Quantum teleportation between atomic ensembles demonstrated for first time The experimental setup for quantum teleportation between two remote atomic ensembles. One of the key components of quantum communication is quantum teleportation, a technique used to transfer quantum states to distant locations without actual transmission of the physical carriers. Quantum teleportation relies on entanglement, and it has so far been demonstrated between single photons, between a photon and matter, and between single ions. Now for the first time, physicists have demonstrated quantum teleportation by entangling two remote macroscopic atomic ensembles, each with a radius of about 1 mm. November 19, 2012 http://phys.org/news/2012-11-quantum-teleportation-atomic-ensembles.html [with comments] [study at http://www.pnas.org/content/early/2012/11/08/1207329109 ]
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First Teleportation from One Macroscopic Object to Another
Physicists have teleported quantum information from one ensemble of atoms to another 150 metres away, a demonstration that paves the way towards quantum routers and a quantum Internet
The Physics arXiv Blog November 15, 2012
One of the enabling technologies behind a quantum internet will be quantum routers capable of transmitting quantum information from one location to another without destroying it.
That's no easy task. Quantum bits or qubits are famously fragile—a single measurement destroys them. So it's not all obvious how macroscopic objects such as routers in a fibre optics network can handle qubits without demolishing them.
However, physicists have a trick up their sleeve to help send qubits safely. This trick is teleportation, a standard tool in any decent quantum optics lab.
It relies on the strange phenomenon of entanglement in which two quantum objects share the same existence. That link ensures that no matter how far apart they are, a measurement on one particle instantly influences the other.
It is this 'influence' that allows physicists to transmit quantum information from one point in space to another without it passing through the space in between.
Of course, teleportation is tricky, but physicists are getting better at it. They've teleported quantum information from one photon to another, from ions to photons and even from a macroscopic ensemble of atoms to a photon.
Today, Xiao-Hui Bao at the University of Science and Technology of China in Hefei and a few buddies say they've added a new and important technique to this box of tricks.
These guys have teleported quantum information from ensemble of rubidium atoms to another ensemble of rubidium atoms over a distance of 150 metres using entangled photons. That's the first time that anybody has performed teleportation from one macroscopic object to another.
“This is interesting as the first teleportation between two macroscopic-sized objects at a distance of macroscopic scale,” say Xiao-Hui and co.
Quite right. The goal in a quantum internet is that ensembles of atoms will sit at the heart of quantum routers, receiving quantum information from incoming photons and then generating photons that pass this information on to the next router.
So clearly the first teleportation from one of these hearts to another is an important advance.
Of course, there are hurdles ahead. Xiao-Hui and co want to increase the probability of success for each instance of teleportation, to increase the amount of time that the atomic ensemble can store quantum information before it leaks away (currently just over 100 microseconds) and to create a chain of atomic ensembles that will better demonstrate the potential of the technique for quantum routing.
None of those challenges seem like showstoppers. Which means that practical quantum routers and the quantum internet that relies on them are just around the corner.
Image: Tommy Moorman, from Best-Kept Secrets by Gary Stix, Scientific American January 2005 Issue.
One of the main objections to QKD has been the expense. It has been necessary to have fibre optic cabling dedicated to the task (so called dark fibre). With dark fibre, in the absence of other data signals, secure key rates exceeding 1 Mb/s and a transmission distance of over 250 km have been achieved. Until now, the idea of leasing such fibres from telecoms providers has put potential users off at the first hurdle.
However, the results of the Cambridge team provide a technique where QKD can be used on a fibre optic cable that is already being used for other communications traffic. The ability to use shared fibres has suddenly, and to many people’s surprise, made QKD an economic prospect, with costs likely to be little different to a corporation fitting a top-end firewall.
QKD and traditional data traffic are at opposite ends of the brightness scale. When beaming traditional data down a fibre, you try to use high power levels in order that it will travel as far as possible without the need to “repeat” the signal. QKD, however, requires light intensities at the lowest imaginable levels. With QKD, we are trying to send and detect single photons of light. Not only that, but we need to be sure that our transmission method isn’t interfering with the “quantum state” of the photon, which is the very property upon which QKD relies.
I describe the task of using shared fibres for QKD as like trying to see the stars whilst staring at the sun. One simply overwhelms the other.
What the Cambridge team have succeeded in doing is switching between the various light sources so rapidly, and so cleanly, that very small numbers of photons can be sent and detected in between the pulses carrying the traditional data. The timing needs to be extraordinarily accurate as the system opens a “gate” to let through single photons for only a tenth of a billionth of a second.
The trick is that the pulse that contains the QKD photons has to be coordinated with the detectors, so that they know when to expect the one QKD photon between the pulses of millions of standard data photons. By using such accurate “time slicing”, it is now possible to at one instant stare at the sun and then momentarily see the stars.
The technique announced is so accurate that the speed at which QKD can be transmitted is not materially different to what was achieved using dark fibre, whilst the rates for standard data can be maintained at Gigabits per second. Currently the method has been used successfully on fibres up to 90km long, but just as distances for QKD over dark fibres have increased up to 250km, it is highly likely that these distances will be extended. After all, this is just the first announcement of the technique, and the one thing we know for sure is that technology has a habit of getting better very quickly.
