MIT Department of Mechanical Engineering

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Draper

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 Do you feel stuck in space and time? Do you want to fast forward to the future, replay the past—be anywhere but here and anywhen but now? Does your work situation seem like a black hole, sucking you remorselessly towards a point where your consciousness is squished into nothingness along with all the known laws of physics? I’m with you on all counts, if only because this article was due last week!
 

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 src="https://www.pbs.org/wgbh/nova/media/images/clocks_510.width-800.jpg"/><figcaption>
 Credit: Floriana/iStock
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 Luckily for you and me, these are good days for teleporting out of here, traveling through time, and escaping from black holes. Hard as it is to bend our minds about such non-standard forms of transportation, the laws of physics allow and even encourage them. The secret lies in a feature of quantum mechanics called entanglement. Two quantum systems are said to be entangled when they have more information about each other than they can have classically: they “know” more about each other than they have any right to know. Entangled systems possess a kind of quantum intimacy that goes beyond anything that is allowed by the classical laws of physics.
 


 Teleportation
 


 Entanglement gives rise to what Einstein called “spooky action at a distance” (
 
 spukhafte Fernwirkung
 
 ), where an action performed on one quantum system seems to have an instantaneous effect on an entangled system. Under normal circumstances, quantum spooky action at a distance doesn’t allow one to communicate instantaneously, but it does allow the quantum effect known as teleportation. Teleportation is a quantum version of the process familiar from Star Trek: the person/molecule/atom to be teleported dematerializes here, and rematerializes over there. For a long time, scientists thought that quantum mechanics didn’t allow teleportation, because if you make measurements on the system to be teleported, quantum mechanics guarantees that those measurements are both destructive and incomplete: They destroy features of the state of the measured system, and they cannot reveal the full quantum state of the system to be teleported. As a result, physicists reasoned, accurate teleportation of any quantum system was impossible.
 

 In 1993, however, a group of quantum physicists realized that entanglement allows one to teleport a quantum system even though measurement is destructive and incomplete. The protocol works as follows. Alice wants to teleport an electron to Bob. Suppose that in addition to the electron that Alice is going to teleport, Alice and Bob share a pair of entangled electrons. Alice makes a measurement on the electron she wants to teleport, as well as her half of the entangled pair. She sends the result of the measurement to Bob. Now, even though Alice’s measurement is completely destructive and, taken on its own, reveals no information about the state of the electron that she wishes to teleport, the result of the measurement contains exactly the information that Bob needs to recreate the original electron from his half of the
 
 entangled pair. (A more detailed description of how quantum teleportation
 
 works can be found
 <a href="http://researcher.watson.ibm.com/researcher/view\_project.php?id=2862">
 here
 </a>
 .)
 

 Shortly after it was proposed, experimentalists made quantum teleportation a reality, teleporting particles of light, or photons, and even larger stuff, like electrons or atoms, over distances which now range beyond a hundred kilometers. Really large quantum systems, like human beings, have proved harder to teleport.
 


 Escape from black holes
 


 Teleportation may be an unorthodox mode of traveling, but in one sense, it is quite conventional: It does not allow one to go faster than the speed of light, because Alice has to send the results of her measurement to Bob in order for him to recreate the original system. Under ordinary conditions, that information can travel no faster than the speed of light. The interior of a black hole does not represent ordinary conditions, however. At the center of the black hole is a singularity, a place where all the known laws of physics break down, and infalling matter and energy are squished to nothingness. Because of the breakdown of known physics at the singularity, we don’t know what happens there. While some might regard such ignorance as an impediment, to a scientist, it represents an opportunity: Since we don’t know what happens at the singularity of a black hole, we are free to postulate any dynamics that we like. That is exactly what theorists Gary Horowitz and Juan Maldacena did to construct
 <a href="http://arxiv.org/abs/hep-th/0310281">
 a theory of escape
 </a>
 from black holes based on quantum teleportation.
 

 Without quantum mechanics, when you fall into a black hole you are doomed: You will be sucked into the singularity in a time no longer than twice the radius of the black hole divided by the speed of light. Before you hit the singularity, you will be ripped apart from tidal forces. Bummer. You might think that a desperate blast on your rockets might at least slow down the inexorable descent, but it turns out that the theory of relativity implies that fighting to blast your way out of a black hole is counterproductive: trying to accelerate away from the singularity actually means it takes you
 
 less
 
 time to arrive there. Black holes are like quicksand: If you fall in, don’t struggle.
 

