Remote connections? Entanglement entanglement in quantum physics

Quantum computers, quantum cryptography and the quantum (insert name here) are often topical these days. Articles about them inevitably make reference to tangle, a property of quantum physics that makes all these magical devices possible.

Einstein described the tangle of "spooky action at a distance," a name that stuck and became more and more popular. Beyond building better quantum computers, understanding and exploiting entanglement are also helpful in other respects.

For example, it can be used to make more accurate measurements of gravitational waves and to better understand the properties of exotic materials. It's also subtle in other places: I'm studying how clanging atoms intertwine to understand how this affects the accuracy of atomic clocks.

But what is tangle? Is there a way to understand this "scary" phenomenon? I will try to explain it by bringing together two notions of physics: the laws of conservation and the quantum superpositions.

Conservation Laws

Conservation laws are among the most profound and widespread concepts of all physics. The law of conservation of energy states that the total amount of energy in an isolated system remains fixed (although it can be converted from electrical energy into mechanical energy into heat, etc. .). This law underlies the operation of all our machines, whether they are steam engines or electric cars. Conservation laws are a kind of accounting state: you can trade pieces of energy, but the total amount has to stay the same.

The conservation of the momentum (the mass being the mass multiplied by the speed) is the reason why, when two ice skaters of different masses are detached, the lightest moves away faster than the heavier one. This law also underlies the famous saying that "every action has an equal and opposite reaction". Conservation of angular The reason why, to get back to ice skaters, is why a spinning figure skater can turn faster by bringing her arms closer to her body

These conservation laws have been experimentally verified to operate on an extraordinary range of scales in the universe, from black holes in distant galaxies to smaller spinning electrons.

Quantum addition

Imagine yourself at a beautiful hike through the woods. You arrive at a junction in the trail, but you struggle to decide if you want to go left or right. The path on the left is dark and gloomy but is deemed to lead to pretty views, while the one on the right is sunny but steep. You finally decide to go right by asking yourself nostalgically about the path that was not taken. In a quantum world, you could have chosen both.

For the systems described by quantum mechanics (that is, elements sufficiently isolated from heat and external disturbances), the rules are more interesting. Like a spinning top, an electron, for example, can be in a state where it rotates clockwise or in another state in which it turns in the direction opposite of the needles of a watch. Unlike a router, it can also be in a state [clockwise spinning] + [anticlockwise spinning].

The states of quantum systems can be added and subtracted from each other. Mathematically, the quantum state combination rules can be described in the same way as the vector addition and subtraction rules. The word for such a combination of quantum states is a superposition. That's really behind the strange quantum effects that you may have heard about, such as the double-slit experiment or the particle-wave duality.

Let's say you decide to force an electron into the [clockwise spinning] + [anticlockwise spinning] state of superposition to give a definitive answer. Then the electron ends at random either in the [clockwise spinning] state or in the [anticlockwise spinning] State. The odds of one outcome over the other are easy to calculate (with a good physics book at hand). The intrinsic hazard of this process may bother you if your worldview demands that the universe behave in a totally predictable way, but … it is the (experimentally tested) compete.

Conservation laws and quantum mechanics

Let's put these two ideas together now and apply the law of conservation of energy to a pair of quantum particles.

Imagine a pair of quantum particles (say atoms) that start with a total of 100 energy units. You and your friend separate the pair by taking one each. You find that yours has 40 energy units. By using the energy conservation law, you deduce that the one your friend has must have 60 energy units. As soon as you know the energy of your atom, you immediately know the energy of your friend's atom. You would know it even if your friend did not reveal any information to you. And you would know even if your friend was on the other side of the galaxy when you measured the energy of your atom. Nothing sinister about it (once you realize that this is just a correlation, not a causal link).

But the quantum states of a pair of atoms can be more interesting. The energy of the pair can be divided in several ways (in keeping with the conservation of energy, of course). The combined state of the pair of atoms may be superimposed, for example:

[your atom: 60 units; friend’s atom: 40 units] + [your atom: 70 units; friend’s atom: 30 units].

It's a entangled state of the two atoms. Neither your atom nor that of your friend has an energy defined in this superposition. Nevertheless, the properties of the two atoms are correlated because of the conservation of the energy: their energies always total 100 units.

For example, if you measure your atom and find it in a state of 70 energy units, you can be sure that your friend's atom has 30 energy units. You would know it even if your friend did not reveal any information to you. And thanks to the energy savings, you would know it even if your friend was on the other side of the galaxy.

Nothing sinister about it.

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