Untangle, Entangle

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Alice and her friends first discover the quantum technologies building because of the poster exhibition-- in undergraduate-speak, free food. It's a cool place, what with the backdrop of glass doors with blue bubble motifs; or, for the more literal minded, the poster proclaiming the low temperatures reached in the macroscopic quantum phenomena labs. Because Alice, Bob, and Eve are the Tom, Dick, and Harry of cryptography, Alice can't help but do a double-take at the quantum cryptography posters.


"But don't you need entangled pairs at this step, since you're testing for the violation of Bell's inequality?" says Luke from physics, spewing crumbs at the presenter in his excitement.


Alice makes little sense of their discussion, with the the '<'s and '>'s encasing the Greek symbols, and the multiplication signs with circles drawn around them. It feels like the Star Wars clips Alice had found online after watching Episode I on television. The names of Alderaan, Dantooine, and Coruscant had just flown by her head, and she'd only remembered the Emperor proclaiming, "The strings of the Force grow taut, Lord Vader, and soon we shall play a tune upon them." Alice had felt like one of the troops: simultaneously clueless and awed.




"Why the frown, Alice? Pining after Bob?" says Luke, when they're using the corridor as a shortcut. In retaliation, Alice mimes a Force choke at him, and her lips quirk up as he pretends to gag.


"Introduction to quantum computing," Luke reads off her smartphone screen. "Signing up?"


"After you guys complained about losing your mental virginity after the physics test screwed everyone?" Alice is wary. It'll be more work, and she's already struggling to maintain her A- average.


"It's quantum computing," Luke says. Before Alice can add that it's worse-- she's never touched a transistor in her life-- he continues, "At that level of abstraction, quantum bits are conceptually more accessible than, say, solving for the wavefunction of the hydrogen atom."


Alice is tempted by the thought that the key to the mysteries in the poster-filled corridor lies within reach. Alice feels the same itch that had led her to watch all the Star Wars movies in one sitting, to understand the enigmatic comments that had felt somehow significant. Besides, the lecturer claims that no prior knowledge of quantum physics is required, so she takes the quantum leap and signs up.




It's fun to have solved the mystery of the angular brackets. Alice stops fearing that her physicist friends are about to conduct a laundry raid when she hears them talking about bras, and she learns to associate kets with vectors rather than furry clawed beasts. 


Alice feels like she's gazing upon the Rosetta stone: one part the quantum concepts of measurements and states, the other familiar linear algebra concepts of matrices and vector spaces. Instead of imagining a tiny ruler next to a nearly-invisible particle, Alice now thinks of Darth Vader rhapsodizing about the infinity of space and the boundlessness of the starscape, wondering if these Star Wars guys were actually closet quantum physicists.


It's not so fun when she can't do the assignment. 


"It's a standard inner-product calculation." Luke says, after a glance. 


"Just because I can parse the hieroglyphs doesn't mean I can speak Egyptian," Alice grumbles. It annoys her, having to plough through the unfamiliar expressions, knowing that the question would be easy in another notation.


The upside is that the formulas enhance precision. Before, Alice's least misleading explanation of entanglement particles was to cast them as each other's voodoo doll: stick a pin into one, and all the others will feel its effect. Now, at least, she knows why quantum teleportation using entanglement is more moving tensor product signs than cable-tying an object to a gleaming metallic machine. 


Whether that represents an improvement in her life is questionable, though.


The night before her numerical linear algebra exam, Alice finds herself commenting on an article describing the effect of a measurement on a particle's "entangled partner".


“It's misleading,” Alice says to Luke over Facebook. “Not all two-photon states are entangled.”


“Neither are all entangled states two-photon states,” Luke agrees.


“The reporter must think entanglement is like the red thread of fate that supposedly ties lovers together," Alice replies, pleased at having made a valid point.


The next day, during the exam, she mistakes the '+' denoting the matrix pseudoinverse for the 'dagger' physicists use to denote the Hermitian. Alice can bid goodbye to her A- average.


She storms through her usual shortcut, furious at the waste of last night, the waste of an entire semester.


But that's the same corridor where Alice had overheard a discussion about a quantum estimation algorithm for plugging holes in gene expression matrices; where Alice'd somehow googled her way to an argument that Bohr's interpretation of quantum physics is consistent with neo-pragmatist irony, and even a film about a quantum love story. She can't ignore the possibility of a state of non-wastage.


