The slim, but real, chance that quantum networking happens
Sometime between now and 2030, the mathematical system that protects all of digital communications may fall victim to a superior quantum system. Preparing for that time may require us to reinvent the network itself.
"The quantum threat is basically going to destroy the security of networks as we know them today," declared Bruno Huttner, who directs strategic quantum initiatives for Geneva, Switzerland-based ID Quantique. No other commercial organization since the turn of the century has been more directly involved in the development of science and working theories for the future quantum computer network.
One class of theory involves cryptographic security. The moment a quantum computer (QC) breaks through the dam currently held in place by public-key cryptography (PKC), every encrypted message in the world will become vulnerable. That's Huttner's "quantum threat".
"A quantum-safe solution," he continued, speaking to the Inside Quantum Technology Europe 2020 conference in late October, "can come in two very different aspects. One is basically using classical [means] to address the quantum threat. The other is to fight quantum with quantum, and that's what we at ID Quantique are doing most of the time."
There is a movement called post-quantum cryptography (PQC), which incorporates efforts to generate more robust classical means to secure encrypted communications, once quantum methods are made reliable. The other method, to which Huttner subscribes, seeks to encrypt all communications through quantum means. Quantum key distribution (QKD) involves the generation of a cryptographic key by a QC, for use in sending messages through a quantum information network (QIN).
Interfacing a QIN with an electronic internet, the way we think about such connections today, is physically impossible. Up until recently, it's been an open question whether any mechanism could be created, however fantastic or convoluted it may become, to exchange usable information between these two systems -- which, at the level of physics, reside on different planes of existence.
Could a quantum Internet connect non-quantum computers?
At IQT Europe, however, there were notes of hope.
"I don't see why you would need a quantum computer," remarked Mathias Van Den Bossche, who directs research into telecommunications and navigation systems for orbital satellite components producer Thales Alenia Space, "to operate a quantum information network. Basically the tasks will be rather simple."
The implications of what Van Den Bossche is implying, during a presentation to IQT Europe, may not be self-evident today, although certainly they will be over the course of history. A quantum information network (QIN) is a theoretical concept, enabling the intertwining of pairs of quantum computers (QC) as though they were physically joined to one another. The product of a QIN connection would be not so much an interfacing of two processors, but a binding of two systems, whose resulting computational limit would be 2 to the power of the sum of their quantum components, or qubits. It would work, so long as our luck with leveraging quantum mechanics the way we've done so far, continues to pan out in our favor.
Van Den Bossche's speculation is not meant to imply that quantum networking could be leveraged to bind together conventional, electronic computers in the same way -- for example, giving any two desktop computers as much combined memory as 2 to the power of the sum of their bytes. Quantum networks are only for quantum computers. But if he's correct, the problem of interfacing a classical computer to a QC's memory system, and communicating large quantities of data over such a system, may be solvable without additional quantum components, which would otherwise make each connected QC more volatile.
"Ultimately, in the future, we would like to make entanglement available for everyone," stated Prof. Stephanie Wehner of Delft University, who leads the Quantum Internet Initiative at the Dutch private/academic partnership QuTech. "This means enabling quantum communications ultimately between local quantum processors anywhere on Earth."
The principal use of a quantum Internet, perhaps permanently, would be to enable QKD to protect all communications. A quantum-encrypted message is protected by physics, not math, so it's not something that can be 'hacked'. Prof. Wehner foresees a time when QKD is applicable to every transaction with the public cloud.
"Here, you should be imagining you have a very simple quantum device -- a quantum terminal, if you wish," she explained, "and you use a quantum Internet to access a remote quantum computer in the cloud, [so] you can perform, for example, a simulation of a proprietary material in such a way that the cloud hosting provider who has the quantum computer cannot find out what your material design actually is."
No part of the cloud server could interfere with the simulation without wrecking it -- in the quantum lexicon, causing it to decohere. That might disrupt your work a bit, but it wouldn't give a malicious actor on the cloud anything useful whatsoever.
Achieving Prof. Wehner's vision of a fully realized quantum Internet would require a respectable number of hurdles having been overcome, and a number of lucky rolls of the dice to come up all box-cars. These good tidings include, but are not limited to, the following:
Classical control systems would need to marshal the exchanges of information to and from the QIN. This is the problem Van Den Bossche is hopeful can be solved: There needs to be some kind of functional waypoint between the two systems that cannot, in and of itself, introduce unreliability, uncertainty, and noise.
Quantum transducers, which would perform a role analogous to repeaters in an electronic network. (You may hear the phrase "quantum repeater" for this reason, although physicists say this is a misnomer.) As Prof. David Awschalom of the University of Chicago, and director of the Chicago Quantum Exchange, asked IQT Europe attendees, "How do you convert light to matter efficiently in the quantum domain, and how do you build a quantum repeater?" Two qubits can share the curious virtue of entanglement when they're linked by optical fiber, but only over a limited distance. A transducer such as Prof. Awschalom described it would handle the strange exchange of states required for entanglement to be effectively handed off, as if by a bucket brigade, enabling the QIN to be chained.
Single photon-emitting qubits, otherwise known as 'better qubits', would make the maintenance of a QIN coupled with classical equipment much more deterministic and manageable. Photons are the signals of a quantum network. A quantum memory system will require high frequencies and heart-stoppingly high bandwidth, which may only be feasible when photon sources can be observed and maintained with precision.
Quantum memory systems (see above) are, at least at present, ideal visions. For now, a high-qubit QC computing element serves as its own memory, and a 53-qubit node may store as much as 253 bits (about 281.5 terabytes), which may seem sufficient enough except that it's completely volatile. It may decohere completely when a calculation is completed, so some type of stable memory system will be required to maintain, say, a database. This is perhaps the tallest order of all.
Available fiber. The 5G Wireless deployment effort could be of assistance here, opening up avenues of connectivity for a photons-only network. Recent experiments conducted by Toshiba Research and the University of Cambridge have shown that telco fiber networks are reliable enough for quantum communications, in places where dark fiber has yet to be laid.
Lasers. Here is the forgotten element of this discussion. We're not talking about reclaimed laser units from unbuilt Blu-ray players, but as Awschalom describes them, "fast, high-power, milliwatt-scale pump lasers that generate high-bandwidth optical photons, to match the wavelengths of these memories."
The current size and breadth of the quantum computing 'ecosystem', if we can call it that, may not yet mandate the investment of billions of dollars, or euros, into the establishment of all the new infrastructure this industry will require. But well before it gets there, we may encounter the point Prof. Huttner talks about, when the quantum threat is more imminent than the quantum bounty. Then, perhaps suddenly, investments may come in spades.