Tag Archives: optics

On Hot Entanglement….

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Quantum Entanglement of 15 Trillion Atoms at 450 Kelvin With “Surprising Results”

By ICFO May 15, 2020

Cloud of Atoms with Pairs of Entangled Particles

Artistic illustration of a cloud of atoms with pairs of particles entangled between each other, represented by the yellow-blue lines. Credit: © ICFO

Quantum entanglement is a process by which microscopic objects like electrons or atoms lose their individuality to become better coordinated with each other. Entanglement is at the heart of quantum technologies that promise large advances in computing, communications and sensing, for example detecting gravitational waves.

Entangled states are famously fragile: in most cases even a tiny disturbance will undo the entanglement. For this reason, current quantum technologies take great pains to isolate the microscopic systems they work with, and typically operate at temperatures close to absolute zero. The ICFO team, in contrast, heated a collection of atoms to 450 Kelvin, millions of times hotter than most atoms used for quantum technology. Moreover, the individual atoms were anything but isolated; they collided with each other every few microseconds, and each collision set their electrons spinning in random directions.

The researchers used a laser to monitor the magnetization of this hot, chaotic gas. The magnetization is caused by the spinning electrons in the atoms, and provides a way to study the effect of the collisions and to detect entanglement. What the researchers observed was an enormous number of entangled atoms — about 100 times more than ever before observed. They also saw that the entanglement is non-local — it involves atoms that are not close to each other. Between any two entangled atoms there are thousands of other atoms, many of which are entangled with still other atoms, in a giant, hot and messy entangled state.

Glass Cell Rubidium Metal

Picture of the glass cell that where the rubidium metal is mixed with nitrogen gas and heated up to 450 degrees Kelvin. At that high temperature, the metal vaporizes, creating free rubidium atoms that diffuse around inside the cell. Credit: © ICFO

What they also saw, as Jia Kong, first author of the study, recalls, “is that if we stop the measurement, the entanglement remains for about 1 millisecond, which means that 1000 times per second a new batch of 15 trillion atoms is being entangled. And you must think that 1 ms is a very long time for the atoms, long enough for about fifty random collisions to occur. This clearly shows that the entanglement is not destroyed by these random events. This is maybe the most surprising result of the work.”

The observation of this hot and messy entangled state paves the way for ultra-sensitive magnetic field detection. For example, in magnetoencephalography (magnetic brain imaging), a new generation of sensors uses these same hot, high-density atomic gases to detect the magnetic fields produced by brain activity. The new results show that entanglement can improve the sensitivity of this technique, which has applications in fundamental brain science and neurosurgery.

As ICREA Prof. at ICFO Morgan Mitchell states, “this result is surprising, a real departure from what everyone expects of entanglement.” He adds “we hope that this kind of giant entangled state will lead to better sensor performance in applications ranging from brain imaging to self-driving cars to searches for dark matter.”

A Spin Singlet and QND

A spin singlet is one form of entanglement where the multiple particles’ spins–their intrinsic angular momentum–add up to 0, meaning the system has zero total angular momentum. In this study, the researchers applied quantum non-demolition (QND) measurement to extract the information of the spin of trillions of atoms. The technique passes laser photons with a specific energy through the gas of atoms. These photons with this precise energy do not excite the atoms but they themselves are affected by the encounter. The atoms’ spins act as magnets to rotate the polarization of the light. By measuring how much the photons’ polarization has changed after passing through the cloud, the researchers are able to determine the total spin of the gas of atoms.

The SERF regime

Current magnetometers operate in a regime that is called SERF, far away from the near absolute zero temperatures that researchers typically employ to study entangled atoms. In this regime, any atom experiences many random collisions with other neighboring atoms, making collisions the most important effect on the state of the atom. In addition, because they are in a hot medium rather than an ultracold one, the collisions rapidly randomize the spin of the electrons in any given atom. The experiment shows, surprisingly, that this kind of disturbance does not break the entangled states, it merely passes the entanglement from one atom to another.

Reference:”Measurement-induced, spatially-extended entanglement in a hot, strongly-interacting atomic system” by Jia Kong, Ricardo Jiménez-Martínez, Charikleia Troullinou, Vito Giovanni Lucivero, Géza Tóth and Morgan W. Mitchell, 15 May 2020, Nature Communications.
DOI: 10.1038/s41467-020-15899-1

from:    https://scitechdaily.com/quantum-entanglement-of-15-trillion-atoms-at-450-kelvin-with-surprising-results/

Time Cloak

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Now You See It, Now You Didn’t: Researchers Cloak a Moment in Time

A laser beam passes through a “split-time lens” – a specially designed waveguide that bumps up the wavelength for a while then suddenly bumps it down. The signal then passes through a filter that slows down the higher-wavelength part of the signal, creating a gap in which the cloaked event takes place. A second filter works in the opposite way from the first, letting the lower wavelength catch up, and a final split-time lens brings the beam back to the original wavelength, leaving no trace of what happened during the gap. (Credit: Gaeta lab)

ScienceDaily (Jan. 6, 2012) — Think Harry Potter movie magic: Cornell researchers have demonstrated a “temporal cloak” — albeit on a very small scale — in the transport of information by a beam of light.

