CATCHING THE LIGHT: An entangled photon can be stored and then retrieved from a solid-state memory, but hurdles remain before the technology is ready for prime time.
Image: Wolfgang Tittel/University of Calgary
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Full-scale quantum computers, with all the number crunching, code cracking and jaw-dropping processing power researchers expect them to deliver, remain a mere twinkle in the eye of physicists and computer scientists. It is a twinkle supported by promising experimental and theoretical work, but a twinkle nonetheless—to date only rudimentary quantum processors have been built.
Such computers would harness the physical properties of quantum bits, or qubits, to expand the reach of computation. Whereas ordinary bits can be 0 or 1, a qubit can be in a superposition of 0 and 1 simultaneously—and qubits can be mutually entangled, meaning that their properties are linked.
Along the road to building a full-fledged quantum computer lies a critical experimental milestone that would be a significant achievement in its own right: the construction of a simpler device called a quantum repeater, essentially a relay station for qubits that could enable quantum communication systems, an analogue to the fiber-optic telecommunications already in wide use. These systems would harness the same properties as quantum computers to transmit information, encoded on photon qubits, over long distances. With a slew of advances in the past year or so, the latest of which are reported in a pair of complementary papers published online January 12 in Nature, researchers now say that quantum repeaters could be within reach within five to 10 years. (Scientific American is part of Nature Publishing Group.)
Two groups of physicists have devised quantum memories from crystals laced with rare earth elements that are capable of storing an entangled photon and then releasing it a short time later. That kind of memory would be useful for long-distance quantum cryptography, for instance, in which individual photons from an entangled pair are sent to two parties who wish to share a unique and secure link. The photons would act as a kind of shared cryptographic key, but the problem is that the individual photons have a maximum range of about 100 kilometers in an optical fiber.
"The solution to extend the range is repeaters," explains Harvard University physicist Mikhail Lukin, who did not contribute to the new studies, "which are essentially small quantum computers that you insert along this channel, and they help to clean up and purify the entangled states along the way." To do that, researchers need a material memory that is capable of taking in a photonic qubit, holding on to it for a quick rejuvenation, and then reemitting it with its quantum properties intact. With a repeater every 10 kilometers, say, each photon could be repeatedly refreshed along its journey before it had a chance to degrade.
Although the two studies differ in specifics, both are notable for shifting an entangled state between two photons into an entangled state between one of those photons and a group of atoms in a solid. Such quantum cross-pollination has been achieved before, but it usually involves highly technical setups such as single trapped atoms cooled by lasers to a fraction of a kelvin (0 kelvin is absolute zero). The crystal memories, on the other hand, need only be cooled to a few K by more conventional means.
"Laser-cooled atoms, I think it's great science, but I have the feeling that it's a little too complicated to move into real applications," says Wolfgang Tittel of the University of Calgary in Alberta, one of the studies' co-authors, who says that the closed-cycle coolers required for crystal memories are much closer to an off-the-shelf technology. "Once you buy it, you have it, and ours was running for an entire year," he says. Mikael Afzelius of the University of Geneva in Switzerland, a co-author of the other study, sounds a similar tone. "Our solid-state approach could prove to be more scalable," he says, "which would be important for future quantum networks."
Tittel's group employed a crystal doped with thulium, a rare earth metal, that can hold on to a photon for seven nanoseconds before releasing it again, quantum state intact. The photon, once absorbed into the crystal memory, creates an excitation in a collection of atoms within the memory that then reproduces the photon a short time later in a sort of optical echo. The group demonstrated that the echo photon retained its entanglement with the original photon's mate.
The other group achieved a similar feat with a crystal doped with neodymium, another rare earth metal. Afzelius and his colleagues achieved longer storage times of up to 200 nanoseconds, but their experiment did not allow them to verify entanglement between the two photons once one of them was stored in and retrieved from the memory.
"I think this is a significant advance," Lukin says, praising in particular the broadband nature of the crystals, which would allow them to receive very rapid pulses of light. He notes that several improvements in the crystal-memory technology would have to be made before a practical quantum memory for a repeater could be implemented: longer storage times, higher efficiencies and the ability to recall a photon on demand rather than after a predetermined time.
