Quantum Weirdness Now a Matter of Time

Bizarre quantum bonds connect distinct moments in time, suggesting that quantum links — not space-time — constitute the fundamental structure of the universe.

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Bill Domonkos for Quanta Magazine

In November, construction workers at the Massachusetts Institute of Technology came across a time capsule 942 years too soon. Buried in 1957 and intended for 2957, the capsule was a glass cylinder filled with inert gas to preserve its contents; it was even laced with carbon-14 so that future researchers could confirm the year of burial, the way they would date a fossil. MIT administrators plan to repair, reseal and rebury it. But is it possible to make it absolutely certain that a message to the future won’t be read before its time?

Quantum physics offers a way. In 2012, Jay Olson and Timothy Ralph, both physicists at the University of Queensland in Australia, laid out a procedure to encrypt data so that it can be decrypted only at a specific moment in the future. Their scheme exploits quantum entanglement, a phenomenon in which particles or points in a field, such as the electromagnetic field, shed their separate identities and assume a shared existence, their properties becoming correlated with one another’s. Normally physicists think of these correlations as spanning space, linking far-flung locations in a phenomenon that Albert Einstein famously described as “spooky action at a distance.” But a growing body of research is investigating how these correlations can span time as well. What happens now can be correlated with what happens later, in ways that elude a simple mechanistic explanation. In effect, you can have spooky action at a delay.

These correlations seriously mess with our intuitions about time and space. Not only can two events be correlated, linking the earlier one to the later one, but two events can become correlated such that it becomes impossible to say which is earlier and which is later. Each of these events is the cause of the other, as if each were the first to occur. (Even a single observer can encounter this causal ambiguity, so it’s distinct from the temporal reversals that can happen when two observers move at different velocities, as described in Einstein’s special theory of relativity.)

Space-time might not be a God-given backdrop to the world, but instead might derive from the material contents of the universe.

 The time-capsule idea is only one demonstration of the potential power of these temporal correlations. They might also boost the speed of quantum computers and strengthen quantum cryptography.

But perhaps most important, researchers hope that the work will open up a new way to unify quantum theory with Einstein’s general theory of relativity, which describes the structure of space-time. The world we experience in daily life, in which events occur in an order determined by their locations in space and time, is just a subset of the possibilities that quantum physics allows. “If you have space-time, you have a well-defined causal order,” said Časlav Brukner, a physicist at the University of Vienna who studies quantum information. But “if you don’t have a well-defined causal order,” he said — as is the case in experiments he has proposed — then “you don’t have space-time.” Some physicists take this as evidence for a profoundly nonintuitive worldview, in which quantum correlations are more fundamental than space-time, and space-time itself is somehow built up from correlations among events, in what might be called quantum relationalism. The argument updates Gottfried Leibniz and Ernst Mach’s idea that space-time might not be a God-given backdrop to the world, but instead might derive from the material contents of the universe.

How Time Entanglement Works

To understand entanglement in time, it helps to first understand entanglement in space, as the two are closely related. In the spatial version of a classic entanglement experiment, two particles, such as photons, are prepared in a shared quantum state, then sent flying in different directions. An observer, Alice, measures the polarization of one photon, and her partner, Bob, measures the other. Alice might measure polarization along the horizontal axis while Bob looks along a diagonal. Or she might  choose the vertical angle and he might measure an oblique one. The permutations are endless.

The outcomes of these measurements will match, and what’s weird is that they match even when Alice and Bob vary their choice of measurement — as though Alice’s particle knew what happened to Bob’s, and vice versa. This is true even when nothing connects the particles — no force, wave or carrier pigeon. The correlation appears to violate “locality,” the rule that states that effects have causes, and chains of cause and effect must be unbroken in space and time.

In the temporal case, though, the mystery is subtler, involving just a single polarized photon. Alice measures it, and then Bob remeasures it. Distance in space is replaced by an interval of time. The probability of their seeing the same outcome varies with the angle between the polarizers; in fact, it varies in just the same way as in the spatial case. On one level, this does not seem to be strange. Of course what we do first affects what happens next. Of course a particle can communicate with its future self.

