Sven Erik Matzen

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Spooky Action at a Distance: Quantum Entanglement from Einstein to the Quantum Internet

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Quantum Physics · 2026-06-18 · approx. 30 minutes

The Hook: Einstein's Greatest Mistake?

Albert Einstein co-founded modern physics — quantum mechanics included. And of all theories, it was this one that would not let go of him until the end of his life, because he held it to be incomplete. His unease crystallized around a single phenomenon, which he labeled with a famous, dismissively intended term: "spooky action at a distance" (in the original, in a 1947 letter to Max Born: "spukhafte Fernwirkungen").

He meant quantum entanglement: two particles can be connected so intimately that a measurement on one particle instantly fixes the state of the other — even if both particles are light-years apart. To Einstein, this was nonsense. Nothing, according to his deep physical conviction, can propagate faster than light, and nothing "here" should instantaneously determine something "there."

Here is the punchline, and it is one of the greatest plot twists in the history of science: Einstein was wrong. Not on some minor point, but precisely on the one thing he took for granted. In 2022, three physicists — Alain Aspect, John Clauser, and Anton Zeilinger — were awarded the Nobel Prize in Physics for experiments that show beyond doubt: entanglement is real, it behaves exactly as "spookily" as quantum mechanics predicts, and there is no hidden, more down-to-earth explanation behind it.

Why should this interest you, beyond intellectual curiosity? Because this seemingly abstract physics is in the process of reshaping your professional world. Entanglement is the physical resource behind quantum computers, quantum teleportation, and an emerging "quantum internet." And a quantum computer of sufficient size would break virtually every encryption scheme on which we currently rely to protect cloud data, signatures, and compliance evidence. The question "Is the cloud secure?" takes on a wholly new urgency through quantum physics (see the cross-reference to Ist die Cloud valide).

This article takes you the full distance: from Einstein's objection, through the decisive experiment that ended the dispute, to the very concrete consequences for cryptography, IT security, and the picture we form of reality.

Part 1: What Entanglement Actually Is

Superposition as the Starting Point

To understand entanglement, you first have to accept superposition. In the classical world, an object has a definite state at every moment: a coin lies heads or tails. In the quantum world, a system can be in an overlay of several states simultaneously before measurement. An electron spin can be "up" and "down" at once — not because we don't know which it is, but because it objectively has no fixed value until it is measured.

This is not merely ignorance of a hidden, already-fixed value. That is exactly the crux of the matter, and exactly where Einstein took his stand.

Two Particles, One Shared State

Entanglement arises when two (or more) particles interact, or are created together, in such a way that their state can no longer be described independently of one another. There is then no complete description of "particle A by itself" and "particle B by itself" — only a joint description of the whole system remains.

The classic example: a process generates two photons with opposite polarization, but it is undetermined which photon carries which polarization. The entangled state says only: "If A is horizontal, then B is vertical, and vice versa" — but neither of them has a fixed value beforehand.

Now measure the polarization of A and obtain "horizontal," and at that same instant it is settled that B is "vertical." It makes no difference whether B is one meter, one kilometer, or one light-second away.

An Honest Analogy — and Its Limit

You often hear the gloves analogy: you mail two parcels, one containing the left glove, the other the right. If you open one in Hamburg and find the left, you instantly know the right one is in Sydney. No spookiness required — the distribution was fixed from the start, you simply didn't know it.

This very analogy is Einstein's position. He said: the quantum particles carry hidden "notes" with their true values; we just don't know them. The apparent action at a distance would then be an illusion born of our ignorance.

And here lies all the drama: the gloves analogy is demonstrably false. Nature behaves measurably differently than any explanation with pre-fixed values would allow. How this could be proven is the dramatic part of this story.

Part 2: The Clash of Giants — EPR and the Incomplete Universe

The EPR Paradox (1935)

In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen published a paper titled "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" — EPR for short. It is one of the most cited and most consequential works in the history of physics.