When asked independently, Lukin, Tittel and Afzelius all give roughly the same answer for how far away a prototype quantum repeater might be: five years, maybe 10. "If you had asked me three or four years ago, I would have said a very long time," Tittel says. But with a handful of major quantum-information science papers appearing just in 2010, things are looking up. "Everything is pretty much there, but everything has to be improved," he says. "From a fundamental point of view, I can see which way to go."
Adds Afzelius: "I think we are looking at five to 10 years before we will see laboratory experiments that have progressed enough to attempt a first practical quantum repeater link between, let's say, two cities. From that point it will be an engineering challenge to make our experimental systems more compact and affordable, but I believe that is within reach with current technologies."
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33 Comments
Add CommentAs far as I understood we will be using photons,ie.light,to store and communicate information, instead of today's technology of electrons,ie. electricty in 10 years.
Reply | Report Abuse | Link to thisThe article quotes Afzelius: "I think we are looking at five to 10 years before we will see laboratory experiments that have progressed enough to attempt a first practical quantum repeater link between, let's say, two cities. From that point it will be an engineering challenge to make our experimental systems more compact and affordable, but I believe that is within reach with current technologies."
Reply | Report Abuse | Link to thisFrom that statement, only experimental laboratory equipment will be available in 5-10 years. Affordability is a subjective matter - is your project funded by the military?
Personally, I remain skeptical that a device that can 'repeat' or 'recharge' a photon without absorbing its momentum (detecting it, breaking entanglement), is possible. The concept sounds like current optical repeaters, but they merely detect photons and transmit new equivalents - easily done.
right.
Reply | Report Abuse | Link to thisO haha. not in scorn.
What physical properties? or should I say, which. or where or when?
the abilities depend on the flash, the opportunities.
Why can't we use entangled photons from starlight to communicate with advanced species on planets in other solar systems.We could do this by both our species looking at a star that is the same distance from both of us.With the possibility of instantaneous communication across many lightyears.
Reply | Report Abuse | Link to thisGeez jack.123, I'd give one of them a call now and set things up but I seem to have forgotten the area code for Alpha Centauri.
Reply | Report Abuse | Link to thisThat sounds strange to me. How can a group of atoms emit a new photon, that will be entangled with the photon that was entangled with the photon that originally entered, or excited that group of atoms. Must be some pretty complicated stuff going on there. I thought entanglement was a thing that happened with proximity. Not something that could be passed down to other photons , or groups of atoms. The way were going the whole world could be entangled some day. once these atoms are effected what happens when the next photon comes along with a different spin etc, and there are a few linked photons floating around in the substrate. Would there be cancellation for example. The system just doesn't sound as straight forward as suggested. There is a big difference between some kind of optical isolation chip, transmitting digital data over fiber optics, and reading/transmitting individually linked photons.
Reply | Report Abuse | Link to thisI agree - these seem to be extraordinary claims - I had to carefully reread:
Reply | Report Abuse | Link to this"Tittel's group employed a crystal doped with thulium, a rare earth metal, that can hold on to a photon for seven nanoseconds before releasing it again, quantum state intact. The photon, once absorbed into the crystal memory, creates an excitation in a collection of atoms within the memory that then reproduces the photon a short time later in a sort of optical echo. The group demonstrated that the echo photon retained its entanglement with the original photon's mate."
"The other group achieved a similar feat with a crystal doped with neodymium, another rare earth metal. Afzelius and his colleagues achieved longer storage times of up to 200 nanoseconds, but their experiment did not allow them to verify entanglement between the two photons once one of them was stored in and retrieved from the memory."
It seems to me that a more likely approach would be slow an entangled photon in a crystal buffer, then some how reamplify the original light wave as it is released into the next transmission fiber. Any 'copying' of photon characteristics into another photon for reemission seems like a violation or simulation of entanglement.
I don't even know why the Afzelius group even reported their results, since they could not even verify that entanglement is preserved.
At any rate, that these methods could someday be employed in precision optical field installable connection units that could provide reliable, robust results seems even more difficult to believe.
By the way, it seems that producing a simple transmission facility using the buffered light amplification methodology means that a second photon signal could not be transmitted until the first photon had been confirmed to have left the _last_ amplifier buffer memory, to avoid collision. This could require an acknowledgement transmission be returned for every photon transmitted to the final destination, severely constraining achievable data throughput.
