Conclusive quantum steering with superconducting transition edge sensors (1111.0829v3)

Abstract: Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted. A conclusive demonstration of steering through the violation of a steering inequality is of considerable fundamental interest and opens up applications in quantum communication. To date all experimental tests with single photon states have relied on post-selection, allowing untrusted devices to cheat by hiding unfavourable events in losses. Here we close this "detection loophole" by combining a highly efficient source of entangled photon pairs with superconducting transition edge sensors. We achieve an unprecedented ~62% conditional detection efficiency of entangled photons and violate a steering inequality with the minimal number of measurement settings by 48 standard deviations. Our results provide a clear path to practical applications of steering and to a photonic loophole-free Bell test.

• The paper demonstrates conclusive quantum steering by closing the detection loophole through high-efficiency superconducting TESs.
• It employs a polarization Sagnac interferometer to generate entangled photons with approximately 62% conditional detection efficiency, achieving violation margins over 200σ.
• The findings pave the way for practical quantum cryptography and loophole-free photonic tests of entanglement using secure, untrusted devices.

Conclusive Quantum Steering with Superconducting Transition Edge Sensors

This paper presents a robust experimental demonstration of quantum steering using superconducting transition edge sensors (TESs) to achieve previously unattainable detection efficiencies. Quantum steering, a fundamental test for the Einstein-Podolsky-Rosen paradox, is crucial because it allows the detection of entanglement even when one party's devices are untrusted. Unlike traditional demonstrations reliant on post-selection—which opens the detection loophole, allowing untrusted devices to discard unfavorable outcomes—this experiment closes this loophole through high detection efficiencies.

The authors employ a highly efficient source of entangled photon pairs using a sophisticated polarisation Sagnac interferometer method. They achieve conditional detection efficiencies of approximately 62%, enabling them to violate a quantum steering inequality by an extensive 48 standard deviations with minimal measurement settings. For experiments using three measurement bases, the violation reaches over 200 standard deviations. This is accomplished by using TESs that offer high detection efficiency, photon-number resolution, and very low dark counts compared to conventional single-photon detectors.

The experiment is conducted with two measurement settings to validate steering for qubits encoded in polarization states of single photons, framed around a quadratic steering inequality. The inequality is carefully matched with Bob's orthogonal measurement settings, ensuring precision in demonstration and accounting for all sources of inefficiencies and imperfections in the measurement process. The implications are not only fundamental but also pave the way for practical applications such as secure quantum communication under the context of untrusted devices, making quantum steering a potential tool in quantum cryptography.

For the community, this paper establishes that quantum steering can be conclusively demonstrated using high-efficiency photon detection. By closing the detection loophole, it provides a critical step toward a loophole-free Bell test for photonic systems, an endeavor that requires overcoming challenges related to detection efficiency, locality, and freedom of choice. The success of the described setup suggests that with further improvements in detection efficiency, particularly with symmetric configurations, full loophole-free quantum tests could soon be a reality, marking a significant advancement in quantum foundational research and communication technologies. The method's achievement in harnessing high conditional detection efficiency is highly instructive for future work focused on practically implementing quantum steering-based protocols and foundational quantum tests.