 With quantum mechanics, however, there is a faint hope. In 1974, Steven Hawking showed that black holes emit a faint, ethereal form of radiation.
 <a href="http://www.pbs.org/wgbh/nova/physics/blog/2013/12/do-black-holes-destroy-information/">
 Just maybe
 </a>
 , the Hawking radiation could contain the bits of information required to assemble any lost travelers who ventured too close to the hole.
 

 How could that be? Hawking radiation consists of entangled pairs of particles, a negative energy particle that falls into the hole, thereby reducing the hole’s mass, and a positive energy particle that escapes off to infinity. So if a would-be teleporter inside the hole were able to make a measurement on you together with the infalling Hawking radiation, and could send the results of that measurement outside of the hole, then that information could be used to recreate you out of the outgoing Hawking radiation. Unfortunately, to send the results of the measurement out of the hole requires faster-than-light communication, so you are still stuck. But there is a way out. Horowitz and Maldacena invite us to consider that the process of being smooshed into nothingness at the singularity is effectively such a measurement, but in contrast to the measurements made in teleportation, which probabilistically yield different outcomes, in the Horowitz-Maldacena model, the measurement made by the singularity always yields the same outcome. Your rescuers outside the hole can therefore recreate you by the same process as in teleportation. You are saved! Although you are still likely to feel a bit smooshed.
 


 Time travel
 


 To escape from a black hole, you must effectively travel faster than the speed of light. But as everyone knows, if you can travel faster than the speed of light, you can also go backwards in time. Althoughit sounds like science fiction, time travel is actually allowed in Einstein’s theory of general relativity: space times can possess closed timelike curves which you can enter in the future and exit in the past. In 2009, my colleagues and I showed that the quantum mechanics of closed timelike curves was essentially the same as that of teleportation and escape from black holes. In addition to providing a novel theory of quantum time travel, we performed an experiment that was the moral equivalent of the famous grandfather paradox of time travel: we sent a photon a few billionths of a second back in time and had it try to kill its former self. What happened? Well, let’s just say that our experiment was not like one of the movies where they say at the end, “No animals were harmed during the making of this movie.” Gajillions of photons died. Luckily there is no society for prevention of cruelty to photons—yet. Ironically, however, the one photon we sent back to perform auto-homocide failed to off its former self.
 

 How exactly did we send the photons back in time? Like escape from a black hole, travel through a closed timelike curve is based on teleportation—in this case, teleportation from the future to the past. Recall that in the Horowitz-Maldacena model, the singularity effectively performs a measurement on you and the infalling Hawking radiation just as you are being smooshed into nothingness. A similar effect occurs at the future entrance to a closed timelike curve: As you enter the curve, you disappear from view of observers in the “normal” spacetime outside the entrance. So far as they are concerned, you are disappearing into nothingness. In our model, as you disappear, the closed timelike curve effectively performs a measurement on you along with curve’s analog of Hawking radiation, called horizon radiation. Just as the effect of the measurement at the singularity of a black hole causes you to reappear outside the hole, the effect of the measurement at the entrance to the closed timelike curve causes you to reappear at the exit to the curve in the past.
 

 Actually teleporting something to the past with certainty would require a true closed timelike curve. (Or maybe a pair of black holes!) Unfortunately, the department of workplace safety at MIT wouldn’t let us construct such an object in the laboratory. At University of Toronto, however, Aephraim Steinberg’s group was able to do the next best thing: perform a teleportation experiment using entangled photons in which some fraction of the time one of the entangled photons in the past was identical to the time traveling photon in the future. The photon returning from the future was tasked with trying to prevent its former self from entering the teleportation device using a device called a photon gun, which was pointed closer and closer at the photon in the past. But the photon from the future couldn’t prevent the photon from the past from performing the teleportation, no matter how directly the photon gun was pointed at its past self. That is, no matter how hard it tried, the photon couldn’t kill its former self. The closer it got to killing itself, the less and less likely the teleporation was to succeed. For a detailed account of the quantum theory of time travel and the results of our experiment, see
 <a href="http://arxiv.org/pdf/1005.2219.pdf">
 here
 </a>
 .
 

