Researchers take two big steps toward quantum computing

November 6, 2015 by Christopher Packham report
A green beacon laser is sent from the Jacobus Kapteyn Telescope on La Palma to Tenerife. Credit: University of Vienna.

(Phys.org)—"Spooky action at a distance," Einstein's famous, dismissive characterization of quantum entanglement, has long been established as a physical phenomenon, and researchers are keen to develop practical applications for entanglement including communication, encryption, and computing.

Quantum entanglement is a phenomenon in which the production or the interactions of a number of particles cannot be described independently of each other, and must instead be described in terms of the whole system's quantum state.

Two recent experiments with entanglement have been reported in the Proceedings of the National Academy of Sciences, one proving that in photons can be preserved even in turbulent atmospheric conditions; the other demonstrating entanglement swapping between qubits over the 143 kilometers between the Canary Islands and Tenerife.

In the first experiment, a group of Austrian researchers sought to transmit complex, quantum-encoded information through turbulent air. The polarization of photons is a robust technique favored for long-distance entanglement research in which two photons with mutually perpendicular polarizations exist in different locations. The state is easily controllable and resistant to . But polarization is a binary state that cannot easily carry complex information. The encoding of more complex information in an entangled system—for instance, a jpeg image of Albert Einstein shaking his fist at entangled photons–would require a much larger space state than the two-dimensional space state of polarization.

Hedy Lamarr Quantum Communication Telescope with opaque slit-mask for measuring twisted photons. Credit: University of Vienna.

The researchers used photons with a twisted phase front, which carry an unbounded amount of orbital angular momentum (OAM)—a much larger space state than polarization. In principle, OAM offers the possibility of practical applications like quantum encryption and the testing of fundamental properties of quantum physics. But OAM is quite sensitive to atmospheric turbulence. However, the researchers succeeded in distributing the quantum entanglement of spatially structured photons over a free-space link across the city of Vienna.

In the experiment, the sender, traditionally known in quantum entanglement experiments as Alice, and the receiver, Bob, were in different physical locations three kilometers apart. Alice was in a 35-meter-high radar tower with a high-fidelity, high-brightness entanglement source that produced entangled pairs. Photon A remained unchanged, while photon B's polarization state was transferred interferometrically to a generated hybrid-entangled quantum OAM state.

Photon A was delayed via a 30 meter fiber cable to ensure that it was not measured before photon B was transferred to Bob, on the other side of the city. After the transfer, the polarization of photon A was measured and each detection event time-stamped. Similarly, the detection of photon B was time-stamped and the two events were correlated at a sub-nanosecond regime. The researchers verified of the pairs through turbulent atmosphere. Though they were still using a two-dimensional space state, the method allows for a quantum link with up to 11 orthogonal channels of OAM.

They write, "Our result clearly shows that entanglement encoded in OAM can be identified after long-distance transmission. It is not fundamentally limited by atmospheric turbulence, as expected in some recent investigation, and thus could be a feasible way to distribute high-dimensional entanglement."

A green beacon laser is sent from the Optical Ground Station on Tenerife to La Palma. Credit: IQOQI Vienna.

In the second experiment, another group that included many of the same researchers teleported an entangled state between two qubits, a process known as entanglement swapping. This is in contrast to the teleportation of a single quantum state from one qubit to another; in essence, the researchers created an entangled state between two qubits located 143 kilometers from each other, and which had never interacted before.

This provides a solution to the no-cloning theorem, which in quantum theory states that it is impossible to create an identical copy of an unidentified . By demonstrating the applicability of the technique outside the laboratory, the experiment opens a door to such practical applications as cloud quantum computing.

The researchers don't shy from the significance of this accomplishment, writing, "Our data demonstrate successful entanglement swapping via a long-distance free-space link under the influence of highly demanding environmental conditions, in fact more challenging than expected for a satellite-to-ground link. This proves the feasibility of a crucial element for realizing a quantum repeater in a future space- and ground-based worldwide quantum internet."

Explore further: Physicists mimic quantum entanglement with laser pointer to double data speeds

More information: Paper 1: Twisted photon entanglement through turbulent air across Vienna. PNAS 2015 ; published ahead of print November 2, 2015, DOI: 10.1073/pnas.1517574112

Abstract
Photons with a twisted phase front can carry a discrete, in principle, unbounded amount of orbital angular momentum (OAM). The large state space allows for complex types of entanglement, interesting both for quantum communication and for fundamental tests of quantum theory. However, the distribution of such entangled states over large distances was thought to be infeasible due to influence of atmospheric turbulence, indicating a serious limitation on their usefulness. Here we show that it is possible to distribute quantum entanglement encoded in OAM over a turbulent intracity link of 3 km. We confirm quantum entanglement of the first two higher-order levels (with OAM=± 1ℏ and ± 2ℏ). They correspond to four additional quantum channels orthogonal to all that have been used in long-distance quantum experiments so far. Therefore, a promising application would be quantum communication with a large alphabet. We also demonstrate that our link allows access to up to 11 quantum channels of OAM. The restrictive factors toward higher numbers are technical limitations that can be circumvented with readily available technologies.