Their argument was razor-sharp. They defined an "element of reality": if you can predict the value of a quantity with certainty without disturbing the system, then that value must really exist. For entangled particles, a measurement on A lets you predict the value of B with certainty without touching B. Therefore — so EPR argued — B must already have possessed that value beforehand.

But since quantum mechanics does not describe this pre-existing value, they concluded: quantum mechanics is incomplete. There must be "hidden variables," a deeper layer of reality that the theory has not yet captured.

In doing so, EPR presupposed two assumptions that sound so reasonable you barely notice them:

  • Realism: physical quantities have definite values, independent of whether we measure them.
  • Locality: an event in one place cannot influence an event in a distant place faster than the speed of light.

Together they yield "local realism" — the worldview that seems self-evident to us from everyday life.

Bohr's Response

Niels Bohr, the spokesman of the "Copenhagen interpretation," disagreed. For him it was meaningless to speak of values that exist independently of measurement. The properties of a quantum system are defined only in the measurement process; before that they simply do not exist. Entangled particles are a single, indivisible system, even when they are spatially separated.

For decades this dispute remained philosophical. Both camps computed with the same quantum mechanics and obtained the same experimental predictions. There seemed to be no way to decide empirically who was right. Many physicists regarded the question as idle — as metaphysics, not physics.

That changed because of a single man.

Part 3: John Bell and the Inequality That Decided Everything

The Brilliant Turn (1964)

In 1964 the Northern Irish physicist John Stewart Bell posed the decisive question differently. Instead of asking "Which interpretation is correct?" he asked: "Do local hidden-variable theories make different measurable predictions than quantum mechanics?"

His answer, Bell's theorem, is one of the deepest results in physics. Bell showed mathematically: any theory based on local realism — no matter how cleverly the hidden variables are constructed — must obey certain statistical limits. These limits can be formulated as Bell's inequality.

The decisive point: quantum mechanics violates this inequality. For certain measurements it predicts correlations that are stronger than any locally realistic theory could ever produce.

With that, the decades-old philosophical dispute suddenly became an experimentally decidable question. One simply had to measure the correlations of entangled particles and see on which side of the limit they fell.

The CHSH Inequality — the Practical Tool

In 1969, Clauser, Horne, Shimony, and Holt cast Bell's idea into an experimentally tractable form, the CHSH inequality. It considers two observers (traditionally called "Alice" and "Bob" in the jargon) who, on their respective particles, choose between different measurement settings and correlate the results.

A locally realistic world allows a certain combined quantity \(S\) to reach at most the value 2. Quantum mechanics allows up to \(2\sqrt{2} \approx 2.83\). This gap between 2 and 2.83 is the measurable fingerprint of "spookiness." If experiments find values above 2, local realism is refuted.

The stage was thus set for three experimental physicists whose life's work would later be crowned with the Nobel Prize.

Part 4: The Three Laureates and the End of the Debate

John Clauser — the First Test (1972)

John Clauser took Bell's abstract idea and built a real experiment from it. Together with Stuart Freedman, in 1972 he generated entangled photons by exciting calcium atoms and measured their polarization correlations. The result: Bell's inequality was violated. The data supported quantum mechanics and contradicted local realism.

At the time, Clauser was working largely against the current — the topic was regarded as unprofitable foundational physics, almost a hobby. But his experiment still had "loopholes": technical gaps through which a stubborn defender of hidden variables could have explained the result away.

Alain Aspect — Closing the Locality Loophole (1982)

The most important loophole was the locality loophole: if the measurement settings on both sides are already fixed while the particles are still in flight, then in theory some (unknown, light-speed) signal could "inform" one side about the setting of the other.

Alain Aspect closed this gap elegantly: he switched the measurement settings only after the entangled photons had already left their source — and did so fast enough that no information traveling at the speed of light could pass from one side to the other before the measurement was complete. The correlations persisted. Quantum mechanics won again, now under markedly stricter conditions.

