Physicists Create Quantum Link Between Photons That Don’t Exist at the Same Time

Now they’re just messing with us. Physicists have long known that quantum mechanics allows for a subtle connection between quantum particles called entanglement, in which measuring one particle can instantly set the otherwise uncertain condition, or “state,” of another particle—even if it’s light years away. Now, experimenters in Israel have shown that they can entangle two photons that don’t even exist at the same time.

“It’s really cool,” says Jeremy O’Brien, an experimenter at the University of Bristol in the United Kingdom, who was not involved in the work. Such time-separated entanglement is predicted by standard quantum theory, O’Brien says, “but it’s certainly not widely appreciated, and I don’t know if it’s been clearly articulated before.”

Entanglement is a kind of order that lurks within the uncertainty of quantum theory. Suppose you have a quantum particle of light, or photon. It can be polarized so that it wriggles either vertically or horizontally. The quantum realm is also hazed over with unavoidable uncertainty, and thanks to such quantum uncertainty, a photon can also be polarized vertically and horizontally at the same time. If you then measure the photon, however, you will find it either horizontally polarized or vertically polarized, as the two-ways-at-once state randomly “collapses” one way or the other.

Entanglement can come in if you have two photons. Each can be put into the uncertain vertical-and-horizontal state. However, the photons can be entangled so that their polarizations are correlated even while they remain undetermined. For example, if you measure the first photon and find it horizontally polarized, you’ll know that the other photon has instantaneously collapsed into the vertical state and vice versa—no matter how far away it is. Because the collapse happens instantly, Albert Einstein dubbed the effect “spooky action at a distance.” It doesn’t violate relativity, though: It’s impossible to control the outcome of the measurement of the first photon, so the quantum link can’t be used to send a message faster than light.

In standard entanglement swapping (top), entanglement (blue shading) is transferred to photons 1 and 4 by making a measurement on photons 2 and 3. The new experiment (bottom) shows that the scheme still works even if photon 1 is destroyed before photon 4 is created. Image: AAAS/Science

Now Eli Megidish, Hagai Eisenberg, and colleagues at the Hebrew University of Jerusalem have entangled two photons that don’t exist at the same time. They start with a scheme known as entanglement swapping. To begin, researchers zap a special crystal with laser light a couple of times to create two entangled pairs of photons, pair 1 and 2 and pair 3 and 4. At the start, photons 1 and 4 are not tangled. But they can be if physicists play the right trick with 2 and 3.

The key is that a measurement “projects” a particle into a definite state — just as the measurement of a photon collapses it into either vertical or horizontal polarization. So even though photons 2 and 3 start out unentangled, physicists can set up a “projective measurement” that asks, are the two in one of two distinct entangled states or the other? That measurement entangles the photons, even as it absorbs and destroys them. If the researchers select only the events in which photons 2 and 3 end up in, say, the first entangled state, then the measurement also entangles photons 1 and 4. (See diagram, top.) The effect is a bit like joining two pairs of gears to form a four-gear chain: Enmeshing two inner gears establishes a link between the outer two.

In recent years, physicists have played with the timing in the scheme. For example, last year a team showed that entanglement swapping still works even if they make the projective measurement after they’ve already measured the polarizations of photons 1 and 4. Now, Eisenberg and colleagues have shown that photons 1 and 4 don’t even have to exist at the same time, as they report in a paper in press at Physical Review Letters.

To do that, they first create entangled pair 1 and 2 and measure the polarization of 1 right away. Only after that do they create entangled pair 3 and 4 and perform the key projective measurement. Finally, they measure the polarization of photon 4. And even though photons 1 and 4 never coexist, the measurements show that their polarizations still end up entangled. Eisenberg emphasizes that even though in relativity, time measured differently by observers traveling at different speeds, no observer would ever see the two photons as coexisting.

The experiment shows that it’s not strictly logical to think of entanglement as a tangible physical property, Eisenberg says. “There is no moment in time in which the two photons coexist,” he says, “so you cannot say that the system is entangled at this or that moment.” Yet, the phenomenon definitely exists. Anton Zeilinger, a physicist at the University of Vienna, agrees that the experiment demonstrates just how slippery the concepts of quantum mechanics are. “It’s really neat because it shows more or less that quantum events are outside our everyday notions of space and time.”

