Jun 17, 2021 11:42 PM
(This post was last modified: Jun 18, 2021 03:53 PM by C C.)
Physicists bring "human-scale" object to near standstill, reaching a quantum state
https://phys.org/news/2021-06-physicists...state.html
INTRO: To the human eye, most stationary objects appear to be just that—still, and completely at rest. Yet if we were handed a quantum lens, allowing us to see objects at the scale of individual atoms, what was an apple sitting idly on our desk would appear as a teeming collection of vibrating particles, very much in motion.
In the last few decades, physicists have found ways to super-cool objects so that their atoms are at a near standstill, or in their "motional ground state." To date, physicists have wrestled small objects such as clouds of millions of atoms, or nanogram-scale objects, into such pure quantum states.
Now for the first time, scientists at MIT and elsewhere have cooled a large, human-scale object to close to its motional ground state. The object isn't tangible in the sense of being situated at one location, but is the combined motion of four separate objects, each weighing about 40 kilograms. The "object" that the researchers cooled has an estimated mass of about 10 kilograms, and comprises about 1x1026, or nearly 1 octillion, atoms.
The researchers took advantage of the ability of the Laser Interfrometer Gravitational-wave Observatory (LIGO) to measure the motion of the masses with extreme precision and super-cool the collective motion of the masses to 77 nanokelvins, just shy of the object's predicted ground state of 10 nanokelvins.
Their results, appearing today in Science, represent the largest object to be cooled to close to its motional ground state. The scientists say they now have a chance to observe the effect of gravity on a massive quantum object.
"Nobody has ever observed how gravity acts on massive quantum states," says Vivishek Sudhir, assistant professor of mechanical engineering at MIT, who directed the project. "We've demonstrated how to prepare kilogram-scale objects in quantum states. This finally opens the door to an experimental study of how gravity might affect large quantum objects, something hitherto only dreamed of."
The study's authors are members of the LIGO Laboratory, and include lead author and graduate student Chris Whittle, postdoc Evan Hall, research scientist Sheila Dwyer, Dean of the School of Science and the Curtis and Kathleen Marble Professor of Astrophysics Nergis Mavalvala, and assistant professor of mechanical engineering Vivishek Sudhir... (MORE)
Quantum Tunnels Show How Particles Can Break the Speed of Light
https://przekroj.pl/en/science/quantum-t...d-of-light
EXCERPTS: “Quantum tunneling” shows how profoundly particles such as electrons differ from bigger things. Throw a ball at the wall and it bounces backward; let it roll to the bottom of a valley and it stays there. But a particle will occasionally hop through the wall. It has a chance of “slipping through the mountain and escaping from the valley,” as two physicists wrote in Nature in 1928, in one of the earliest descriptions of tunneling.
Physicists quickly saw that particles’ ability to tunnel through barriers solved many mysteries. It explained various chemical bonds and radioactive decays and how hydrogen nuclei in the sun are able to overcome their mutual repulsion and fuse, producing sunlight.
[...] The first tentative calculation of tunneling time appeared in print in 1932. ... It wasn’t until 1962 that ... Thomas Hartman wrote a paper that explicitly embraced the shocking implications of the math.
Hartman found that a barrier seemed to act as a shortcut. When a particle tunnels, the trip takes less time than if the barrier weren’t there. Even more astonishing, he calculated that thickening a barrier hardly increases the time it takes for a particle to tunnel across it. This means that with a sufficiently thick barrier, particles could hop from one side to the other faster than light traveling the same distance through empty space.
In short, quantum tunneling seemed to allow faster-than-light travel, a supposed physical impossibility. [...] The discussion spiraled for decades ... But the tunneling-time question is making a comeback, fueled by a series of virtuoso experiments that have precisely measured tunneling time in the lab.
In the most highly praised measurement yet, reported in Nature in July, Aephraim Steinberg’s group in Toronto used what’s called the Larmor clock method to gauge how long rubidium atoms took to tunnel through a repulsive laser field.
[...] The recent experiments are bringing new attention to an unresolved issue. In the six decades since Hartman’s paper, no matter how carefully physicists have redefined tunneling time or how precisely they’ve measured it in the lab, they’ve found that quantum tunneling invariably exhibits the Hartman effect. Tunneling seems to be incurably, robustly superluminal.
“How is it possible for [a tunneling particle] to travel faster than light?” Igor Litvinyuk said. “It was purely theoretical until the measurements were made.”
[...] In quantum theory, a particle has a range of possible locations and speeds. From among these options, definite properties somehow crystallize at the moment of measurement. How this happens is one of the deepest questions.