Anton Zeilinger — From Violation to Application

Anton Zeilinger drove the field from demonstration to utilization. His group performed especially clean Bell tests and was the first to use entanglement for practical quantum information processing. In 1997 his team demonstrated the first quantum teleportation — the transfer of a quantum state from one particle to another (more on this shortly). Later, his work made possible entanglement experiments over large distances and even via satellite.

The 2022 Nobel Prize

In 2022, Aspect, Clauser, and Zeilinger were jointly awarded the Nobel Prize in Physics — "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science."

Its significance can hardly be overstated. It was the official scientific confirmation that, at the most fundamental level, the world is not locally realistic. One of our deepest everyday intuitions — that things have fixed properties and are influenced only by local contact — is false. Moreover, between 2015 and 2017 several groups (including in Delft, Vienna, and Boulder) carried out loophole-free Bell tests that also closed the last technical back doors. The case is, as far as physics can ever be "closed," closed.

Part 5: What Entanglement Is NOT — Three Persistent Misconceptions

Precisely because the topic is so fascinating, popular-science misconceptions circulate. Clearing them up cleanly sharpens understanding enormously.

Misconception Reality
"Entanglement allows communication faster than light." No. The no-signaling theorem forbids it.
"Measuring A actively changes B across the distance." Careful — there is no transmitted effect, only correlation.
"It's like entangled dice that secretly conspired in advance." Bell refuted exactly that: no hidden plan of agreement is sufficient.

The No-Signaling Theorem — Why No Faster-Than-Light Radio Is Possible

This is the most important point, and it is misrepresented almost everywhere. Yes, Alice's measurement instantly fixes what Bob will measure. But Alice cannot control which result she gets — her outcome is purely random. On his side, Bob likewise sees only a random sequence of results.

Only when the two compare their lists of outcomes — and this comparison requires a classical communication channel, which is at most light-speed — do the mysterious correlations appear. Before that comparison, each side, taken in isolation, sees only noise. So no information can be transmitted by entanglement alone. Relativity remains untouched; on this particular point, Einstein is right.

This coexistence is subtle and beautiful: the world is non-local in its correlations, but local in its signal transmission. Both at once.

Correlation Is Not Causation

It is tempting to imagine that measuring A "sends" something to B. But there is no "something" that travels, no measurable effect that propagates. What exists is a non-local correlation without a locally traceable mechanism. Some physicists therefore prefer the sober term "non-separability": the system simply cannot be decomposed into independent parts.

Part 6: From Curiosity to Technology

For a long time entanglement was regarded as a philosophical cabinet-of-curiosities piece. Today it is a resource — something you generate, distribute, "consume," and keep on the books like energy. Three fields of application stand out.

1. Quantum Computing

A classical bit is 0 or 1. A qubit can be in a superposition of both states. But the real leverage arises only through entanglement: if you entangle \(n\) qubits, their joint state describes up to \(2^n\) amplitudes simultaneously. With 300 entangled qubits, that exceeds the number of atoms in the observable universe.

Quantum algorithms such as Shor's algorithm (factoring large numbers) or Grover's algorithm (search) exploit entanglement and interference to solve certain problems dramatically faster than any classical computer. Without entanglement, a quantum computer would be no more than an expensive random-number generator.

Important for a sober assessment: today's quantum computers are "noisy" and small (the NISQ era — Noisy Intermediate-Scale Quantum). A cryptographically relevant, error-corrected quantum computer does not yet exist. But the direction is clear — and security planning must already account for it today (see Part 7).

2. Quantum Teleportation

The term is unfortunately marketing-tainted. No matter and no energy is teleported, and nothing faster than light either. What is teleported is a quantum state: the complete quantum-mechanical information of a particle is transferred onto another, distant particle, while the original is destroyed in the process.

The recipe: Alice and Bob share an entangled pair in advance. Alice performs a joint measurement on the particle she wants to teleport and on her half of the pair. In doing so she obtains two classical bits, which she sends to Bob over a normal channel (at most light-speed). With these two bits, Bob reconstructs the exact original state on his particle.