Reply | Report Abuse | Link to thisjtdwyer-It happens with every phone call you make and with the note you just sent.Every hundred miles or so a repeater amplifies a group of photons and sends it the information along its way down a fiber optic.Even starlight that has traveled a million light years has an entangled partner arriving somewhere else a million light years from here.
Reply | Report Abuse | Link to thisI'm not a communications expert, but I think that the simple detection and reemission of analog or digital electrical and optical signals does require an acknowledgement protocol delay for every bit transmitted. Whether some radical pacing delay or acknowledgement protocol is used, I think that collision avoidance is critical as collision would destroy entanglement.
Reply | Report Abuse | Link to thisI can't imagine any entangled starlight communications methods...
I don't know what they mean by entanglement or echo photon in this experiment, but it makes sense to me to use some light-absorbing material to store information sent by photons in each atom of the material as 1 or 0 and then decode it by reading it with a repeater. It is different from current technology of using magnets to store information with electrons.
Reply | Report Abuse | Link to thisI suggest you begin by referring to:
Reply | Report Abuse | Link to thishttp://en.wikipedia.org/wiki/Quantum_entanglement
The fundamental idea, as I understand it, it that rather than the charge state of a set of atoms determining a binary value, the state of a characteristic property (spin) of photon particles, for example, can probabilistically be determined to be some value between 0 and 1.
Moreover, (a pair of) photons can be entangled such that they will probabilistically represent the _same_ value despite being separated by photon transmission distances of up to around 140 km (each). The objective of the research discussed in this article is to extend the transmission distance of entangled particles.
As I personally understand it, entanglement of particles is achieved by splitting a single light wave (even if its emission duration would normally ensure eventual detection of only a single, quantum, photon) into two independently directed wave fronts such that two photons can eventually be detected, both with, probabilistically, the same (spin) state.
jtdwyer-you just explained how to use starlight entanglement to communicate with.The 144km limit is because of the use of a fiber optics and the signal passing through air.Starlight has no such interference,you just throw in a double two slit experiment on each end,and find the rapid wave function drop,and start talking.The problem is finding that first signal.Right now to my knowledge no one is looking for that first signal.and it will never be found till someone does.I realize that the hundreds of telescopes are busy with important work,but I challenge one them to take a few minutes a day to attempt the task.The most you would lose is a few minutes,wherein what you could gain is a Nobel prize,If there is anybody out there they have already detected us by seeing the effect caused by our use of electronic devices looking at the stars,and causing a wave function drop that effects their devices.Who knows maybe our mere looking at the stars could have the effect.
Reply | Report Abuse | Link to thisYou certainly do have a point re. starlight propagation distance vs. light attenuation through a glass fiber or the atmosphere.
Reply | Report Abuse | Link to thisAs you say, the trick would be to identify the photon(s) encoded with the encryption key value, so that we could decode the remaining message, when identified. Successful detection and decoding would be an improbable challenge!
the article says " Although the two studies differ in specifics, both are notable for shifting an entangled state between two photons into an entangled state between one of those photons and a group of atoms in a solid", which means absorbing a single photon (or its luminosity) by an atom and decoding it later via x-ray reader. I dont know how you can make a computer out of non-binary system.
Reply | Report Abuse | Link to thisFrankly, there is so much that is either not well explained in this article or is misleading information that I can't tell if anyone knows what they're talking about.
Reply | Report Abuse | Link to thisFor example, the crystal buffer is referred to as a 'crystal memory'. I think it is highly inappropriate to refer to a device that slows a light wave for a few nanoseconds to be referred to as though it is a memory device.
I strongly suspect that the crystal devices used here amplify the originally emitted light wave, not absorbing its momentum to detect a photon particle. This is a crucial distinction that allows the preservation of the eventually detected particle state property. In my view, these devices can only be correctly described as optical amplifiers.
In the paragraph you quoted, the article goes on to state:
"The photon, once absorbed into the crystal memory, creates an excitation in a collection of atoms within the memory that then reproduces the photon a short time later in a sort of optical echo."
If this is a valid description of the physical process being applied to a quantum photon, then the photon' momentum is being somehow absorbed by a quartz atom's electrons, then being reemitted to be absorbed by the next atom, etc., until the last quartz atom emits the new photon, somehow reflecting all of the initial photons characteristic states and properties.
The article continues:
"Such quantum cross-pollination has been achieved before, but it usually involves highly technical setups such as single trapped atoms cooled by lasers to a fraction of a kelvin (0 kelvin is absolute zero)."
