Quantum steering is a striking form of quantum nonlocality, where the measurement choices made on one part of an entangled system can nonclassically affect the state of another distant system.
First discussed by Erwin Schrödinger in response to the famous Einstein-Podolsky-Rosen (EPR) paradox, quantum steering reveals a unique asymmetry in quantum correlations:
- One party (say, Alice) can “steer” the state of another (Bob) by choosing different measurements on her part of an entangled pair.
- Bob, even without trusting Alice’s device, can verify that his system’s responses are consistent with being entangled—not just classically correlated.
Quantum steering sits between entanglement and Bell nonlocality:
- All steerable states are entangled,
- But not all entangled states are steerable,
- And not all steerable states violate Bell inequalities.
This phenomenon has practical importance in quantum cryptography, particularly in semi-device-independent protocols, where one party’s devices can be untrusted.
In short, quantum steering demonstrates that one system’s measurements can remotely influence the set of possible outcomes of another—a deep and surprising consequence of entanglement in the quantum world.