FROM the photocopier to the fax to a computer鈥檚 鈥渃opy and paste鈥� function, the ability to replicate information is something we take for granted. At the quantum scale, however, making exact copies should be impossible. Now there鈥檚 a way to dodge this cloning ban that could also help map the border between the 鈥渃lassical鈥�, everyday world and the quantum realm 鈥� a goal illustrated by the Schr枚dinger鈥檚 cat paradox.
The ban arises because, unlike large objects, two or more quantum particles can be entangled, a weird state in which they remain intimately linked even when physically separated. If entangled objects could also be cloned, it would be possible to use them to send a message across space: changing one object would automatically cause an identical change in the other. And this quantum-telegraph message would travel faster than light, violating Einstein鈥檚 relativity and posing problems for causality.
Luckily for physicists, in the 1980s a mathematical proof emerged showing that some of the information stored in the original quantum state is always lost: the best you can do is make a sort of grainy, black-and-white photocopy. 鈥淐opying classical information is a really important thing that we do all the time,鈥� says of the University of Calgary in Alberta, Canada. 鈥淪o it鈥檚 interesting that for quantum information, you can鈥檛 do it.鈥�
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One way to make the quantum equivalent of the grainy photocopy is to shine a single photon at a crystal. If the photon is in a particular, excited state, it can prod the atoms into emitting many more photons that all approximate the original, but none of which is identical.
In 2008, a group led by of the Sapienza University of Rome, Italy, made about 10,000 of these imperfect clones in order to probe a different question raised by quantum mechanics: how big can an object be yet still show quantum activity?
Their original photon shared its quantum state with another photon via entanglement. The team claimed that the whole army of imperfect clones was therefore also . Such large-scale entanglement is reminiscent of Schr枚dinger鈥檚 cat. In that thought experiment, a cat sits in a box with a radioactive atom linked to a poisoning device. If the atom decays, the cat dies; if not, the cat lives. Being a quantum object, until someone opens the box the atom can exist in both states at once 鈥� physicists see examples of such quantum behaviour in little things all the time. But then the cat must also be both dead and alive, and no one ever sees evidence of a cat-scale object in such a superposition of states.
鈥淪chr枚dinger originally was using that to say, 鈥業f you take quantum physics to this extreme size, it鈥檚 absurd鈥�,鈥� Simon says. 鈥淣owadays, we鈥檙e not so sure.鈥�
鈥淪chr枚dinger used his cat to say, 鈥業f you take quantum physics to this extreme, it鈥檚 absurd鈥�. We鈥檙e not so sure鈥�
De Martini鈥檚 group argued that they had attacked this problem because the army of imperfect clones is a macroscopic object behaving in a quantum way, by being entangled with a single photon. But it proved tough to show that all the clones were entangled with the original photon鈥檚 entangled partner.
Now Simon and his colleagues suggest a way to settle this, by converting the ensemble of imperfect clones back into an exact copy of the original photon that spawned them. This would provide a way to dodge the ban on quantum cloning without creating a relativity-defying quantum telegraph, they suggest.
The key to their proposal is a second crystal that acts as an inverted version of the first 鈥� instead of amplifying the original photon and distributing its quantum state, it takes the clone army and boils it back down into a single photon, ideally preserving all the information in the process (see diagram).
This could be done using lasers, says Simon. A laser is already used to turn the first crystal into an amplifier, which then awaits the initial single photon. His team proposes pumping the second crystal with a laser of the opposite phase to prime it to do exactly the opposite 鈥� convert the clones back into a single photon ().
The new set-up also provides an easier way to test whether a single photon can be entangled with the entire clone army, as De Martini鈥檚 team claimed. Only if this is possible can the second emitted photon be entangled with the first photon鈥檚 quantum partner.
So to prove entanglement between the quantum and macroscopic realms, all you need to do is measure entanglement between two photons, not thousands. 鈥淚f you can show that after all of this you still have entanglement, then that implies you had entanglement in between,鈥� Simon says.
To discover where the laws of the macroscopic world start dominating over those of quantum mechanics, his team suggest increasing the size of the clone army one photon at a time and noting when entanglement breaks down. This would help to explain why we see atoms but not cats in two or more states at once.
De Martini鈥檚 colleague , who recently started testing the reverse-cloning idea, points out that the more photons you add to the intermediate army, the harder it gets. His group has only managed to do it with three photons, for example, and he suspects success may only be possible with an army of hundreds.
There鈥檚 yet another reason to boost the clone army. The technique might also have a spin-off application: taking non-destructive images of delicate cells.
If a cell sits between the two crystals, it will absorb or scatter some of the clone photons, and so the final photon, emitted from the second crystal, will be less well-entangled with the first photon鈥檚 entangled partner. Figuring out how much entanglement was lost tells you exactly how many photons passed through the cell 鈥� and so could be used to construct an image of it. This should require much less light than traditional imaging, says Simon, and so could avoid damaging the cell.
