Quantum ‘spookiness’ passes toughest test yet

Author: Zeeya Merali
(Image Credit: freakingnews.com)

It’s a bad day both for Albert Einstein and for hackers. The most rigorous test of quantum theory ever carried out has confirmed that the ‘spooky action at a distance’ that the German physicist famously hated — in which manipulating one object instantaneously seems to affect another, far away one — is an inherent part of the quantum world.

The experiment, performed in the Netherlands, could be the final nail in the coffin for models of the atomic world that are more intuitive than standard quantum mechanics, say some physicists. It could also enable quantum engineers to develop a new suite of ultrasecure cryptographic devices.

“From a fundamental point of view, this is truly history-making,” says Nicolas Gisin, a quantum physicist at the University of Geneva in Switzerland.

Einstein’s annoyance

In quantum mechanics, objects can be in multiple states simultaneously: for example, an atom can be in two places, or spin in opposite directions, at once. Measuring an object forces it to snap into a well-defined state. Furthermore, the properties of different objects can become ‘entangled’, meaning that their states are linked: when a property of one such object is measured, the properties of all its entangled twins become set, too.

This idea galled Einstein because it seemed that this ghostly influence would be transmitted instantaneously between even vastly separated but entangled particles — implying that it could contravene the universal rule that nothing can travel faster than the speed of light. He proposed that quantum particles do have set properties before they are measured, called hidden variables. And even though those variable cannot be access, he suggested that they pre-program entangled particles to behave in correlated ways.

1982 CERN John Bell 8206265web
John Bell at CERN, Geneva, Switzerland circa 1982. (Image Credit: CERN)

In the 1960s, Irish physicist John Bell proposed a test that could discriminate between Einstein’s hidden variables and the spooky interpretation of quantum mechanics1. He calculated that hidden variables can explain correlations only up to some maximum limit. If that level is exceeded, then Einstein’s model must be wrong.

The first Bell test was carried out in 19812, by Alain Aspect’s team at the Institute of Optics in Palaiseau, France. Many more have been performed since, always coming down on the side of spookiness — but each of those experiments has had loopholes that meant that physicists have never been able to fully close the door on Einstein’s view. Experiments that use entangled photons are prone to the ‘detection loophole’: not all photons produced in the experiment are detected, and sometimes as many as 80% are lost. Experimenters therefore have to assume that the properties of the photons they capture are representative of the entire set.

To get around the detection loophole, physicists often use particles that are easier to keep track of than photons, such as atoms. But it is tough to separate distant atoms apart without destroying their entanglement. This opens the ‘communication loophole’: if the entangled atoms are too close together, then, in principle, measurements made on one could affect the other without violating the speed-of-light limit.

Entanglement swapping

In the latest paper3, which was submitted to the arXiv preprint repository on 24 August and has not yet been peer reviewed, a team led by Ronald Hanson of Delft University of Technology reports the first Bell experiment that closes both the detection and the communication loopholes. The team used a cunning technique called entanglement swapping to combine the benefits of using both light and matter. The researchers started with two unentangled electrons sitting in diamond crystals held in different labs on the Delft campus, 1.3 kilometres apart. Each electron was individually entangled with a photon, and both of those photons were then zipped to a third location. There, the two photons were entangled with each other — and this caused both their partner electrons to become entangled, too.

Ronald Hanson and his group at Delft University (Image Credit: Michel van Baal)
Ronald Hanson and his team at Delft University (Image Credit: Michel van Baal)

This did not work every time. In total, the team managed to generate 245 entangled pairs of electrons over the course of nine days. The team’s measurements exceeded Bell’s bound, once again supporting the standard quantum view. Moreover, the experiment closed both loopholes at once: because the electrons were easy to monitor, the detection loophole was not an issue, and they were separated far enough apart to close the communication loophole, too.

“It is a truly ingenious and beautiful experiment,” says Anton Zeilinger, a physicist at the Vienna Centre for Quantum Science and Technology.

