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Posts Tagged ‘quantum mechanics’

Firing up the fission fragment rocket engine

Each winning proposal will receive roughly $100,000 with which to bring their ideas closer--albeit only slightly closer--to reality. Many of the ideas, if developed fully, would cost well into the billions of dollars to develop (entanglement-assisted quantum communications systems for deep space missions don’t come cheap, even in this economy).

Hopefully by the time any of these projects matures (and NASA itself says the horizon for any of these proposals is ten years out) the agency will have a little more money to spend. Because--and this is the whole point of this exercise--NASA hopes these ideas will incubate and grow into technologies that will underpin the future of long-duration spaceflight as well as pay technological dividends elsewhere.

And who knows? We might just get that ambient plasma wave propulsion engine we’ve been waiting for. See the whole list of winning proposals here.

[NASA]

According to quantum theory, empty space is, well, not that empty after all. Rather it is full of virtual particles – particles that quickly blip in and out of existence. Theory states that a mirror can absorb energy from some of these virtual photons, and re-emit it as actual photons. Of course, this only works if the mirror is traveling through the vacuum at nearly the speed of light, making it difficult to prove, to say the least.

Per Delsing and his team of physicists worked around this by using something called a superconducting quantum interference device, or SQUID. The SQUID, which is very sensitive to magnetic fields, acted as a mirror in the researchers’ superconducting circuit. They passed a magnetic field through the SQUID, switching the field’s direction a few billion times per second, which caused the SQUID to move back and forth at about 5 percent of the speed of light. Microwave photons were then observed. Consistent with the theory, the frequency of the released photons was about half the frequency of the mirror’s wiggling.

The researchers aren’t talking about their findings until their work has been peer-reviewed, but they will be presenting at a workshop next week in Italy.

[Nature]

But don't wipe your hard drives just yet

The phenomenon here has to do with basic rules about knowledge and the lack of knowledge, and it is rooted firmly in the definition of entropy and how information theory, thermodynamics, and quantum theory define it differently (and also in the same way). But the idea is thus: If it were technologically possible (and it should be, perhaps someday) to quantum-mechanically entangle the bits to be deleted with an observer, the observer could actually withdraw heat from the system while deleting the bits.

This is where the headaches start, and I’m not going to pretend to understand the nuts and bolts here. But conceptually, it comes down to knowledge. In information theory, entropy describes information density. In thermodynamics, entropy describes disorder in systems. What this new paper claims to prove is that in both cases, entropy basically describes a lack of knowledge.

An object doesn’t really possess entropy, but rather its entropy is dependent on the observer. So the idea is that if there are two observers deleting data from a memory, and one observer has more knowledge of the data, that observer will perceive the memory has lower entropy, and thus can delete it with less energy expenditure.

Now, enter quantum mechanics. In quantum theory, when calculated from the information theory standpoint entropy can actually be negative. So here’s the magic idea: quantum correlations (like entanglement) are stronger than classical correlations. So if an observer has perfect classical knowledge of a memory, then he or she would perceive its entropy as zero. And if the two were quantum-mechanically entangled, the entropy would be perceived as even less--less than zero, or negative entropy.

So with perfect classical knowledge of a system, deletion of data could theoretically happen with no energy at all. And if the deleter of the system has more than complete classical knowledge (i.e., is entangled with the system at the quantum level), then the deletion will actually remove heat from the system.

Right? It’s a stretch for the brain but it makes sense conceptually. In the future, such mind-bending could in theory lead to supercomputers with capacities that are not restrained by heat as they are today, allowing them to reach full potential while cooling themselves as they compute. We’re a long way from that, but considering the rapid pace at which researchers are learning to manipulate entangled systems, it’s certainly not out of the question.

[Science Daily]

As Technology Review’s KFC cleverly points out, “getting eight photons exactly where you want them at the same time is the quantum mechanical equivalent of herding cats (clearly of the Schrodinger variety).” Manipulating individual particles at this level is difficult enough, and that’s before you create that quantum link. Once you’ve entangled two or more particles, manipulating the entangled system without breaking the link is even more daunting.

How do you entangle this many photons? You start with one photon from a high energy beam, and you split it with a nonlinear crystal. You now have two weaker photons that are entangled--any exertion on one will affect the other. You put one photon aside in an apparatus and you then split the other, put one of those aside and split the other, etc.

But each split weakens the beam, and previously it was difficult--and time consuming--to produce to a manipulable eight-photon entangled system, so difficult that it hadn’t been achieved. The Chinese team, from the University of Science and Technology of China in Hefei, used a much brighter UV laser capable of churning out more entangled pairs much faster than smaller lasers. Then they figured out how to manipulate them.

That’s significant on a variety of fronts, not least of which is quantum computing. An eight-photon system would allow researchers to probe the quantum world at higher resolutions than was previously possible, demonstrating key pieces of the technology puzzle that should someday enable quantum computers to work as we’ve envisioned them.

For more, check the arXiv.

