From Spooky Action to Quantum Internet: Communication Takes to the Air

Frequent claims reporting successful experiments aimed at leveraging the strange concepts of quantum mechanics in real-world applications risk making what are genuinely remarkable achievements appear almost routine. This is happening with quantum computing (QC) but also with quantum communication, the backbone of a future quantum internet.

Among the properties of the quantum world-superposition, entanglement, and coherence-entanglement is perhaps the least intuitive. Yet it is also one of the most useful. The first-of-its-kind demonstration by researchers at Brookhaven National Laboratory and Stony Brook University ¹ provides a good example: entangled photons were transmitted through free space over a 13-mile (21-km) link, opening up new possibilities for extending quantum networks beyond the physical limitations of optical fiber.

The experiment goes beyond a mere demonstration of quantum physics. It illustrates an important transition: quantum communication is beginning to face the same practical networking problems encountered with the classical internet, including how to connect distant nodes, how to combine different transmission technologies, and how to extend networks over very long distances.

The foundations of quantum mechanics

QC, a more established application field, shares foundational principles with the quantum internet; however, distinguishing between the two is essential for further development.

Superposition causes a quantum system to exist as a combination of different states simultaneously-each one associated to a probability amplitude-until a measurement makes it "collapse" into a single state, with the likelihood of each outcome expressed by the squared magnitude of this amplitude.

Coherence preserves the phase relationship between the components of a quantum superposition, allowing them to interfere: if the relative phase relationship is maintained, interference can be constructive, reinforcing an outcome and making it more likely. If it is destructive, it makes an outcome less likely or even impossible. A loss of coherence breaks down this phase relationship.

Entanglement, famously defined as "spooky action at a distance," creates strong correlations between two quantum systems-like two photons-which persist even when they are spatially separated.

Why entanglement matters

To understand why entanglement is so important, it is useful to go beyond the popularization of quantum mechanics. In classical communication, information is encoded in physical states that, not being directly correlated, can, at least in principle, be measured independently. A quantum system behaves differently because its state can be a superposition of different possibilities.

Dirac introduced a powerful notation. A quantum state can be written as a ket (from bra-ket), such as

A ket corresponds to a column vector, and a bra to a row vector:

This notation greatly simplifies the expression of inner product between a bra and a ket-a routine operation in quantum mechanics.

A two-level quantum system can be represented as a linear combination of two basis states (or basis vectors)

with complex coefficients α and β, whose squared magnitudes determine the probabilities of obtaining the corresponding measurement outcomes, either |0> or |1>. This relationship also defines a quantum qubit.

For two quantum particles, however, the combined state sometimes cannot be expressed as two independent states. An entangled state can instead take the form:

A measurement on this 2-qubit (0 and 1) state returns |00> or |11> with 50% probability, as the squared magnitude of each coefficient is 0.5. But regardless of the final state reached, the 2-qubit measurement is correlated: it is impossible to know beforehand the outcome, but if the measurement of the first qubit is 0, the measurement of the second qubit will also return 0; the same happens if the first qubit measured is 1.

There are other 2-qubit states that share the same property, collectively called Bell states. The + sign indicates the relative phase between the two basis states.

This is the essence of quantum entanglement: the two particles must be treated as parts of a single quantum state and measuring one establishes a corresponding outcome for the other.

This mechanism, however, doesn't mean that messages can be transmitted instantaneously, violating a postulate of special relativity that no information can travel faster than the speed of light. The practical value of entanglement lies instead in the correlations and quantum resources that can be exploited in protocols such as quantum key distribution (i.e., secure key exchange), and teleportation of quantum states-not matter as featured in the Star Trek series!

How to generate entangled photons

The Brookhaven-Stony Brook experiment addresses a core challenge: how to move quantum states between physically separated nodes in a network-the prerequisite for quantum teleportation.

The researchers generated entangled photon pairs in the Stony Brook University physics laboratory.

The entangled pairs are produced through spontaneous parametric down-conversion (SPDC). This is a non-linear optical process, where an incoming photon from a strong laser beam transforms, by virtue of energy and momentum conservation, into a photon-pair. Under suitable conditions, the two photons emerge in an entangled Bell state.

In many photonic quantum experiments, including this one, quantum information is encoded in polarization, with horizontal and vertical polarization representing the two basis states of a qubit.

