Invisible quantum tripwire senses intruders that don't intercept photons

In an interaction-free measurement apparatus, the interference of a single photon with itself in a Mach-Zehnder interferometer is used to detect the presence or absence of an object in one of the interferometer's arms without the photon ever actually striking the object; however, the setup must be perfect, and even so, it only works 25% of the time. Researchers at Louisiana State University (Baton Rouge, LA) have designed a more practical version that tolerates—and even thrives—on misalignments and lossiness.

Their invisible quantum tripwire (IQT) consists of a polarization interferometer through which a photon repeatedly cycles. In fact, the amount of loss becomes one of the parameters to adjust in optimizing the IQT, along with the single-pass polarization-rotation angle. The device, which exists at present only in a simulation, depends on a nonlinear form of the quantum Zeno effect that appears only when losses are included. In the model, the photons must pass through the device at least 13 times before invisible detection of an intruding object becomes possible. While detection at 100% certainty will never be achieved, an optimized real IQT could be useful for some security applications, say the researchers. Contact Petr Anisimov at[email protected].

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