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New approach uses ‘twisted light’ to increase efficiency of quantum cryptography systems

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Researchers demonstrate how to encode 2.05 bits per photon, doubling existing systems that use light polarization

Researchers at the University of Rochester and their collaborators have developed a way to transfer 2.05 bits per photon by using "twisted light." This remarkable achievement is possible because the researchers used the orbital angular momentum of the photons to encode information, rather than the more commonly used polarization of light. The new approach doubles the 1 bit per photon that is possible with current systems that rely on light polarization and could help increase the efficiency of quantum cryptography systems.

Quantum cryptography promises more secure communications. The first step in such systems is quantum key distribution (QKD), to ensure that both the sender and receiver - usually referred to as Alice and Bob - are communicating in such a way that only they know what is being sent. They are the only ones who hold the "key" to the messages, and the systems are set up in such a way that the presence of any eavesdropper would be identified.

In the paper, published in New Journal of Physics today, Mohammad Mirhosseini and his colleagues describe a proof-of-principle experiment that shows that using OAM to encode information rather than polarization opens up the possibility of high-dimensional QKD. Mirhosseini, a Ph.D. student in Robert W. Boyd's group at the University of Rochester's Institute of Optics, explains that they were able to encode a seven dimensional "alphabet" - that is, seven letters or symbols - using both the orbital angular momentum (OAM) of the photons and their angular position (ANG). These two properties of the photons form what physicists refer to as mutually unbiased bases, a requirement for QKD. Using mutually unbiased bases, the correct answer is revealed only if Alice encodes the information using a particular basis and Bob measures in that same basis.

In QKD, once they have generated a long, shared key, Alice and Bob publicly announce the basis (or "alphabet") they have used for each symbol in the key. They then compare what alphabet was used for sending and which one for receiving. They only keep the part of the key in which they have used the same "alphabet." The letters they keep produce a secure key, which they can use to encrypt messages and transmit these with regular encryption without the need for quantum cryptography.

If for any reason their communication is intercepted, because of a fundamental property of quantum mechanics, there will be discrepancies between Alice and Bob's keys. To check for this, Alice and Bob sacrifice a short part of their key. They share this publicly and identify any discrepancies. This lets them know whether their connection is secure and, if not, they will stop the communication.

The researchers showed that using their system they were able to generate and detect information at a rate of 4kHz and with 93% accuracy. A long term goal of the research is to realize secure communications at GHz transmission rates, which is desirable for telecommunication applications.

"Our experiment shows that it is possible to use "twisted light" for QKD and that it doubles the capacity compared to using polarization," said Mirhosseini. "Unlike with polarization, where it is impossible to encode more than one bit per photon, "twisted light" could make it possible to encode several bits, and every extra bit of information encoded in a photon means fewer photons to generate and measure."

In a previous experiment using a strong laser beam instead of single photons, Boyd's team were able to measure up to 25 modes of OAM and ANG. This is equivalent to having 25 letters available in your "alphabet" rather than 7. This shows the potential for a system like the one described in the new paper to have the capacity to transmit and measure 4.17 bits per photon using more sophisticated equipment.

Mirhosseini acknowledges that the real-world challenges are not straightforward to overcome but when it comes to QKD, he is excited about the possibilities their system opens up.

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New approach uses 'twisted light' to increase efficiency of quantum cryptography systems

Swedish Prosecutor to Interview Julian Assange in London …

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WikiLeaks founder Julian Assange has spent nearly three years in an Ecuadorian embassy in London in an effort to avoid both charges from the United States regarding the dissemination of classified information as well as a nearly five-year investigation that alleges Assange committed sex crimes in Sweden. With the statue of limitations in the latter case set to expire this August, Swedish prosecutors hope to travel to London to interview Assange in the near future in a last-ditch effort to decide whether to pursue charges.

"My view has always been that to perform an interview with him at the Ecuadorean embassy in London would lower the quality of the interview, and that he would need to be present in Sweden in any case should there be a trial in the future," Lead prosecutor Marianne Ny said in a statement (via The Hollywood Reporter). "Now that time is of the essence, I have viewed it therefore necessary to accept such deficiencies in the investigation and likewise take the risk that the interview does not move the case forward."

