Superposition, entanglement, qubits, and Google's big announcement.
According to last week's news, the world took a giant step into the brave new world of quantum computing.
Researchers at Google announced that a machine at their labs had crossed the much-heralded milestone known as quantum supremacy. The announcement made headlines in pretty much every news outlet I came across.
Given that much coverage, I thought now would be a good time to think about which parts of quantum mechanics are involved with quantum computing.
“Quantum supremacy" is a term invented by the famous quantum physicist John Preskill in a 2012 paper. The idea he was trying to express was the moment when a quantum computer could do a calculation which a normal (or “classical") computer could not. In practice, this means building a quantum computer that can compute something (like a random number) in a time much shorter than a classical computer could achieve. The idea is that there will be tasks which even a solar system–sized classical computer would take a billion years to finish—but a quantum machine could finish in a millisecond. That, in principle, is the kind of transition which happened last week at Google labs.
So, what's going on that lets quantum computers do things that the kind of machine you're reading this on now can't? The answer turns on two particular aspects of quantum mechanics that pretty much stomp our commonsense ideas about the world into the ground.
The first aspect is what's called superposition. In classical physics (and common-sense experience), things have definite properties. Take location, for example. I am either in the bedroom or I am not. How about speed? I am either traveling at 60 miles an hour or I'm not. Now perhaps there might be some slop associated with your measurements, so that you might not be able to tell if I'm traveling 60 miles an hour or 58 miles an hour. That's just a detail, however. Whatever its actual value might be, most of us are sure that there is a real and definite speed that I'm traveling. But in quantum physics, this is not the case.
Before a measurement of some property—of, say, an electron—is made, quantum physics tells us it will be in a superposition of “states" for that property. Using the example of location again, before a measurement is made, the electron will be in the bedroom and in the kitchen and upstairs and in the family room of the house down the street—in all of these places at the same time. Physicists say the locations are “superposed"—that the electron doesn't have just one value of a property, but has many at once.
Wait, It gets even weirder
The second kind of weirdness at the heart of quantum computing is what's called entanglement. Normally we imagine that two objects that are widely separated can't instantaneously affect each other. A ringing alarm clock in Singapore shouldn't wake a sleeping dog in Denver. If the objects are so far apart that a light signal (the fastest signal possible) can't make it between them in the time they are affecting each other, then such an effect is physically impossible.
But in quantum mechanics, particles can be “entangled." If you entangle two electrons and move them to opposite sides of the solar system, an operation (i.e., a measurement) made on one of the pair will instantly change the state of the other. Entanglement means that, in a very real sense, the two electrons represent a kind of whole that a separation does not disturb. It's like those stories of identical twins who can magically feel each other's pain even if they are continents apart. In this case, however, it's not magic, it's quantum mechanics. (And no, you can't use entanglement to send signals faster than light speed. Sorry.)
So, if superposition and entanglement seem really weird to you, that's good. They are weird to us physicists. And if you want to know what's really going in terms of how the world is built such that superposition and entanglement are possible, well then that's good too, because most of us physicists want to know that as well. Mostly we're clueless, though, which would lead to a whole discussion about quantum interpretations . . . which we'll forgo for now.
The final question, of course, is how does any of this lead to quantum computers being supreme in terms of, you know, computing? The answer is actually simple. A classical computer is built on manipulating information in the form of bits, each of which can be 1 or 0. But a quantum computer uses qubits, which are superpositions of bits. That means a qubit is both 1 and 0 at the same time (until a measurement is made on it). That's twice as much information in a single “unit" of computation. But that doubling is just the beginning. I can entangle qubits. That means if I have 2 qubits, I get 2×2 more information. If I have 5 qubits that increase goes up to 2x2x2x2x2 or 2 = 32. With just 50 entangled qubits, the information I'm working with goes up about a thousand billion times above what's available with 50 classical bits. See? That's pretty supreme supremacy.
The trick is getting anything done with those qbits since they are notoriously sensitive and tend to kind of fall apart at the slightest nudge. But that's is another story which will have to wait for another day.
A new study challenges what we understand about the workings of time.
Quantum physics has spawned its share of strange ideas and hard-to-grasp concepts - from Einstein’s “spooky action at a distance” to the adventures of Shroedinger’s cat. Now a new study lends support to another mind-bender - the idea of retrocausality, which basically proposes that the future can influence the past and the effect, in essence, happens before the cause.
At this point, retrocausality does not mean that you get to send signals from the future to the past - rather that an experimenter’s measurement of a particle can influence the properties of that particle in the past, even before making their choice.
The new paper argues that retrocausality could be a part of quantum theory. The scientists expound on the more traditionally accepted concept of time symmetry and show that if that is true, then so should be retrocausality. Time symmetry says that physical processes can run forward and backwards in time while being subject to the same physical laws.
The scientists describe an experiment where time symmetry would require processes to have the same probabilities, whether they go backwards or forward in time. But that would cause a contradiction if there was no retrocausality, as it requires these processes to have different probabilities. What the paper shows is that you can’t have both concepts be true at the same time.
Eliminating time symmetry would also get rid of some other sticky problems of quantum physics, like Einstein’s discomfort with entanglement which he described as “spooky action at a distance.” He saw challenges to quantum theory in the idea that entangled or connected particles could instantly affect each other even at large distances. In fact, accepting retrocausality could allow for a reinterpretation of Bell tests that were used to show evidence of “spooky action”. Instead, the tests could be supporting retrocausailty.
The paper, published in the Proceedings of the Royal Society A, was authored by Matthew S. Leifer at Chapman University in California and Matthew F. Pusey at the Perimeter Institute for Theoretical Physics in Ontario. The scientists hope their work can lead towards a fuller understanding of quantum theory.
"The reason I think that retrocausality is worth investigating is that we now have a slew of no-go results about realist interpretations of quantum theory, including Bell's theorem, Kochen-Specker, and recent proofs of the reality of the quantum state," said Leifer to Phys.org. "These say that any interpretation that fits into the standard framework for realist interpretations must have features that I would regard as undesirable. Therefore, the only options seem to be to abandon realism or to break out of the standard realist framework.”
Are we going to have time travel as a result of this? In one idea proposed by Richard Feynman, existence of retrocausality could mean that positrons, antimatter counterparts of electrons, would move backwards in time so that they could have a positive charge. If this was proven to be true, time travel could involve simply changing the direction of moving particles in the single dimension of time.
Leifer doesn’t go as far as time travel in his explanation, but speculates that if retrocausality does exist in the universe, then there could be evidence of it in the cosmological data, saying that “there are certain eras, perhaps near the big bang, in which there is not a definite arrow of causality.”
Is this idea ready for the big time? It is supported by Huw Price, a philosophy professor at the University of Cambridge who focuses on the physics of time and is a leading advocate of retrocausality. Leifer and Pusey are taking things in stride, however, realizing that much more work needs to be done.
"There is not, to my knowledge, a generally agreed upon interpretation of quantum theory that recovers the whole theory and exploits this idea. It is more of an idea for an interpretation at the moment, so I think that other physicists are rightly skeptical, and the onus is on us to flesh out the idea,” said Leifer.
There are no experiments underway by the physicists to test their theory, but they hope this work will question the assumptions of quantum mechanics and lead to new discoveries down the line.