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Quantum computer that 'computes without running' sets efficiency record
PhysOrg ^ | 8/31/15 | Lisa Zyga

Posted on 09/01/2015 10:33:43 PM PDT by LibWhacker

(Phys.org)—Due to quantum effects, it's possible to build a quantum computer that computes without running—or as the scientists explain, "the result of a computation may be learned without actually running the computer." So far, however, the efficiency of this process, which is called counterfactual computation (CFC), has had an upper limit of 50%, limiting its practical applications.

Now in a new paper, scientists have experimentally demonstrated a slightly different version called a "generalized CFC" that has an efficiency of 85% with the potential to reach 100%. This improvement opens the doors to realizing a much greater variety of applications, such as low-light medical X-rays and the imaging of delicate biological cells and proteins—in certain cases, using only a single photon.

The researchers, led by Prof. Jiangfeng Du at the University of Science and Technology of China and Prof. Liang Jiang at Yale University in the US, have published a paper on the high-efficiency counterfactual computing method in a recent issue of Physical Review Letters.

"The main keys to achieving high-efficiency CFC include the utilization of exotic quantum features (quantum superposition, quantum measurement, and the quantum Zeno effect), as well as the use of a generalized CFC protocol," Du told Phys.org.

How counterfactual computing works

By "not running," the scientists mean that the computer—which can operate in either an "on" subspace or an "off" subspace—stays in its "off" subspace for the entire computation. Physically maintaining the computer in the "off" subspace, in this scheme, involves controlling the spin properties of a diamond system, which acts as a quantum switch. Some of the spins must be kept in a superposition state, in which they occupy two states at the same time.

To control the spin superposition, the physicists took advantage of the quantum Zeno effect, in which frequent measurements on a system can "freeze" the system in its current state. By applying a sequence of pulses to the system, the scientists could keep the system in its "off" subspace, and so keep it from running.

"The procedure comprises a quantum switch and a quantum register," Jiang explained. "For each repetition, we prepare the quantum switch into a quantum superposition state, including two coherent parts ('on' and 'off'). Then the 'algorithm,' a NOT gate on the quantum register in our case, is performed in the 'on' subspace. Although it seems the computer has run in this step, a consequent projective measurement will remove all the changes in the 'on' subspace, since the probability of the whole system collapsing into the 'off' subspace during the measurement is very large (approaches 100% as the number of repetitions tends to infinity utilizing the quantum Zeno effect)."

The researchers explain that the "on" and "off" states can be thought of as the two paths of an interferometer, where a photon may take one path or the other, but not both.

"Such a situation is very similar to the case of a photon passing through a two-way interferometer," Jiang said. "When a detector on one of the paths catches the photon, then one says the photon does not go on the other path. Similarly, when the whole system collapses into the 'off' subspace, one can conclude that the computer does not run. After each repetition, the state changes slightly. It finally evolves to a certain value after N repetitions from its initial value. By detecting its state, we get the information that is 'programmed' in the computer, although the computer has not run."

Breaking the efficiency limit

Previous experimental CFC protocols have faced a counterfactual efficiency limit of 50%, where the counterfactual efficiency is defined as "the average probability of learning the result of a computation without running the computer." But the generalized CFC (first proposed by G. Mitchison and R. Jozsa in 2001) does not face this limit, which allowed the researchers in the new study to experimentally demonstrate an efficiency of 85% at 17 pulse repetitions.

"The key difference between the two protocols is that the 'off' subspace of the generalized CFC is dependent on the choice of the 'algorithm' (Ur), whereas it is independent in the controlled-Ur CFC," said coauthor Chenyong Ju at the University of Science and Technology of China. "As a consequence of this fact, the sum of the 'volume' of each 'off' subspace, which has a direct relation to the counterfactual efficiency, is much larger for the generalized CFC than the controlled-Ur CFC."

The higher efficiency opens up the possibility of developing highly efficient yet very low-light imaging technology. This technology could be useful in any situation in which light may damage or destroy the illuminated sample, which makes the method particularly relevant for biological imaging. Applications may include imaging green fluorescent proteins that might be bleached under laser light, as well as UV imaging of cells and safe X-ray imaging. In some situations, these applications might be performed using only a single photon.

