Spin entanglement is one kind of quantum entanglement, not a competing phenomenon. Quantum entanglement describes a joint state that cannot be separated into independent states of its parts; spin entanglement means that the entangled property is spin. The same general idea can apply to other degrees of freedom, including spatial wave functions and photon polarization.
What distinguishes the two terms?
“Quantum entanglement” names a general relationship between quantum subsystems: their joint state is nonseparable, so it cannot be fully described as a product of independent states for each subsystem. “Spin entanglement” narrows that description to cases where the relevant property is the particles’ spin.
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So the useful comparison is about which property is involved and how it is represented and measured—not about two rival theories. For spin, states are often described in a discrete spin basis and tested by measuring spin components along chosen axes. For spatial entanglement, the state is described using wave functions over position; other degrees of freedom require measurements suited to those properties. In every case, the key question is whether the joint state factors across the chosen subsystems.
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Can a pair of spins be unentangled?
Yes. A system involving two spins is not automatically entangled. For two spin-1/2 particles, such as two electrons, the four coupled states consist of one singlet and three triplets. Whether any particular state is entangled depends on whether it can be written as a product of individual-particle states.
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The singlet
The spin singlet is nonfactorizable, so the two particles are entangled. When their spins are measured along the same axis, the outcomes are anticorrelated: one is found up and the other down relative to that axis.
Triplet states
Triplet states are symmetric under exchange of the particles, but that symmetry alone does not settle whether a state is entangled. For example, the state in which both spins are up is a product of individual spin states, so it is not entangled. Other states must be assessed by checking whether they factorize.
How does spin entanglement compare with other kinds?
Entanglement is not limited to spin. Daniel V. Schroeder’s 2017 American Journal of Physics article notes that it can occur in spatial wave functions as well as in discrete systems such as spins. Photon polarization is another commonly discussed degree of freedom. Spin examples are mathematically simple and useful for introducing Bell’s theorem and quantum information; spatial-wave-function examples help connect the concept to wave mechanics.
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What do entangled-spin correlations mean?
Correlations depend on both the state and the measurement. In a singlet, measurements along the same axis are anticorrelated; changing the measurement axes changes the correlation pattern. Bell’s work shows that these statistics cannot be explained by local hidden-variable accounts that satisfy the relevant assumptions.
That does not make entanglement a faster-than-light messaging channel. Caltech’s Science Exchange explains that quantum physics cannot be used for faster-than-light communication. As Caltech professor Thomas Vidick puts it, “There can be correlation without communication.” The observed correlations do not let one observer choose an outcome and use it to send a controllable message to another.
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What do real-world demonstrations show?
University of Zurich reports two historical examples involving entangled photons. In 2016, it described satellite transmission of entangled photons over more than 1,200 kilometers. That is a reported demonstration from that year, not a current performance benchmark.
The university also reported a 2017 quantum telephone call between Vienna and Beijing, describing it as “tap-proof.” That characterization belongs to the university’s account; it should not be treated as an unconditional guarantee of security.
Quick Recap
At a glance
| Question | Quantum entanglement | Spin entanglement |
|---|---|---|
| What does the term identify? | Nonseparability between quantum subsystems, across any relevant degree of freedom. | Nonseparability involving spin degrees of freedom. |
| Is it a separate phenomenon? | It is the general concept. | No; it is a specific case of quantum entanglement. |
| What might be measured? | The property appropriate to the systems, such as position or photon polarization. | Spin components along selected axes. |
| Does every pair in that category have to be entangled? | No. A joint state may or may not be separable. | No. A two-spin product state, such as both spins up, is separable. |
Further reading
- Daniel V. Schroeder’s 2017 article on entanglement beyond spin.
- Caltech Science Exchange’s explanation of quantum entanglement and communication.
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