[Technology Review]

D-Wave’s technology is something they call a “quantum annealing processor,” and without going to deep into the inner workings of the thing (because I can’t), it is basically a means of finding solutions to “combinatorial optimization problems.” In other words, rather than dealing in ones and zeros, the processor taps the processing power of qubits--or quantum bits--which are multidimensional analogs to the analog bit.

The fundamental advantage here is that a qubit can be in more than one state at the same time, unlike the classical bit. And so quantum computers can, in theory, consider multiple possible solutions to a problem at the same time in essence. That makes them vastly more powerful and much, much faster than today’s conventional supercomputers.

D-Wave has come under some scrutiny from the quantum community, where other researchers have claimed that their “quantum optimizers” can really solve useful problems. But Lockheed seems to think they can. No word on what the company aims to do with their quantum computer, but D-Wave claims it is going to be used to address the Lockheed’s “most challenging computation problems.”

[Kurzweil AI]

For those that need a primer, entanglement is that strange quantum phenomenon that links two particles across distances such that any any measurements carried out on one particle immediately changes the properties of the other--even if they are separated by the entire universe. Einstein called it “spooky action at a distance.” And indeed it is weird.

Nicolas Gisin at U. of Geneva noted that Italian physicists had previously done an interesting thing with entangled photons. Rather than entangling just a few as experimenters usually do, the Italian team had entangled a pair of photons and then amplified one of them to create a photon shower containing thousands of particles, all linked to the single other photon from the original pair. That is, there was one “microscopic” photon, and a shower of “macroscopic” photons, all tied together at the quantum level.

Gisin realized that while the naked eye can’t see a single photon, it can certainly see thousands. So he used a setup similar to the Italians’, but rather than putting a photon detector in front of the macroscopic photons he put himself and his colleagues there. The beam of photons produced by the amplifier would appear in one of two positions in their darkened room, depending on the polarization state given to their microscopic single photon. Time after time, when the human results were tested against photon detectors, they got a positive result.

It may sounds like a bunch of scientists sitting in a dark room looking at blinking lights, but it represents the first time quantum entanglement has been directly observed with the naked eye.

Sort of. The Swiss team also found that what they were looking at wasn’t necessarily macro-micro entanglement. Even when they deliberately broke the quantum link between micro and macro and then ran their “human detector” experiment, they still got a positive result. This is due to the imperfection of detectors (even human ones) and a loophole in what’s known as the Bell Test (which, in a nutshell, is used to measure entanglement) that’s negligible in small quantities of photons but grows along with their quantity. This introduces a degree of uncertainty (for a better explanation of this, click through the Nature link below).

What the Swiss team does know is this: when they started, they had two entangled photons. Even though flaws may have been introduced in the amplification process, they could still “see” the effects of entanglement. A new method is being devised by the original Italian researchers (who also detected this flaw in their research) to verify micro-macro entanglement with lasers. Unfortunately, humans can’t be used as detectors for these experiments, as the highly focused beams of light would be the last thing those humans would see.

[Nature]

For those that need a primer, entanglement is that strange quantum phenomenon that links two particles across distances such that any any measurements carried out on one particle immediately changes the properties of the other--even if they are separated by the entire universe. Einstein called it “spooky action at a distance.” And indeed it is weird.

Nicolas Gisin at U. of Geneva noted that Italian physicists had previously done an interesting thing with entangled photons. Rather than entangling just a few as experimenters usually do, the Italian team had entangled a pair of photons and then amplified one of them to create a photon shower containing thousands of particles, all linked to the single other photon from the original pair. That is, there was one “microscopic” photon, and a shower of “macroscopic” photons, all tied together at the quantum level.

Gisin realized that while the naked eye can’t see a single photon, it can certainly see thousands. So he used a setup similar to the Italians’, but rather than putting a photon detector in front of the macroscopic photons he put himself and his colleagues there. The beam of photons produced by the amplifier would appear in one of two positions in their darkened room, depending on the polarization state given to their microscopic single photon. Time after time, when the human results were tested against photon detectors, they got a positive result.

It may sounds like a bunch of scientists sitting in a dark room looking at blinking lights, but it represents the first time quantum entanglement has been directly observed with the naked eye.

Sort of. The Swiss team also found that what they were looking at wasn’t necessarily macro-micro entanglement. Even when they deliberately broke the quantum link between micro and macro and then ran their “human detector” experiment, they still got a positive result. This is due to the imperfection of detectors (even human ones) and a loophole in what’s known as the Bell Test (which, in a nutshell, is used to measure entanglement) that’s negligible in small quantities of photons but grows along with their quantity. This introduces a degree of uncertainty (for a better explanation of this, click through the Nature link below).

What the Swiss team does know is this: when they started, they had two entangled photons. Even though flaws may have been introduced in the amplification process, they could still “see” the effects of entanglement. A new method is being devised by the original Italian researchers (who also detected this flaw in their research) to verify micro-macro entanglement with lasers. Unfortunately, humans can’t be used as detectors for these experiments, as the highly focused beams of light would be the last thing those humans would see.

[Nature]