From fiber to free space

Trials were conducted at night, when entangled photon pairs were sent through fiber from the physics laboratory to the Quantum Watchtower, on the rooftop of Stony Brook's Health Sciences Center. From there, they were coupled into the 13-mile FSO (free-space optical) link.

The photons were then transmitted over the air to the Quantum Lighthouse at Brookhaven National Laboratory with a clear line of sight-demonstrating the feasibility of adding a wireless quantum link to the existing fiber-based network. The researchers then compared the measurement results to verify that the photons' quantum correlations survived the free-space propagation. The team subsequently demonstrated daytime transmission, validating single-photon optical tracking and beam stabilization under far more severe conditions.

Calling the system "wireless" requires some clarification. Unlike cellular networks, which encode information through modulation of radio waves, this system carries quantum information in the quantum states of photons traveling through the atmosphere. The key difference is that the optical channel does not require a physical fiber between the two endpoints.

That distinction is technically important because atmospheric propagation is not without challenges. Air turbulence can distort the optical beam, while background light can interfere with the detection of extremely weak quantum signals. Sunlight exacerbates both problems, while blinding the detectors with background photons.

The two teams therefore had to deeply optimize optical equipment as well as quantum sources and detectors, drawing on technologies traditionally adopted in high-sensitivity astronomical telescopes.

And beyond fiber?

While optical fiber already supports long-distance quantum experiments, it is not necessarily the universal answer.

One limitation concerns wavelength. Conventional telecommunications infrastructure relies on specific optical wavelengths, whereas quantum equipment may operate naturally at different wavelengths. The researchers say the new free-space links allow them to investigate the infrared spectrum that is native inside some quantum processors.

Free-space links also offer a potentially important route toward connecting remote locations or, ultimately, quantum systems in space.

The next step in the Brookhaven-Stony Brook-Yale project illustrates this ambition. A third facility at Yale has been built, and the scientists plan to establish a 48-km free-space connection across Long Island Sound.

As stated by Brookhaven's associate laboratory director for discovery technologies, Gabriella Carini, one of their longer-term goals is to transmit quantum information through Earth's atmosphere to orbiting satellites.

From local-area network to quantum internet

A quantum internet is not simply a faster or more secure version of today's internet; moreover, successfully transmitting photons is only part of the job.

Quantum states are fragile, and losses build up rapidly as photons travel through non-ideal channels. Future networks will require novel technologies such as quantum memories, entanglement distribution, quantum repeaters, efficient single-photon sources and detectors, as well as sophisticated synchronization and control.

The Brookhaven experiment addresses one of the challenges: establishing a reliable optical, fiber-less connection between separated quantum nodes. But there remain system-level hurdles on how to integrate optics, controls, communications, and quantum sources and detectors, much like classical internet, where networking evolved through the combination of different physical links rather than through a universal transmission technology.

Space can be a game changer

The satellite component could prove the real breakthrough, dramatically extending the reach of the project.

Free-space quantum communication could connect locations where deploying dedicated fiber is impractical. Satellites could potentially provide long-distance links between otherwise disconnected terrestrial quantum networks.

At the same time, the engineering challenges are enormous. Atmospheric propagation is hampered by turbulence, pointing accuracy, photon losses, background radiation, as well as the need to preserve delicate quantum states.

The road to quantum internet

Quantum communication is not just demonstrating that quantum effects are reproducible. It also delineates a path to build systems capable of exploiting them reliably outside the laboratory.

The Brookhaven and Stony Brook experiment is fundamental because it connects quantum physics with this practical networking challenge. Entanglement is becoming a resource that engineers are learning to generate, distribute, preserve, and eventually manage across heterogeneous networks.

The FSO link does not make fiber obsolete. Instead, it adds another layer to the emerging architecture of quantum networks, potentially enabling different wavelengths, longer-distance connections, and finally satellite-based links.

There is still a long way ahead before entangled photons in a laboratory are routinely used in a global quantum network. But as quantum links extend from fiber to open air and into space, the concept begins to look less like a theoretical-physics experiment and more like an engineering reality.

References

¹ https://www.bnl.gov/newsroom/news.php?a=123096

The post From Spooky Action to Quantum Internet: Communication Takes to the Air appeared first on Embedded.