The trick is to create a gap in the beam of light, have the hidden event occur as the gap goes by and then stitch the beam back together. Alexander Gaeta, Cornell professor of applied and engineering physics, and colleagues report their work entitled “Demonstration of temporal cloaking,” in the journal Nature (Jan. 5, 2012.)

The researchers created what they call a time lens, which can manipulate and focus signals in time, analogous to the way a glass lens focuses light in space. They use a technique called four-wave mixing, in which two beams of light, a “signal” and a “pump,” are sent together through an optical fiber. The two beams interact and change the wavelength of the signal. To begin creating a time gap, the researchers first bump the wavelength of the signal up, then by flipping the wavelength of the pump beam, bump it down.

The beam then passes through another, very long, stretch of optical fiber. Light passing through a transparent material is slowed down just a bit, and how much it is slowed varies with the wavelength. So the lower wavelength pulls ahead of the higher, leaving a gap, like the hare pulling ahead of the tortoise. During the gap the experimenters introduced a brief flash of light at a still higher wavelength that would cause a glitch in the beam coming out the other end.

Then the split beam passes through more optical fiber with a different composition, engineered to slow lower wavelengths more than higher. The higher wavelength signal now catches up with the lower, closing the gap. The hare is plodding through mud, but the tortoise is good at that and catches up. Finally, another four-wave mixer brings both parts back to the original wavelength, and the beam emerges with no trace that there ever was a gap, and no evidence of the intruding signal.

None of this will let you steal the crown jewels without anyone noticing. The gap created in the experiment was 15 picoseconds long, and might be increased up to 10 nanoseconds, Gaeta said. But the technique could have applications in fiber-optic data transmission and data processing, he added. For example, it might allow inserting an emergency signal without interrupting the main data stream, or multitasking operations in a photonic computer, where light beams on a chip replace wires.

The experiment was inspired by a theoretical proposal for a space-time cloak or “history editor” published by Martin McCall, professor of physics at Imperial College in London, in the Journal of Optics in November 2010. “But his method required an optical response from a material that does not exist. Now we’ve done it in one spatial dimension. Extending it to two [that is, hiding a moment in an entire scene] is not out of the realm of possibility. All advances have to start from somewhere,” Gaeta says.

Funding for the research: The Defense Advanced Research Project Agency (DARPA) and by the Cornell Center for Nanoscale Systems, which is supported by the National Science Foundation and the New York State Division of Science, Technology and Innovation (NYSTAR).

from:    http://www.sciencedaily.com/releases/2012/01/120106111312.htm

Temporal Cloak Tested

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Erasing History? Temporal Cloaks Adjust Light’s Throttle to Hide an Event in Time

ScienceDaily (Oct. 13, 2011) — Researchers from Cornell University in Ithaca, N.Y., have demonstrated for the first time that it’s possible to cloak a singular event in time, creating what has been described as a “history editor.” In a feat of Einstein-inspired physics, Moti Fridman and his colleagues sent a beam of light traveling down an optical fiber and through a pair of so-called “time lenses.” Between these two lenses, the researchers were able to briefly create a small bubble, or gap, in the flow of light. During that fleetingly brief moment, lasting only the tiniest fraction of a second, the gap functioned like a temporal hole, concealing the fact that a brief burst of light ever occurred.

By sending a beam of light down an optical fiber and through a pair of “time lenses”, researchers have demonstrated for the first time that it’s possible to cloak a singular event in time. (Credit: © 555images / Fotolia)

The team is presenting their findings at the Optical Society’s (OSA) Annual Meeting, Frontiers in Optics (FiO) 2011 (http://www.frontiersinoptics.com/), taking place in San Jose, Calif. next week.

Their ingenious system, which is the first physical demonstration of a phenomenon originally described theoretically a year ago by Martin McCall and his colleagues at Imperial College London in the Journal of Optics, relies on the ability to use short intense pulses of light to alter the speed of light as it travels through optical materials, in this case an optical fiber. (In a vacuum, light maintains its predetermined speed limit of 180,000 miles per second.) As the beam passes through a split-time lens (a silicon device originally designed to speed up data transfer), it accelerates near the center and slows down along the edges, causing it to balloon out toward the edges, leaving a dead zone around which the light waves curve. A similar lens a little farther along the path produces the exact but opposite velocity adjustments, resetting the speeds and reproducing the original shape and appearance of the light rays.

To test the performance of their temporal cloak, the researchers created pulses of light directly between the two lenses. The pulses repeated like clockwork at a rate of 41 kilohertz. When the cloak was off, the researchers were able to detect a steady beat. By switching on the temporal cloak, which was synchronized with the light pulses, all signs that these events ever took place were erased from the data stream.

to read more, go to:    http://www.sciencedaily.com/releases/2011/10/111012113554.htm