"The use of one photon is just for the special case that the object to be imaged has only one pixel being transparent, whereas the other pixels are opaque," said coauthor Fei Kong at the University of Science and Technology of China. "To image the object with our protocol, one may imagine that the situation in which a photon is absorbed by an opaque pixel is just like the computer evolving into the 'on' subspace. Such a process is effectively avoided in our protocol. The photon will eventually 'find' the transparent pixel and pass through it. Through a detector below, one can locate this pixel and hence accomplish the imaging with just one photon. The number of photons needed is proportional to the number of transparent pixels, whereas normal imaging methods need [many more] photons."

In the future, the researchers also plan to investigate potential applications of counterfactual computing for secure communication.

"We are looking forward to exploring more realistic applications of the generalized CFC," Du said. "There are several recent works on the topic of counterfactual quantum cryptography and communication. Employing the counterfactual quantum phenomenon, several groups have proposed and demonstrated a new model of secret communication in which no physical signal particles are transmitted, which provides practical security advantages. [For example, see here, here, and here.] We wonder what the potential of the generalized CFC is in this area."


TOPICS: Business/Economy; Computers/Internet; Military/Veterans; Science
KEYWORDS: cfc; computation; computer; computing; counterfactual; quantum; quantumcomputer; quantumcomputing; stringtheory
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Next... Force a quantum computer that hasn't been built yet to compute. Then you'll have something! j/k
1 posted on 09/01/2015 10:33:44 PM PDT by LibWhacker
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To: LibWhacker

But will it run Doom 3?

(Old Slashdot joke)


2 posted on 09/01/2015 10:37:42 PM PDT by Crazieman (Article V or National Divorce. The only solutions now.)
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To: LibWhacker

interesting but its over my head.. lol


3 posted on 09/01/2015 10:38:58 PM PDT by GeronL (Cruz is for real, 100%)
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To: GeronL

Mine too. That’s why I’m reduced to cracking jokes about it!


4 posted on 09/01/2015 10:42:08 PM PDT by LibWhacker
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To: LibWhacker
Speaking of jokes:

Schrodinger's cat

5 posted on 09/01/2015 10:58:16 PM PDT by dorothy ( "When injustice becomes law, resistance becomes duty." - Thomas Jefferson)
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To: LibWhacker
".......the result of a computation may be learned without actually running the computer."


6 posted on 09/01/2015 11:00:24 PM PDT by Viking2002 (The Avatar is back by popular request.)
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To: LibWhacker

When they have a cardboard box with a unattached screen that can turn on by itself and play SplinterCell Double Agent, THEN I’ll be impressed.


7 posted on 09/01/2015 11:03:00 PM PDT by Greetings_Puny_Humans (I mostly come out at night... mostly.)
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To: LibWhacker

[Employing the counterfactual quantum phenomenon, several groups have proposed and demonstrated a new model of secret communication in which no physical signal particles are transmitted, which provides practical security advantages.]

A new challenge for the nsa, cia, dia, and fbi.


8 posted on 09/01/2015 11:05:38 PM PDT by Vision Thing ("Community Organizer" is a shorter way of saying "Commie Unity Organizer".)
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To: LibWhacker

From what I’ve read they don’t even really know how it works. It sends information into the quantum realm (or another dimension) and then the answer comes back like magic. Makes you wonder just who or what is supplying the answer.


9 posted on 09/01/2015 11:07:18 PM PDT by Sir Gawain
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To: LibWhacker
[Article]The researchers, led by Prof. Jiangfeng Du at the University of Science and Technology of China and Prof. Liang Jiang at Yale University in the US, have published a paper on the high-efficiency counterfactual computing method in a recent issue of Physical Review Letters.

God damn it.

Where are our guys?

This is top-drawer theoretical physics. Do you know what this means for 15 years down the road?

How about military technologies we can only dream of, illuminating U.S. submarines and aircraft and decrypting our best cryptosystems in a flash?

What is this collaboration with Yale -- a thinktank pipeline sucking our best people dry and channeling their insights to Chinese skunk works?

I really didn't need to read something like this, not when we've got America's biggest enemy in the White Hut.