The strangeness comes through in an experiment conceived by Robert Spekkens, a physicist who studies the foundations of quantum mechanics at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. Spekkens and his colleagues carried out the experiment in 2009. Alice prepares a photon in one of four possible ways. Classically, we could think of these four ways as two bits of information. Bob then measures the particle in one of two possible ways. If he chooses to measure the particle in the first way, he obtains Alice’s first bit of information; if he chooses the second, he obtains her second bit. (Technically, he does not get either bit with certainty, just with a high degree of probability.) The obvious explanation for this result would be if the photon stores both bits and releases one based on Bob’s choice. But if that were the case, you’d expect Bob to be able to obtain information about both bits — to measure both of them or at least some characteristic of both, such as whether they are the same or different. But he can’t. No experiment, even in principle, can get at both bits — a restriction known as the Holevo bound. “Quantum systems seem to have more memory, but you can’t actually access it,” said Costantino Budroni, a physicist at the University of Siegen in Germany.

The photon really does seem to hold just one bit, and it is as if Bob’s choice of measurement retroactively decides which it is. Perhaps that really is what happens, but this is tantamount to time travel — on an oddly limited basis, involving the ability to determine the nature of the bit but denying any glimpse of the future.

Another example of temporal entanglement comes from a team led by Stephen Brierley, a mathematical physicist at the University of Cambridge. In a paper last year, Brierley and his collaborators explored the bizarre intersection of entanglement, information and time. If Alice and Bob choose from just two polarizer orientations, the correlations they see are readily explained by a particle carrying a single bit. But if they choose among eight possible directions and they measure and remeasure the particle 16 times, they see correlations that a single bit of memory can’t explain. “What we have proven rigorously is that, if you propagate in time the number of bits that corresponds to this Holevo bound, then you definitely cannot explain what quantum mechanics predicts,” said Tomasz Paterek, a physicist at Nanyang Technological University in Singapore, and one of Brierley’s co-authors. In short, what Alice does to the particle at the beginning of the experiment is correlated with what Bob sees at the end in a way that’s too strong to be easily explained. You might call this “supermemory,” except that the category of “memory” doesn’t seem to capture what’s going on.

What exactly is it about quantum physics that goes beyond classical physics to endow particles with this supermemory? Researchers have differing opinions. Some say the key is that quantum measurements inevitably disturb a particle. A disturbance, by definition, is something that affects later measurements. In this case, the disturbance leads to the predicted correlation.

In 2009 Michael Goggin, a physicist who was then at the University of Queensland, and his colleagues did an experiment to get at this issue. They used the trick of spatially entangling a particle with another of its kind and measuring that stand-in particle rather than the original. The measurement of the stand-in still disrupts the original particle (because the two are entangled), but researchers can control the amount that the original is disrupted by varying the degree of entanglement. The trade-off is that the experimenter’s knowledge of the original becomes less reliable, but the researchers compensate by testing multiple pairs of particles and aggregating the results in a special way. Goggin and his team reduced the disruption to the point where the original particle was hardly disturbed at all. Measurements at different times were still closely correlated. In fact, they were even more closely correlated than when the measurements disturbed the particle the most. So the question of a particle’s supermemory remains a mystery. For now, if you ask why quantum particles produce the strong temporal correlations, physicists basically will answer: “Because.”

Quantum Time Capsules

Things get more interesting still — offering the potential for quantum time capsules and other fun stuff — when we move to quantum field theory, a more advanced version of quantum mechanics that describes the electromagnetic field and other fields of nature. A field is a highly entangled system. Different parts of it are mutually correlated: A random fluctuation of the field in one place will be matched by a random fluctuation in another. (“Parts” here refers both to regions of space and to spans of time.)

Even a perfect vacuum, which is defined as the absence of particles, will still have quantum fields. And these fields are always vibrating. Space looks empty because the vibrations cancel each other out. And to do this, they must be entangled. The cancellation requires the full set of vibrations; a subset won’t necessarily cancel out. But a subset is all you ever see.

If an idealized detector just sits in a vacuum, it will not detect particles. However, any practical detector has a limited range. The field will appear imbalanced to it, and it will detect particles in a vacuum, clicking away like a Geiger counter in a uranium mine. In 1976 Bill Unruh, a theoretical physicist at the University of British Columbia, showed that the detection rate goes up if the detector is accelerating, since the detector loses sensitivity to the regions of space it is moving away from. Accelerate it very strongly and it will click like mad, and the particles it sees will be entangled with particles that remain beyond its view.

