Source-linked AI summary
Observation of genuine one-way Einstein-Podolsky-Rosen steering
Sabine Wollmann, Nathan Walk, Adam J. Bennet, Howard M. Wiseman, G. J. Pryde
TL;DR
The paper addresses whether one-way steering exists for arbitrary POVMs beyond the restricted Gaussian setting. It analyzes EPR states using non-Gaussian measurements and finds that a 3 dB state is two-way steerable for T ≳0.3, showing that Gaussian-restricted one-way steering can disappear with more general measurements.
Problem
The paper addresses whether one-way steering exists beyond the restricted setting of Gaussian measurements, where EPR states can appear one-way steerable.
Method
The paper analyzes an EPR state parameterized by χ using infinite-dimensional analogues of Pauli operators and a nonlinear steering inequality.
Results
For 3 dB of squeezing, the EPR state is two-way steerable for T ≳0.3, so Gaussian-measurement one-way steering can vanish with more general measurements.
Takeaways & Limitations
The result shows that observing one-way steering can depend on restricting the allowed measurements to Gaussian measurements.
Abstract
from arXiv · showhide
Within the hierarchy of inseparable quantum correlations, Einstein-Podolsky-Rosen steering is distinguished from both entanglement and Bell nonlocality by its asymmetry -- there exist conditions where the steering phenomenon changes from being observable to not observable, simply by exchanging the role of the two measuring parties. Whilst this one-way steering feature has been previously demonstrated for the restricted class of Gaussian measurements, for the general case of positive-operator-valued measures even its theoretical existence has only recently been settled. Here, we prove, and then experimentally observe, the one-way steerability of an experimentally practical class of entangled states in this general setting. As well as its foundational significance, the demonstration of fundamentally asymmetric nonlocality also has practical implications for the distribution of the trust in quantum communication networks.
Appendix A: Steering Gaussian states with non-Gaussian measurements
The appendix analyzes an EPR state with non-Gaussian measurements and a nonlinear steering inequality, showing that Gaussian-restricted one-way steering can disappear for more general measurements.
- State and measurements: The canonical EPR state is expressed in the number basis and parameterized by χ, which describes the entanglement strength.χ is related to the variance of the squeezed resource states.
- State and measurements: For a lossy channel with transmission T, Gaussian quadrature measurements allow Alice to steer Bob for any T, while Bob can steer Alice only for T > 1/2.
- Non-Gaussian steering test: The analysis instead uses infinite-dimensional analogues of Pauli operators and applies them to a nonlinear steering inequality with dichotomic outcomes.The measurements have outcomes 1 or -1.
- Non-Gaussian steering test: The steering inequality is evaluated using Alice’s reported outcomes and Bob’s conditional expectation values, with a state-dependent bound when Alice is honest.
- Result: For 3 dB of squeezing, the EPR state is two-way steerable for T ≳0.3, showing that Gaussian-measurement one-way steering can vanish with more general measurements.
Appendix B: Determination of the experimental Werner parameter µ
The appendix characterizes the experimentally created states as Werner-like and compares two-way steering with the Gaussian-measurement case as transmission to Bob varies.
- State characterization: The created quantum states are Werner-like and are described using a single-qubit unitary transformation together with a Werner-state density matrix.
- Transmission dependence: For 3 dB of initial pure two-mode squeezing, the state is demonstrably two-way steerable when transmission to Bob satisfies T ≳0.3.
- Transmission dependence: The figure compares the two-way steering region for the general test with the narrower region obtained using Gaussian measurements.