Posts Tagged ‘Schrodinger’s Cat’

A new uncertainty principle holds that quantum objects can be at two temperatures at once, which is similar to the famous Schrödinger’s cat thought experiment, in which a cat in a box with a radioactive element can be both alive and dead.

By Meredith Fore

The famous thought experiment known as Schrödinger’s cat implies that a cat in a box can be both dead and alive at the same time — a bizarre phenomenon that is a consequence of quantum mechanics.

Now, physicists at the University of Exeter in England have found that a similar state of limbo may exist for temperatures: Objects can be two temperatures at the same time at the quantum level. This weird quantum paradox is the first completely new quantum uncertainty relation to be formulated in decades.

Heisenberg’s other principle
In 1927, German physicist Werner Heisenberg postulated that the more precisely you measure a quantum particle’s position, the less precisely you can know its momentum, and vice versa — a rule that would become the now-famous Heisenberg uncertainty principle.

The new quantum uncertainty, which states that the more precisely you know temperature, the less you can say about energy, and vice versa, has big implications for nanoscience, which studies incredibly tiny objects smaller than a nanometer. This principle will change how scientists measure the temperature of extremely small things such as quantum dots, small semiconductors or single cells, the researchers said in the new study, which was published in June in the journal Nature Communications.

In the 1930s, Heisenberg and Danish physicist Niels Bohr established an uncertainty relation between energy and temperature on the nonquantum scale. The idea was that, if you wanted to know the exact temperature of an object, the best and most precise scientific way to do that would be to immerse it in a “reservoir” — say, a tub of water, or a fridge full of cold air — with a known temperature, and allow the object to slowly become that temperature. This is called thermal equilibrium.

However, that thermal equilibrium is maintained by the object and the reservoir constantly exchanging energy. The energy in your object therefore goes up and down by infinitesimal amounts, making it impossible to define precisely. On the flip side, if you wanted to know the precise energy in your object, you would have to isolate it so that it could not come into contact with, and exchange energy with, anything. But if you isolated it, you would not be able to precisely measure its temperature using a reservoir. This limitation makes the temperature uncertain.

Things get weirder when you go to the quantum scale.

A new uncertainty relation
Even if a typical thermometer has an energy that goes up and down slightly, that energy can still be known to within a small range. This is not true at all on the quantum level, the new research showed, and it’s all due to Schrödinger’s cat. That thought experiment proposed a theoretical cat in a box with a poison that could be activated by the decay of a radioactive particle. According to the laws of quantum mechanics, the particle could have decayed and not decayed at the same time, meaning that until the box was opened, the cat would be both dead and alive at the same time — a phenomenon known as superposition.

The researchers used math and theory to predict exactly how such superposition affects the measurement of the temperature of quantum objects.

“In the quantum case, a quantum thermometer … will be in a superposition of energy states simultaneously,”Harry Miller, one of the physicists at the University of Exeter who developed the new principle, told Live Science. “What we find is that because the thermometer no longer has a well-defined energy and is actually in a combination of different states at once, that this actually contributes to the uncertainty in the temperature that we can measure.”

In our world, a thermometer may tell us an object is between 31 and 32 degrees Fahrenheit (minus 0.5 and zero degrees Celsius). In the quantum world, a thermometer may tell us an object is both those temperatures at the same time. The new uncertainty principle accounts for that quantum weirdness.

Interactions between objects at the quantum scale can create superpositions, and also create energy. The old uncertainty relation ignored these effects, because it doesn’t matter for nonquantum objects. But it matters a lot when you’re trying to measure the temperature of a quantum dot, and this new uncertainty relation makes up a theoretical framework to take these interactions into account.

The new paper could help anyone who’s designing an experiment to measure temperature changes in objects below the nanometer scale, Miller said. “Our result is going to tell them exactly how to accurately design their probes and tell them how to account for the additional quantum uncertainty that you get.”



By Tia Ghose

Bizarrely behaving light particles show that the famous Schrödinger’s cat thought experiment, meant to reveal the strange nature of subatomic particles, can get even weirder than physicists thought.

Not only can the quantum cat be alive and dead at the same time — but it can also be in two places at once, new research shows.

“We are showing an analogy to Schrödinger’s cat that is made out of an electromagnetic field that is confined in two cavities,” said study lead author Chen Wang, a physicist at Yale University. “The interesting thing here is the cat is in two boxes at once.”

The findings could have implications for cracking unsolvable mathematicalproblems using quantum computing, which relies on the ability of subatomic particles to be in multiple states at once, Wang said.

Cat experiment

The famous paradox was laid out by physicist Erwin Schrödinger in 1935 to elucidate the notion of quantum superposition, the phenomenon in which tiny subatomic particles can be in multiple states at once.

In the paradox, a cat is trapped in a box with a deadly radioactive atom. If the radioactive atom decayed, the cat was a goner, but if it had not yet decayed, the cat was still alive. Because, according to the dominant interpretation of quantum mechanics, particles can exist in multiple states until they are measured, logic dictated that the cat would be both alive and dead at the same time until the radioactive atom was measured.

Cat in two boxes

The setup for the new study was deceptively simple: The team created two aluminum cavities about 1 inch (2.5 centimeters) across, and then used a sapphire chip to produce a standing wave of light in those cavities. They used a special electronic element, called a Josephson Junction, to superimpose a standing wave of two separate wavelengths of light in each cavity. The end result was that the cat, or the group of about 80 photons in the cavities, was oscillating at two different wavelengths at once — in two different places. Figuring out whether the cat is dead or alive, so to speak, requires opening both boxes.

Though conceptually simple, the physical setup required ultrapure aluminum and highly precise chips and electromagnetic devices to ensure that the photons were as isolated from the environment as possible, Wang said.

That’s because at large scales, quantum superposition tends to disappear almost instantaneously, as soon as these superimposed subatomic particles whose fates are linked interact with the environment. Most of the time, this so-called decoherence would happen so quickly that researchers would have no time to observe the superposition, Wang said. So devices that keep coherence (or keep the particles in superposition) for long periods of time, known as the quality factor, is extremely important, Wang added.

“The quality of these things determines once you put a single excitation into the system, how long does it live, or does it die away,” Wang told Live Science.

If the excitation of the system — the production of the electromagnetic standing wave — is similar to the swing of a pendulum, then “our pendulum swings essentially tens of billions of times before it stops.”

The new findings could make for easier error correction in quantum computing, Wang said. In quantum computing, bits of information are encoded in the fragile superposition states of particles, and once that superposition is lost or corrupted, the data is also corrupted. So most quantum computing concepts involve a lot of redundancy.

“It’s well understood that 99 percent of computation or more will be done to correct for errors, rather than computation itself,” Wang said.

Their system could conceivably get around this problem by encoding the redundancy in the size of the cavity itself rather than in separate, calculated bits, Wang said.

“Demonstrating this cat in a ‘two boxes state’ is basically the first step in our architecture,” Wang said.

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