Physicists set quantum record by using photons to carry messages from electrons almost 2 kilometers apart

November 25, 2015 by Bethany Augliere

Researchers from Stanford have advanced a long-standing problem in quantum physics – how to send "entangled" particles over long distances.

Their work is described in the online edition of Nature Communications.

Scientists and engineers are interested in the practical application of this technology to make networks that can send highly secure information over long distances – a capability that also makes the technology appealing to governments, banks and militaries.

Quantum entanglement is the observed phenomenon of two or more particles that are connected, even over thousands of miles. If it sounds strange, take comfort knowing that Albert Einstein described this behavior as "spooky action."

Consider, for instance, entangled electrons. Electrons spin in one of two characteristic directions, and if they are entangled, those two electrons' spins are linked. It's as if you spun a quarter in New York clockwise, an entangled second coin in Los Angeles would start to spin clockwise. And likewise, if you spun that quarter counter-clockwise, the second coin would shift its spin as well.

Electrons are trapped inside atoms, so entangled electrons can't talk directly at long distance. But photons – tiny particles of light – can move. Scientists can establish a necessary condition of entanglement, called quantum correlation, to correlate photons to electrons, so that the photons can act as the messengers of an electron's spin.

In his previous work, Stanford physicist Leo Yu has entangled photons with electrons through fiber optic cables over a distance of several feet. Now, he and a team of scientists, including Professor Emeritus Yoshihisa Yamamoto, have correlated photons with over a record distance of 1.2 miles (1.93 kilometers).

This nonlinear optical wave guide converts the wavelength of a single-photon signal to a common telecom wavelength.

"Electron spin is the basic unit of a quantum computer," Yu said. "This work can pave the way for future that can send highly secure data around the world."

To do this, Yu and his team had to make sure that the correlation could be preserved over – a key challenge given that photons have a tendency to change orientation while traveling in optical fibers.

Photons can have a vertical or horizontal orientation (known as polarization), which can be referenced as a 0 or a 1, as in digital computer programming. But if they change en route, the connection to the correlated electron is lost.

This information can be preserved in another way, Yu said. He created a time-stamp to correlate arrival time of the photon with the electron spin, which provided a sort of reference key for each photon to confirm its correlation to the source electron.

To eventually entangle two that had never met over great distances, two photons, each correlated with a unique source electron, had to be sent through fiber optic cables to meet in the middle at a "beam splitter" and interact. Photons do not normally interact, just two flashlights beams passing through one another, so the researchers had to mediate this interaction called the "two-photon interference."

To ensure the two-photon interference, they had another issue to overcome. Photons from two different sources have different characteristics, like color and wavelength. If they have different wavelengths, they cannot interfere, Yu said. Before traveling along the fiber optic cable, the photons passed through a "quantum down-converter," which matched their wavelengths. The down-converter also shifted both to a wavelength that can travel farther within the designed for telecommunications.

Quantum supercomputers promise to be exponentially faster and more powerful than traditional computers, Yu said, and can communicate with immunity to hacking or spying. With this work, the team has brought the quantum networks one step closer to reality.

Explore further: Producing spin-entangled electrons

More information: Leo Yu et al. Two-photon interference at telecom wavelengths for time-bin-encoded single photons from quantum-dot spin qubits, Nature Communications (2015). DOI: 10.1038/ncomms9955

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antialias_physorg
4.2 / 5 (5) Nov 25, 2015
Physicists set quantum record by using photons to carry messages from electrons almost 2 kilometers apart

Careful. A message is a very rigidly defined term in information theory (Something where you set information, transport it and then decode it).
Entanglement does not let you do the first part (set the information). That is why spooky action at a distance does not constitute information transmission...and consequently does not violate the speed of light limit for information transmission.

a capability that also makes the technology appealing to governments, banks and militaries.

Military? yes. Banks? Yes. Governments? Not so much. They don't like it when they can't snoop on their citizens.

It's as if you spun a quarter in New York clockwise, an entangled second coin in Los Angeles would start to spin clockwise.

Bad analogy since you cannot choose the spin only measure it. That would be setting information. And that is not allowed (see above).
Jim4321
3 / 5 (2) Nov 25, 2015
This is very confusing, antialias. Your point about setting information seems valid. However, the authors say "The quantum-networking operations that we demonstrate will enable practical communication between solid-state spin qubits across long distances."
Perhaps they used the word "communication" loosely and didn't mean what the common language implies. Maybe they meant something like, we can set the initial conditions for the two separated electrons and they measure their evolution at some later time, which has implications for computing. Any ideas?
antialias_physorg
4.2 / 5 (5) Nov 25, 2015
not rated yet 16 minutes ago
This is very confusing, antialias. Your point about setting information seems valid. However, the authors say "The quantum-networking operations that we demonstrate will enable practical communication between solid-state spin qubits across long distances."

