Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

by Ananthaswamy, Anil

  At the least, Einstein would likely have been enthralled by the experiments done on mountaintops, given his own penchant for hiking in the Swiss Alps ( in 1913, he crossed the nearly 1,800-meter-high Maloja Pass on foot, with Marie Curie and her daughters for company). One mountaintop experiment, a particularly intricate and involved variant of the double-slit experiment done by Zeilinger and his team, combined the two elements of quantum mechanics that made Einstein insist on the theory’s incompleteness: wave-particle duality and nonlocality. The origins of this line of inquiry lie in a thought experiment dreamt up by a physicist who came to be called the “Quantum Cowboy,” for his pioneering research on the nature of reality and beef cattle production.

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  During the American Civil War, a confederate officer named Robert P. Salter cultivated cotton on a farm that lies midway between Houston and Dallas. He bought guns with the cotton he grew. Today, Marlan Scully studies sustainable farming on parts of that historic farm. “ The mystery is not that I’m interested in farming, but that I’m interested in quantum physics” has been Scully’s response to questions about why a quantum physicist took up farming. Scully grew up in rural Wyoming and married into a farming family.

  He went to Yale to do his graduate studies, where he pursued the great experimentalist Willis Lamb nearly every day. “ A dumb kid from Wyoming, I didn’t know that the Nobel Prize physicist at Yale wasn’t there for me.” Lamb always obliged with his time. After his PhD, Scully continued as an instructor at Yale. Within two years he moved to MIT, and then soon after to the University of Arizona. A decade later he moved to the University of New Mexico, and when he was there, he collaborated with Kai Drühl, a postdoc based in Munich, Germany, to come up with one of the most famous thought experiments in quantum physics: the quantum eraser.

  The “quantum eraser is qualitatively, conceptually, intellectually, much deeper than the Young [double-slit] experiment,” Scully told me during a phone conversation. Still, at its core, it is yet another type of double-slit experiment, albeit a very sophisticated one.

  Scully and Drühl targeted a key aspect of the debate between Einstein and Bohr: whether or not experiments themselves disturb quantum systems in ways that enforce complementarity. In the early days of their deliberations, Bohr had argued that the uncertainty principle would prevent us from seeing the wave nature and the particle nature of reality simultaneously. These were complementary aspects that were forever separated by the clumsiness of our classical measurements, and the uncertainty principle was the enforcer. But as Aspect showed with his implementation of Wheeler’s delayed-choice experiment, even when you could not point the finger at disturbances caused by the measuring apparatus and hence at the uncertainty principle, complementarity still reigned. It was a deeper principle than anyone had realized. Scully and Drühl pushed the argument much further.

  They imagined collecting information about which slit a particle goes through without disturbing the particle. The particle continues to do what it normally does, and yet somehow, it leaves behind information about the path it takes through the double slit. According to quantum mechanics, the mere presence of such information should destroy the interference pattern. As if that isn’t surprising enough, Scully and Drühl then asked a deeper question: What if this information is erased? Will the interference pattern come back?

  With their thought experiment, the duo was trying to refine the notion of measurement in quantum physics. In the 1930s, John von Neumann developed the rigorous mathematical formalism for quantum mechanics (in the very book in which he supposedly proved that there can be no hidden variable theories). This formalism emerged from axioms that gave measurements center stage: measurements caused a wavefunction to collapse. But there was no precise definition of what constitutes a measurement. Bohr, for example, merely divided up the world into the big and small, and measurement apparatuses were “big,” while the things they were measuring were “small.” The boundary between the classical and the quantum was entirely unclear—nothing in the formalism suggested where such a boundary might lie.

  Yet, the practical use of the theory implied such a boundary. A quantum system, described by its wavefunction, evolves according to the Schrödinger equation and then suddenly, upon measurement, the wavefunction collapses. The process of collapse does not follow the same laws as the ones governing the evolution of the wavefunction. In fact, there is no law, so to speak, that governs collapse. It’s something ad hoc, attributed to measurement. So a particle that, until the measurement, was in a superposition of multiple states is reduced to being in just one of the many possible states. What is it that determines when and how this collapse happens?

  This question was pushed to, some would say, its logical conclusion by Eugene Wigner, a Nobel Prize–winning physicist and von Neumann’s contemporary at Princeton University in the early to mid-1930s. Wigner, after a careful analysis of von Neumann’s formalism, concluded that the laws of quantum mechanics did not draw a line between the quantum and the classical. Everything—the quantum system, the measuring apparatus, everything—should evolve according to the same laws. The only thing, he reasoned, that could be responsible for the collapse of the wavefunction was consciousness. The act of perception by a conscious observer, Wigner argued, is the nail in the coffin for the wavefunction. In 1961, he wrote: “ When the province of physical theory was extended to encompass microscopic phenomena, through the creation of quantum mechanics, the concept of consciousness came to the fore again: it was not possible to formulate the laws of quantum mechanics in a fully consistent way without reference to the consciousness.” But by 1970, Wigner changed his mind, doubting his own claims of consciousness playing a role in causing collapse.

