From: Michael Nielsen <mnielsen@theory.caltech.edu>

Date: Wed Jun 02 1999 - 15:29:58 PDT

Date: Wed Jun 02 1999 - 15:29:58 PDT

Quantum entanglement is believed by many people to be the key resource

allowing quantum systems to do tasks like quantum computation

that are impossible with classical systems.

Entanglement is a resource of a distinctly non-classical nature;

iron in the classical world's bronze age. It is a crucial ingredient in

quantum teleportation, superdense coding, quantum algorithms, and most

other quantum information processing tasks.

An obvious question to ask is whether or not there are quantum resources

other than entanglement which take us beyond the classical

world.

In a very interesting preprint, Jonathan and Plenio show that there are

information processing tasks in which an essentially new type of

resource is being used:

http://xxx.lanl.gov/abs/quant-ph/990507

The idea is a bit technical. For those who are interested: two people,

Alice and Bob, share a quantum state which we'll call psi. Alice and Bob

can talk to each other classically, and they can both do whatever

operations they like on their own systems, but they have no quantum

communications channel between them by which to communicate quantum

states. (The lingo is that Alice and Bob can apply local operations and

classical communication to their state psi.)

A natural question to ask is what states phi can Alice and Bob generate

using these resources?

What Jonathan and Plenio noticed is the following: there are quantum

states psi and phi with the following two properties:

(a) Alice and Bob cannot transform psi into phi using only local

operations and classical communication.

(b) Suppose a third party, Scrooge, lends a _catalysing_ state, gamma,

to be shared by Alice and Bob. With the aid of this catalysing state it

becomes possible to transform psi into phi. Moreover, the catalysing

state is _left intact_ by this process.

These catalysing states are pretty interesting for two reasons. First,

they enable an information processing task of some interest. Second, and

more important, Jonathan and Plenio show that so-called "maximally

entangled" states are no good as catalysts. Thus, entanglement is not the

key property being used for catalysis; it is a new type of quantum

resource.

At this point in time it's pretty difficult to see what these "catalysing

states" are useful for other than the catalysing process described above.

However, the history of quantum computation and quantum information shows

that identifying new types of physical resources is of the utmost

importance; I hope that eventually we'll find all sorts of uses for these

funny new types of states.

Michael Nielsen

Ph: 626 395 8431 Fax: 626 793 9506

Email: mnielsen@theory.caltech.edu

Received on Wed Jun 2 15:32:21 1999

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