The crucial point for IT practitioners: without the classical channel, nothing works. Teleportation too respects the speed of light. It is not "beaming," but a protocol for state transfer — and a basic building block of the quantum internet.

3. Quantum Communication and the Quantum Internet

Entanglement in principle allows eavesdropping-proof communication. In the E91 protocol (Artur Ekert, 1991), two parties use entangled pairs to generate a secret key. If an eavesdropper tries to listen in, their measurement destroys the entanglement — and the Bell correlations measurably break down. Eavesdropping thus becomes physically visible, not merely computationally harder. More on this in the next part.

Part 7: The Bridge into Your World — Post-Quantum Cryptography

Now it gets concrete for IT security and compliance. This is the section that connects the abstract physics with your professional practice.

The Threat: "Harvest Now, Decrypt Later"

Today's public-key encryption (RSA, Diffie-Hellman, elliptic curves) rests on the fact that certain mathematical problems — such as factoring very large numbers — are practically unsolvable for classical computers. Shor's algorithm on a sufficiently large quantum computer would solve exactly these problems efficiently and thereby break RSA and ECC.

The crux for risk assessment: attackers can capture and store encrypted data today in order to decrypt it as soon as a powerful quantum computer exists. This strategy is called "harvest now, decrypt later." For data with long protection horizons — health data, contracts, state secrets, intellectual property — the threat is therefore already real now, even without the quantum computer yet existing.

Answer 1: Post-Quantum Cryptography (PQC)

The main defense is not exotic quantum hardware but new classical algorithms that are also hard for quantum computers to crack. In August 2024, after an eight-year selection process, the U.S. NIST published the first three final standards:

  • FIPS 203 – ML-KEM (based on CRYSTALS-Kyber): a method for secure key exchange (key encapsulation), as a replacement for RSA/ECDH. The expected workhorse standard.
  • FIPS 204 – ML-DSA (based on CRYSTALS-Dilithium): digital signatures, NIST's recommended standard for most applications (code signing, certificates, authentication).
  • FIPS 205 – SLH-DSA (based on SPHINCS+): a hash-based signature as a conservative fallback, whose security does not rest on lattice assumptions.

ML-KEM and ML-DSA rest on lattice-based problems, against which no efficient quantum attack is so far known. For you as a consultant, this means: PQC is no longer a research topic but a standard and a migration task. Cryptographic inventory (which systems use which algorithms?), crypto-agility (can the methods be swapped out?), and migration roadmaps now belong in every serious security and compliance strategy. Government agencies and regulators (BSI, EU) are already moving in this direction.

Answer 2: Quantum Key Distribution (QKD)

Here the circle back to entanglement closes. QKD (Quantum Key Distribution) uses quantum-physical properties directly to exchange keys. Any eavesdropping attempt provably disturbs the quantum system — security rests on physics, not on computational hardness.

An important clarification — and a frequent exam mistake: QKD is NOT part of the NIST standards FIPS 203–205. It is regarded as a complementary method that, on top of everything, requires special hardware (often fiber or satellite) and has its own practical weaknesses. The BSI and the NSA are rather reserved toward pure QKD and prioritize PQC. For practice today: PQC is the main path, QKD a niche for special cases.

Cross-reference: These considerations directly extend the discussion from Ist die Cloud valide. Anyone protecting data in the cloud must weigh the protection horizon against the time horizon of quantum-safe attacks — "harvest now, decrypt later" changes the risk calculation for any long-lived cloud data storage.

