique is called quantum teleportation (despite this name, for which I am to blame, quantum teleportation cannot be used for transporting material objects).

**But could we ever hope for quantum communication in which no wires are needed – but entanglement handles everything?**

The most common misconception about entanglement is that it can be used to communicate—transmit information from a sender to a receiver—perhaps even instantaneously. In fact it cannot communicate at all, except when assisted by a classical or quantum channel, neither of which communicate faster than the speed of light. So a future Internet will need wires, radio links, optical fibers, or other kinds of communications links, mostly classical, but also including a few quantum channels.

**How soon before the quantum internet could arrive?**

I don’t think there will ever be an all-quantum or mostly-quantum internet. Quantum cryptographic systems are already in use in a few places, and I think can fairly be said to have proven potential for improving cybersecurity. Within a few decades I think there will be practical large-scale quantum computers, which will be used to solve some problems intractable on any present or foreseeable classical computer, but they will not replace classical computers for most problems. I think the Internet as a whole will continue to consist mostly of classical computers, communications links, and data storage devices.

Given that the existing classical Internet is not going away, what sort of global quantum infrastructure can we expect, and what would it be used for? Quantum cryptographic key distribution, the most mature quantum information application, is already deployed over short distances today (typically < 100 km). Planned experiments between ground stations and satellites in low earth orbit promise to increase this range several fold. The next and more important stage, which depends on further progress in quantum memory and error correction, will probably be the development of a network of quantum repeaters, allowing entanglement to be generated between any two nodes in the network, and, more importantly, stockpiled and stored until needed. Aside from its benefits for cybersecurity (allowing quantum-generated cryptographic keys to be shared between any two nodes without having to trust the intermediate nodes) such a globe-spanning quantum repeater network will have important scientific applications, for example allowing coherent quantum measurements to be made on astronomical signals over intercontinental distances. Still later, one can expect full scale quantum computers to be developed and attached to the repeater network. We would then finally have achieved a capacity for fully general processing of quantum information, both locally and globally—an expensive, low-bandwidth quantum internet if you will—to be used in conjunction with the cheap high-bandwidth classical Internet when the unique capabilities of quantum information processing are needed.

“coherent quantum measurements to be made on astronomical signals over intercontinental distances”

How much of an improvement would that yield? Could you give a reference?

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Charlie is thinking of my paper with Thomas Jennewein and Sarah Croke. Quantum repeaters allow optical interferometric telescope arrays to operate with arbitrarily long baselines (distance between telescopes), whereas without there is a severe practical limit to the baseline. Current state-of-the-art without repeaters is about 300 meters. Note, though, that the application requires extremely high bandwidth quantum repeaters.

The reference is:

D. Gottesman, T. Jennewein, S. Croke, “Longer-Baseline Telescopes Using Quantum Repeaters,” Phys. Rev. Lett. 109, 070503 (2012) [5 pages], DOI: 10.1103/PhysRevLett.109.070503. Also arXiv:1107.2939 [quant-ph].

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Thanks Dan. Will look at it with much interest.

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What about post-quantum cryptography?

How come IBM has chosen to pursue with vim the buidling of a quantum computer but is basically a no show when it comes to commercial quantum cryptography?

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