Is quantum entanglement faster than light? The correlations between entangled particles appear instantly, however far apart the particles are, but no message, signal or usable information travels faster than light. Experiments only show that if a hidden influence linked the particles, it would have to be at least about ten thousand times faster than light, provided Earth moves slower than about one-thousandth of light speed in the hypothetical privileged frame (Salart et al.; Yin et al.). Theory suggests even that explanation does not work.
This article walks through what entanglement is, how physicists tried to put a speed on “spooky action at a distance,” what the numbers mean, and why relativity remains intact.
The short answer: correlations look instant, messages don’t
When two particles are entangled, measuring one gives a result that is correlated with the result on the other. Those correlations hold even when the two measurements happen so close together in time that no light-speed signal could have passed between them.
That sounds like a violation of relativity, but it isn’t. The Nobel Prize committee’s popular explainer for the 2022 physics prize puts it this way: a signal cannot travel faster than light, yet in quantum mechanics there does not seem to be any need for a signal linking the different parts of an extended system.
So the honest summary has two parts:
- The correlations are real and cannot be explained by information the particles carried from the start.
- Nobody can use them to send a message, so nothing usable outruns light.
What entanglement is, and what Bell tests ruled out
The classic objection, going back to Einstein, was that entangled particles might simply carry pre-set instructions, so-called hidden variables. The Nobel explainer uses an analogy with coloured balls: under hidden variables, the balls always contained hidden information about which colour to show. Quantum mechanics says they were grey until someone looked at them. Our article on the EPR paradox, from Einstein’s 1935 doubt to the 2022 Nobel covers that history in more detail.
John Bell turned the question into a measurable inequality. In the commonly used CHSH form, local hidden-variable theories require |S| ≤ 2, while quantum theory predicts |S| = 2√2, as summarised in a 2023 Scientific Reports paper.
Bell test experiments that settled it
The Nobel Prize in Physics 2022 went to Alain Aspect, John Clauser and Anton Zeilinger for experiments with entangled particles:
- In 1972, Clauser and doctoral student Stuart Freedman showed a clear violation of a Bell inequality that agreed with quantum mechanics.
- Aspect closed an important loophole, and his result showed that quantum mechanics is correct and there are no hidden variables.
- Zeilinger’s group carried out the first quantum teleportation experiment in 1997 and first demonstrated entanglement swapping in 1998.
In 2015, Hensen and colleagues reported a loophole-free Bell test using entangled electron spins 1.3 km apart. Efficient spin readout closed the detection loophole, while fast random basis selection plus the distance ensured locality. Across 245 trials they found S = 2.42 ± 0.20, with a p-value of 0.039 for a local-realist model. The Stanford Encyclopedia of Philosophy notes that the small number of trials limited the statistical significance to about 2 standard deviations.
How do you measure the speed of spooky action at a distance?
If the correlations cannot come from pre-set instructions, one alternative is that some hidden influence travels from one particle to the other, just very fast. Physicists asked: how fast would it need to be?
The difficulty is that “simultaneous” depends on the observer’s reference frame in relativity. A hypothetical faster-than-light influence would need some universal, privileged frame in which its speed is defined. You do not know which frame that would be, so a single measurement cannot pin it down.
Eberhard proposed a workaround: run a continuous, space-like Bell test for 12 hours over a long baseline oriented east to west. Because the Earth rotates, the measurement would sweep through all possible reference frames, making the resulting speed bound universal. This idea is described in both the Yin et al. paper and the 2023 Scientific Reports study.
The logic of these tests is important. They never measure a speed. They show that the correlations stayed intact even when the two detections were so nearly simultaneous that any influence would have needed at least a certain speed to connect them. The result is a lower bound, and it depends on assumptions about Earth’s motion in the hypothetical privileged frame.
Is quantum entanglement faster than light? What speed tests found
Three experiments illustrate how the bounds have been set.
| Experiment | Setup | Lower bound on a hidden influence |
|---|---|---|
| Salart et al., Nature (2008) | Two villages 18 km apart, roughly east–west, more than 24 hours | At least four orders of magnitude above light speed, if Earth moves at less than 10⁻³ c in the privileged frame |
| Yin et al., Phys. Rev. Lett. (2013) | Qinghai Lake region, China; sites 15.3 km apart, east–west, 12 hours continuous | 1.38 × 10⁴ times light speed (assuming Earth’s speed about 10⁻³ c); 7× light speed even at 0.9c |
| Tabletop test, Scientific Reports (2023) | Sharp east–west baseline, 11 minutes | About 3.3 × 10⁴ c, tested only in the CMB frame |
Two villages, 18 km apart
Salart, Baas, Branciard, Gisin and Zbinden ran a Bell test for more than 24 hours between two villages 18 km apart, oriented approximately east–west, with the photon source precisely in the middle. Using Earth’s rotation, they set a lower bound for any hypothetically privileged frame. For example, if Earth moves at less than 10⁻³ times light speed in that frame, the influence would have to exceed light speed by at least four orders of magnitude.
Qinghai Lake, 12 hours of continuous violation
Yin and colleagues, in Jian-Wei Pan’s group, sent entangled photons over a free-space optical link to two receiving sites 15.3 km apart (the paper elsewhere describes them as 16 km apart), with the sender in the middle. They observed a 12-hour continuous Bell violation and closed the locality loophole, with fast electro-optic modulators addressing the freedom-of-choice loophole. Like most photonic Bell tests, they relied on the fair-sampling assumption for the detection loophole.