 Quantum mechanics is famously weird, and quantum weirdness opens up opportunities for funky methods for getting from point A to point B, even if point A is inside a black hole, and point B is outside, or point A lies in the present, and point B is in the past. To date, only teeny things have been teleported and effectivelysent backwards in time. Since big things are made of teeny things, they too obey the laws of quantum mechanics, and are candidates for funky quantum transportation. In the not so distant future, we too will be quantum commuters.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 NOVA:
 <a href="http://www.pbs.org/wgbh/nova/physics/fabric-of-cosmos.html#fabric-quantum">
 Fabric of the Cosmos: Quantum Leap
 </a>
 
 Explore entanglement and teleportation in this episode from the NOVA series Fabric of the Cosmos.
 

 NOVA:
 <a href="http://www.pbs.org/wgbh/nova/physics/spooky-action-distance.html">
 Spooky Action at a Distance
 </a>
 
 In this essay, physicist Brian Greene explores the history of quantum entanglement, and explains why Einstein found it so “spooky.”
 

 The Open University:
 <a href="http://www.youtube.com/watch?v=Y6RjjaEy59I">
 The Grandfather Paradox – 60-Second Adventures in Thought
 </a>
 
 This this brief animated video, discover the history of the grandfather paradox of time travel.
 
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Quantum entanglement may make it possible to travel through time, escape from a black hole, and teleport across space.

Trap Doors in Time and Space: Teleportation, Time Travel, and Escape from Black Holes

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 A quantum computer is a serious piece of hardware. My colleagues and I build quantum computers from superconducting systems, quantum dots, lasers operating on nonlinear crystals, and the like. Although the part of a quantum computer that actually performs the calculation is too small to be seen even under a microscope, the apparatus used to address and control the quantum computer typically takes up an entire laboratory full of equipment. In order to keep their sensitive components shielded from the environment, many quantum computers have to operate at very low temperatures, sometimes a few thousandths of a degree above absolute zero.
 

 So in the spring of 2007 when the New York Times reported that green sulphur-breathing bacteria were performing quantum computations during photosynthesis, my colleagues and I laughed. We thought it was the most crackpot idea we had heard in a long time. Closer examination of the
 <a href="http://www.nature.com/nature/journal/v446/n7137/full/nature05678.html">
 paper
 </a>
 , published in Nature, however, showed that something decidedly non-crackpot was going on.
 

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 Photosynthesis converts light from the Sun into chemically useful energy inside cells. In photosynthesis, particles of light called photons are absorbed by light-sensitive molecules called chromophores (“light carriers” in ancient Greek), which are arranged in a tightly bound structure called an antenna photocomplex. When a photon is absorbed, a quantum particle of energy called an exciton is generated. (An exciton isn’t a particle in the traditional sense, but it acts enough like a particle that physicists find it useful to treat it as one. Such mathematical likenesses are called “quasi-particles.”) The exciton hops from chromophore to chromophore inside the photocomplex until it arrives at the reaction center, an agglomeration of molecules that take in the exciton and transform its energy into a form that the living system can put to use to perform cellular metabolism, grow, and reproduce. The great majority of the energy used by living systems once came from photosynthesis: Every calorie that you consume came originally from excitons that hopped through the antenna photocomplex of a photosynthetic organism.
 

 By zapping complexes of photosynthetic molecules with lasers, the authors of the paper were able to show that the excitons use quantum mechanics to make their journey through the photocomplex more efficient. The experimental evidence was strong and compelling. The authors also speculated that the excitons were performing a particular quantum computation algorithm called a quantum search, in which the wave-like nature of propagation allows the excitons to zero in on their target. As it turns out, the excitons were performing a different kind of quantum algorithm called a quantum walk, but the “crackpot” fact remained: Quantum computation was helping the bacteria move energy from point A to point B.
 