  Very few physicists today put stock in Wigner’s ideas. Scully and Drühl too weren’t concerned about consciousness and its role; they wanted a sharper understanding of measurement and the nature of collapse. They asked whether measurement could itself be something quantum mechanical. If so, the measurement device would also evolve according to the Schrödinger equation. Its wavefunction would not collapse, and so could be made to reverse its evolution in a manner that undid the measurement. “ We propose and analyze an experiment such that the presence of information accessible to an observer and the subsequent ‘eraser’ of this information should qualitatively change the outcome of our experiment.”

  They designed their thought experiment to show that one could in principle acquire which-way information about a photon’s path through a double slit by using an entangled partner photon. As long as this which-way information (or the welcher-weg information, in German) remains accessible to an observer, no interference can be detected in the patterns made by the photons going through the double slit. But if this information were to be erased, Scully and Drühl showed that one would observe interference. Their paper on the quantum eraser was published in 1982.

  By 1995, Zeilinger and colleagues carried out a version of the quantum eraser experiment, as did a few other teams, but none of the experiments were quite the ideal gedankenexperiment that Scully and Drühl were after. Scully eventually joined hands with Yoon-Ho Kim of the University of Maryland in Baltimore, and his colleagues, and in January 2000 they published the results of an experiment that was closest in spirit to the original idea.

  The experiment uses an atom that can be made to emit entangled photons when hit with a laser pulse. Imagine two such atoms, A and B. Each atom emits a pair of photons. The atoms are arranged such that one of the entangled pair of photons goes toward a screen. Let’s call it the “system” photon. Both A and B can emit a system photon. The two atoms are placed side by side, such that their system photons appear to be coming through a double slit. So the double slit in this scenario is virtual; all we have are the two atoms sending out system photons. If all we had were system photons and we had no other information (so ignoring the entangled photons for now), then the system photons would create an interference pattern on the screen.
That’s because any system photon that lands on the screen could have come from either atom A or atom B, or from one or the other slit (assuming we have no way of telling which atom the photon came from).

  But that’s not all we have. For each system photon that an atom emits, it emits another photon in the opposite direction; let’s call it the “environment” photon, which is entangled with its system photon. The environment photon contains information about which atom (or analogously, which slit) the system photon came from. The key now is to either preserve or destroy this information, and see what happens with the system photon that lands on the screen. Does it act like a wave or a particle?

  Take a pair of photons emitted by atom A. The system photon goes toward the right of the screen, where it’s recorded on a photographic plate. The environment photon heads left toward a set of beam splitters designed to either preserve or erase the which-way information. It first encounters beam splitter BSA. At BSA, the photon can be either transmitted to a detector D3 or reflected toward another beam splitter, BS, whereupon it can be reflected to D1 or transmitted to D2. Similarly, an environment photon from atom B will end up at D1 or D2 or D4.

  It’s clear that D3 will click only if the environment photon came from atom (slit) A. So a click at D3 constitutes which-way information about its partner system photon (note that this information is obtained without physically disturbing the system photon in any way). Similarly, if D4 clicks, we know the corresponding system photon came from atom (slit) B.

  However, once an environment photon gets reflected at either BSA or BSB and then goes past the central beam splitter BS and hits either detector D1 or D2, it’s impossible to tell whether it came from atom (slit) A or B, because environment photons from both atoms (slits) can trigger either D1 or D2. The which-way information for the corresponding system photon is erased.

  Now imagine that you have collected the pattern made by a large set of system photons, and the clicks made by the environment photons. If you consider only those environment photons that ended up at D3 and D4, and looked at the pattern that the corresponding system photons made on the photographic plate, you don’t see an interference pattern. That’s because for each of those photons, we know which slit it came through. We get particle-like behavior.

  But if you look at the pattern made only by those system photons whose corresponding environment photons were detected at D1, something strange happens: you see interference fringes. The same goes for environment photons detected at D2. The detection at D1 or D2 has erased the which-way information. At the photographic plate, there’s no way to tell whether the corresponding system photons came from the left or the right slit. The paths become indistinguishable, setting the stage for superposition and for interference.

  One of the most astonishing facts about the quantum eraser experiment is that the act of erasing the which-way information can be delayed for an arbitrary length of time. Say the system photons were detected almost immediately on the screen and their positions recorded. The environment photons, however, were allowed to travel, maybe for kilometers, before they went through the various beam splitters and onward to the detectors. If you analyzed the patterns made by all the system photons while all the environment photons were still in flight, you would not see any interference (because, in principle, you still have access to which-way information).

  But once the environment photons encounter the beam splitters and hit the final detectors, and if you then selectively analyze the measurements already made of the system photons, you will see something entirely different. Pick only those environment photons that reach D3 or D4 and check their corresponding system photons—you will not see an interference pattern. But pick those system photons whose corresponding environment photons caused either D1 or D2 to click—and hence erased the which-way information—and you will see an interference pattern. Was the pattern always there? Or did it reappear?

  If the idea of waiting and waiting before choosing what to do with the environment photons seems like theoretical fantasy, no one bothered to tell some physicists in Austria. A little more than a decade after Scully and Yoon-Ho Kim did their experiment, Zeilinger and colleagues were ready to test such fantasies across the mountaintops on La Palma and Tenerife.