“I wouldn’t be surprised if in the next few years we see one of the authors of this paper, along with some of the older experiments, Aspect’s and others, named on a Nobel prize,” says Matthew Leifer, a quantum physicist at the Perimeter Institute in Waterloo for Theoretical Physics, Ontario. “It’s that exciting.”

A loophole-free Bell test also has crucial implications for quantum cryptography, says Leifer. Companies already sell systems that use quantum mechanics to block eavesdroppers. The systems produce entangled pairs of photons, sending one photon in each pair to the first user and the other photon to the second user. The two users then turn these photons into a cryptographic key that only they know. Because observing a quantum system disrupts its properties, if someone tries to eavesdrop on this process it will produce a noticeable effect, setting off an alarm.

The final chink

But loopholes, and the detection loophole in particular, leave the door open to sophisticated eavesdroppers. Through this loophole, malicious companies could sell devices that fool users into thinking that they are getting quantum-entangled particles, while they are instead being given keys that the company can use to spy on them. In 1991, quantum physicist Artur Ekert observed4 that integrating a Bell test into the cryptographic system also would ensure that the system uses a genuine quantum process. For this to be valid, however, the Bell test must be free of any loopholes that a hacker could exploit. The Delft experiment “is the final proof that quantum cryptography can be unconditionally secure”, Zeilinger says.

Austrian quantum physicist, Anton Zeilinger (Image Credit: Jacqueline Godany)
Austrian quantum physicist, Anton Zeilinger (Image Credit: Jacqueline Godany)

In practice, however, the entanglement-swapping idea will be hard to implement. The team took more than week to generate a few hundred entangled electron pairs, whereas generating a quantum key would require thousands of bits to be processed per minute, points out Gisin, who is a co-founder of the quantum cryptographic company ID Quantique in Geneva.

Zeilinger also notes that there remains one last, somewhat philosophical loophole, first identified by Bell himself: the possibility that hidden variables could somehow manipulate the experimenters’ choices of what properties to measure, tricking them into thinking quantum theory is correct.

Leifer is less troubled by this ‘freedom-of-choice loophole’, however. “It could be that there is some kind of superdeterminism, so that the choice of measurement settings was determined at the Big Bang,” he says. “We can never prove that is not the case, so I think it’s fair to say that most physicists don’t worry too much about this.”

Author: Zeeya Merali

Permission to Reprint Copyright Number: 3697741491732

Is time really passing?

What Does “Happy New Year” Even Really Mean?


When Albert Einstein’s good friend Michele Besso died in 1955, just a few weeks before Einstein’s own death, Einstein wrote a letter to Besso’s family in which he put forward a scientist’s consolation: “This is not important. For us who are convinced physicists, the distinction between past, present, and future is only an illusion, however persistent.”

The idea that time is an illusion is an old one, predating any Times Square ball drop or champagne celebrations. It reaches back to the days of Heraclitus and Parmenides, pre-Socratic thinkers who are staples of introductory philosophy courses. Heraclitus argued that the primary feature of the universe is that it is always changing. Parmenides, foreshadowing Einstein, countered by suggesting that there was no such thing as change. Put into modern language, Parmenides believed the universe is the set of all moments at once. The entire history of the universe simply is.


Today we would call this the “eternalist” or “block universe” view—thinking of space and time together as a single four-dimensional collection of events, rather than a three-dimensional world that evolves over time. Besides Parmenides and Einstein, this picture is shared by the Tralfamadorians, an alien race who appear in Kurt Vonnegut’s novel Slaughterhouse-Five. To a being from Tralfamadore, visiting the past is no harder than walking down the street.

This “timeless” view of the universe goes against our usual thinking. We perceive our lives as unfolding. But it has adherents even in contemporary physics. The laws of nature, as we currently understand them, treat all moments as equally real. No one is picked out as special; the laws simply say how any moment relates to the previous one and to the next.