Of course, nothing is ever quite as simple as it appears. Fibre optic cable suffers from effects that can work against this time slicing technique, no matter how accurate it is. One such phenomenon is called Raman Scattering [ http://en.wikipedia.org/wiki/Raman_scattering ]. This is different from the more usual Rayleigh Scattering [ http://en.wikipedia.org/wiki/Rayleigh_scattering ] that people tend to envisage with fibre optic cabling, as they imagine the photons bouncing their way down the length of a glass fibre. In Raman Scattering, rather than bouncing inside the glass crystal of the fibre “elastically” so that the photons maintain their energy and wavelength, a very small proportion of the photons (1 in 10million) are scattered so that their frequency is changed. This causes detection problems.
However, the phenomenon is well understood, the models predicted to a high degree what was observed by the Cambridge team, and their system coped well, and was able to do the necessary error correction. Indeed, the fact that the Raman scattering modelling was so well modelled in these tests suggests that the team have an environment that they can control successfully. If you can predict the level of errors likely to occur, then you can make the necessary corrections. You treat the errors as noise, which in the field of communications engineering is a well-trodden path.
More prosaic matters are what will hold up this technology. For domestic use, there are few homes that have fibre running all the way to the homes router. But, certainly in the UK, fibre is making an appearance at the end of many streets in cities across the nation, even though the final leg of the network is good old fashioned copper. Governments are vying to roll out superfast broadband in their countries, which can only mean that fibre based networks will proliferate. Perhaps with them will come “super secure” networking in the form of QKD.
Meanwhile, fibre networks are the basis for most Internet backbone providers, and many corporations. With the costs now of the same order as much of the other equipment they buy for securing their networks, we must surely be about to see QKD become at least part of the network security landscape.
It appears the Cambridge research team have made a significant step forward, and whilst the journey is far from complete, I believe this further demonstrates that QKD is no longer just of academic interest. It’s about to become a mainstream method of securing data networks.
Alan Woodward is a Professor at the Department of Computing, University of Surrey, where he specialises in cyber security, computer forensics, cryptography and steganography. Alan began with a degree in physics but did his postgraduate research in signal processing at a time when computing power began to enable some radical changes to what was possible. He began his career working for the UK government, was involved in delivering some of the most challenging IT developments of the past 20 years for a variety of organisations and has for the past 10 years since when he has also been Chief Technology Officer a company called Charteris which he helped to build from a start up and float on the London Stock Exchange. As well as writing extensively in the UK on technology as well as presenting on current affairs issues relating to technology for the likes of BBC TV and radio, Alan remains actively involved in the daily battles that occur in cyberspace.
The views expressed are those of the author and are not necessarily those of Scientific American.
Stanford’s quantum entanglement device brings us one step closer to quantum cryptography
The spin-photon entanglement experimental apparatus
By Graham Templeton on November 19, 2012 at 9:56 am
Researchers at Stanford University have taken another major step toward using quantum entanglement for communication, streamlining the process by which two particles can be forced into an entangled state. Once entangled, each should react to changes in the other’s quantum spin — if one switches from up-spin to down-spin, the other should hypothetically do the same, instantly and regardless of the distance between them. The study demonstrates a technique in which each particle is induced to emit a photon entangled to its parent. By funneling these photons down a fiber optic cable so that they collide somewhere in the middle, the system can force the two parents (still held at their respective sources) to become entangled to one other. While the pipe dream of a latency-free internet is enticing enough, a much more immediate application could be the next generation of data encryption.
In cryptography, there is a classic thought experiment: How can I secure a message such that only the intended recipient (you) can read it, without ever having to compromise security by sending you the key? The traditional solution has been to send the message twice — I lock the message in a box and send it to you; you lock it again and send the box back; I use my key to remove my own lock; and when you receive the message the second time you have only your own lock left to remove. The problem with this approach is that while it’s easy to remove padlocks out of order, there is only one known way of similarly undoing complex mathematical operations without garbling the information they protect. As a result, electronic cryptography has remained static for years now. PGP [ http://en.wikipedia.org/wiki/Pretty_Good_Privacy ] and similar encryption standards are still theoretically unbreakable but, perhaps understandably, national and even personal security agencies are becoming increasingly paranoid that it will break any day now. Or perhaps it already has.
This means that we could fearlessly send an unencrypted message via quantum communicator, but the finite nature of entangled particles mean that we will likely need to be extremely conservative about how much information we transmit — which brings us back to our thought experiment. In the quantum future, I could send a highly encrypted message over conventional channels, unbreakable by both you and the eavesdropper, then send you the key via our mutually entangled bundles of particles. This minimizes the amount of information sent via entanglement, while completely negating the problem of key transmission. According to quantum theory, interception of our key is quite simply impossible.
The Stanford study [ http://news.stanford.edu/news/2012/november/toward-quantum-cryptography-111512.html ] used a quantum dot array; a stamp-sized semiconductor with millions of electrons held immobile on its surface. This German innovation provides a handy source of photons, and is designed to give rise to precisely the right frequency of photon for transmission along fiber optic cable. It’s all very tidy, though still years away from practical application. You can be sure that military agencies around the world have long ago taken notice of this technology, however, and that they are actively performing similar experiments of their own.
The first military to acquire invulnerable data transmission, instant and without the need for satellites to bounce the signal around the Earth, will enjoy a decisive advantage in virtually every sector. The implications for civilian life are much more difficult to predict.
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