For now, looking at the posters, Alice can at least say that it's been like extending a hand to the towering, hairy bipeds of an alien planet without having her arm torn out at the socket or clawed open at the point where palm meets paw. It'd been scary, but now, looking at the references to  Bell's inequalities and Bloch spheres, she gets to think, "Ah, we've met." 


It's like having a single dragline that stretches across the web of knowledge, the one thread in the tangle of fate that Alice can follow, in the words of the young queen from Star Wars. It amuses Alice, the notion of a correspondence principle between quantum physics and Star Wars. It's as if beyond one of the glass doors lies a distant galaxy, where the queen is forging an unprecedented alliance between the two historically antagonistic races of the land, towards a victory that lies just slightly beyond the horizon.

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Quantum Theories

P is for ... Probability

Quantum mechanics is a probabilistic theory: it does not give definite answers, but only the probability that an experiment will come up with a particular answer. This was the source of Einstein’s objection that God “does not play dice” with the universe.

W is for ... Wave-particle duality

It is possible to describe an atom, an electron, or a photon as either a wave or a particle. In reality, they are both: a wave and a particle.

D is for ... Decoherence

Unless it is carefully isolated, a quantum system will “leak” information into its surroundings. This can destroy delicate states such as superposition and entanglement.

B is for ... Bose-Einstein Condensate (BEC)

At extremely low temperatures, quantum rules mean that atoms can come together and behave as if they are one giant super-atom.

E is for ... Entanglement

When two quantum objects interact, the information they contain becomes shared. This can result in a kind of link between them, where an action performed on one will affect the outcome of an action performed on the other. This “entanglement” applies even if the two particles are half a universe apart.

H is for ... Hidden Variables

One school of thought says that the strangeness of quantum theory can be put down to a lack of information; if we could find the “hidden variables” the mysteries would all go away.

S is for ... Superposition

Quantum objects can exist in two or more states at once: an electron in superposition, for example, can simultaneously move clockwise and anticlockwise around a ring-shaped conductor.

T is for ... Tunnelling

This happens when quantum objects “borrow” energy in order to bypass an obstacle such as a gap in an electrical circuit. It is possible thanks to the uncertainty principle, and enables quantum particles to do things other particles can’t.

A is for ... Act of observation

Some people believe this changes everything in the quantum world, even bringing things into existence.

F is for ... Free Will

Ideas at the heart of quantum theory, to do with randomness and the character of the molecules that make up the physical matter of our brains, lead some researchers to suggest humans can’t have free will.

Q is for ... Quantum biology

A new and growing field that explores whether many biological processes depend on uniquely quantum processes to work. Under particular scrutiny at the moment are photosynthesis, smell and the navigation of migratory birds.

R is for ... Randomness

Unpredictability lies at the heart of quantum mechanics. It bothered Einstein, but it also bothers the Dalai Lama.

N is for ... Nonlocality

When two quantum particles are entangled, it can also be said they are “nonlocal”: their physical proximity does not affect the way their quantum states are linked.

Z is for ... Zero-point energy

Even at absolute zero, the lowest temperature possible, nothing has zero energy. In these conditions, particles and fields are in their lowest energy state, with an energy proportional to Planck’s constant.

W is for ... Wavefunction

The mathematics of quantum theory associates each quantum object with a wavefunction that appears in the Schrödinger equation and gives the probability of finding it in any given state.

U is for ... Universe

To many researchers, the universe behaves like a gigantic quantum computer that is busy processing all the information it contains.

B is for ... Bell's Theorem

In 1964, John Bell came up with a way of testing whether quantum theory was a true reflection of reality. In 1982, the results came in – and the world has never been the same since!

O is for ... Objective reality

Niels Bohr, one of the founding fathers of quantum physics, said there is no such thing as objective reality. All we can talk about, he said, is the results of measurements we make.

C is for ... Computing

The rules of the quantum world mean that we can process information much faster than is possible using the computers we use now.

H is for ... Hawking Radiation

In 1975, Stephen Hawking showed that the principles of quantum mechanics would mean that a black hole emits a slow stream of particles and would eventually evaporate.

U is for ... Uncertainty Principle

One of the most famous ideas in science, this declares that it is impossible to know all the physical attributes of a quantum particle or system simultaneously.