Paper 2: Teleportation of entanglement over 143 km. PNAS 2015 ; published ahead of print November 2, 2015, DOI: 10.1073/pnas.1517007112

Abstract
As a direct consequence of the no-cloning theorem, the deterministic amplification as in classical communication is impossible for unknown quantum states. This calls for more advanced techniques in a future global quantum network, e.g., for cloud quantum computing. A unique solution is the teleportation of an entangled state, i.e., entanglement swapping, representing the central resource to relay entanglement between distant nodes. Together with entanglement purification and a quantum memory it constitutes a so-called quantum repeater. Since the aforementioned building blocks have been individually demonstrated in laboratory setups only, the applicability of the required technology in real-world scenarios remained to be proven. Here we present a free-space entanglement-swapping experiment between the Canary Islands of La Palma and Tenerife, verifying the presence of quantum entanglement between two previously independent photons separated by 143 km. We obtained an expectation value for the entanglement-witness operator, more than 6 SDs beyond the classical limit. By consecutive generation of the two required photon pairs and space-like separation of the relevant measurement events, we also showed the feasibility of the swapping protocol in a long-distance scenario, where the independence of the nodes is highly demanded. Because our results already allow for efficient implementation of entanglement purification, we anticipate our research to lay the ground for a fully fledged quantum repeater over a realistic high-loss and even turbulent quantum channel.

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Spaced out Engineer
not rated yet Nov 06, 2015
Can someone explain to me how 11 orthorgonal channels are possible? Does this imply at least some of String/Twistor theory? Does anyone have any good books on this or bi-photons? I understand light has spin-1 and in low luminescences can have deterministic quantum interactions, but how are they able to even get definitive dimensionality? I get that the OAM maybe riemann surfaces, but how do you quantify orthogonality? I thought we were limited logically and experimentally by imaginable processes. Is orthogonality information and dimensionality dependant?

I have read, A road to reality, The eloquent universe, and the Island of Knowledge, but my executable mathematics ended at partial differential equations.

Even though it is quantum information can we not manipulate the systems' entropic referentiality? Is there a way to induce ordered or categorically chaotic clustering for actual information vs data?
shavera
5 / 5 (4) Nov 06, 2015
Spaced: "Twisted phase front" is the key point above. Namely that they give a photon orbital angular momentum around its direction of travel.

Here's the wiki: https://en.wikipe...l_vortex

So the 11 orthogonal channels are probably 1 no orbital angular momentum, and 5 levels of momentum in either handedness around the beamline.

I'd also not entirely recommend thinking of this as a single photon spiriling around in a corkscrew. The photon just occupies a superposition of several possible phases that constructively and destructively interfere so that you will most likely find it in a cylindrical "shell" along the beam axis with some net orbital angular momentum value.
shavera
5 / 5 (3) Nov 06, 2015
other neat thing from the paper:
the receiver is the Hedy Lamarr Quantum Communication Telescope

Read up on Hedy Lamarr. Really cool story about a famous actress who contributed a lot to our knowledge of signal encoding.

https://en.wikipe...y_Lamarr

(oh I guess this is mentioned in the picture caption above. Still neat)
shavera
5 / 5 (2) Nov 06, 2015
Sorry, last one, Here's a better article, linked from the Optical Vortex one, about light with orbital angular momentum: https://en.wikipe...of_light
swordsman
not rated yet Nov 07, 2015
Yes, and this contradicts Einstein's assertion that electromagnetic radiation is "spherical" in nature. Which also contradicts his famous theory. Nothing new there, as measurements from as far back as 1936 illustrated.
Osiris1
not rated yet Nov 07, 2015
The neatest thing about QComm is its 'non-locality'. Just gotta love being able to talk without delays with,say, our System Ship..a large 'base' ship with crew of maybe 60 complete with staterooms, offices, engineering spaces, science labs, observatories, and artificial spin gravity in the multidecked ring section. All powered by Boeing's Fusion thruster and its' magnetoelectrodynamic secondary recovery nozzle. That could have space also to house the quantumtronics and provide repeaters to local ship based and docked exploration shuttles. Yeah I watch Star Trek, however if one bypasses the chaff for the ideas of value, we COULD make a way to go to space, enjoy it, eat well,......and.....survive!.... with also space for refuges from space weather, micrometeor strikes...and autoclosing bulkheads, multiple redundant systems, excess air, recycling, hydroponics spaces, etc. We need to go to space to LIVE.
Osiris1
not rated yet Nov 07, 2015
Hedy Lamar also graced the cover art of the Manual, Disk, and intro art of Corel's CorelDraw8. It was said that she had found the large insect in an early computer that was malfunctioning, thereby 'fixing' it. She said, the story goes, that she found the bug in the computer causing its programs not to run. Thereafter any problem in computers and especially software was referred to as a 'bug'. Her contribution to computer science in the early days was exceptional.
gmcgill
not rated yet Nov 08, 2015
Does this mean that we will transmit the message after it has been received?
And what does this have to do with computing? Isn't this about quantum communications?
Questions, questions, questions.
TiagoTiago
not rated yet Nov 09, 2015
Can they control what data they transmit, or is it still just a random number generator that will produce the same results in two different locations at once?
gmcgill
not rated yet Nov 10, 2015
I think they are using entangled photons. So they keep one and send one. Or maybe not.
TiagoTiago
not rated yet Nov 10, 2015
I think they are using entangled photons. So they keep one and send one. Or maybe not.

But can they know what they got without undoing the entanglement?
gmcgill
not rated yet Nov 10, 2015
Seems likely. Trick is sender manipulates the photons after reception? With the "spooky forces acting at a distance" the receiver's photons will reflect the changes. If I'm even close, it's starting to make sense.
TiagoTiago
not rated yet Nov 10, 2015
I thought "reading" the results would break the entanglement...

And what's the point of selecting which particle to send instead of just sending a regular particle created to spec?
gmcgill
not rated yet Nov 12, 2015
"I thought "reading" the results would break the entanglement."
I think something like that happens. It seems that they might be able to use the second phase of the experiment to solve the sending bottleneck . (As quoted below)
"In the second experiment, another group that included many of the same researchers teleported an entangled state between two qubits, a process known as entanglement swapping. This is in contrast to the teleportation of a single quantum state from one qubit to another; in essence, the researchers created an entangled state between two qubits located 143 kilometers from each other, and which had never interacted before."
This idea sends me back to square one.

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