Quantum computing: Entanglement may not be necessary

Dec 05, 2008 By Miranda Marquit feature

(PhysOrg.com) -- It is a truth universally acknowledged that quantum computing must have entanglement.

“Entanglement,” Andrew White tells PhysOrg.com, “is normally considered a non-negotiable part of quantum information processing. In fact, if you told me a couple of years ago that you could do quantum computing without entanglement, I would have been pretty skeptical – to say the least!”

White says that he first heard the idea of non-entanglement quantum computing from Carl Caves. “I was intrigued when Professor Caves, on sabbatical here in Australia from New Mexico, mentioned that there were sober predictions that entanglement wasn’t always necessary.”

White leads a team of young experimental scientists at the University of Queensland in Brisbane, Australia. Ben Lanyon, Marco Barbieri, Marcelo Almeida and White have been studying deterministic quantum computing with only one pure qubit (DQC1). “Entanglement is not the final story on what makes quantum information processing powerful,” White insists. The Australian team’s results can be found in Physical Review Letters: “Experimental Quantum Computing without Entanglement.”

“Normally, in order for quantum computing to work,” White explains, “we need to encode the information into quantum bits—qubits—which are in a noise-free pure state. It’s known that the entanglement between these is what makes standard quantum computing powerful.” He continues, “With a DQC1 scheme, you only have to have one pure qubit, and the rest can be noisy or mixed.” The idea behind quantum information processing using entanglement is that noiselessness has to be applied in order to provide a substantial advantage over classical computing. DQC1, though, could potentially offer a more efficient and less resource-intensive method of quantum computing, since entanglement would no longer be a necessity.

“For this demonstration,” White says, “we used the smallest possible example: a circuit with just two qubits, one pure and one mixed. We ran a phase-estimation algorithm as a small example, and found in every setting there was zero entanglement, but that most of the states couldn’t be described efficiently in a classical manner.”

White points out that this is suggestive that there are other possibilities, beyond entanglement, that contribute to the power provided by quantum information processing. “We’re still chewing through the implications,” he says.

“This is not a universal panacea,” White admits. “For some problems and algorithms you just need pure qubits and entanglement, problems such as Shor’s algorithm. However, there are applications and problems where the DQC1 method will work quite well, and will be more efficient than trying to get qubits that are all pure.”

With so many different architectures and schemes for quantum computing – all of them trying to create a system in which all the qubits are pure – it is rare to see a group looking to find applications for a quantum information system that makes allowances for impurity and the introduction of noise – insisting that entanglement is not necessary. “The fact is that certain classes of problems don’t need entanglement, and they don’t need all of the purity. In some cases, all that is needed is one pure qubit and the rest could be mixed. Really, with DQC1, you don’t have to work as hard as you think you do.”

We are starting to build more complicated algorithms to get an idea of where this could go. Regardless, the idea that entanglement may not be necessary for some types of quantum computing is big news.”

More information: B. P. Lanyon, M. Barbieri, M. P. Almeida, and A. G. White. “Experimental Quantum Computing without Entanglement.” Physical Review Letters (2008). Available online: link.aps.org/abstract/PRL/v101/e200501 .

Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

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User comments : 13

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1.8 / 5 (5) Dec 05, 2008
3.7 / 5 (3) Dec 05, 2008
Well, technically, are they not entangled? It's just that the state of entanglement is not strictly (purely as they put it) known.

Kind of like a hologram how the entire image is encoded in a smaller piece. Instead of looking at the entire picture, you look at the small piece of derive the larger picture without knowing it first.

crazy r i
4.3 / 5 (4) Dec 05, 2008
Um...ok. So the news is that something that is still being developed might be developed differently. Interesting, but you would think they would come up with something more tangible before declaring it news-worhty.
3 / 5 (4) Dec 05, 2008

The real issue here is they are trying to describe a highly technical theoretical circuit in "laymans terms", and without equations or diagrams. Which basicly makes this entire article useless.
3.5 / 5 (2) Dec 06, 2008
I agree with Quantum, while the article is intriguing, it really contains essentially no actual information.
3 / 5 (4) Dec 06, 2008
I agree with Quantum, while the article is intriguing, it really contains essentially no actual information.

moi, aussi

1.8 / 5 (5) Dec 06, 2008
quantum computers for real applications is still a 'science fiction'.
3.7 / 5 (3) Dec 06, 2008
There's no real breakthrough here. If they had a working model, then it would be a breakthrough. It is very facinating to read the actual theories without the watering down of laymans terms, but I still think it's all useless until there is at least some kind of expirimentation with those theories with some sort of hope-inducing results. As it is now, the whole thing is a bunch of guys at a chalk board designing the experiment for later. Interesting when presented in detail? Yes. Remotely useful or news worthy? Not really.
3 / 5 (6) Dec 06, 2008
Experimental Quantum Computing without Entanglement
B. P. Lanyon, M. Barbieri, M. P. Almeida, and A. G. White

Department of Physics and Centre for Quantum Computer Technology, University of Queensland, Brisbane 4072, Australia
(Received 15 August 2008; published 13 November 2008)

Deterministic quantum computation with one pure qubit (DQC1) is an efficient model of computation that uses highly mixed states. Unlike pure-state models, its power is not derived from the generation of a large amount of entanglement. Instead it has been proposed that other nonclassical correlations are responsible for the computational speedup, and that these can be captured by the quantum discord. In this Letter we implement DQC1 in an all-optical architecture, and experimentally observe the generated correlations. We find no entanglement, but large amounts of quantum discord%u2014except in three cases where an efficient classical simulation is always possible. Our results show that even fully separable, highly mixed, states can contain intrinsically quantum mechanical correlations and that these could offer a valuable resource for quantum information technologies.
2 / 5 (4) Dec 07, 2008
One day, one day we'll grasp the concept that there is in fact a deterministic process underlying the supposed mystical universe of quantum mechanics. Define something as a closed system (i.e. say ANYTHING is one of ANYTHING... e.g. ONE atom, ONE particle, ONE waveform, ONE etc.)... understanding prime numbers will unravel the mystery, understanding what happens when you quantize implicitly by virtue of the fact you have alloted ONE of anything... the math is directly linked to quantum physics, as we're starting to see now. Quantum computing won't come about until we have this fundamental understanding of math. Math is all. Lets go back to school kids... Math > Physics > Chemistry > Biology
not rated yet Dec 30, 2008
"One day, one day we'll grasp the concept that there is in fact a deterministic process underlying the supposed mystical universe of quantum mechanics."
Why do you think that the reality of the universe should be so boring simple and predictable?
/Kim Michelsen
not rated yet Jan 01, 2009
Those imperious little atomic structures!

Or, in the words of Obi-Wan, Should we say that, "we would like to avoid any Imperial entanglements?"

Star Wars Groaner. Hah! Got yah.
not rated yet Jan 01, 2009
On a more serious note, anyone interested in this sort of work should take a look at the works of 'Dr. Dan Burisch' (copy - google search).

Be prepared to have your sensibilities assaulted to some degree, depending on where your understanding of 'how the world really works' sits. Specifically, you'll be looking at his work (incidental reports-and for good reasons!)on dimensional vortexes found in organic matter. (inorganic as well? dig deep for that particular aspect)

I will warn that it will take some time(put in the effort to let the framework and connections build, that will let it slowly become clear) to get to the core of it as the framework and fleshing out of it will assault the sensibilities of some folks here, but some are intrepid enough explorers that they can go there.

Good luck to you. If you make it - Welcome to the next level.