Perhaps the most energetic and persistent advocate of the claim that time is illusory is the British physicist Julian Barbour.

Impressively, Barbour has managed to do interesting research in physics for decades now without any academic position, publishing dozens of papers in respected journals. He has supported himself in part by translating technical papers from Russian to English—in his spare time, tirelessly investigating the idea that time does not exist, constructing theoretical models of classical and quantum gravity in which time plays no fundamental role.

We have to be a little careful about what we mean by “time does not exist.” Even Parmenides or Barbour would acknowledge the existence of clocks, or of the concept of being late. At issue is whether each subsequent moment is brought into existence from the previous moment by the passage of time. Think of a movie, back in the days when most movies were projected from actual reels of film. You could watch the movie, see what happened and talk sensibly about how long the whole thing lasted. But you could also sneak into the projection room, assemble the reels of the film, and look at them all at once.


The anti-time perspective says that the best way to think about the universe is, similarly, as a collection of the frames.

There has, predictably, been some push back. Tim Maudlin, a philosopher, and Lee Smolin, a physicist, have argued vociferously that time is real, and that the passage of time plays what we might call a generative role: It indeed brings the future into existence. They think of time as an active player rather than a mere bookkeeping device.

Lee Smolin’s simple maxim: “There is nothing outside the universe” which he described as the “first principle of cosmology”. This means there can be no absolute coordinate system for space or time outside the universe by which object positions and times can be defined. Instead, the position of every object in the universe must be defined solely in terms of the position of other objects in the universe

Whereas traditional topology uses regions of space as fundamental building blocks, Maudlin takes worldlines (paths of particles through time) as the most basic object. From there, time evolution seems like a central feature of physics.

Both researchers have been developing new mathematical tools and physical models to buttress their views. Maudlin’s novel approach focuses on the topology of spacetime itself—how different points in the universe are sewn together.

(Credit: John D. Norton Gauge Workshop, University of Pittsburgh)
“Still, going from Mars to Earth is not the same as going from Earth to Mars. The difference, if you will, is how these sequences of states are oriented with respect to the passage of time.” Maudlin (Credit: J. D. Norton, University of Pittsburgh, Wikipedia)

Whereas traditional topology uses regions of space as fundamental building blocks, Maudlin takes worldlines (paths of particles through time) as the most basic object. From there, time evolution seems like a central feature of physics.

Smolin, in contrast, has suggested that the laws of physics themselves are evolving with time. We wouldn’t notice this from moment to moment, but over cosmological time scales, the parameters we think of as fixed may eventually take on very different values.

There is, perhaps, a judicious middle position between insisting on the centrality of time and denying its existence. Something can be real—actually existing, not merely illusory—and yet not be fundamental. Scientists used to think that heat, for example, was a fluid like substance, called “caloric,” that flowed from hot objects to colder ones.

The world’s first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat involved in various chemical changes. (Credit: Wikipedia)
The world’s first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat involved in various chemical changes. (Credit: Wikipedia)

These days we know better: Heat is simply the random motions of the atoms and molecules out of which objects are made. Heat is still real, but it’s been explained at a deeper level. It emerges out of a more comprehensive understanding.

Perhaps time is like that. Someday, when the ultimate laws of physics are in our grasp, we may discover that the notion of time isn’t actually essential. Time might instead emerge to play an important role in the macroscopic world of our experience, even if it is nowhere to be found in the final Theory of Everything.

In that case, I would have no trouble saying that time is “real.” I know what it means to grow older or to celebrate an anniversary whether or not time is “fundamental.” And either way, I can still wish people a Happy New Year in good conscience.


By Sean M. Carroll for Smithsonian Magazine

Sean M. Carroll is a research professor in physics at the California Institute of Technology. He is the author of From Eternity to Here, Spacetime and Geometry and The Particle at the End of the Universe, which won the Winton Prize from the Royal Society.

Credit: Smithsonian Magazine