Satellite quantum communication circles closer
15:03 29 July 2010 by Eugenie Samuel Reich
Communications protected by quantum encryption systems offer unconditional security – if you know which way is up. A new quantum protocol is the first that promises to work independently of orientation, which will prove vital if quantum communications are ever to be sent via satellites.
Many quantum encryption protocols work by measuring the "up" or "down" spins on pairs of entangled photons shared between a sender, conventionally called Alice, and a receiver called Bob. The two members of an entangled pair of photons always have an opposite spin from one another. If an eavesdropper were to intercept one, the very act of reading it would affect the entangled pair in a detectable way.
The distance record for quantum encrypted communications between two sites on Earth is 144 kilometres. If quantum encryption is to go global the data must be sent via satellite links, and here the conventional method hits a snag: a spinning satellite's sense of up and down changes over time, making it harder to interpret a photon's spin and establish a key.
Clockwise corkscrew
A team at the University of Bristol in the UK has invented a protocol independent of orientation that exploits the fact that photons can have an entangled circular polarisation as well as entangled spin.
Circularly polarised light can be imagined to corkscrew either clockwise or anticlockwise along its axis of travel. The two forms are readily identified regardless of the receiver's orientation.
Some modern 3D-movie projector systems already polarise light in this way to differentiate the two images used to form the 3D illusion. Doing so ensures that a cinemagoer wearing polarised glasses sees the 3D effect even if they tilt their head.
A 3D system that uses horizontally and vertically polarised light to differentiate the two images only works if the viewer's glasses are orientated in the same up-and-down direction as the theatre projector – in other words, only if the glasses and the projector share the same physical frame of reference.
Circular argument
If information is encoded in the circular aspect of photon entanglement, it is possible for Alice and Bob to establish a quantum encryption key even if they lack a shared physical frame of reference.
The relatively simple system still leaves the problem of detecting eavesdroppers, but Anthony Laing, one of the team in Bristol, says that there are ways around that. Although the lack of a shared frame of reference precludes the use of conventional up/down spin-measurements to establish an encryption key, the sender and receiver can still measure them.
And a combination of such measurements on the string of photons used to encrypt the communication channel is enough to detect an eavesdropper. The mathematics is tricky, but "physics provides an inherent way to do it", Laing claims.
Norbert Lütkenhaus at the University of Waterloo in Ontario, Canada, says that the new protocol is likely to be a very useful tool in developing quantum information technology.
But he adds that the biggest issues in Earth-satellite communication are the fact that photons tend to get lost over long distances, reducing the efficiency of the communication, and that photon detectors sometimes register detections when no photons are present, which can cause errors in the data.
"There are other barriers for Earth-satellite quantum communication that may be more challenging," agrees Hoi-Kwong Lo at the University of Toronto in Canada.
Journal reference: Physical Review A, DOI: 10.1103/PhysRevA.82.012304
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Thanks to Lim for sending these links on synthetic telepathy in Persinger's Lab experiment using coils.
The website is Skeptico
http://www.skeptiko.com/michael-persinger-discovers-telepathic-link/
The audio recording is at middle of the page.
Or click this link :
http://www.skeptiko.com/upload/skeptiko-89-Michael-Persinger.mp3
Telepathy link, quantum entanglement were observed in Michael Persinger's lab experiment.
Digital quantum simulator realized
September 2, 2011 by Editor
http://www.kurzweilai.net/digital-quantum-simulator-realized?utm_so...
The mathematical description of a phenomenon to be investigated is programmed using a series of laser pulses to perform a quantum calculation with atoms (credit: H. Ritsch)
University of Innsbruck physicists have created a digital quantum simulator that can potentially be programmed to simulate any physical system efficiently.
They used a digital approach to quantum simulation in a system of trapped ions. They encoded the desired initial state of the system in qubits and then implemented the operation sets by laser pulses. They then demonstrated this method in two experiments, using up to 100 gates and 6 qubits.
The physicists said they were able to simulate interactions and dynamics not even present in the quantum computer. They simulated the full-time dynamics of a range of spin systems. Interactions beyond those naturally present in the simulator were accurately reproduced, and quantitative bounds were provided for the overall simulation quality.
They demonstrated the key principles of digital quantum simulation and provided evidence that the level of control required for a full-scale quantum device is within reach, the physicists said.
Ref.: B. P. Lanyon, et al., Universal digital quantum simulation with trapped ions, Science Express, 2011; [DOI: 10.1126/science.1208001]
Topics: Computers/Infotech/UI | Physics/Cosmology | Quantum
Physicists Build a Memory that Stores Entanglement
The first quantum memory that stores and releases entanglement has been built by researchers in Switzerland.
http://www.technologyreview.com/blog/arxiv/25718/
Entanglement is the strange, ghostly phenomenon in which quantum particles share the same existence (actually, the same wave function). So a measurement on one instantaneously influences the other, no matter how far apart they might be.
So-called action-at-a-distance lies at the heart of many of modern physic's most dramatic new technologies: quantum cryptography, quantum teleportation and quantum computation all rely on it.
That makes entanglement important stuff.
"Stuff" is the way many physicists are beginning to think of entanglement: as a resource, rather like water or energy, to be called upon when needed in the new quantum world. These physicists want to be able to create entanglement, use it and store it whenever they need to.
The first two of these--creating and using entanglement--has been the subject of intense research for the last 30 or 40 years. But the ability to store entanglement in a useful way has eluded physicists. Until now.
Today, Christoph Clausen and buddies at the University of Geneva demonstrate not only how to store entanglement but how to release it again in fully working order.
Their device consists of a load of neodymium atoms buried in a crystal of ytterbium silicate, which when cooled, can absorb and store photons. The question that Clausen and co attempt to answer is whether this device can store entanglement too.
So they created a pair of entangled photons, sent one into the crystal and waited until it was emitted again. They were then left with this new photon and the original member of the pair. They then carried out a standard experiment, known as a Bell test, and proved that the pair were still entangled.
That's impressive for several reasons. For a start, for the entanglement to be preserved, the entire crystal has to be involved. This crystal is about a centimetre in size and the idea that entanglement can be exchanged between a photon and an object of this size is amazing.
Next is the ability to transfer entanglement form a flying qubit--the photon--to a stationary one, the crystal. And to do it with photons with a wavelength of 1338nm, the so-called telecommunications wavelength that can pass easily through fibre optic cables. Any other wavelengths are interesting but practically useless for communications.
But the most exciting aspect of all this is that the entanglement survives the process of storage and release at all. Notoriously fragile, entanglement leaks into the environment like water through a sieve. Being able to store and release it is the enabling technology that could make devices such as quantum repeaters work.
There's not shortage of uses for this kind of ability. The quantum internet, to name just one, will require the ability to store and send on entangled photons. At one time, it looked more or less impossibile to do this. Entanglement was just too fragile. Now it looks merely a matter of time before we'll have it on tap.