There's a couple of things in this:
1) qbits have to be set (i.e. the entanglement has to be distributed to two locations). This is done via photons at the speed of light (so no FTL information transmission happening)
2) The entanglement means the states of the qbits is then correlated (lets call the two possible states 'horizontal' and 'vertical'). While you don't know which will have the 'horizontal' and which will have the 'vertical' state when measuered you do know that if one has 'horizontal' the other has 'vertical'. Thus this is very good for synchronization (or encryption, which does not add information, and therefore does not constitute information transmission, either)
antialias_physorg
4.2 / 5 (5) Nov 25, 2015
we can set the initial conditions for the two separated electrons

That doesn't work, as a setting is a measurement - and that immediately breaks entanglement.
You can set electrons to an entangled state. But that always means that you do not know which of your electrons will come up 'horizontal' and which 'vertical' until measured (i.e. until entaglement is broken).

Addendum re. encryption: This is actually the most beautiful proof (because it isn't mathematical but follows directly from physics!) that encryption does not add information to a message. (Which is somewhat surprising/counterintuitive, but when you go into information theory it turns out to be true.) It's one of these things that shows us that describing reality via math/physics is actually a valid approach.
Jim4321
not rated yet Nov 25, 2015
There is an interpretation of quantum mechanics called "the Ithaca interpretation" by Mermin. It basically does away with the idea of events and works as far as I know with conditional probabilities; if this, then that. Sort of like Heisenberg's derivation of the matrix mechanics. It also goes under the rubric "correlations without correlata". Perhaps, the confusion with these EPR type measurements is that we introduce the "metaphysical" construct of an event. No wonder Einstein was upset.
jalmy
1 / 5 (3) Nov 25, 2015
I have a feeling that time has more to do with entanglement than people realize.
baudrunner
not rated yet Nov 25, 2015
The spin of the electron is the information, where horizontal spin could represent the 1, or "on" state, and vertical spin the 0, or "off" state. This is not modulation of the electron shell of a particle (atom/molecule) in the propagating medium, as occurs in normal light transmission, where the modulated information represents the characteristics of reflecting surfaces, but control of the spin of entangled particles to send information under controlled conditions. Therefore, entangled particles can indeed be information carriers, so long as the entangled state can be maintained.
NoStrings
3 / 5 (2) Nov 25, 2015
baudrunner, @all. Yes, everyone says that entangled particles can be information carriers. I don't know about anyone sending any MESSAGE, even using Morse code.
Please, if someone has a link to a study that passes information, not just 'can', please paste it here. Otherwise, this study is only proving the obvious for the thousands time, that entangled particles remain entangled even half way across the universe - until the measurement occurs, at which point we can also know the state of the second particle. Or until a particle interacts with something that ruins entanglement, just like measurement does.
antialias_physorg
3 / 5 (4) Nov 26, 2015
Therefore, entangled particles can indeed be information carriers, so long as the entangled state can be maintained.

Careful: "Quantum information carriers" not "information carriers". For it to be information carriers you would have to be able to choose which one is in the 1 position and which one is in the 0 position (you need to be able to *encode* the message). But for quantum information carriers you only know that they are entangled - not which of the entities carries the 1 and which carries the 0. This is why...
I don't know about anyone sending any MESSAGE, even using Morse code.

...quantum information cannot be used for message passing (only for things that do not constitute information transmission like encryption)

It's a little confusing because "quantum information" and "information" sound so much alike. But they are two entirely different critters
https://en.wikipe...ormation
Osiris1
not rated yet Nov 28, 2015
I believe that 'measurement ruining entanglement' is probably an assumption arising from the inability of science of some time in the past of high enough impedance measurements. Since any measurement of a system puts the measuring system in line WITH the system being measured, a low or relatively low impedance measuring system will load the total system too much for its measurements to be of any value. It is pointless to argue against quantum communication inasmuch as it has been an accomplished fact for years
antialias_physorg
5 / 5 (1) Nov 28, 2015
It is pointless to argue against quantum communication inasmuch as it has been an accomplished fact for years

Care to point to a source for this? If it's been 'accomplished fact for years' that shouldn't be a problem, right?
Steelwolf
not rated yet Nov 29, 2015
Actually, for quantum computation, would it not be possible to do this with, instead of using a B-E Condensate, use the type of setup in this article:

http://phys.org/n...tem-menu

And figure that the plasmon surface is going to interact in ways similar to how the effects form linearly within the BEC, while with a graphene sheet one should be able to set up and run different 3D inputs to the graphene sheet that produce the plasmon waves and the ways that those waves intersect and create nodal points (which n the above article they are using to detach electrons) But by controlled input and the use of moire tech to be able to read it via the nodal patterns inputs, how they reinforce or damp each other out, so that you would have the quantum type computer, reading it on a flat surface rather than the single line of BEC for computing. Allowing for much wider range of input as well.

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