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Teleportation in a Quantum World

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  • Teleportation in a Quantum World

    THE FABRIC of the COSMOS, Brian Greene, 2004
    ```(annotated and with added bold highlights by Epsilon=One)
    Chapter 15 - Teleporters and Time Machines
    Teleportation in a Quantum World
    In conventional science fiction depictions, a teleporter (or, in Star Trek lingo, a transporter) scans an object to determine its detailed composition and sends the information to a distant location, where the object is reconstituted. Whether the object itself is "dematerialized," its atoms and molecules being sent along with the blueprint for putting them back together, or whether atoms and molecules located at the receiving end are used to build an exact replica of the object, varies from one fictional incarnation to another. As we'll see, the scientific approach to teleportation developed over the last decade is closer in spirit to the latter category, and this raises two essential questions. The first is a standard but thorny philosophical conundrum: When, if ever, should an exact replica be identified, called, considered, or treated as if it were the original? The second is the question of whether it's possible, even in principle; to examine an object and determine its composition with complete accuracy so that we can draw up a perfect blueprint with which to reconstitute it.

    In a universe governed by the laws of classical physics, the answer to the second question would be yes. In principle, the attributes of every particle making up an object — each particle's identity, position, velocity, and so on — could be measured with total precision, transmitted to a distant location, and used as an instruction manual for recreating the object. Doing this for an object composed of more than just a handful of elementary particles would be laughably beyond reach, but in a classical universe, the obstacle would be complexity, not physics.

    In a universe governed by the laws of quantum physics — our universe — the situation is far more subtle. We've learned that the act of measurement coaxes one of the myriad potential attributes of an object to snap out of the quantum haze and take on a definite value. When we observe a particle, for example, the definite features we see do not generally reflect the fuzzy quantum mixture of attributes it had a moment before we looked. 1 Thus, if we want to replicate an object, we face a quantum Catch-22. To replicate we must observe, so we know what to replicate. But the act of observation causes change, so if we replicate what we see, we will not replicate what was there before we looked. This suggests that teleportation in a quantum universe is unattainable, not merely because of practical limitations arising from complexity, but because of fundamental limitations inherent in quantum physics. Nevertheless, as we'll see in the next section, in the early 1990s an international team of physicists found an ingenious way to circumvent this conclusion.

    As for the first question, regarding the relationship between replica and original, quantum physics gives an answer that's both precise and encouraging. According to quantum mechanics, every electron in the universe is identical to every other, in that they all have exactly the same mass, exactly the same electric charge, exactly the same weak and strong nuclear force properties, and exactly the same total spin. Moreover, our well-tested quantum mechanical description says that these exhaust the attributes that an electron can possess; electrons are all identical with regard to these properties, and there are no other properties to consider. In the same sense, every up-quark is the same as every other, every down-quark is the same as every other, every photon is the same as every other, and so on for all other particle species. As recognized by quantum practitioners many decades ago, particles may be thought of as the smallest possible packets of a field (e.g., photons are the smallest packets of the electromagnetic field), and quantum physics shows that such smallest constituents of the same field are always identical. (Or, in the framework of string theory, particles of the same species have identical properties because they are identical vibrations of a single species of string.)

    What can differ between two particles of the same species are the probabilities that they are located at various positions, the probabilities that their spins are pointing in particular directions, and the probabilities that they have particular velocities and energies. Or, as physicists say more succinctly, the two particles can be in different quantum states. But if two particles of the same species are in the same quantum state — except, possibly, for one particle having a high likelihood of being here while the other particle has a high likelihood of being over there — the laws of quanturn mechanics ensure that they are indistinguishable, not just in practice but in principle. They are perfect twins. If someone were to exchange the particles' positions (more precisely, exchange the two particles' probabilities of being located at any given position), there'd be absolutely no way to tell.
    Thus, if we imagine starting with a particle located here,* and somehow  put another particle of the same species into exactly the same quanturn state (same probabilities for spin orientation, energy, and so on) at some distant location, the resulting particle would be indistinguishable from the original and the process would rightly be called quantum teleportation. Of course, were the original particle to survive the process intact, you might be tempted to call the process quantum cloning or, perhaps, quantum faxing. But as we'll see, the scientific realization of these ideas does not preserve the original particle — it is unavoidably modified during the teleportation process — so we won't be faced with this taxonomic dilemma.

    A more pressing concern, and one that philosophers have considered closely in various forms, is whether what's true for an individual particle is true for an agglomeration. If you were able to teleport from one location to another every single particle that makes up your DeLorean, ensuring that the quantum state of each, including its relationship to all others, was reproduced with 100% fidelity, would you have succeeded in teleporting the vehicle? Although we have no empirical evidence to guide us, the theoretical case in support of having teleported the car is strong. Atomic and molecular arrangements determine how an object looks and feels, sounds and smells, and even tastes, so the resulting vehicle should be identical to the original DeLorean — bumps, nicks, squeaky left wing-door, musty smell from the family dog, all of it — and the car should take a sharp turn and respond to flooring the gas pedal exactly as the original did. The question of whether the vehicle actually is the original or, instead, is an exact duplicate, is of no concern. If you'd asked United Quantum Van Lines to ship your car by boat from New York to London but, unbeknownst to you, they teleported it in the manner described, you could never know the difference — even in principle.

    But what if the moving company did the same to your cat, or, having sated your appetite for airplane gastronomy, what if you decided on teleportation for your own transatlantic travel? Would the cat or person who steps out of the receiving chamber be the same as the one who stepped into the teleporter? Personally, I think so. Again, since we have no relevant data, the best that I or anyone can do is speculate. But to my way of thinking, a living being whose constituent atoms and molecules are in exactly the same quantum state as mine is me. Even if the "original" me still existed after the "copy" had been made, I (we) would say without hesitation that each was me. We'd be of the same mind — literally — in asserting that neither would have priority over the other. Thoughts, memories, emotions, and judgments have a physical basis in the human body's atomic and molecular properties; an identical quantum state of these elementary constituents should entail an identical conscious being. As time went by, our experiences would cause us to differentiate, but I truly believe that henceforth there'd be two of me, not an original that was somehow "really" me and a copy that somehow wasn't.

    In fact, I'm willing to be a bit looser. Our physical composition goes through numerous transformations all the time — some minor, some drastic — but we remain the same person. From the Haagen-Dazs that inundates the bloodstream with fat and sugar, to the MRI that flips the spin axes of various atomic nuclei in the brain, to heart transplants and liposuction, to the trillion atoms in the average human body that are replaced every millionth of a second, we undergo constant change, yet our personal identity remains unaffected. So, even if a teleported being did not match my physical state with perfect accuracy, it could very well be fully indistinguishable from me. In my book, it could very well be me.

    Certainly, if you believe that there is more to life, and conscious life in particular, than its physical makeup, your standards for successful teleportation
    will be more, stringent than mine. This tricky issue — to what extent is our personal identity tied to our physical being? — has been debated for years in a variety of guises without being answered to everyone's satisfaction. While I believe identity all resides in the physical, others disagree, and no one can claim to have the definitive answer.

    But irrespective of your point of view on the hypothetical question of teleporting a living being, scientists have now established that, through the wonders of quantum mechanics, individual particles can be — have been—teleported.

    Let's see how.
    "Since teleportation starts with something here and seeks to make it appear at a distant location, in this section I will often speak as if particles have definite positions. To be more precise, I should always say, "starting with a particle that has a high likelihood of being located here" or "starting with a particle with a 99 percent chance of being located here," with similar language used where the particle is teleported, but for brevity's sake I will use the looser language.
    Last edited by Reviewer; 10-14-2012, 10:09 PM.