The British may have to bail us out on this growing -- not technology, but hierarchically higher theoretical knowledge -- disparity, which threatens us like no money-slurping low-level spy ever did.

This is top-level espionage, conducted out in the open "in the name of science".

10 posted on 09/01/2015 11:08:50 PM PDT by lentulusgracchus ("If America was a house , the Left would root for the termites." - Greg Gutfeld)
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To: GeronL
interesting but its over my head.. lol

I'll make it easy. The Chinese are swarming our top universities and technology companies, sucking our brains and passing the take back to China, where 40,000,000 Chinese coolies are busy digging our graves.

11 posted on 09/01/2015 11:17:03 PM PDT by lentulusgracchus ("If America was a house , the Left would root for the termites." - Greg Gutfeld)
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To: LibWhacker
MAGIC
12 posted on 09/01/2015 11:22:21 PM PDT by Swordmaker ( This tag line is a Microsoft insult free zone... but if the insults to Mac users continue...)
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To: LibWhacker

Potentiated photonic cryptography via axial polarization! Brilliant! Thanks for posting this.


13 posted on 09/01/2015 11:23:29 PM PDT by davius (You can roll manure in powdered sugar but that don't make it a jelly doughnut.)
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To: LibWhacker

I must confess, it is rather quaint to observe the people of your world being fascinated by such basic discoveries...


14 posted on 09/01/2015 11:39:35 PM PDT by Norm Lenhart
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To: Sir Gawain
From what I’ve read they don’t even really know how it works. It sends information into the quantum realm (or another dimension) and then the answer comes back like magic. Makes you wonder just who or what is supplying the answer.

What's the old Sci/Fi movie line: ”Some Things Man Was Not Meant to Know”, ..............Just saying :^)

15 posted on 09/01/2015 11:39:57 PM PDT by The Cajun (Ted Cruz, Sarah Palin, Mark Levin, Mike Lee, Louie Gohmert....Nuff said.)
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To: LibWhacker
"How counterfactual computing works By "not running," the scientists mean that the computer—which can operate in either an "on" subspace or an "off" subspace—stays in its "off" subspace for the entire computation. Physically maintaining the computer in the "off" subspace, in this scheme, involves controlling the spin properties of a diamond system, which acts as a quantum switch. Some of the spins must be kept in a superposition state, in which they occupy two states at the same time. To control the spin superposition, the physicists took advantage of the quantum Zeno effect, in which frequent measurements on a system can "freeze" the system in its current state. By applying a sequence of pulses to the system, the scientists could keep the system in its "off" subspace, and so keep it from running." Ahh, now I get it!
16 posted on 09/01/2015 11:57:16 PM PDT by aquila48
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To: Sir Gawain

The concept, as far as I understand it, is essentially a quantum analog computer. A classical analog computer emulated differential equations with voltages and currents. In the quantum analog computer, different sorts of analogies are made, but it still relies on physical laws, in this case quantum laws, to determine the final state, and the output of the computation. Personally, I think the whole thing is a crock. They’ve been touting this idea for decades, and whatever success they may eventually proclaim, it won’t amount to a hill of beans.


17 posted on 09/02/2015 12:01:54 AM PDT by dr_lew
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To: lentulusgracchus

“The researchers, led by Prof. Jiangfeng Du at the University of Science and Technology of China and Prof. Liang Jiang at Yale University”

So the leading researchers are a chinese in China and a chinese in the US.

If anybody should be upset it should be the chinese for making this cutting edge technology public.

But I guess they figure that whatever comes out of it, they’ll be building it and exporting it to us.


18 posted on 09/02/2015 12:06:26 AM PDT by aquila48
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To: lentulusgracchus

i’ve been screaming about the impact of H-1bs on the sciences for years.

i’m glad i’m not the only one that noticed it in this brief.


19 posted on 09/02/2015 12:08:54 AM PDT by sten (fighting tyranny never goes out of style)
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To: dr_lew

I have a feeling that whatever is made public is about 10 to 20 years behind what tech the black sites currently have.


20 posted on 09/02/2015 12:20:51 AM PDT by Sir Gawain
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