In 2011 Olson and Ralph showed that much the same thing happens if the detector can be made to accelerate through time. They described a detector that is sensitive to photons of a single frequency at any one time. The detector sweeps through frequencies like a police radio scanner, moving from lower to higher frequencies (or the other way around). If it sweeps at a quickening pace, it will scan right off the end of the radio dial and cease to function altogether. Because the detector works for only a limited period of time, it lacks sensitivity to the full range of field vibrations, creating the same imbalances that Unruh predicted. Only now, the particles it picks up will be entangled with particles in a hidden region of time — namely, the future.

“We cannot really explain these correlations,” said Baumeler. “They don’t really fit into our notion of space-time.”

Olson and Ralph suggest constructing the detector from a loop of superconducting material. Tuned to pick up near-infrared light and completing a scan in a few femtoseconds (10–15 second), the loop would see the vacuum glowing like a gas at room temperature. No feasible detector accelerating through space could achieve that, so Olson and Ralph’s experiment would be an important test of quantum field theory. It could also vindicate Stephen Hawking’s ideas about black-hole evaporation, which involve the same basic physics.

If you build two such detectors, one that accelerates and one that decelerates at the same rate, then the particles seen by one detector will be correlated with the particles seen by the other. The first detector might pick up a string of stray particles at random intervals. Minutes or years later, the second detector will pick up another string of stray particles at the same intervals — a spooky recurrence of events. “If you just look at them individually, then they’re randomly clicking, but if you get a click in one, then you know that there’s going to be a click in the other one if you look at a particular time,” Ralph said.

These temporal correlations are the ingredients for that quantum time capsule. The original idea for such a contraption goes back to James Franson, a physicist at the University of Maryland, Baltimore County. (Franson used spacelike correlations; Olson and Ralph say temporal correlations may make it easier.) You write your message, encode each bit in a photon, and use one of your special detectors to measure those photons along with the background field, thus effectively encrypting your bits. You then store the outcome in the capsule and bury it.

At the designated future time, your descendants measure the field with the paired detector. The two outcomes, together, will reconstitute the original information. “The state is disembodied for the time between [the two measurements], but is encoded somehow in these correlations in the vacuum,” Ralph said. Because your descendants must wait for the second detector to be triggered, there’s no way to unscramble the message before its time.

The same basic procedure would let you generate entangled particles for use in computation and cryptography. “You could do quantum key distribution without actually sending any quantum signal,” Ralph said. “The idea is that you just use the correlations that are already there in the vacuum.”

The Nature of Space-Time

These temporal correlations are also challenging physicists’ assumptions about the nature of space-time. Whenever two events are correlated and it’s not a fluke, there are two explanations: One event causes the other, or some third factor causes both. A background assumption to this logic is that events occur in a given order, dictated by their locations in space and time. Since quantum correlations — certainly the spatial kind, possibly the temporal — are too strong to be explained using one of these two explanations, physicists are revisiting their assumptions. “We cannot really explain these correlations,” said Ämin Baumeler, a physicist at the University of Italian Switzerland in Lugano, Switzerland. “There’s no mechanism for how these correlations appear. So, they don’t really fit into our notion of space-time.”

Building on an idea by Lucien Hardy, a theoretical physicist at the Perimeter Institute, Brukner and his colleagues have studied how events might be related to one another without presupposing the existence of space-time. If the setup of one event depends on the outcome of another, you deduce that it occurs later; if the events are completely independent, they must occur far apart in space and time. Such an approach puts spatial and temporal correlations on an equal footing. And it also allows for correlations that are neither spatial nor temporal — meaning that the experiments don’t all fit together consistently and there’s no way to situate them within space and time.

Brukner’s group devised a strange thought experiment that illustrates the idea. Alice and Bob each toss a coin. Each person writes the result of his or her own toss on a piece of paper, along with a guess for the other person’s outcome. Each person also sends the paper to the other with this information. They do this a number of times and see how well they do.

