A secure communication channel that relies on quantum entanglement survives despite the noisy break up of the entanglement itself. July 1, 2013 The simplest example of how this can work involves two entangled pulses of light, each containing just one photon. “Alice” (the sender) keeps one pulse and sends the other one towards her target, “Bob.” When Bob sends back the pulse, Alice interferometrically recombines it with the light she kept. Here is where the difference between classical and quantum signals becomes important: With classical light, time and frequency can’t both be simultaneously localized, as the Fourier transform of a pulse that is sharply localized in time is spread out over all frequencies. In contrast, if the sent and retained signals are truly entangled, they will be simultaneously strongly correlated in both arrival time and frequency. The much stronger initial correlation of the entangled beams allows reflected photons to be distinguished from background photons with a much higher signal to noise when they are “decoded” by recombining them with the retained signal. (The decoder is basically the reverse of the original entangler—a sort of “disentangler”—which only lets through the tiny residual correlation that matches the original entanglement.) Even though the entanglement doesn’t survive, a classical correlation survives that is stronger than would exist in the absence of entanglement in the first place. The enhancement in signal to noise is by a factor d , where d is the number of optical modes involved in the entanglement. In this way, the presence (or absence) of an object can be determined with far less light than a classical experiment would require. Instead of using quantum illumination to enhance a measurement, Zhang et al. [1] apply the method to making a secure transmission channel, based on an idea from group leader Jeffrey Shapiro [3]. Suppose Bob controls if the object is present, while Alice has to use her entangled photons to detect it. Upon receiving one photon, Bob encodes a “1 ” if the object is present and a “0 ” if it is not. Using the quantum illumination technique, Alice’s measurement of whether Bob sent a “0 ” or “1 ” has a high signal to noise. She can therefore determine the presence or absence of the object with confidence, even with low levels of light. physics.aps.org/articles/v6/74
Posted on: Wed, 31 Jul 2013 12:52:21 +0000
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