The upshot is that until a particle strikes a detector, it’s everywhere and nowhere in particular. This makes it really hard to say how long the particle previously spent somewhere, such as inside a barrier. “You cannot say what time it spends there,” Litvinyuk said, “because it can be simultaneously two places at the same time.”
[...] Experts generally feel confident that tunneling doesn’t really break causality, but there’s no consensus on the precise reasons why not. “I don’t feel like we have a completely unified way of thinking about it,” Steinberg said. “There’s a mystery there, not a paradox.”
Some good guesses are wrong. Luiz Manzoni, on hearing about the superluminal tunneling issue in the early 2000s, worked with a colleague to redo the calculations. They thought they would see tunneling drop to subluminal speeds if they accounted for relativistic effects (where time slows down for fast-moving particles). “To our surprise, it was possible to have superluminal tunneling there too,” Manzoni said. “In fact, the problem was even more drastic in relativistic quantum mechanics.”
[...] Researchers stress that superluminal tunneling is not a problem as long as it doesn’t allow superluminal signaling. It’s similar in this way to the “spooky action at a distance” that so bothered Einstein. Spooky action refers to the ability of far-apart particles to be “entangled,” so that a measurement of one instantly determines the properties of both. This instant connection between distant particles doesn’t cause paradoxes because it can’t be used to signal from one to the other.
Considering the amount of hand-wringing over spooky action at a distance, though, surprisingly little fuss has been made about superluminal tunneling. “With tunneling, you’re not dealing with two systems that are separate, whose states are linked in this spooky way,” said Grace Field, who studies the tunneling-time issue at the University of Cambridge. “You’re dealing with a single system that’s traveling through space. In that way it almost seems weirder than entanglement.”
In a paper published in the New Journal of Physics in September, Eli Pollak and two colleagues argued that superluminal tunneling doesn’t allow superluminal signaling for a statistical reason: Even though tunneling through an extremely thick barrier happens very fast, the chance of a tunneling event happening through such a barrier is extraordinarily low. A signaler would always prefer to send the signal through free space.
Why, though, couldn’t you blast tons of particles at the ultra-thick barrier in the hopes that one will make it through superluminally? Wouldn’t just one particle be enough to convey your message and break physics? Steinberg, who agrees with the statistical view of the situation, argues that a single tunneled particle can’t convey information. A signal requires detail and structure, and any attempt to send a detailed signal will always be faster sent through the air than through an unreliable barrier.
Pollak said these questions are the subject of future study. “I believe the experiments of Steinberg are going to be an impetus for more theory. Where that leads, I don’t know.” (MORE - details)
https://phys.org/news/2021-06-physicists...state.html
INTRO: To the human eye, most stationary objects appear to be just that—still, and completely at rest. Yet if we were handed a quantum lens, allowing us to see objects at the scale of individual atoms, what was an apple sitting idly on our desk would appear as a teeming collection of vibrating particles, very much in motion.
In the last few decades, physicists have found ways to super-cool objects so that their atoms are at a near standstill, or in their "motional ground state." To date, physicists have wrestled small objects such as clouds of millions of atoms, or nanogram-scale objects, into such pure quantum states.
Now for the first time, scientists at MIT and elsewhere have cooled a large, human-scale object to close to its motional ground state. The object isn't tangible in the sense of being situated at one location, but is the combined motion of four separate objects, each weighing about 40 kilograms. The "object" that the researchers cooled has an estimated mass of about 10 kilograms, and comprises about 1x1026, or nearly 1 octillion, atoms.
The researchers took advantage of the ability of the Laser Interfrometer Gravitational-wave Observatory (LIGO) to measure the motion of the masses with extreme precision and super-cool the collective motion of the masses to 77 nanokelvins, just shy of the object's predicted ground state of 10 nanokelvins.
Their results, appearing today in Science, represent the largest object to be cooled to close to its motional ground state. The scientists say they now have a chance to observe the effect of gravity on a massive quantum object.
"Nobody has ever observed how gravity acts on massive quantum states," says Vivishek Sudhir, assistant professor of mechanical engineering at MIT, who directed the project. "We've demonstrated how to prepare kilogram-scale objects in quantum states. This finally opens the door to an experimental study of how gravity might affect large quantum objects, something hitherto only dreamed of."
The study's authors are members of the LIGO Laboratory, and include lead author and graduate student Chris Whittle, postdoc Evan Hall, research scientist Sheila Dwyer, Dean of the School of Science and the Curtis and Kathleen Marble Professor of Astrophysics Nergis Mavalvala, and assistant professor of mechanical engineering Vivishek Sudhir... (MORE)
Quantum Tunnels Show How Particles Can Break the Speed of Light
https://przekroj.pl/en/science/quantum-t...d-of-light
EXCERPTS: “Quantum tunneling” shows how profoundly particles such as electrons differ from bigger things. Throw a ball at the wall and it bounces backward; let it roll to the bottom of a valley and it stays there. But a particle will occasionally hop through the wall. It has a chance of “slipping through the mountain and escaping from the valley,” as two physicists wrote in Nature in 1928, in one of the earliest descriptions of tunneling.