Imbedded in the above statement is a reference to 'single trapped atoms' which links to a 2008 article, " Quantum Computing with Ions";
http://www.scientificamerican.com/article.cfm?id=quantum-computing-with-ions
That article seems to have nothing at all to do with the amplification process being reported for these quartz devices.
With so much misinformation, the entire article seems highly suspect to me. But then, I don't know where you got the idea for some later decoding of atoms using an x-ray reader, either. Perhaps I'm just confused.
Thank you sharing your ideas with me. I got the idea of x-ray reader from the characteristics of the element called "thulium", which they used in this experiment. As my reading shows it is rare earth metal that absorbs and emit x-rays. As magnetic resonance devices that work with light shows it is possible to absorb light and later read it (even in organic matter, ie. kirlian photography) with such readers
Reply | Report Abuse | Link to thisThanks for explaining - a very interesting idea brought on, I think, by the article's invalid use of the term 'memory' rather than 'amplifier'. I apologize for being overly caustic - my only justification is that eventually we all become tired. Sorry.
Reply | Report Abuse | Link to thisI believe that most comments are missing the basic value of entanglement; that particle A will instantly reflect a change to particle B even though they are separated by great (or small, for that matter) distances. The accomplishment here is the ability to get a photon to a remote location with its entanglement intact. Once it is there then the entanglement of the two particles can be tested and the success of the test can be taken as proof of authenticity.
Reply | Report Abuse | Link to thisI agree with you to some extent, although I think confirmation of remotely detected photon entanglement is a difficult process.
Reply | Report Abuse | Link to thisThe article states:
"The other group achieved a similar feat with a crystal doped with neodymium, another rare earth metal. Afzelius and his colleagues achieved longer storage times of up to 200 nanoseconds, but their experiment did not allow them to verify entanglement between the two photons once one of them was stored in and retrieved from the memory."
I don't know why they could not verify entanglement.
IMO, there is no processes that 'retrieves' photons from 'memory'. I think the quartz devices are signal amplifiers, (like guitar players use) - photons are transmitted through the device, which incidentally delays the signal as it is amplified.
I think the article contains a great deal of confusing misinformation.
The earliest type of computer 'memories' were devices that did exactly the same thing. 'Bits' were 'stored' as an impulse traveling from one end of a tray of mercury to the other. The bit was stored for the time that the wave took to propagate to the other end of the tray and then it was either 'read' or cycled back and stored for another interval. The consumer of the data doesn't know that the data is continuously retrieved and re-stored, only that it will be waiting when it is needed.
Reply | Report Abuse | Link to thisThis article is about repeater units necessary to extend the range of entangled photon transmission, which is now limited to less than 150 km.
Reply | Report Abuse | Link to thisIn digital optical fiber communication systems, repeaters essentially consist of a receiver and a transmitter, to boost the optical signal for another (~100 km) transmission leg. The requirement is to retransmit the digital signal as quickly as possible to avoid collision.
There is no requirement to store the information for any duration except perhaps to ensure successful reception, to ensure that no further retransmission will be necessary. Again, a protocol supporting this error recovery capability for entangled photon transmissions would incur transmission delays and is not likely possible with current technology.
IMO, in the entangled photon repeater development units being discussed in this article, once the entangled photon has traversed the quartz amplifier unit, it's gone. There is no possibility for retention of the photon for later retrieval.
I haven't been following quantum computing - has any persistent photonic memory device been demonstrated? Even without supporting quantum entanglement, that alone will be an extremely difficult challenge. As I understand, this is not an attempt to achieve persistent storage.
jtdwyer-I agree that entanglement communications between the stars is problematic,but the rewards are well worth the investment even if it cost billions.Making one connection would change the world as we know it.As for deciphering any message,I believe the sender would make it as simple as possible.The exception of course would be a very advanced civilization who would make things difficult so they were communicating with someone of their own level,but a species close to our level even maybe a thousand years ahead of us would be just as eager for contact as we are and thus would make it as easy as possible.
Reply | Report Abuse | Link to thisThere does not seem to be so many distinguished scientists saying quantum computing is impossible these days.
Reply | Report Abuse | Link to thisStill a few hanging on the cliff of , no quantum brain.
O dear, cold fusion, impossible, impossible.
Precognition, you must be Mad.
