Loophole-free Einstein-Podolsky-Rosen experiment via quantum steering (1111.0760v3)

Abstract: Tests of the predictions of quantum mechanics for entangled systems have provided increasing evidence against local realistic theories. However, there still remains the crucial challenge of simultaneously closing all major loopholes - the locality, freedom-of-choice, and detection loopholes - in a single experiment. An important sub-class of local realistic theories can be tested with the concept of "steering". The term steering was introduced by Schr\"odinger in 1935 for the fact that entanglement would seem to allow an experimenter to remotely steer the state of a distant system as in the Einstein-Podolsky-Rosen (EPR) argument. Einstein called this "spooky action at a distance". EPR-Steering has recently been rigorously formulated as a quantum information task opening it up to new experimental tests. Here, we present the first loophole-free demonstration of EPR-steering by violating three-setting quadratic steering inequality, tested with polarization entangled photons shared between two distant laboratories. Our experiment demonstrates this effect while simultaneously closing all loopholes: both the locality loophole and a specific form of the freedom-of-choice loophole are closed by having a large separation of the parties and using fast quantum random number generators, and the fair-sampling loophole is closed by having high overall detection efficiency. Thereby, we exclude - for the first time loophole-free - an important class of local realistic theories considered by EPR. As well as its foundational importance, loop-hole-free steering also allows the distribution of quantum entanglement secure from an untrusted party.

• The paper closes traditional locality, freedom-of-choice, and detection loopholes using polarization-entangled photons and fast quantum random number generators.
• It achieves a steering parameter of 1.049 ± 0.002, exceeding the classical bound by over 20 standard deviations, validating quantum non-locality.
• This loophole-free setup paves the way for secure quantum communication and further exploration of robust entangled systems.

Insights on Loophole-Free EPR Experiment via Quantum Steering

This research paper presents a pivotal experiment in the field of quantum mechanics by addressing longstanding loopholes associated with testing entangled systems through a novel quantum steering approach. The primary aim of the study is the experimental demonstration of Einstein-Podolsky-Rosen (EPR) steering without the conventional loopholes that have historically challenged the validity of such quantum mechanical predictions.

The fundamental challenge in previous experiments has been the closure of three critical loopholes: locality, freedom-of-choice, and detection. The present study exemplifies a loophole-free scenario by simultaneously addressing these pivotal concerns. The researchers utilized polarization entangled photons shared between geographically separated laboratories, achieving a noteworthy violation of a three-setting quadratic steering inequality.

Experiment Considerations

1. Locality Loophole: This was closed by creating substantial spatial separation between the entangled parties and ensuring all relevant events occurred in a space-like separation. The implementation of fast quantum random number generators (QRNGs) ensured independence between measurement parameter choices and photon creation.
2. Freedom-of-Choice Loophole: Spatio-temporal arrangements were meticulously designed so that any potential causal influence between the QRNG's setting choice and the photon generation was obliterated.
3. Detection Loophole: The experiment achieved high detection efficiency, exceeding the required 1/3 efficiency threshold with an average of 38.3% on Alice's side. This addressed concerns of biased sample representations in the detection, thus affirming the reliability of the steering measurements.

Experimental Findings

The experiment realized a steering parameter $S$ of 1.049 ± 0.002, a significant exceedance of the steering bound of 1 by over 20 standard deviations. This result substantiates the presence of steering without the need for the fair-sampling assumption, marking a substantial advancement in closing the detection loophole for photon-based systems. Alice’s detection efficiencies and the mutual independence of measurement bases played a pivotal role in ensuring the results were unequivocally against local realistic theories.

Theoretical Implications

By resolving these loopholes, this experiment effectively excludes a major subclass of local realistic theories postulated by EPR. The violation of the steering inequality translates into a practical demonstration of non-local influences in quantum systems, correlating with contextual predictions of quantum mechanics.

Practical Implications and Future Directions

The implications of this research extend to secure quantum communication protocols. The loophole-free nature of this steering approach enables the development of quantum cryptographic systems that are resilient to adversarial attacks from untrusted parties.

Future experimental research could focus on refining detection efficiencies further and exploring applications of loophole-free protocols in real-world quantum communication technologies. Moreover, anchoring this setup in more complex systems or materials can explore intricate facets of entanglement and quantum non-locality.

In summary, the experiment signifies a notable step forward in quantum research by providing a robust verification of quantum predictions, bypassing local hidden variable theories. This positions the field closer to realizing reliable quantum information applications and deepens our understanding of quantum non-locality.