Envisioning quantum networks: Photonics history fuels hopes of achieving quantum’s bold promise

Applied quantum mechanical theory has taken us from light bulbs to lasers. But the deeper and stranger levels of quantum mechanics offer much more. Mastery of quantum superposition and entanglement are enabling development of quantum computing, and already a next step is envisioned—quantum networking to include a quantum version of the internet. Much as today’s digital internet connects computers all over the world, quantum networks will distribute entanglement to connect multiple devices no matter how widely separated.

Researchers say that quantum interactivity could enable such capabilities as networks of quantum clocks to more precisely measure phenomena like gravitational waves, or networks of quantum enabled telescopes that might be linked to sharpen and enhance their aggregate images. Other possibilities include quantum metrology for more precise microscopy or optical and electromagnetic imaging.

The promise of quantum interconnectivity was a key impetus for US passage of the National Quantum Initiative (NQI) Act in 2018 to “to accelerate quantum research and development for the economic and national security of the United States.” Planners envision distributed quantum computing networks linking many smaller quantum computers to provide computing capacity at a scale that stretches the limits of imagination.

Early adopters of quantum network applications will likely be found in areas like national security, banking, and energy delivery infrastructure. Other application areas may include health services, security for government services (think elections), and gaming.

A notional hybrid (repeaterless) quantum network. Photo credit: P.G. Kwiat

Taking these next steps toward quantum networking is part of the bold, 20-year science program created by NQI and its partners. Although quantum entanglement has already been demonstrated over hundreds of kilometers between the ground and a satellite, it’s still a pioneering effort. The first practical step toward use of quantum networks is likely to be point-to-point connections to entangle two quantum computers in the same building. In the longer term, quantum networks might be the heart of large sensor systems such as seismic arrays deployed for earthquake prediction in high-risk areas like California.

 If 20 years sounds like a long lead time for quantum networks to be developed, remember that Charles Kao proposed fiber-optic networks more than half a century ago. Now NQI, with its whole-of-government approach, is working with the National Institute of Standards and Technology (NIST) and the private-sector Quantum Economic Development Consortium (QED-C) to jumpstart a quantum networking industry. They’re borrowing their playbook from Bellcore, later renamed Telcordia, which from the 1980s into the 2000s masterminded the integration of data, voice, and video networks into today’s global fiber-optic network, says Carl Williams, former head of the Quantum Information Program at NIST and now head of CJW Quantum Consulting.

The starting point for quantum networking is developing standards for terminology and measurement tools for industry to use when evaluating testbeds and comparing results. Specific goals include new techniques to entangle matter with matter and photons with photons, developing quantum repeaters to transfer entanglement over long distances, improving quantum sources, and getting everything in future quantum networks to work together.

Interconnecting digital computers enhanced their processing power and information access. Building quantum networks promises even greater improvements for quantum computers because of the nature of quantum science.

Digital bits can have only two states, zero or one. Quantum bits, or qubits, are superpositions of two quantum states, so they can have values anywhere in between zero and one. That offers qubits much more information capacity, but there’s no direct comparison between digital bits and qubits, and qubit capacities can differ widely.

“If you have a quantum computer that’s got 100 qubits, it’s already solving some interesting problems,” says Krister Shalm of NIST. “If you go to 1,000 qubits, it’s just way more powerful for certain applications and puts you in a different computing regime.” But quantum computers aren’t there yet.

A qubit is an individual particle with quantum states, for example, an electron, usually kept isolated at temperatures near absolute zero to stabilize its quantum state. That state can be encoded as the quantum state of a photon, such as its polarization or spin, which the photon can transfer to another qubit. That transfer “entangles” the two qubits so both have the same quantum properties and identity even when they are far apart.

Quantum networks are intended to connect quantum computers with each other and with other nodes or devices on the network. Their main function, says Paul Kwiat of the University of Illinois at Urbana-
Champaign (UIUC), “is to entangle qubits across nodes.” The connections are made by photons which carry the quantum information through optical fibers or free space. When they reach their destination, the photons entangle the quantum devices at the two ends of the link, making their quantum properties indistinguishable.

Entanglement requires only a single photon, but that turns out to be a problem. It’s easy to send a single photon across a laboratory, but over longer distances they can be lost before reaching and entangling the target node. In fibers, photons are lost to attenuation; in free space, light acts as waves and radiates outwards, decreasing in power. The longer the distance between nodes, the harder it is to entangle them. The problem, says Kwiat, “is the no-cloning rule, which says you can’t copy a quantum state.”

The optical amplifiers used in today’s digital networks amplify incoming photons by emitting new photons with the same energy going in the same direction. That amplifies light pulses carrying digital bits, which contain huge numbers of photons. But a quantum network carries only one photon at a time, and there is no way to copy or replace the quantum properties of that photon if it is lost before entanglement.

A node can stay entangled a long time if completely isolated, but noise can break the entanglement. Sending a series of single photons in the same quantum state could renew entanglements disrupted by noise, but the longer the distance, the longer the time needed for a new photon to arrive and the harder it is to renew the entanglement.

Entanglement is not limited to occurring between pairs of qubits or quantum computers. “You can take entangled states and entangle them to thousands [of nodes or qubits] rather than just one,” says Shalm. “The purely quantum thing is that they all lose their individual identity.” Spreading entanglement among qubits like that could greatly increase the power of a quantum computer so much that Williams thinks “all quantum computers will need [quantum] networking.” However, we’re still learning how to distribute entanglement.