One day modern science and scientists may catch up with the great scientists who all shouted "keep an open mind"
Good idea Jack123, I like to go still further if you don't mind , we are observating with the PLANCK sattelite the first emitted (entangled) light of our known universe, in this way we could communicate direct with all possible species that have the same entanglement as our universe, you could imagine that when other scientists in our universe also observating this source of light emitted 380.000 years after 10 force -43 second (PLANCK time) when they observe the same "image" (for exemple the same cosmological constant)as the one we observe,this image could act as a "mirror" in which we see them and they see us, and as the observation ends the superposition on both ends of the communication, we could communicate as you indicated simultanuesly.
Reply | Report Abuse | Link to thisEntangled photons are similar to waves.Why not forget photons and speak and write about electromagnetic waves?
Reply | Report Abuse | Link to thisAs we are talking in the quantum entanglement, it means we have a superposition of particle and wave function, once you observe you deal with a photon, the wave function is applicable for all the "distance" between origine and goal (observer), thats why no time is involved.
Reply | Report Abuse | Link to thisYes and now all we need is to do it,and thanks for for pointing out the duality of light.When the wave function drops it is the particle function we are seeing,and this is why this theory will work.Glad to see some other's have the same ideas as I.
Reply | Report Abuse | Link to thisDoing it is an action, an action is a causal effect, the cause is our knowledge of the entanglement, the entanglement is not causal , but in fact a timeless superposition of states that will form a sequence of events in our conscience, once our conscience in this fourdimensional causal world becomes "aware" of this way of communication we ARE communicating ...
Reply | Report Abuse | Link to thisAs I said earlier our mere looking at the stars may have caused wave function drop at any equal distance luminous object through out the universe.There may other civilizations that have seen our watching the stars for millions of years waiting for a response from a humanoid species on Earth that has finally found out about entanglement.And here we are almost a century later and still not making a concerted effort to look for such a signal.It my sincere hope that somebody like you Wilhelmus de Wilde has some influence over the powers that be,so this search can begin.For I am but layman with with little influence.
Reply | Report Abuse | Link to thisBeing layman in this world means that one is not an expert in some special field,(I am an architect(specialisation) not a physicist(highly interested) so everybody in a way is a layman, mankind will always stay this way, to reach the total "truth" is asymptotic, we cannot in the stage of our scientific evolution being specialist in every field (unlike NEWTON in his time), but now as a total we have enough force (LHC, Planck satelite)to continue our struggle for understanding, and now beginning to understand that it not merely formula's and technique (so called quantum weirdnes like entanglement , superposition of states) that perhaps we have to look for other (at this time still called trancendental) ways of consciesness that can lead us to new "techniques" and "handling", for example the article states that we have to place repeaters every 100km with single trapped atoms etc., but once we have arranged an entanglement in fact it is timeless and not space-bounded, the wave function of the entanglement travels at the speed of light, so sent it not to an earth bound goal (repeater) but to a sattelite, you can sent it everywhere, from one quantum computer to another via satelites, in fact once when we finished a quantum computer it contains all the answers of our future and past questions, so we created a knowledge that is approachable for ALL , this however is in vieuw of several human disagreements not applicable...
Reply | Report Abuse | Link to thisI agree, sounds hard to believe, but the creation of an 'echo' is how it get passed,
Reply | Report Abuse | Link to this"Tittel's group employed a crystal doped with thulium, a rare earth metal, that can hold on to a photon for seven nanoseconds before releasing it again, quantum state intact. The photon, once absorbed into the crystal memory, creates an excitation in a collection of atoms within the memory that then reproduces the photon a short time later in a sort of optical echo. The group demonstrated that the echo photon retained its entanglement with the original photon's mate."
and if you want to wander into the ESP connection, one can see that the far past mastered this type of communication
I agree, sounds hard to believe, but the creation of an 'echo' is how it get passed,
Reply | Report Abuse | Link to this"Tittel's group employed a crystal doped with thulium, a rare earth metal, that can hold on to a photon for seven nanoseconds before releasing it again, quantum state intact. The photon, once absorbed into the crystal memory, creates an excitation in a collection of atoms within the memory that then reproduces the photon a short time later in a sort of optical echo. The group demonstrated that the echo photon retained its entanglement with the original photon's mate."
and if you want to wander into the ESP connection, one can see that the far past mastered this type of communication