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  The delayed-choice quantum eraser experiment done by Zeilinger’s team is among the most sophisticated of all the variations of the double-slit experiment. Rupert Ursin, once a student of Zeilinger’s, now a senior member of the team, recalled the travails of the seven-hour flight from Vienna to the Canaries. They were carrying almost two-thirds of a ton of equipment. For Europeans used to borderless travel, getting the equipment past customs in La Palma wasn’t trivial. “Believe it or not, the Canaries are outside of the European Union,” Ursin told me, sounding somewhat miffed. Actually, the islands are an autonomous part of Spain but still require customs checks for tax reasons.

  The team had a logistics company lug the equipment up to the summit of Roque de los Muchachos, where the scientists proceeded to set up an extremely sensitive experiment. They began working cheek to jowl. “You better have good friends before you start [such an] experiment, because you’ll hate them when you finish the experiment,” Ursin said.

  The experiment, in principle, is much the same as the one described in the previous section. But the practical details differ enormously. The experiment was spread over two physical locations: one atop the mountain in La Palma and the other near Mount Teide in Tenerife, 144 kilometers away as the crow—or in this case, the photon—flies. Most of the equipment was at La Palma, including a source of entangled photons.

  In the previous experiment, two atoms were positioned such that when one of the atoms emitted a pair of entangled photons, the system photon behaved as if it came through a double slit, and the environment photon went the other way, carrying information about which (virtual) slit the system photon came through. In the Canary Islands experiment, there is only one source of entangled photons. It emits a system photon and an environment photon. The system photon is sent into a Mach-Zehnder interferometer at La Palma and is detected immediately at either detector D1 or detector D2. The first beam splitter in the interferometer is somewhat different from the beam splitters we have seen so far; instead of randomly sending a photon one way or the other, this so-called polarizing beam splitter (PBS) sends the photon one way if it’s, say, horizontally polarized, and the other way if it’s polarized vertically (there are experimental subtleties about what is done to the photon after it crosses the PBS, but we can leave that aside). So, if you know the polarization of the photon, you know which path it takes through the interferometer.

  The entangled environment photon, however, is sent toward a telescope at Tenerife. The photons are entangled in their polarization states. The polarization of the environment photon can be used to tell which path the system photon takes through the interferometer at La Palma. Or the polarization of the environment photon can be scrambled, which is tantamount to erasing the which-way information about the corresponding system photon. This is the quantum eraser part of the experiment.

  The delayed-choice part comes in because the decision to erase or not to erase is made only when the environment photon reaches Tenerife—well after the partner system photon has been detected at La Palma, and thus well after it has ostensibly already behaved like a wave or a particle.

  To precisely control the length of the two arms of the interferometer at La Palma, the team used the vibrations of a tiny piezoelectric crystal. Such delicate control is hard enough in a temperature-controlled lab in the basement of a university building. On Roque de los Muchachos, it was a phenomenal feat. The laboratory was essentially a steel shipping container that was being buffeted by winds and was subject to constant day-night temperature fluctuations. “To stabilize such an interferometer in a mountain hut [at] 2,500 meters altitude is not easy,” said Ursin. “This is not a nice environment.”

  His colleague Xiao-Song Ma, who wa
s also then a student of Zeilinger’s, recalled how once someone merely opened the door of the shipping container and the resulting acoustic vibrations changed the interference patterns. So what did they have to do to ensure that the testbed was stable and free of noise? “Everything, literally everything,” Ma told me. “Even the breath of a human being or a stamp of the feet in the lab will . . . [destroy] the interference.”

  The numerous beaches on the islands somewhat made up for the stress of working on the summits. The team would work through the night and go to bed at sunrise, sleep for a few hours, and then head to a beach in the afternoon. I asked Ursin about which beach they frequented. “All of them,” he quipped. They did prefer, however, two in Tenerife: Las Teresitas, an artificial beach built of sand brought over from the Sahara Desert, with swaying palms and calm waters made possible by a breakwater, and contrastingly, El Bollullo, one of the island’s best natural beaches.

  But they had to get back to the summit before sunset and start experimenting all over again.

  Transmitting the environment photon from La Palma and detecting it at Tenerife was a serious challenge. The work of aiming the source at the receiving telescope had to be done in near complete darkness. While the entanglement between the system and environment photons could survive all the optical equipment (lenses, mirrors, and the like), it couldn’t survive moonlight, let alone sunlight. The photons from the moon would interact with the environment photons as they flew to Tenerife, causing them to lose their entanglement with the system photons. So the researchers worked with only stars for company in an otherwise dark sky.

  To receive the photon at the Observatorio del Teide in Tenerife, the team used the European Space Agency’s optical ground station, with its 1-meter telescope, ordinarily used for communicating with satellites. Near complete darkness was essential. Once, one of Ursin’s colleagues stood near the source, smoking a cigarette. The infrared photons from the glowing cigarette at La Palma completely saturated the receiver on Tenerife, overwhelming the signal of the lone environment photon.