“What we would like to do is build a quantum repeater,” Shalm says. Before optical amplifiers were available, early fiber-optic developers used electro-optic repeaters to extend transmission distances. Those repeaters converted the input optical signal into electrical form, fed it into an electronic amplifier, and used the amplified electronic signal to drive a laser that emitted the amplified signal as light to travel the next span of fiber.

What you need for quantum repeaters, Shalm says, is “some kind of memory to store the quantum bits for a while, and you need a very simple quantum processor to do a very limited set of operations. You also need really good photon detectors.” Those functions would all have to work to store the entanglement and transfer it through a series of links, each powered by a quantum repeater to span a long distance. It’s similar to spacing optical amplifiers to minimize transmission errors, but the effects of errors in the quantum system are not well understood.

Integrated optic chip uses thin-film lithium niobate waveguides to route telecom photons through a tunable unbalanced Mach-Zehnder interferometer, which can be used to create or analyze time-bin encoded qubit states. Photo Credit P.G. Kwiat

“No one has a good quantum repeater at the moment,” says Kwiat, “It’s a very hard technical challenge, but things are improving. The good news is that detector efficiency is only going to go up, and noise is going to go down.”

However, Williams adds, “it’s not just the repeater you need, you also need the memory... [and] a bunch of other technologies that don’t require dilution refrigerators,” which operate near absolute zero and are now used in most qubits and quantum computers. Those challenges may seem formidable today, but so did submarine single-mode fiber cables, fiber amplifiers, and high-power diode lasers in 1975.

Currently, the only commercial use of quantum communications is for quantum key distribution (QKD), introduced in the mid-2000s, to entangle two nodes for physically secure distribution of keys to decode encrypted messages. Successful tapping of the QKD encryption link would break the entanglement, alerting the users but also denying both them and the intruder access to the key needed to decrypt the coded message sent on the digital connection. Several companies now offer QKD commercially, claiming it secures channels “by the laws of physics.” However, these systems are not considered quantum networking because they only entangle two nodes. Williams is not impressed because QKD breaks the connection entirely rather than blocking only the intruder while allowing continued operation by the users. The US National Security Agency does not certify QKD as secure.

The QED-C is focused on laying the groundwork for quantum networks. “Networks demand interoperability,” and need standards to make them work together, said Elliot Mason, a former MIT quantum researcher turned patent attorney, at a recent QED-C online seminar on quantum networking. It’s a lesson the classical computing industry learned the hard way. Early mainframes used different standards for character encoding, and that legacy persisted into the personal computer era. Apple and Microsoft computers, for example, for example, for a long time couldn’t “talk” to each other. It took years to integrate voice, data, and video signals for transmission on the same backbone network.

“[Quantum] network concepts are emerging, language is emerging, and we are trying to get us all on the same page,” said Mason. Interoperability is essential for the industrial testbeds now being developed to demonstrate quantum communications and networking. Like the hero experiments that companies used to test and develop concepts for long-haul fiber transmission, current quantum testbeds are one-off designs to explore capabilities. However, as quantum technology matures, planners want to standardize testbeds to make measurements more comparable, eventually leading to interoperability.

It’s important to realize it is still early days for quantum networks. “We are at the smoke-signal stage of quantum networking,” consultant Mark Wippich said at the QED-C seminar, referring to the primitive state of the technology. Shalm thought it fair to compare the maturity of quantum networks to fiber optic communications in 1975. At that time, commercial diode lasers were so new that many doubted they could be made durable enough for use in the phone system. The best fibers had loss of 2 dB/km, and the goal was for fiber to carry 45 Mb/second for 10 miles between local switching offices.

The first practical quantum networks will not be massive, says Williams. “They will use point-to-point technology that can work over distances of 50 or 100 meters, not kilometers.” He thinks the next class of applications for quantum networks after that might be in large sensor systems where quantum processing could work much better than digital processing. Potential applications, besides environmental monitoring, include fundamental physics experiments and advanced navigation systems.

Quantum networking is not going to replace the traditional digital internet because it will do different things. If quantum networking does come to the internet, “we are going to have to interconnect it with the existing infrastructure,” says Williams. Although the resulting system might look integrated, the digital and quantum parts would remain distinct. The quantum network might process information much faster, but digital networks could deliver large volumes of data, like super-high-resolution 8K movies, faster and over longer distances than possible on the quantum network.

“People hear that quantum computers are going to be exponentially faster than classical computers, which they could be for some classes of problems,” says Kwiat. “They think that means they are going to be able to download Game of Thrones exponentially faster. And that is just not true.”

“The one thing you will probably never use a quantum network for is just to send classical [digital] information,” says Kwiat. A single-photon source could send one bit at a time, but it’s as inefficient as cooking rice by dropping one grain at a time into a pan of water. It’s more efficient to send digital information as digital bits. “You can send a [digital] pulse with millions of photons that can tolerate much, much higher loss. And when the signal gets weaker, you just go through an amplifier,” he says.

Quantum networks can do surprising things because processing quantum data is fundamentally different from digital data. Quantum computing won’t balance your checkbook, but it will detect patterns in the information a quantum network collects from a large array of sensors that digital computers would miss.

Williams confesses he doesn’t know everything quantum networks would be useful for. “One of the most important things [for the research program] to do is to figure out what is the value of a quantum network,” he says. That’s a crucial point. Quantum networks are still in the exploration stage, like fiber optics was in 1975. We can see the potential of short connections, just like Charles Kao could see that 10-mile fiber links could carry voice telephone traffic better than copper wire. And as Kao realized fibers could go much further, we realize that quantum networks have more potential, even if we don’t yet know exactly what they would do and how they would do it.

Jeff Hecht is an SPIE Member and freelancer who writes about science and technology.