# The Physics Of Quantum Information: Quantum Cry...

If that sounds like gobbledygook, don't worry. We got in touch with one of the researchers, physicist Andreas Wallraff, of the Quantum Device Lab at the Swiss Federal Institute of Technology Zurich, to explain how his team and a team based at the University of Tokyo were able to reliably teleport quantum states from one place to another.

## The Physics of Quantum Information: Quantum Cry...

Quantum teleportation relies on something called an entangled state. An entangled state, in the words of Wallraff, is a "state of two quantum bits that share correlations." In other words, it's a state that can't be separated.

"The sender takes one of the bits of the entangled pair, and the receiver takes the other," says Wallraff. "The sender can run a quantum computing program measuring his part of the entangled pair as well as what he wants to transport, which is a qubit in an unknown state."

Dividing or multiplying numbers is fairly easy for any computer, but determining the factors of a really large 500- or 600-digit number is next to impossible for classical computers. But quantum computers can process these numbers easily and simultaneously.

Credit card companies, for instance, assign users a public key to encode credit card information. The key is the product of two large prime numbers, which only the website seller knows. Without a quantum computer, it would be impossible to figure out the two prime numbers that are multiplied together to make the key-which protects your information from being shared. (For more info, read this really useful guide about the basics of quantum computing from the University of Waterloo.)

We hope that the reader will enjoy this special issue, which can be useful to experts working in all domains of quantum physics and quantum information theory, ranging from experimenters, to theoreticians and philosophers.

1935 Albert Einstein, Boris Podolsky, and Nathan Rosen publish a paper highlighting the counterintuitive nature of quantum superpositions and arguing that the description of physical reality provided by quantum mechanics is incomplete

1980 Paul Benioff of the Argonne National Laboratory publishes a paper describing a quantum mechanical model of a Turing machine or a classical computer, the first to demonstrate the possibility of quantum computing

1981 In a keynote speech titled Simulating Physics with Computers, Richard Feynman of the California Institute of Technology argues that a quantum computer had the potential to simulate physical phenomena that a classical computer could not simulate

1994 Peter Shor of Bell Laboratories develops a quantum algorithm for factoring integers that has the potential to decrypt RSA-encrypted communications, a widely-used method for securing data transmissions

2014 Physicists at the Kavli Institute of Nanoscience at the Delft University of Technology, The Netherlands, teleport information between two quantum bits separated by about 10 feet with zero percent error rate

2018 The National Quantum Initiative Act is signed into law by President Donald Trump, establishing the goals and priorities for a 10-year plan to accelerate the development of quantum information science and technology applications in the United States

2019 Google claims to have reached quantum supremacy by performing a series of operations in 200 seconds that would take a supercomputer about 10,000 years to complete; IBM responds by suggesting it could take 2.5 days instead of 10,000 years, highlighting techniques a supercomputer may use to maximize computing speed

Three scientists jointly won this year's Nobel Prize in physics Tuesday for proving that tiny particles could retain a connection with each other even when separated, a phenomenon once doubted but now being explored for potential real-world applications such as encrypting information. googletag.cmd.push(function() googletag.display('div-gpt-ad-1449240174198-2'); ); Frenchman Alain Aspect, American John F. Clauser and Austrian Anton Zeilinger were cited by the Royal Swedish Academy of Sciences for experiments proving the "totally crazy" field of quantum entanglements to be all too real. They demonstrated that unseen particles, such as photons, can be linked, or "entangled," with each other even when they are separated by large distances.

Clauser, 79, was awarded his prize for a 1972 experiment, cobbled together with scavenged equipment, that helped settle a famous debate about quantum mechanics between Einstein and famed physicist Niels Bohr. Einstein described "a spooky action at a distance" that he thought would eventually be disproved.

In quantum entanglement, establishing common information between two photons not near each other "allows us to do things like secret communication, in ways which weren't possible to do before," said David Haviland, chair of the Nobel Committee for Physics.