K is for ... Kaon

These are particles that carry a quantum property called strangeness. Some fundamental particles have the property known as charm!

D is for ... Dice

Albert Einstein decided quantum theory couldn’t be right because its reliance on probability means everything is a result of chance. “God doesn’t play dice with the world,” he said.

J is for ... Josephson Junction

This is a narrow constriction in a ring of superconductor. Current can only move around the ring because of quantum laws; the apparatus provides a neat way to investigate the properties of quantum mechanics.

T is for ... Teleportation

Quantum tricks allow a particle to be transported from one location to another without passing through the intervening space – or that’s how it appears. The reality is that the process is more like faxing, where the information held by one particle is written onto a distant particle.

X is for ... X-ray

In 1923 Arthur Compton shone X-rays onto a block of graphite and found that they bounced off with their energy reduced exactly as would be expected if they were composed of particles colliding with electrons in the graphite. This was the first indication of radiation’s particle-like nature.

S is for ... Schrödinger’s Cat

A hypothetical experiment in which a cat kept in a closed box can be alive and dead at the same time – as long as nobody lifts the lid to take a look.

L is for ... Large Hadron Collider (LHC)

At CERN in Geneva, Switzerland, this machine is smashing apart particles in order to discover their constituent parts and the quantum laws that govern their behaviour.

A is for ... Atom

This is the basic building block of matter that creates the world of chemical elements – although it is made up of more fundamental particles.

R is for ... Radioactivity

The atoms of a radioactive substance break apart, emitting particles. It is impossible to predict when the next particle will be emitted as it happens at random. All we can do is give the probability that any particular atom will have decayed by a given time.

G is for ... Gravity

Our best theory of gravity no longer belongs to Isaac Newton. It’s Einstein’s General Theory of Relativity. There’s just one problem: it is incompatible with quantum theory. The effort to tie the two together provides the greatest challenge to physics in the 21st century.

V is for ... Virtual particles

Quantum theory’s uncertainty principle says that since not even empty space can have zero energy, the universe is fizzing with particle-antiparticle pairs that pop in and out of existence. These “virtual” particles are the source of Hawking radiation.

I is for ... Information

Many researchers working in quantum theory believe that information is the most fundamental building block of reality.

C is for ... Cryptography

People have been hiding information in messages for millennia, but the quantum world provides a whole new way to do it.

I is for ... Interferometer

Some of the strangest characteristics of quantum theory can be demonstrated by firing a photon into an interferometer: the device’s output is a pattern that can only be explained by the photon passing simultaneously through two widely-separated slits.

R is for ... Reality

Since the predictions of quantum theory have been right in every experiment ever done, many researchers think it is the best guide we have to the nature of reality. Unfortunately, that still leaves room for plenty of ideas about what reality really is!

L is for ... Light

We used to believe light was a wave, then we discovered it had the properties of a particle that we call a photon. Now we know it, like all elementary quantum objects, is both a wave and a particle!

M is for ... Multiverse

Our most successful theories of cosmology suggest that our universe is one of many universes that bubble off from one another. It’s not clear whether it will ever be possible to detect these other universes.

M is for ... Many Worlds Theory

Some researchers think the best way to explain the strange characteristics of the quantum world is to allow that each quantum event creates a new universe.

S is for ... Schrödinger Equation

This is the central equation of quantum theory, and describes how any quantum system will behave, and how its observable qualities are likely to manifest in an experiment.

Y is for ... Young's Double Slit Experiment

In 1801, Thomas Young proved light was a wave, and overthrew Newton’s idea that light was a “corpuscle”.

P is for ... Planck's Constant

This is one of the universal constants of nature, and relates the energy of a single quantum of radiation to its frequency. It is central to quantum theory and appears in many important formulae, including the Schrödinger Equation.

A is for ... Alice and Bob

In quantum experiments, these are the names traditionally given to the people transmitting and receiving information. In quantum cryptography, an eavesdropper called Eve tries to intercept the information.

G is for ... Gluon

These elementary particles hold together the quarks that lie at the heart of matter.

Q is for ... Qubit

One quantum bit of information is known as a qubit (pronounced Q-bit). The ability of quantum particles to exist in many different states at once means a single quantum object can represent multiple qubits at once, opening up the possibility of extremely fast information processing.