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