Part 8: Where Research Stands in 2024–2026

Entanglement is leaving the laboratory. A few current milestones:

  • Teleportation over existing internet infrastructure: In late 2024, it became possible to teleport a quantum state over roughly 30 km of optical fiber — alongside ongoing, classical internet traffic on the same line. This is practically significant because it shows that quantum networks and classical networks could coexist, rather than requiring completely separate infrastructure.
  • Higher fidelity: Newer experiments achieve teleportation fidelities above 90 %, compared with the 75–85 % typical earlier. Fidelity is the key indicator of how faithfully the transferred state arrives.
  • Entanglement between independent sources: At the University of Stuttgart, the state of a photon from one quantum dot was teleported onto a photon from a second, independent quantum dot — an important step toward scalable network nodes.
  • Satellite-based ranges: Through satellite experiments (pioneering work from China, among others), entanglement and keys were distributed over intercontinental distances of several thousand kilometers.

The vision behind it is the quantum internet: a network that distributes entangled states between distant nodes and thereby enables eavesdropping-proof communication, distributed quantum computing, and highly precise networked sensing. We are roughly where the classical internet stood in the early 1970s: the basic building blocks exist, and scaling is the great open task.

Part 9: The Philosophical Dimension — What Remains of Reality?

Here it is worth stepping back, for entanglement strikes at the heart of our worldview (a lovely point of contact with philosophy).

Bell's theorem forces an uncomfortable choice. At least one of the following familiar assumptions must be false:

  • Realism — the idea that properties exist independently of observation.
  • Locality — the idea that distant things cannot influence one another instantaneously.
  • Free will / statistical independence — the idea that experimenters choose their measurement settings freely and independently of the state of the particles.

Something must be given up. Which answer you choose defines different interpretations of quantum mechanics:

  • The Copenhagen interpretation gives up naive realism: properties come into existence only through measurement.
  • The many-worlds interpretation saves realism and locality, but sacrifices the uniqueness of reality in exchange — every measurement splits the universe into branches.
  • Bohm's pilot-wave theory retains definite particle positions, but buys this with explicit non-locality.
  • Superdeterminism (disliked by Bell himself, pursued by few) sacrifices the statistical independence of the measurement choice.

There is no consensus on which interpretation is "correct" — the mathematics and all predictions are identical. What is settled: the cozy picture of a world made of little spheres with fixed properties, interacting only by contact, is physically refuted. That is perhaps the deepest lesson of quantum entanglement — deeper even than any technology that follows from it.

In my view, this point in particular is especially valuable for someone interested in epistemology and in the limits of formal systems: entanglement shows that even "self-evident" basic assumptions are empirically falsifiable — if only one poses the right, precise question (as Bell did).

The Central Takeaway

Quantum entanglement is not an esoteric fringe topic but a threefold lever:

  1. Scientifically it is the experimentally secured proof that the world is not locally realistic — one of the most robust and at the same time most counterintuitive facts of modern physics (2022 Nobel Prize).
  2. Technologically it is the resource behind quantum computing, teleportation, and the emerging quantum internet.
  3. Practically it compels you, as an IT professional, to act already today: the migration to post-quantum cryptography (NIST FIPS 203/204/205, a standard since August 2024) is no longer a distant option but an ongoing compliance and architecture task.

A concrete prompt to act on this week: Ask yourself (or a client) a single question — "Which of our data must remain confidential for longer than ten years, and with which algorithms do we protect it today?" Wherever a long protection horizon meets RSA/ECC, there is a concrete "harvest now, decrypt later" risk. That is the most honest entry point into a quantum-safe roadmap — and it costs nothing but a clear thought.

A reflection question: If even the assumption that things "possess" fixed properties can be physically false — which "self-evident" basic assumptions in your own architecture and security decisions have you last genuinely questioned, rather than merely presupposed?

Cross-References in the Vault

  • Ist die Cloud valide – Data security and questions of trust in the cloud; the PQC migration extends this discussion into the quantum era.
  • Der Wettlauf mit der (um die) KI – Quantum computing and AI are the two great "exponential" technology waves; both fundamentally reshape the spaces of threat and possibility.

Sources and Further Reading

Created as part of the daily learning workflow. Field of interest: Quantum Physics. Estimated reading time: ~30 minutes.

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