In 1981 physicist Nick Herbert leveraged strange features of quantum mechanics to design a superluminal communication system. The quest to uncover its subtle flaw led to a profound new understanding of the quantum world
?| May 29, 2012
Herbert's FLASH system?the acronym stood for "first laser-amplified superluminal hookup"?employed a source that emitted pairs of photons in opposite directions. The scheme focused on photons' polarization?that is, the directions along which their associated electric fields oscillated. The photons could be plane-polarized, with the electric fields oscillating either horizontally (H) or vertically (V). Or the photons could be circularly polarized, with the electric fields tracing out helical patterns in either a right-handed (R) or left-handed (L) orientation.
Physicists had long known that the two flavors of polarization?plane or circular?were intimately related. Plane-polarized light could be used to create circularly polarized light, and vice versa. For example, a beam of H-polarized light consisted of equal parts R- and L-polarized light, in a particular combination, just as a beam of R-polarized light could be broken down into equal parts H and V. Likewise for individual photons: a photon in state R, for example, could be represented as a special combination of states H and V. If one prepared a photon in state R but chose to measure plane rather than circular polarization, one would have an equal probability of finding H or V: a single-particle version of Schr?dinger?s cat.
In Herbert's imagined set-up, one physicist, Alice ("Detector A" in the illustration), could choose to measure either plane or circular polarization of the photon headed her way [1]. If she chose to measure plane polarization, she would measure H and V outcomes with equal probability. If she chose to measure circular polarization, she would find R and L outcomes with equal probability.
In addition, Alice knows that because of the nature of the source of photons, each photon she measures has an entangled twin moving toward her partner, Bob. Quantum entanglement means that the two photons behave like two sides of a coin: if one is measured to be in state R, then the other must be in state L; or if one is measured in state H, the other must be in state V. The kicker, according to Bell's theorem, is that Alice's choice of which type of polarization to measure (plane or circular) should instantly affect the other photon, streaming toward Bob [2]. If she chose to measure plane polarization and happened to get the result H, then the entangled photon heading toward Bob would enter the state V instantaneously. If she had chosen instead to measure circular polarization and found the result R, then the entangled photon instantly would have entered the state L.
Next came Herbert's special twist. Before the second photon made its way to Bob's detectors, it entered a laser gain tube [3]. Lasers had been around for 20 years by that time, and as the leading textbooks routinely touted, the output from a laser had the same polarization as the input signal. That suggested that the laser should release a burst of photons in the complementary state to whatever Alice had found at her side. Bob could then split the beam [4], sending half toward a detector to measure plane polarization [5] and half toward a detector to measure circular polarization [6].
If Alice chose to measure circular polarization and happened to find L, then the entangled photon heading toward Bob would instantly go into the state R prior to entering the laser gain tube. Out of the laser would burst a stream of R photons heading toward Bob. He could then send half the beam toward a detector to measure plane polarization and half toward a detector to measure circular polarization. In this case, Herbert concluded, Bob would find half the photons in state R, none in state L, and a quarter each in states H and V. In an instant, Bob would know that Alice had chosen to measure circular polarization. Alice's choice?plane or circular polarization?would function like the dots and dashes of Morse code. She could signal Bob simply by alternating her choice of what type of polarization to measure. Bob could decode each bit of Alice?s code faster than light could have traveled between them.
As GianCarlo Ghirardi, Tullio Weber, Wojciech Zurek, Bill Wootters and Dennis Dieks each clarified, Herbert?s device would not actually allow superluminal signaling. A photon in state R, for example, would exist as a combination of equal parts H and V. Each of those underlying states would be amplified by the laser. Hence the output would be a superposition of two states: one in which all the photons were in state H, and the other in which all the photons were in state V, each with a probability of 50 percent. Bob would never find half in H and half in V at the same time, just as physicists would never find Schr?dinger's cat to be both half-dead and half-alive upon opening the box. Thus, Bob would receive only noise no matter what setting Alice had chosen on her end. Moment by moment, Bob's detectors would flash H with R or V with L or H with L and so on, in random combinations. He would never find H and V with R, and hence he would have no way to determine what Alice had been trying to tell him. Quantum entanglement and relativity could coexist after all.
This discovery became known as the "no-cloning theorem": a powerful statement about the ultimate foundations of quantum theory. An arbitrary or unknown quantum state cannot be copied without disturbing the original state. No one had ever recognized that fundamental feature of quantum theory before the cat-and-mouse game had unfolded between Nick Herbert's thought experiment and his talented detractors. The fact that quantum theory sets an ultimate limit on the ability of anyone?including a potential eavesdropper?to seize individual quantum particles and make copies of them soon became the bedrock for quantum encryption, and today is at the heart of the flourishing field of quantum information science.
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