Their bound was 1.38 × 10⁴ times light speed, assuming Earth moves at about 10⁻³ c, similar to its speed relative to the cosmic microwave background. Even if Earth’s relative speed reached 0.9c, the influence would still have to be 7 times faster than light.
A tabletop version
A 2023 experiment published in Scientific Reports began on 20 December 2021 and lasted 11 minutes, using a baseline with a sharp east–west orientation. All 97 measurements of S were reported, and almost all violated the Bell–CHSH inequality by at least 3 standard deviations. The resulting bound was about 3.3 × 10⁴ c, tested only in the CMB frame, which the authors say more than doubles the Yin et al. bound in that frame.
The authors are clear about the limits: they did not address the loopholes problem, and they describe the work as a feasibility test. They also note that earlier kilometre-scale experiments gave bounds ranging from 10⁴ c to 5 × 10⁶ c.
Why entanglement can’t be used for faster-than-light communication
Even if the correlations look instantaneous, they cannot carry a message. Three separate arguments point the same way.
The no-communication theorem
The no-communication theorem states that during measurement of an entangled quantum state, one observer cannot transmit information to another, regardless of distance. Statistically, Bob cannot tell the difference between what Alice did and a random measurement, or whether she did anything at all.
Each side sees what looks like random results. The correlation only becomes visible when the two lists are brought together and compared, and that comparison needs an ordinary channel limited by light speed.
Quantum teleportation and the speed of light
Teleportation is often cited as a counterexample, but it isn’t one. As APS Physics Magazine explains in its Quantum Milestones piece on Bennett et al. (1993), teleportation needs both quantum information carried by the entangled pair and classical information sent over classical channels, which can go no faster than light. Without the classical message, the receiver cannot reconstruct the state.
Teleportation also does not copy. Alice’s measurement destroys her original unknown state, and the Nobel explainer likewise notes that the transferred properties are destroyed in the original.
The deeper twist: hidden FTL influences would break relativity anyway
The speed bounds above leave open, in principle, a hidden influence that is very fast but finite. In 2012, Bancal, Pironio, Acín, Liang, Scarani and Gisin showed in Nature Physics that models explaining the correlations with hidden influences moving at a finite speed v > c predict correlations that could themselves be used for faster-than-light communication.
That communication would not even require access to hidden quantities, only manipulation of measurement devices. Since faster-than-light messaging is ruled out, so is this class of explanation. The upshot is that quantum correlations cannot be understood as a finite-speed signal at all, however fast.
What this means for quantum networks and secure communication
For engineers, the practical lesson is that entanglement is a resource for correlation and security, not for speed. Every quantum network still needs classical links, and those links set the pace.
Distance is a separate problem. According to the Nobel explainer, entangled pairs can’t be amplified like ordinary light signals, because amplifying them breaks the entanglement. Entanglement swapping is one way to extend the distance between nodes. Our piece on how far quantum key distribution can reach looks at the current range records and their caveats.
Loophole-free Bell tests matter here too: Hensen and colleagues wrote that their result paves the way for device-independent quantum-secure communication and randomness certification.
Quantum communication is only one response to the quantum era. Most deployed systems are instead moving to post-quantum cryptography that runs on ordinary hardware. Hadamard, for example, builds BB84, a hash-based, ECDSA-free Layer 2 rollup on Ethereum designed for post-quantum security, which needs no entanglement at all. The post-quantum migration tracker follows how blockchains, browsers and standards are adapting.
FAQ
If measuring one entangled particle instantly fixes the other, why can’t that be used to send a message faster than light?
Because each observer sees only random results and cannot choose which outcome appears. The correlation shows up only when both sets of results are compared, which requires an ordinary, light-speed channel.
Has anyone actually measured the speed of quantum entanglement?
No speed has been measured. Experiments set lower bounds: if a hidden influence existed, and Earth moves slower than about 10⁻³ c in its privileged frame, the influence would have to be at least about 10⁴ times faster than light (Yin et al. still get 7× light speed even at 0.9c). A tabletop test gave about 3.3 × 10⁴ c, tested only in the CMB frame.
Does quantum teleportation move information faster than light?
No. Teleportation requires a classical message sent over a normal channel, which travels no faster than light, and the original state is destroyed in the process.
Does entanglement mean Einstein’s relativity is wrong?
No. Entanglement rules out local hidden variables, but it does not allow faster-than-light signalling, which is what relativity forbids. Models with finite-speed hidden influences would allow such signalling, which is why physicists reject them.
Sources
- Popular information: Nobel Prize in Physics 2022, NobelPrize.org (retrieved 26 Aug 2026)
- Santamaria Amato et al., tabletop Bell test speed bound, Scientific Reports 13, 8201, Nature Portfolio (21 May 2023)
- Salart et al., Testing the speed of spooky action at a distance, Nature 454, 861–864, Nature (2008)
- Yin et al., lower bound on the speed of spooky action, Phys. Rev. Lett. 110, 260407, arXiv (submitted 4 Mar 2013; v2 18 Jun 2013)
- Bancal et al., Quantum non-locality based on finite-speed causal influences leads to superluminal signalling, Nature Physics 8, 867–870, Nature Physics (28 Oct 2012)
- Hensen et al., Loophole-free Bell inequality violation using electron spins separated by 1.3 km, arXiv (24 Aug 2015); published in Nature 526, 682–686 (2015)
- Bell’s Theorem, Stanford Encyclopedia of Philosophy (retrieved 7 Nov 2026)
- Quantum Milestones, 1993: Teleportation Is Not Science Fiction, APS Physics Magazine (original story 8 Jan 2010)
- No-communication theorem, Wikipedia (retrieved 14 Sep 2026)