Amar Vutha, * University of Toronto *

Quantum computers, quantum cryptography and Quantum (insert name here) are common in the news these days. Articles about them inevitably refer to the entanglement of * * a property of quantum physics that enables all these magical devices.

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Einstein called the entanglement "creepy long-range effect", a name that becomes stuck and becomes more and more popular. Apart from building better quantum computers, interlacing and using entanglement is also useful in other ways.

For example, it can be used to make more accurate measurements of gravitational waves and to better understand the properties of exotic materials. It also subtly shows itself in other places: I've studied how atoms that bump into each other are involved in understanding how this affects the accuracy of atomic clocks.

But what is * * entanglement? Is there a way to understand this "scary" phenomenon? I will try to explain it by bringing together two terms from physics: conservation laws and quantum overlays.

## Conservation laws

Conservation laws are some of the deepest and most pervasive concepts in physics. The Energy Conservation Act states that the total amount of energy remains fixed in an isolated system (although it can be converted from electrical energy to mechanical energy to heat). This law is based on the operation of all our machines, be it steam engines or electric cars. Conservation rates are a kind of accounting statement: you can exchange a bit of energy, but the total amount must stay the same.

The conservation of momentum (momentum is mass times velocity) is the reason why two skaters with different masses are pushing each other off, the lighter one moves away faster than the heavier one. This law is also based on the famous dictum that "every action has an equal and opposite reaction". The conservation of * angular * impulse is the reason that a swirling figure skater, by pulling her arms, can go faster when she resorts to skaters closer to her body.

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

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## Quantum Addition

Imagine a beautiful hike through the forest. You come to a fork in the road, but you do not have to decide if you want to go left or right. The way to the left looks dark and gloomy, but should lead to beautiful views, while the right looks sunny but steep. They finally decide to drive to the right and wonder wistfully that the road will not be used. In a quantum world you could have chosen both.

For systems described by quantum mechanics (ie, things that are sufficiently well insulated against heat and external disturbances), the rules are more interesting. For example, as with a gyro, an electron may be in a state of rotating clockwise or in another state of rotating counter-clockwise. However, unlike a gyro, it may also be in a state that is *[clockwise spinning] + [anticlockwise spinning]* .

* The states of quantum systems can be added and subtracted from each other *. Mathematically, the rules for combining quantum states can be described in the same way as the rules for adding and subtracting vectors. The word for such a combination of quantum states is a * overlay *. This is really what stands behind strange quantum effects of which you may have heard, such as the double-slit experiment or the particle-wave duality.

Suppose you decide to force an electron in the superposition state *[clockwise spinning] + [anticlockwise spinning]* to get a definite answer. Then the electron ends randomly either in the state *[clockwise spinning]* or in the state * [anticlockwise spinning]*. The chances of one result compared to the other are easy to calculate (with a good physics book at hand). The intrinsic randomness of this process may bother you if the universe requires a completely predictable behavior of the universe, but … * c & # 39; est la * (experimentally tested) * vie *. Conservation Laws and Quantum Mechanics

Let us now summarize these two ideas and apply the Law of Energy Conservation to a quantum particle pair.

Imagine a quantum particle pair (say atoms) starting with a sum of 1

But the quantum states of a nuclear pair may be more interesting. The energy of the couple can be split in many different ways (of course, in line with the energy savings). For example, the combined state of the pair of atoms may be superimposed:

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

This is a * entangled state * of the two atoms. Neither your atom nor your friend's has any energy in this superimposition. Nevertheless, the properties of the two atoms are related due to energy conservation: their energy always adds up to 100 units.

For example, if you measure your atom and find it in a state of 70 energy units, you can do that. Make sure your friend's Atom has 30 energy units. You would also know if your friend has never revealed information to you. And thanks to the conservation of energy, you would also know that when your friend is on the other side of the galaxy.

There is nothing scary about that.

Amar Vutha, Assistant Professor of Physics, * University of Toronto *

This article has been published under The Conversation under a Creative Commons License. Read the original article.

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