Normally the rules of the game are set up so that Alice and Bob do this in a certain sequence. Suppose Alice is first. She can only guess at Bob’s outcome (which has yet to occur), but she can send her own result to Bob. Alice’s guess as to Bob’s flip will be right 50 percent of the time, but he will always get hers right. In the next round, Bob goes first, and the roles are reversed. Overall the success rate will be 75 percent. But if you don’t presume they do this in a certain sequence, and if they replace the sheet of paper with a quantum particle, they can succeed 85 percent of the time.

If you try to situate this experiment within space and time, you’ll be forced to conclude that it involves a limited degree of time travel, so that the person who goes second can communicate his or her result backward in time to the one who goes first. (The Time Patrol will be relieved that no logical paradoxes can arise: No event can become its own cause.)

Brukner and his colleagues at Vienna have performed a real-world experiment that is similar to this. In the experiment, Alice-and-Bob manipulations were carried out by two optical filters. The researchers beamed a stream of photons at a partially silvered mirror, so that half the photons took one path and half another. (It was impossible to tell, without measuring, which path each individual photon went down; in a sense, it took both paths at once.) On the first path, the photons passed through Alice’s filter first, followed by Bob’s. On the second path, the photons navigated them in reverse order. The experiment took quantum indeterminacy to a whole new level. Not only did the particles not possess definite properties in advance of measurement, the operations performed on them were not even conducted in a definite sequence.

On a practical level, the experiment opens up new possibilities for quantum computers. The filters corresponding to Alice and Bob represent two different mathematical operations, and the apparatus was able to ascertain in a single step whether the order of those operations matters — whether A followed by B is the same as B followed by A. Normally you’d need two steps to do that, so the procedure is a significant speedup. Quantum computers are sometimes described as performing a series of operations on all possible data at once, but they might also be able to perform all possible operations at once.

Now imagine taking this experiment a step further. In Brukner’s original experiment, the path of each individual photon is placed into a “superposition” — the photon goes down a quantum combination of the Alice-first path and the Bob-first path. There is no definite answer to the question, “Which filter did the photon go through first?”— until a measurement is carried out and the ambiguity is resolved. If, instead of a photon, a gravitating object could be put into such a temporal superposition, the apparatus would put space-time itself into a superposition. In such a case, the sequence of Alice and Bob would remain ambiguous. Cause and effect would blur together, and you would be unable to give a step-by-step account of what happened.

Only when these indeterminate causal relations between events are pruned away — so that nature realizes only some of the possibilities available to it — do space and time become meaningful. Quantum correlations come first, space-time later. Exactly how does space-time emerge out of the quantum world? Brukner said he is still unsure. As with the time capsule, the answer will come only when the time is right.

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  • Have you read Wolfram's thoughts on space-time as a network?
    Seems like what he's describing would fit well with these observations.

  • @Duncan: Thanks for your remarks. I do discuss the idea of space as a network in my recent book, based on work on quantum gravity, though not Wolfram's ideas per se.

  • As a Science enthusiast, but only a physician, the entanglement conundrum has kept me hoping for a better understanding of the universe. I'm sitting in the bench-warming area, cheering for the team players — in "unpredictable" sequence. 🙂

  • If this was proven to be true (which I believe is the case) than it would show that all time (and space) exists all at once. This would imply that future and past all exist at once and affect each other forwards or backwards in time meaning that something you are about to do was affected by something that already happened (what we usually experience) and something that you already did was affected by what is happening right now(this will be troubling for some to comprehend). This can be manipulated to satisfy all 8 possible combinations of past, present, and future. It is as if future and past events collapse into a single moment that leads to now. Memory would be like a worm hole to the past events and plans or predictions would be a worm hole into the future. The future becomes the now when all valid information received proves that this is a future event that became now. All the information that we are able to retain (in memory or in any form) will become the past. The now would be the purest form of information. Thus the limit to 100 %prediction of past and present and the now is memory.

  • Spectacular article. I feel that it completes the image of a singular phenomenon from different angles, a phenomenom made of parts such as the photons that interact with themselves. We may call it the superposition or coherence. The correlations feel like the coordination among the photon paths. The unavoidable self-interaction is perhaps what comes before the foundational step at which the spacetime appears as the separation between the photons and between the other entities.