Quantum information "has broad and potential implications in areas such as secure information transfer, quantum computing and sensing technology," said Eva Olsson, a member of the Nobel committee. "Its predictions have opened doors to another world, and it has also shaken the very foundations of how we interpret measurements."

While quantum entanglement is "incredibly cool" security technologist Bruce Schneier, who teaches at Harvard, said it is fortifying an already secure part of information technology where other areas, including human factors and software are more of a problem. He likened it to installing a side door with 25 locks on an otherwise insecure house.

The Nobel committee said Clauser developed quantum theories first put forward in the 1960s into a practical experiment. Aspect was able to close a loophole in those theories, while Zeilinger demonstrated a phenomenon called quantum teleportation that effectively allows information to be transmitted over distances.

Alain Aspect, John Clauser and Anton Zeilinger have each conducted groundbreaking experiments using entangled quantum states, where two particles behave like a single unit even when they are separated. Their results have cleared the way for new technology based upon quantum information.

The ineffable effects of quantum mechanics are starting to find applications. There is now a large field of research that includes quantum computers, quantum networks and secure quantum encrypted communication.

One key factor in this development is how quantum mechanics allows two or more particles to exist in what is called an entangled state. What happens to one of the particles in an entangled pair determines what happens to the other particle, even if they are far apart.

For a long time, the question was whether the correlation was because the particles in an entangled pair contained hidden variables, instructions that tell them which result they should give in an experiment. In the 1960s, John Stewart Bell developed the mathematical inequality that is named after him. This states that if there are hidden variables, the correlation between the results of a large number of measurements will never exceed a certain value. However, quantum mechanics predicts that a certain type of experiment will violate Bell's inequality, thus resulting in a stronger correlation than would otherwise be possible.

John Clauser developed John Bell's ideas, leading to a practical experiment. When he took the measurements, they supported quantum mechanics by clearly violating a Bell inequality. This means that quantum mechanics cannot be replaced by a theory that uses hidden variables.

Using refined tools and long series of experiments, Anton Zeilinger started to use entangled quantum states. Among other things, his research group has demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance.

"It has become increasingly clear that a new kind of quantum technology is emerging. We can see that the laureates' work with entangled states is of great importance, even beyond the fundamental questions about the interpretation of quantum mechanics," says Anders IrbÃ¤ck, Chair of the Nobel Committee for Physics.

Using groundbreaking experiments, Alain Aspect, John Clauser and Anton Zeilinger have demonstrated the potential to investigate and control particles that are in entangled states. What happens to one particle in an entangled pair determines what happens to the other, even if they are really too far apart to affect each other. The laureates' development of experimental tools has laid the foundation for a new era of quantum technology.

The fundamentals of quantum mechanics are not just a theoretical or philosophical issue. Intense research and development are underway to utilise the special properties of individual particle systems to construct quantum computers, improve measurements, build quantum networks and establish secure quantum encrypted communication.

Many applications rest upon how quantum mechanics allow two or more particles to exist in a shared state, regardless of how far apart they are. This is called entanglement, and has been one of the most debated elements of quantum mechanics ever since the theory was formulated. Albert Einstein talked about spooky action at a distance and Erwin SchrÃ¶dinger said it was quantum mechanics' most important trait.

When two particles are in entangled quantum states, someone who measures a property of one particle can immediately determine the result of an equivalent measurement on the other particle, without needing to check.

What makes quantum mechanics so special is that its equivalents to the balls have no determined states until they are measured. It is as if both the balls are grey, right up until someone looks at one of them. Then, it can randomly take either all the black the pair of balls has access to, or can show itself to be white. The other ball immediately turns the opposite colour.

Quantum mechanics' entangled pairs can be compared to a machine that throws out balls of opposite colours in opposite directions. When Bob catches a ball and sees that it is black, he immediately knows that Alice has caught a white one. In a theory that uses hidden variables, the balls had always contained hidden information about what colour to show. However, quantum mechanics says that the balls were grey until someone looked at them, when one randomly turned white and the other black. Bell inequalities show that there are experiments that can differentiate between these cases. Such experiments have proven that quantum mechanics' description is correct. 041b061a72