"The Alternate View" columns of John G. Cramer

Published in the December-1993 issue of

This column was written and submitted 5/17/93 and is copyrighted ©1993 by John G. Cramer.

All rights reserved. No part may be reproduced in any form without the explicit permission of the author.

Teleportation is familiar idea in science fiction. It permeated the SF
literature of the Golden Age, providing the basis for SF classics like Van
Vogt's **World of Null-A**** **(1945),
Budry's * Rogue Moon*(1960),
and many others. The Star Trek series' transporter has also "beamed"
the concept of teleportation to our pop culture. But modern hard-SF has largely
abandoned teleportation as a concept that has more to do with fantasy and
parapsychology than with real science.

Imagine my surprise then, to discover an article on the subject of teleportation
in the March 29, 1993 issue of * Physical
Review Letters*, that "hot-news" flagship physics journal of the
American Institute of Physics. The article, by an international collaboration of
physicists hereafter designated by the initials BBCJPW, does not provide a plan
for constructing a transporter. Instead it describes an in-principle procedure
for copying and transporting a quantum state from one location and one observer
to another by a process that is characterized as teleportation. The BBCJPW
scheme exploits some of the peculiarities of quantum physics and reveals some of
the rules of the game in that exotic domain. Let us begin by discussing these
rules.

First, we will focus on the quantum *state
vector* (also known as the
wave function). Any quantum system, for example an electron with the
characteristics of position, energy, momentum, and a spin vector pointing in
some direction, is completely described by the state vector, as denoted by the
symbol **|****y****>**. Anything
that is knowable about the electron is mathematically encoded within **|****y****>**.
An essential rule of the quantum world is that the state vector can never be
completely known because no measurement can determine it completely (except in
the special case that it has been prepared in some particular state or some
member of a known "basis" group of states in advance). In general, the
quantum state coded within **|****y****>** can
only be "glimpsed" by a measurement of one of the properties of the
quantum system. In the act of pinning down one particular property of **|****y****>**,
the measurement destroys any opportunity to determine some of the other
properties of the quantum state. The quantum state can be preserved unchanged
only by refraining from making any measurements of its properties. This
frustrating aspect of quantum mechanics is the essence of Heisenberg's
uncertainty principle.

A second ground rule of quantum mechanics is that a pair of spatially separated quantum sub-systems that are parts of an overall quantum system can be "entangled". This bizarre property of quantum systems was discovered in the formalism of quantum mechanics by Albert Einstein and his coworkers Podolsky and Rosen and is known as EPR non-locality. A measurement on one of the entangled sub-systems not only forces it into a particular state but also, across space-time and even backwards in time, forces the sub-system with which it is entangled into a corresponding state. For example, measuring the polarization of one of a pair of entangled photons precipitates the other photon, which may be light years away, into the same state of polarization as that which was measured for its entangled twin.

The state vector, however, does not have to describe a microscopic system like a photon or an electron. It can describe large collections of atoms: chemical compounds, human beings, planets, stars, galaxies. There have even been recent papers in quantum cosmology by Stephen Hawking and others in which the properties of the state vector of the entire universe are discussed.

But what has this to do with teleportation? The basic operation of teleportation can be described as determining the total quantum state of some largish system, transmitting this state information from one place to another, and making a perfect reconstruction of the system at the new location. However, since it is not even in-principle possible to measure the complete state vector of even a very simple quantum system because of the uncertainty principle, as discussed above, this would seem to rule out teleportation on even small quantum systems as physically impossible.

"Not so!" say BBCJPW. There is a way around this quantum roadblock
which exploits the peculiarities of EPR nonlocality to transmit the complete
description of the state of a quantum system over nature's privileged
communication channel without performing measurements that extract a complete
description of the state vector as information. They propose a multistep
procedure by which any quantum state **|****y****>** can
be teleported intact from one location to another (but only at a transmission
speed that is less than or equal to the velocity of light). The BBCJPW procedure
goes like this:

*Step
1: *Prepare a pair of quantum
sub-systems **| a>** and

*Step
2: *Transport one of these
entangled quantum subsystems (**| a>**)
to the location of the teleport transmitter (the authors call the transmitter
operator Alice) and the other subsystem (

*Step
3: *Alice brings the
teleported state **|***y***>** into
contact with the entangled state **| a>** and
performs a quantum measurement on the combined system (

*Step
4: *Using a conventional
communication channel, Alice transmits to Bob a complete description of the
outcome of measurement she has performed.