Ny asked permission from both Assange's legal team and the Ecuadorian embassy to interview the WikiLeaks founder, as well as requesting a DNA sample. Per Samuelson, one of Assange's defense attorneys, told The Associated Press, "This is something we've demanded for over four years. Julian Assange wants to be interviewed so he can be exonerated."

In a 2012 interview with Rolling Stone's Michael Hastings, Assange couldn't discuss the ongoing investigation, butbriefly opened up about the 2010 allegations that he had sexually molested two women in Sweden. "It is very difficult, being in the position where you can't tell your version of events," Assange said. "People can read the allegations for themselves. They're not correct, but even as stated, they are absurd. What the prosecution successfully managed to do is use the word 'rape.' Although I've not been charged and technically what they are investigating is called 'minor rape,' a Swedish concept that hasn't stopped our opponents from constantly referring to 'rape charges,' which is false."

When asked by Hastings why hedidn't just deny the allegations and face his accusations against him head-on in Sweden, Assangereplied, "I have no faith in the Swedish justice system being just. The International Prison Chaplains Association says that Swedish prisons are the worst prisons in Europe In addition, if you criticize matters, such as that Swedes have the worst prison system in all of Europe, then it would be the worse for you, because the Swedish justice system will take its revenge."

Assange also dismissed the widely held belief that the Swedish charges were planted by the CIA as a means of discrediting him, and in Ny'scomments Friday, she also reiterated that the U.S. is not involved with the Swedish investigation.

Following Ny's decision to venture to London to talk with Assange, WikiLeaks spokesman Kristinn Hrafnsson said, "I think it's absolutely outrageous that it took the Swedish prosecutor four-and-a-half years to come to this conclusion after maintaining that she couldn't come to London because it would be illegal to do so. Obviously that was a bogus argument."

Even if Assange is exonerated of the charges, it's still unlikely the WikiLeaks proprietor will be able to return to Sweden because there is nothing in place to prevent Sweden from extraditing him to the United States, where he'd likely face countless charges in the wake of Cablegate and the ChelseaManning trial.

In the Rolling Stone interview, Assange says he asked a "Western intelligence source" whether he'd ever be able to return to his native Australia and freely move around the globe. "He told me I was fucked," Assange said.

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Swedish Prosecutor to Interview Julian Assange in London ...

Julian Assange offered interview with Swedish prosecutors

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Swedish prosecutors' surprise offer to question WikiLeaks founder Julian Assange at Ecuador's London embassy over rape allegations offers a clear breakthrough in the deadlocked case, Ecuadorean Foreign Minister Ricardo Patino says.

Assange took refuge in the London embassy in 2012 to avoid extradition to Sweden and has been ensconced there ever since.

"This allows us to see a light at the end of the tunnel," Patino told reporters in Washington on Thursday (local time).

"That light is still not close, but this is the first time there is a movement in the Swedish judicial system suggesting that it may proceed with an interview."

The top Ecuadorean diplomat said he welcomed the Swedish move, but regretted it had not taken place earlier.

Up to now, Swedish prosecutors have refused to go to London to question the 43-year-old Australian former hacker over the allegations.

Assange has refused to go to Sweden to be questioned over the allegations, which he has vehemently denied, saying the sexual encounters were consensual.

But the prosecutor in charge of the case, Marianne Ny, said last week she was dropping her opposition as some of the alleged offences will reach their statute of limitations in August.

Sweden issued an arrest warrant for Assange in 2010 following allegations by two women in Sweden, one who claimed rape and another who alleged sexual assault.

Assange fears that Sweden will pass him on to the United States, where an investigation is ongoing into his anti-secrecy website's release in 2010 of 500,000 classified military files on the wars in Afghanistan and Iraq and 250,000 diplomatic cables which embarrassed Washington.

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Julian Assange offered interview with Swedish prosecutors


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