Physicists quickly saw that particles’ ability to tunnel through barriers solved many mysteries. It explained various chemical bonds and radioactive decays and how hydrogen nuclei in the sun are able to overcome their mutual repulsion and fuse, producing sunlight.
[...] The first tentative calculation of tunneling time appeared in print in 1932. ... It wasn’t until 1962 that ... Thomas Hartman wrote a paper that explicitly embraced the shocking implications of the math.
Hartman found that a barrier seemed to act as a shortcut. When a particle tunnels, the trip takes less time than if the barrier weren’t there. Even more astonishing, he calculated that thickening a barrier hardly increases the time it takes for a particle to tunnel across it. This means that with a sufficiently thick barrier, particles could hop from one side to the other faster than light traveling the same distance through empty space.
In short, quantum tunneling seemed to allow faster-than-light travel, a supposed physical impossibility. [...] The discussion spiraled for decades ... But the tunneling-time question is making a comeback, fueled by a series of virtuoso experiments that have precisely measured tunneling time in the lab.
In the most highly praised measurement yet, reported in Nature in July, Aephraim Steinberg’s group in Toronto used what’s called the Larmor clock method to gauge how long rubidium atoms took to tunnel through a repulsive laser field.
[...] The recent experiments are bringing new attention to an unresolved issue. In the six decades since Hartman’s paper, no matter how carefully physicists have redefined tunneling time or how precisely they’ve measured it in the lab, they’ve found that quantum tunneling invariably exhibits the Hartman effect. Tunneling seems to be incurably, robustly superluminal.
“How is it possible for [a tunneling particle] to travel faster than light?” Igor Litvinyuk said. “It was purely theoretical until the measurements were made.”
[...] In quantum theory, a particle has a range of possible locations and speeds. From among these options, definite properties somehow crystallize at the moment of measurement. How this happens is one of the deepest questions.
The upshot is that until a particle strikes a detector, it’s everywhere and nowhere in particular. This makes it really hard to say how long the particle previously spent somewhere, such as inside a barrier. “You cannot say what time it spends there,” Litvinyuk said, “because it can be simultaneously two places at the same time.”
[...] Experts generally feel confident that tunneling doesn’t really break causality, but there’s no consensus on the precise reasons why not. “I don’t feel like we have a completely unified way of thinking about it,” Steinberg said. “There’s a mystery there, not a paradox.”
Some good guesses are wrong. Luiz Manzoni, on hearing about the superluminal tunneling issue in the early 2000s, worked with a colleague to redo the calculations. They thought they would see tunneling drop to subluminal speeds if they accounted for relativistic effects (where time slows down for fast-moving particles). “To our surprise, it was possible to have superluminal tunneling there too,” Manzoni said. “In fact, the problem was even more drastic in relativistic quantum mechanics.”
[...] Researchers stress that superluminal tunneling is not a problem as long as it doesn’t allow superluminal signaling. It’s similar in this way to the “spooky action at a distance” that so bothered Einstein. Spooky action refers to the ability of far-apart particles to be “entangled,” so that a measurement of one instantly determines the properties of both. This instant connection between distant particles doesn’t cause paradoxes because it can’t be used to signal from one to the other.
Considering the amount of hand-wringing over spooky action at a distance, though, surprisingly little fuss has been made about superluminal tunneling. “With tunneling, you’re not dealing with two systems that are separate, whose states are linked in this spooky way,” said Grace Field, who studies the tunneling-time issue at the University of Cambridge. “You’re dealing with a single system that’s traveling through space. In that way it almost seems weirder than entanglement.”
In a paper published in the New Journal of Physics in September, Eli Pollak and two colleagues argued that superluminal tunneling doesn’t allow superluminal signaling for a statistical reason: Even though tunneling through an extremely thick barrier happens very fast, the chance of a tunneling event happening through such a barrier is extraordinarily low. A signaler would always prefer to send the signal through free space.
Why, though, couldn’t you blast tons of particles at the ultra-thick barrier in the hopes that one will make it through superluminally? Wouldn’t just one particle be enough to convey your message and break physics? Steinberg, who agrees with the statistical view of the situation, argues that a single tunneled particle can’t convey information. A signal requires detail and structure, and any attempt to send a detailed signal will always be faster sent through the air than through an unreliable barrier.
Pollak said these questions are the subject of future study. “I believe the experiments of Steinberg are going to be an impetus for more theory. Where that leads, I don’t know.” (MORE - details)