*Step
5: *Bob subjects his quantum
subsystem to a set of linear transformations (for example rotations through 90^{o}.)
that are dictated by the outcome of Alice's measurement. After these
transformations, Bob's quantum subsystem is no longer in state **| b>** and
is now in a state identical to the original quantum state

The BBCJPW scheme for teleportation requires* both* a
normal sub-light-speed communication channel and a nonlocal EPR channel to send
the quantum state vector from one location to another, and also requires
considerable pre-arrangement of entangled states and measurement procedures to
make the transfer possible. It transfers the quantum system without having
completely measured its initial state. The initial state **|***y***>** is
in effect destroyed at Alice's location and recreated at Bob's location.

BBCJPW analyze the information flow implicit in the process and show that
Alice's measurement does not provide any information about the quantum state **|***y***>**.
All of the state information is passed by the "privileged" EPR link
between the entangled states. The measurement results can be thought of as
providing the code key that permits the EPR information to be decoded properly
at Bob's end. And because the measurement information must travel on a
conventional communications channel, the decoding cannot take place until the
code key arrives, insuring that no faster-than-light teleportation is possible.

It should be mentioned that while it is quite feasible to produced entangled quantum states of the kind needed in the BBCJPW teleportation scheme, as demonstrated by the pioneering EPR experiment of Freedman and Clauser and the subsequent experiments of Aspect and his co-workers a decade later, these states have up to now involved particles (photons) that are separating at the velocity of light and cannot be "stored until needed" as would be desirable for convenient implementation of the BBCJPW scheme.

The BBCJPW procedure, as mentioned above, is not a design for a machine that teleports macroscopic objects, e.g., human beings, from one location to another. It is concerned with the teleportation only of quantum states of elementary particles. However, since this is a SF magazine I am permitted to indulge in some speculation on the science-fictional implications of the BBCJPW work. The key element of the scheme is the entangled quantum state that provides the nonlocal link between the transmitter and receiver. It is difficult to visualize a complex entangled sub-system that could interact properly with a macroscopic object for the purposes of teleportation. It would have to contain the same ensemble of atoms in nearly the same arrangements as the object transmitted. For physicists, that's a mere engineering detail now that the in-principle feasibility of teleportation has been demonstrated.

So let's consider how macroscopic BBCJPW teleportation might be used in an SF context.

* Example
1:* an advance
terrestrial civilization sends out an exploration ship filled with macroscopic
entangled state that are twins of similar states held in storage on Earth. If
the ship travels at 1% of

** Example
2: **a wormhole link has
been established with a neighboring bubble universe, but it is discovered that
the other universe is dominated by antimatter rather than normal matter. Is
exploration within the antimatter universe possible? Sure. We know that
entangled matter-antimatter subsystems can be produced, for example an electron
and positron produced as a pair in an electromagnetic interaction. With
macroscopic entangled subsystems that are matter-antimatter twins, the
antimatter subsystem could be transported through the wormhole, after which
explorers from our world might be teleported through the wormhole and converted
to antimatter at the same time. A return trip with other matter-antimatter
subsystems would reverse the process, converting the explorers back to matter
and returning them to our universe simultaneously.

I'm sure that there are many other possibilities for SF themes based on quantum teleportation. We now have a physics basis for teleportation. Its uses in SF are limited only by our imagination.

**References:**

**Quantum
Teleportation:**

"Teleporting an Unknown Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels", C. H. Bennett, G. Brassard, C. Crépeau, R. Jozsa, A. Peres, and W. K Wootters,

Phys. Rev. Letters70, 1895 (1993).

*Entangled
States and EPR Nonlocality:*

S. J. Freedman and J. F. Clauser,

Phys. Rev. Letters28, 938 (1972);

A. Aspect, J. Dalibard, and G. Roger,Phys. Rev. Letters49,91 (1982);

John G. Cramer,Reviews of Modern Physics58,647 (1986);

John G. Cramer,.Intl. J. Theor. Phys27, 227 (1988);

C. H. Bennett and S. J. Wiesner,Phys. Rev. Letters69, 2881 (1992).

** SF
Novels by John Cramer: **
my two hard SF novels,

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*This page was created by John G. Cramer
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