Using degrees of freedom to get hyperentanglement

January 27, 2010 By Miranda Marquit, feature

( -- One of the biggest challenges scientists are grappling with today is the creation of an efficient quantum computer. There are a number of models out there, and hundreds of scientists and researchers around the world are constantly coming up with theories and experiments in an effort to make quantum computing possible. One of the main issues is that of entanglement. Being able to entangle photons has long been thought a necessity of quantum computing.

“It’s important to manipulate an increasing number of qubits,” Wei-bo Gao tells “One way to extend the number of qubits is to be able to entangle a greater number of particles. However, usually it is very difficult. So far, up to eight atoms have been demonstrated. The other way is to create with more degrees of freedom. It is called hyper-entanglement, and it is vital for efficient .”

Gao is a scientist in Jian-wei Pan’s group at the University of Science and Technology of China, located in Hefei. He has been working with a team of scientists from the University of Science and Technology, as well as scientists from the Österrichische Akademie der Wissenshaften and the Universität Innsbruck in Austria, the Universidad de Sevilla in Spain, and Ruprecht-Karls-Universität Heidelberg in Germany. They have succeeded in demonstrating an optical controlled-NOT gate based on a six qubit cluster state, and the results can be seen in Physical Review Letters: “Experimental Realization of a Controlled-NOT Gate with Four-Photon Six-Qubit Cluster States.”

“We created a cluster state that is universal for quantum computing,” Gao says. “We also show that spatial qubits can help in quantum computing. Our method uses different degrees of freedom, including polarization and spatial modes of , to help achieve hyper-entanglement. We have recently done up to 10 qubits, using five photons and two degrees of freedom for each photon, even though this paper only demonstrates six qubits.”

Gao and his peers generated the six-qubit state using photons, and then used single qubit measurements to apply the CNOT gate to arbitrary single input qubits, which is thought to be one of the keys to creating universal quantum computation. The team also took care to show that this optical method of quantum computation could not be reproduced using local operations and classical communication techniques.

The next step is to see whether hyper-entanglement can be achieved using even more photons and degrees of freedom. “We’d like to create seven photon entanglement, and use more degrees of freedom,” Gao explains. “Now that we have shown that spatial qubits can be used, we’d like to see if arrival time and angular momentum could also be used. This would help increase the number of qubits and provide scalability to quantum computing.”

Part of the challenge to increase the number of photons used is the collection process. “We will need to increase collection efficiency so that we can actually get more photons. That will be a bit of a challenge in making efficient optical quantum computers, but it is something that many scientists are working on.”

Gao is sure that scalable quantum computing can be achieved at some point in the relatively near future. “With more power, more degrees of freedom, and the developing technique of atomic memory, I am confident that more efficiency in optical can be achieved as we have more qubits.”

Explore further: Creating a six-qubit cluster state

More information: Wei-Bo Gao, et. al., “Experimental Realization of a Controlled-NOT Gate with Four-Photon Six-Qubit Cluster States,” Physical Review Letters (2010). Available online:


Related Stories

Creating a six-qubit cluster state

November 2, 2009

( -- Many scientists believe that quantum entanglement is required in order for effective quantum computing. Entanglement takes place when there is a connection that exists between two objects - even when they ...

Physicists Demonstrate Quantum Memory with Matter Qubits

July 3, 2009

( -- For the first time, scientists have successfully operated a quantum gate between two remote particles of matter, marking an important step toward the development of a quantum computer. In previous experiments, ...

Shining light in quantum computing

September 12, 2006

University of Queensland scientist Devon Biggerstaff is investigating ways to manipulate light in a process that will help shape future supercomputers and communication technology.

A solid case of entanglement

January 11, 2010

Physicists have finally managed to demonstrate quantum entanglement of spatially separated electrons in solid state circuitry.

Straightening messy correlations with a quantum comb

November 23, 2009

Quantum computing promises ultra-fast communication, computation and more powerful ways to encrypt sensitive information. But trying to use quantum states as carriers of information is an extremely delicate business. Now ...

Recommended for you

CMS gets first result using largest-ever LHC data sample

February 15, 2019

Just under three months after the final proton–proton collisions from the Large Hadron Collider (LHC)'s second run (Run 2), the CMS collaboration has submitted its first paper based on the full LHC dataset collected in ...

Gravitational waves will settle cosmic conundrum

February 14, 2019

Measurements of gravitational waves from approximately 50 binary neutron stars over the next decade will definitively resolve an intense debate about how quickly our universe is expanding, according to findings from an international ...


Adjust slider to filter visible comments by rank

Display comments: newest first

3.5 / 5 (2) Jan 27, 2010
This article doesn't seem to have a point beyond saying that more entanglement in any form is a good thing for quantum computing. Isn't that kind of obvious?
4.3 / 5 (3) Jan 27, 2010
What they are saying is that if you can get more relationships with fewer particles, then that is better. They're doing that by entangling each particle in more than one way (degrees of freedom).

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.