Physicists demonstrate teleportation-based optical quantum entangling gate

December 8, 2010 By Lisa Zyga feature
This diagram shows the experimental set-up of a quantum CNOT gate. Image credit: Wei-Bo Gao, et al. ©2010 PNAS.

( -- Taking a step toward the realization of futuristic quantum technologies, a team of physicists from China and Germany has demonstrated a key element – an entangling gate – of a quantum teleportation scheme proposed more than 10 years ago. The entangling gate serves as a fundamental building block for applications such as long-distance quantum communication and practical quantum computers.

The scientists, Wei-Bo Gao, Jian-Wei Pan, and coauthors from the University of Science and Technology of China in Hefei, Anhui, China, and the University of Heidelberg in Heidelberg, Germany, have published their study in an early edition of the .

The work builds on earlier research by D. Gottesman and I. L. Chuang, who theoretically showed in 1999 that a quantum gate can be built by teleporting qubits (the basic units of quantum information) with the help of certain entangled states. In quantum teleportation techniques, unknown quantum states are transferred from one location to another through the use of entanglement. One of the key requirements of the "GC scheme" is the ability to perform single-qubit for quantum computations.

In the new study, Gao, Pan, and coauthors have experimentally demonstrated the feasibility of the GC scheme by demonstrating a logic gate based on quantum teleportation for two photonic qubits. Further, the scientists demonstrated the entangling gate using two different methods – one with a six-photon interferometer to realize controlled-NOT gates, and the other with four-photon hyperentanglement to realize controlled-Phase gates.

“The logic gate is the basic unit in a quantum computer, as a classical logic gate is in a classical computer,” coauthor Yu-Ao Chen of the University of Science and Technology of China told “The difference with the quantum logic gate is that it involves quantum superposition. The controlled-NOT gate, together with single-qubit operations, is sufficient to perform all logic operations needed for quantum computation. In classical computation, it is similar, and it can be easily achieved by measurement and feedforward. However, in the quantum world, the states cannot be measured directly without irrevocably changing them. This makes the demonstration of a CNOT gate more difficult.”

In their study, the scientists showed how each of the two methods could be used to entangle qubits, transfer qubit states, and enable quantum logic operations. The researchers also confirmed that both demonstrate genuine entanglement, and showed that the controlled-Phase gate achieves quantum parallelism, meaning that the gate cannot be reproduced by local operations and classical communications.

While the experiments here represent an important step toward the realization of practical quantum computers, there are still many other components to work on in order to develop an advanced optical system in the future. As the scientists explained, one of the advantages of the current scheme is that it is inherently fault-tolerant.

“Our experiment shows a proof-of-principle that the GC scheme can be implemented to construct a linear optical computational hierarchy,” Chen said. “The GC scheme is a protocol that could potentially be implemented in a real-world optical quantum computer because it is inherently fault-tolerant, which is a very basic request for any large-scale computation protocol.”

As Chen noted, some of the potential applications of quantum computing include the efficient simulation of complex quantum physics, chemistry systems, or materials systems; the ability to quickly solve certain mathematical problems; and database searching.

Explore further: Measuring quantum information without destroying it

More information: Wei-Bo Gao, et al. “Teleportation-based realization of an optical quantum two-qubit entangling gate.” PNAS Early Edition. DOI:10.1073/pnas.1005720107


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4 / 5 (1) Dec 08, 2010
I'm going to want a video to explain this step by step without the gibberish picture.
not rated yet Dec 08, 2010
thats an awfull lot of complicated logic to build a single gate, better hope one gate can process a million values, because you need the size of a house to stuff a million of these gates
not rated yet Dec 08, 2010
Anyone reminded of the _Enders_Game_ series?
not rated yet Dec 09, 2010
I was thinking Rendering and lighting Voxels and game physics. Of course of Unlimited Detail is not a hoax, I guess the Voxel problem is solved.
not rated yet Dec 14, 2010
I just have a question possibly related to entanglement. If I created a rod of some very strong material, titanium for example, and placed it in space and it was very long, say 186,000 miles and I pushed it one inch on January 1st 2011 at 12:00.01 am EST at what time would the other end move 1 inch.
not rated yet Dec 15, 2010
I just have a question possibly related to entanglement. If I created a rod of some very strong material, titanium for example, and placed it in space and it was very long, say 186,000 miles and I pushed it one inch on January 1st 2011 at 12:00.01 am EST at what time would the other end move 1 inch.

Depends how stiff your rod is. But no matter how stiff it is, I don't see how you'll achieve entanglement. For one thing, it's too long.

I could go on.
not rated yet Dec 18, 2010
did you want to know when the other end would move, or when the light reflecting off of that end would hit your eyes? i think it probably would not move. force is proportional to mass, so even a rod in negligible gravity would require a huge force to accelerate to a one inch displacement. i think the force would compress the rod instead of move it, creating heat instead of potential energy. it would melt, not move linearly. just my hypothesis...
lets say the rod was in motion already. both ends move with the same displacement in the same amount of time, but the light from the far end would take a year to reach you.
not rated yet Dec 18, 2010
oops. i meant one second not year, considering the length. i was thinking in terms of c, but that is 186000 miles per *second*
big difference...

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