A quantum correlation where a measurement on one particle can remotely influence the state of another.Quantum steering is a quantum correlation phenomenon where a measurement on one particle can remotely influence the state of another particle, even when they are far apart. It sits between entanglement and Bell nonlocality in strength: it is stronger than simple entanglement but weaker than full Bell inequality violations. The term 'steering' captures the idea that one party (Alice) can 'steer' the state of the other party's (Bob's) system by choosing different measurement settings.
The concept was introduced by Erwin Schrödinger in 1935, shortly after the Einstein-Podolsky-Rosen paper, as a way to describe the 'spooky action at a distance' that Einstein objected to. It was formalized mathematically much later, in 2007, by Howard Wiseman, Steve Jones, and Andrew Doherty.
In a steering scenario, Alice and Bob share an entangled pair. Alice performs one of several possible measurements on her particle. Depending on her measurement choice and outcome, Bob's particle is projected into a specific set of possible states. If Bob cannot explain the observed correlations using a local hidden state model—where his particle had a pre-existing state independent of Alice's choice—then steering is demonstrated. This is verified through steering inequalities, which bound the correlations achievable by classical means.
Quantum steering is used in one-sided device-independent quantum cryptography, where one party's devices are untrusted but the other's are trusted, enabling secure key distribution with fewer assumptions.
Schrödinger originally called it 'steering' because he imagined Alice could 'steer' Bob's system into different states, but he did not prove it was a distinct phenomenon—that took over 70 years.
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