  • I'm not a physicist!
    But absolute fascinated with what has been occurring on the physics's world!
    Great article!
    And, please keep up this great work of give us in tune !
    Best wishes,

  • I assume it's already given that we can encrypt information in a similar fashion to be read only in a specific space, but would it be possible to somehow encrypt information that can only be read in a specific space at a specific time?

  • My son sent me this article, and I read it with surely more delight than comprehension. And it has led to my recently subscribing to Quanta. I devoutly believe in entanglement, but I have still been bothered about how/why the entanglement of two widely separated particles can somehow instantly permit effects over any enormous distance between them. Sure, this happens because the particles are in the same quantum state. But lacking an understanding of what quantum mechanics (including entanglement) is really telling us about the nature of Reality, we can only ask what my question really "means"? The implication in the work described in this excellent article that entanglement may be more fundamental than space (or time) strongly reduces my anxiety about this question, since this will probably be resolved when we understand how space-time emerges from entanglement.

  • If there was a big bang event because of which all matter resulted, were all of the particles that formed entangled with one another, resulting from the same quantum event? And does any part of that entanglement continue when those particles are involved in subsequent quantum events?

  • Time present and time past
    Are both perhaps present in time future
    And time future contained in time past.
    If all time is eternally present
    All time is unredeemable……
    What might have been and what has been
    Point to one end, which is always present.

    T.S. Eliot Burnt Norton (Four Quartets)

  • it would seem to me that Brukner's: “If you have space-time, you have a well-defined causal order,” said Časlav Brukner, a physicist at the University of Vienna who studies quantum information. But “if you don’t have a well-defined causal order,” he said — as is the case in experiments he has proposed — then “you don’t have space-time.” is just a demonstration that "spacetime" and "causal order" are co-terminus; ie, they share a common boundary, in regard to which they are (in that sense) equivalent. For logically speaking, it is quite possible that, were the concept of causal order contained within the logic of spacetime, that there could be examples of noncausally-ordered systems which nevertheless lay within the logic of spacetime. Brukner says that the two realms are coterminous; you cant have one without the other. Surely Whitehead discusses this; I would start there in sorting out what is at philosophical stake here.

  • “What we have proven rigorously is that, if you propagate in time the number of bits that corresponds to this Holevo bound, then you definitely cannot explain what quantum mechanics predicts…"

    Wow. Not only counterintuitive but unexplainable by definition. Wow. Physics is getting more Alice-in-Wonderland all the time. What next?

  • Mathematically, what this suggests is that some elements of a matrix are coupled, such as in a tensor.

  • How does this notion of quantum time entanglement relate to our confidence in the operation of the second law of thermodynamics?

  • I am not a physicist or a scientist but these articles continue to intrigue me as they reveal the complexity of existence. I am a painter and potter and also a student of literature. It is amusing as I plough through Proust's Remembrance to see how the imaginative mind can suggest the ideas that science unearths. There may be a worm hole that Matcel Proust traveled thru that suggested to him the circularity of time. Thanks for the wonderful articles that help me begin to know what is going on in the quest to know what is going on.

  • "Building on an idea by Lucien Hardy, a theoretical physicist at the Perimeter Institute, Brukner and his colleagues have studied how events might be related to one another without presupposing the existence of space-time." Normally, an "event" indicates that something has happened, i.e., that the state of something is different than it was an instant before. If you do not presuppose the existence of space-time, would this not imply that you also do not presuppose the sequential passage of time? If so, then what constitutes an "event" in this formalism?

  • @Wil R: An "event" generalizes the idea of location to spacetime. It is a place in space at a moment in time. It does not, on its own, presuppose a change or occurrence. Hardy's operational framework does have a localized notion of time, though: it talks about experiments, which are by their very nature a process involving a setup and an outcome. But the framework doesn’t presume how different experiments relate to one another.

  • "clicking away like a Geiger counter in a uranium mine"
    Unrelated comment to the fascinating main question, but I hope everyone is aware that any reasonably sensitive Geiger counter clicks away all the time, everywhere on Earth. Radiation is not confined to exotic places like mines or nuclear facilities

  • "You write your message, encode each bit in a photon, and use one of your special detectors to measure those photons along with the background field, thus effectively encrypting your bits. You then store the outcome in the capsule and bury it.