 How could tiny bacteria be performing the kind of sophisticated quantum manipulations that it takes human beings a room full of equipment to perform? Natural selection is a powerful force. Photosynthetic bacteria have been around for more than a billion years, and during that time, if a little quantum hanky panky allowed some bacteria to process energy and reproduce more efficiently than other bacteria, then quantum hanky panky stuck around for the next generation. Nature is also the great nanotechnologist. Living systems operate on the basis of molecular mechanisms, where atoms and energy are channeled systematically through molecular complexes within the cell. The molecules in turn are assembled using the laws of quantum mechanics—quantum weirdness is always lurking just around the chemical corner. These quantum changes can either help or hinder energy transport. Natural selection ensures that the role of quantum weirdness in cellular energy transport is a beneficial one.
 

 How can quantum weirdness assist in energy transport? The answer lies in a phenomenon called wave-particle duality. Wave-particle duality means that waves—light and sound—are at bottom composed of particles—photons and phonons. Conversely, things that we think of as particles, such as electrons, atoms, or for that matter soccer balls, have waves associated with them.
 

 The quantum wave of a soccer ball is about the same size as the ball itself, and doesn’t extend halfway down the soccer field (although the ball can sometimes seem to be on Lionel Messi’s left and right foot at the same time). But the wave corresponding to a particle can be much larger than the particle itself. While a single exciton consists of an excited electron within a chromophore, the wave corresponding to a propagating exciton can extend over many chromophores.
 

 Does that make sense? Of course not! Quantum mechanics is fundamentally strange and counterintuitive: Quantum particles don’t behave like soccer balls.
 

 To see how the wavelike nature of excitons can assist in their propagation, first visualize a classical kind of exciton dynamics. Imagine that each chromophore is a lilypad, and the exciton is a frog hopping randomly between neighboring lilypads. The frog starts at the edge of the circular lilypond. How long does it take to get to the pond’s center? Because the frog is hopping from lilypad to lilypad at random, it sometimes moves towards the center of the pond, but it is equally likely to move to the left or right, or even backward. The frog will land on a substantial fraction of all the lilypads in the pond on its way to the center. The number of hops the frog has to take to get to the center is proportional to the number of lilypads in the pond.
 

 Now consider a quantum frog. The frog’s initial wave is circularly symmetric at the edge of the lilypond and propagates inward, like a backward version of the wave created when you drop a stone in the center of a pond. The time it takes for the wave to travel from the shore to the center of the pond is proportional to the radius of the lilypond. But the radius of the lilypond goes as the
 
 square root
 
 of the number of lilypads in the pond, because the number of lilypads is proportional to the area of the pond, i.e., the radius squared. The wavelike nature of propagation in quantum mechanics—the “quantum hop”—has the potential to get the frog to the center of the pond much more quickly than the “classical hop.” So, for example, if the frog hops once a minute, and a classical frog takes 100 minutes to get to the center, then the quantum frog takes only 10 minutes.
 

 In green sulphur-breathing bacteria, the antenna photocomplex through which the excitons propagate is like the lilypond for the quantum frog: The waves corresponding to the excitons are spread out over many chromophores, and wavelike propagation allows excitons to move more quickly from chromophore to chromophore than classical hopping would allow.
 

 Together with Alan Aspuru-Guzik and Patrick Rebentrost at Harvard, my MIT colleague Masoud Mohseni and I constructed a
 <a href="http://arxiv.org/abs/0805.2741">
 general theory of how quantum walks in photosynthesis can use the wavelike nature of quantum mechanics to attain maximum efficiency
 </a>
 . It turns out that wavelike transport is not always the best strategy. To understand why, suppose that the lilypond is full of rocks sticking up out of the water. As the wave moves through the pond, it scatters off the rocks. As a result, the wave never reaches the middle of the pond, which remains calm and protected. This is a phenomenon called destructive interference. Although the wave can propagate a short distance, eventually the random waves scattered off the rocks interfere with the overall wave’s propagation, effectively stopping it in its tracks. The quantum frog becomes completely stuck: A classical hopping strategy would have been more efficient. In the antenna photocomplex, the “rocks” are microscopic irregularities and molecular disorder that scatter the quantum wave as it tries to pass through.
 

