- 3297 - QUANTUM MECHANICS - enters new dimensions? A new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways. The new tool could be useful in applications as diverse as mapping the electrical impulses inside a firing neuron, characterizing new magnetic materials, and probing exotic quantum physical phenomena.
------------- 3297 - QUANTUM MECHANICS - enters new dimensions?
- Quantum mechanics is really confusing. All the rules of physics that we're used to simply do not work in the quantum realm.
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- In classical thinking, you can measure the momentum and position of something to an arbitrary degree of precision. Not so in the quantum world. The more you know about one, the less you know about the other.
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- Is something a wave or a particle? According to the classical viewpoint, you can pick one and only one. But in quantum mechanics it can be both at the same time.
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- The quantum world is hard to understand Yet at some point the rules of the subatomic give way to the rules of the macroscopic that we thought we understood. But how does this transition occur? We're not exactly sure, and it's been a long, strange journey in trying to answer that question.
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- The first person to put some useful labels on the quantum world was physicist Niels Bohr. In the early 1900s, scientists around the world were beginning to awaken to the strange and unexpected behavior of atomic and subatomic world.
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- They had, after decades of grueling work, realized that certain properties, like energy, come in discrete packets of levels named "quanta." And while physicists were beginning to sketch out a mathematical foundation to explain these experiments, nobody had yet developed a complete, consistent framework.
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- Bohr was one of the first to attempt did lay some serious groundwork. He also promoted some ideas that would become the cornerstones of modern quantum theory.
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- The first appeared in his early attempt to model the atom. In the 1920s, we had known through a variety of very cool experiments that the atom is made of a heavy, dense, positively charged nucleus surrounded by a swarm of tiny, light, negatively charged electrons. We also knew that these atoms could only absorb or emit radiation at very specific energies.
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- Bohr put the electrons "in orbit" around the nucleus, circling around that dense core like planets in an tiny ‘solar system“. In a real solar system, the planets can have whatever orbit they like. But in Bohr's atom the electrons were stuck on little tracks. They could only have certain predefined orbital distances.
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- By jumping from one orbit to another, the atom could receive or emit radiation at specific energies. Its quantum nature was thus “encoded“.
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- But Bohr added one more interesting twist. There are a lot of potential ways to construct a quantum model of the atom. Why should this one be used? He found that when the electrons orbited very far away from the nucleus, their quantum nature disappeared and the atom could be perfectly described by classical electromagnetism. Just like two charged particles.
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- This was called the “Correspondence Principle“, and it was Bohr's argument that his model of the atom. You can have any quantum theory you want, but the right ones are the ones that give way to classical physics under some limit. In the case of his atom, when the electrons got far away from the nucleus.
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- Bohr's model of the atom would later be replaced by the “valence shell model” that remains to this day. Bohr argued that, even though this Correspondence Principle allowed a connection between the quantum and classical worlds, those two worlds are not the same.
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- Around the same time Werner Heisenberg came up with his soon-to-be-famous “Uncertainty Principle“. Try to measure the position of a tiny particle, and you'll end up losing information about its momentum. Go for the opposite, trying to pin down its momentum, and you'll become ignorant about its position.
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- Bohr saw Heisenberg's Uncertainty Principle as a part of a much larger facet of the quantum world: that everything comes in pairs. Consider the most famous pair in the quantum world, the wave and the particle.
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- In classical systems, something is either purely a wave or purely a particle. You can pick one or the other to classify some behavior. But in quantum mechanics, these two properties are paired up: everything is simultaneously both a particle and a wave and always exhibits some properties of both.
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- Quantum Mechanic’s rules rely on “probabilities“. Quantum mechanics only reproduces classical physics on average. Based on these two insights, Bohr argued that a quantum theory can never explain classical physics.
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- Atoms operate under one set of rules, and trains people to operate on another set of rules. The rules connected via the Correspondence Principle, but otherwise they live separate and parallel lives.
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- How do we study this quantum world? A new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways. The new tool could be useful in applications as diverse as mapping the electrical impulses inside a firing neuron, characterizing new magnetic materials, and probing exotic quantum physical phenomena.
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- The new technique builds on a platform already developed to probe magnetic fields with high precision, using tiny defects in diamond called “nitrogen-vacancy centers“. These defects consist of two adjacent places in the diamond's orderly lattice of carbon atoms where carbon atoms are missing; one of them is replaced by a nitrogen atom, and the other is left empty.
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- This leaves missing bonds in the structure, with electrons that are extremely sensitive to tiny variations in their nitrogen-variant environment, be they electrical, magnetic, or light-based.
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- Previous uses of single nitrogen-variant centers to detect magnetic fields have been extremely precise but only capable of measuring those variations along a single dimension, aligned with the sensor axis.
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- The new method is using a secondary oscillator provided by the nitrogen atom's nuclear spin. The sideways component of the field to be measured nudges the orientation of the secondary oscillator.
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- By knocking it slightly off-axis, the sideways component induces a kind of wobble that appears as a periodic fluctuation of the field aligned with the sensor, thus turning that perpendicular component into a wave pattern superimposed on the primary, static magnetic field measurement. This can then be mathematically nitrogen-variant inverted back to determine the magnitude of the sideways component.
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- In order to read out the results, the researchers use an “optical confocal microscope” that makes use of a special property of the nitrogen-variant centers: When exposed to green light, they emit a red glow, or fluorescence, whose intensity depends on their exact spin state. These nitrogen-variant centers can function as qubits, the quantum-computing equivalent of the bits used in ordinary, 1 or zero, computing.
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- We can tell the spin state from the fluorescence. If it's dark," producing less fluorescence, "that's a 'one' state, and if it's bright, that's a 'zero' state. If the fluorescence is some number in between then the spin state is somewhere in between 'zero' and 'one.'
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- The primary field is indicated by the overall, steady brightness level, whereas the wobble introduced by knocking the magnetic field off-axis shows up as a regular, wave-like variation of that brightness, which can then be measured precisely.
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- This new magnetic sensor system can work well at ordinary room temperature making it feasible to test biological samples without damaging them.
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- This is the first step toward “vector nanoscale magnetometry“. It remains to be seen whether this technique can indeed be applied to actual samples, such as molecules or condensed matter systems.
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- This is a new way of measuring atomic-scale magnetic fields with great precision, not only up and down but sideways as well. The new tool could be useful in applications as diverse as mapping the electrical impulses inside a firing neuron, characterizing new magnetic materials, and probing exotic quantum physical phenomena.
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- The more we learn the more we need to know.
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- October 7, 2021 QUANTUM MECHANICS - new dimensions? 3297
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