    At the designated future time, your descendants measure the field with the paired detector."

    I would love to know how exactly you decide which encryption 'belongs' to the time you want to specify as the point at which the message can be decrypted. Does that not mean you need to look into the future to know what encryption to use ?

  • The Wheeler-DeWitt equation shows that Time is not real, but only a metrical record (past), or projection (future), based on the current state of the universe. Minkowski's "loaf" (whereby all time and space exist simultaneously) is a misguided fantasy. "Time" manifests as the description of changes in relative positions of charge interactions at the quantum scale and thus is "inextricably" bound up with the notion of space, as Einstein stated. Information reception rates based upon different relative velocities of observers viewing a light beam from different positions will clarify the dilation in mass, length and time associated with the special relativistic transformation.

  • I believe Jim Farned's comment above (January 22, 2016 at 9:41 pm) is incorrect. What Brukner is saying is no more than the observation: "if p then q" is logically equivalent to "if not-q then not-p."

    For example, having a well-defined causal order can be true and having space-time could be false, yet both statements "if you have space-time, you have a well-defined causal order" and "if you don't have a well-defined causal order, you don't have space-time" will still be logically true. So space-time and a well-defined causal order are not the same thing in general.

  • Here is another (possibly related) and published explanation for the speedup of quantum computers over classical computers:

  • @Bob Collins: In my reading, given the “logic” of spacetime, it is possible to define both causally
    and non-causally ordered systems. Sort of like, given the logic of “geometry”, it is possible to define both euclidean and non-euclidean geometries. Thus, in my reading, the traditional usage “logically implies” (necessitates) should be rendered with “logically permits” (allows a consistent definition of). Thus, when Brukner says: 'when you've got spacetime you have causal ordering', I understand him as saying 'when you've got the logic necessary to talk consistently about spacetime, you automatically have what you need to talk about causal orderings”. You may have all you need to talk about non-causal ordering as well; ie, that given the logical underpinnings for discourse of of spacetime, it is possible to define both causal and non-causal orderings. Thus, that both causal and non-causal orderings are “permitted” (ie, are definable) in the logic of spacetime.. I think these questions might be approached in socalled “model theory”, wherein the famed Lowenheim-Scholem theorem regarding nonstandard models lurks.öwenheim–Skolem_theorem

  • What if waves existed in the fourth dimension … Wouldn't that explain these effects without requiring travel backwards in time? Perhaps the same information would just repeat whenever the wave crested (or whatever waves would do in the 4th dimension)? And why wouldn't waves exist in dimensions 4-10, simply not in a way in which the human mind could ever fully understand? I see that the author says that the particles aren't connected by a wave, but this type of wave would not be observable …

  • Wouldn't it be possible to communicate from the future into the past?

    To use the time capsule example. When someone uses the Geiger counter to decrypt the information in the future, couldn't they alter they alter the particle in some manner, which would also alter the particle in the past?

  • From a position of neurological solipsism both space and time are part of our brain function. Take sight for example: photons are reflected from an object's surface, go into the eyes and the resultant neurological processing creates the best example of virtual reality we have. The brain constructs from and in itself the 'external' visual world that we experience. This process is a one way street. Everything we see, including the space in which it is placed, is internal to the brain. Our sensory systems all work like this. So, as everything you see in the world is inside your head (including your head) things do not really have a distance between them and you because they are you. If distance in our individual neurological reality is not real then how long does it take for an object to travel a mile. Turtles all the way down. A real external reality that is external to this creative process is unknowable directly.

  • Perhaps our very notions of "past" and "future" are but illusions. What if all that exists (or can ever exist) is the "now"? What we call the "past" is merely a previous version of "now" that no longer exists, whilst what we call the "future" is a version of "now" that has yet to come into effect. The "Now" (big 'n') encompasses all 3 versions of "now" (small 'n') I.E. what we usually term "past", "present" and "future" are actually 3 flavours of the same "Now" that always was. In this contect it would be hardly surprising that the Now would be influenced by all 3 nows.

  • But is it possible to make it absolutely certain that a message to the future won’t be read before its time?
    Yes send it into space on an orbit that matches the length of time required.

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