 By constructing detailed quantum mechanical models, my collaborators and I were able to identify the optimal strategy for the interplay between wavelike propagation and classical hopping in photosynthesis. Over short distances, the wavelike propagation is more effective than random hopping. The exciton travels like a wave right up to the distance at which destructive interference causes it to get stuck. At this point, the fact that living systems are hot, wet environments comes into play: The environment effectively gives the exciton a whack that gets it unstuck and makes it perform a classical hop, which frees up the exciton to propagate again. (The technical term for this whack is “decoherence.”) Then the process repeats. The wave propagates until it gets stuck; the environment gives it a whack; the exciton hops. Eventually, the exciton reaches the reaction center in the minimum possible time. Expressed in terms of our quantum frog, the rule is simple: Wave until you get stuck, then hop.
 


 The birds, the bees, and the fruit flies
 


 Photosynthetic plants and bacteria are masters of the minutiae of quantum mechanics, manipulating quantum coherence and decoherence to attain almost 100% energy transport efficiency. If quantum hanky panky is so effective, are there other living beings that take advantage of quantum effects to live better and have more offspring? Only in the case of photosynthesis have scientists actually found the smoking gun (or maybe the smoking photon). However, there are several other organisms in which quantum mechanisms apparently play an important role.
 

 European robins are sensitive to the Earth’s magnetic field, which helps them during migration. Do they have a tiny compass in their heads, a piece of magnetite that swivels back and forth to point out magnetic north? Apparently not. Instead the evidence suggests another light-activated quantum mechanism. A photon excites an electron, which swivels around in the Earth’s magnetic field; the rate at which the electron decays from its excited state depends on how far it has swiveled. Since the robins need light to detect magnetic north, the next time you see a flock of them stumbling around at night, you know what is going on.
 


 I smell a quantum
 


 Quantum mechanics may also be involved in the sense of smell. Scientists have long believed that smell operates via a lock and key mechanism, an idea Linus Pauling first proposed more than 50 years ago. The molecule to be sniffed—the “odorant”—locks into a receptor in the olfactory apparatus that can only fit that particular key. The receptor then unlocks or changes its configuration, leading to a flow of ions sufficiently large to trigger a neuron to fire.
 

 The problem with this model is that olfactory receptors are not very specific: They can be unlocked by many different keys. This has led some researchers to propose that the receptors are sensitive not only to the shape of the odorant molecule, but to its vibrational frequencies—that is, its “sound.” The combination of shape and sound provides a unique signature for the molecule. For the vibrational theory of smell to hold, however, the underlying dynamics of the molecule in the receptor must be intrinsically quantum mechanical: The receptor must respond to individual phonons—quasi-particles of sound—generated by the molecule. Experiments show that fruit flies are indeed sensitive to vibrational frequencies of molecules, supporting the hypothesis of quantum smell. When trained to sense a molecule with a particular vibrational frequency they are attracted to it like flies to…phonons.
 


 Who is quantum?
 


 Where else might quantum mechanics play a role in life? Because light is made up of photons, interactions between living systems and light represent a good place to look. Our eyes are capable of detecting single photons by a highly quantum mechanical mechanism: A molecule in the retina absorbs a single photon, and uses its energy to release the flow of tens of thousands of ions, stimulating a neural response. Neural impulses in the brain are probably too coarse and classical to support the wave-like quantum dynamics that hold sway in photosynthesis, but at the level of individual synapses, the neurotransmitter binding mechanism might well benefit from the same types of quantum dynamics that apparently enhance smell.
 

 As scientists delve deeper into the details of molecular dynamics in living systems, they are likely to see more examples of quantum mechanics at work. We don’t yet know exactly what aspects of biology benefit from quantum mechanics. But we do know one thing: The unquantized life is not worth living.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 Nature:
 <a href="http://www.nature.com/news/2011/110615/full/474272a.html">
 Physics of life: The dawn of quantum biology
 </a>
 
 In this Nature news feature, asks whether quantum biology should be treated as a new scientific discipline.
 

 World Science Festival:
 <a href="http://worldsciencefestival.com/webcasts/quantum_biology">
 Quantum Biology
 </a>
 
 In this 90-minute webcast, Seth Lloyd, Thorsten Ritz, and Paul Davies discuss the intersection of biology and quantum mechanics.
 
</section>

Physicists are investigating how quantum mechanics may be involved in biological processes.

Seth Lloyd

Articles

Trap Doors in Time and Space: Teleportation, Time Travel, and Escape from Black Holes

Quantum Biology: Better Living Through Quantum Mechanics