So what’s the advance good for? Physicists hope to create quantum networks in which protocols like entanglement swapping are used to create quantum links among distant users and transmit uncrackable (but slower than light) secret communications. The new result suggests that when sharing entangled pairs of photons on such a network, a user wouldn’t have to wait to see what happens to the photons sent down the line before manipulating the ones kept behind, Eisenberg says. Zeilinger says the result might have other unexpected uses: “This sort of thing opens up people’s minds and suddenly somebody has an idea to use it in quantum computing or something.”

This story provided by ScienceNOW, the daily online news service of the journal Science.

http://www.wired.com/wiredscience/2013/05/quantum-linked-photons/

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idlnmclean:

If your brain were a computer, how much storage space would it have?

The comparison between the human brain and a computer is not a perfect one, but it does lend itself to some interesting lines of inquiry. For instance: what is the storage capacity of your brain?

The answer to the first question – how much storage space is there inside the average human head? – varies considerably, depending on who you ask. Some estimates come in as low as 1 terabyte, or approximately 1,000 gigabytes. These days, you can purchase an external hard drive with twice that capacity for under a hundred bucks.

The reasoning behind the 100-terabyte estimate has its flaws. It assumes, for example, that each synapse stores 1 byte of information. In reality, each one could conceivably store more or less than that.

Most of the computer chips that we use to model brain activity operate in this binary fashion – but the brain probably doesn’t work this way.

Consider, also, that synapses are often interdependent, and will rely on one another to convey a single piece of information. While it’s logical to assume that the brain’s extensive neural networks greatly improve its processing speed (a couple years ago, researchers writing in Science concluded that the number of nerve impulses executed by one human brain per second is “in the same ballpark [as] the 6.4*10¹⁸ instructions per second that human kind [could] carry out on its general purpose computers in 2007”), it’s also possible that they do so at the expense of storage capacity.

So, which is it? One terabyte? 100 terabytes? 2.5-thousand terabytes? Or can you fit an entire human consciousness into just 300 megabytes (approximately 60 3-minute MP3s), as suggested in an episode of Caprica? Perhaps these questions are irrelevant. As Reber himself says: “if your brain worked like a digital video recorder, 2.5 petabytes would be enough to hold three million hours of TV shows.” We’ve already established that our brains don’t work like DVRs, or the vast majority of computers, for that matter, and so down the rabbit hole we go: how much brain-space does a memory occupy? Does a more detailed memory take up more space than a foggy one? Have forgotten memories been deleted, or have they been relegated to some forgotten subfolder in the dusty corners of your consciousness? Does a deeply rooted, subconscious bias take up more space than a transient dream? Is each encoded in different file format? And while we’re exploring the brain/computer/file-size/file-type metaphor: what is the cognitive equivalent of a GIF, anyway?

Perhaps a better question is whether the size of memories and the storage capacity of the human mind are things that can be measured at all. Reason would suggest that the brain’s capacity is, in fact, limited, and therefore can be measured. Determining what it’s limited by, exactly, and how to quantify those limits, would be a significant boon to fields as diverse as neuroscience, robotics and computer science – especially where the three overlap.

If a human mind is partially computable and the brain is the major CNC of the mind then memory is a matter of compression. Where there is compression there are at least two classes of memory: lossless and lossy compression. JPEG file formats for instance are a kind of lossy compression of image data which produces compression artifacts due to data loss. Lossless compression preserves a given cluster of compressed data invariably. From the above, a “foggy” memory sounds a lot like a lossy form of compression. Furthermore, we’d need to talk about data streams like our sensory organs, and how data is filtered before conscious integration and compression.

needcoffee:

Wayhomer Review #151 for Fast & Furious 6, in which our protagonist marvels at the tenacity and testicular fortitude of this particular franchise, is happy to see more going on than cars and writhing women at street races, and apologies to Ludacris for getting his name wrong. To be fair, I was punished for doing so by gravity’s karma.

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