Quantum Mechanical Interaction-Free Measurement

Foundations of Physics, Vol. 23, No. 7, 1993

Avshalom C. Elitzur and Lev Vaidman
Received August 17, 1992; revised January 2, 1993

"A novel manifestation of nonlocality of quantum mechanics is presented. It is
shown that it is possible to ascertain the existence of an object in a given region
of space without interacting with it. The method might have practical applications
for delicate quantum experiments. ...

We shall describe a measurement which, when successful,
is capable of ascertaining the existence of an object
in a given region of space, though no particle and no light "touched" this
object. This is a new type of an interaction-free quantum measurement
which has no classical analog. ...

If an object is charged or has an electric (magnetic) moment, then its
existence in a given region can be inferred without any particle passing
through that region, but rather by the measurement of the electric
(magnetic) field that the object creates outside the region. Quantum
mechanics allows inferring the existence of an object in a nonlocal way via
the Aharonov-Bohm effect (31 even when the object creates no electro-
magnetic field outside a certain space region, but only an electromagnetic
potential.

Even if the object creates no detectable change at a distance, i.e., it
interacts with the external world only locally, its location can often be
found in a simple nonlocal interaction-free measurement (i.e., without
interacting with the object). For example, assume it is known that an
object is located in one out of two boxes. Looking and not finding it in one
box tells us that the object is located inside the other box. A more
sophisticated example of obtaining information in a nonlocal way is the
measurement performed on a system prepared in the Einstein-Podolsky-
Rosen state. If two objects are prepared in an eigenstate of relative position,
the measurement of the position of one object yields the position of
the other.

In the above cases, what allowed us to infer that an object is located
in a given place by performing an interaction-free measurement was the
information about the object prior to the measurement. In the first example
we knew that the object is located inside one of the two boxes, and in the
second example we knew about the correlation between the position of one
object and that of another. The question we address in this article is this:
Is it possible to obtain knowledge about the existence of an object in a
certain place using interaction-free measurements without any prior information
about the object? The answer is, indeed, in the affirmative as we
proceed to show. ...

Our method allows one to detect the existence of any unstable system
without disturbing its internal quantum state. It might, therefore, have
practical applications. For example, one might select atoms in a specific
excited metastable state. Let us assume that the atom has a very high cross
section for absorbing photons of certain energy while it is in one out of
several metastable states into which it can be "pumped" by a laser, and
that the atom is practically transparent for these photons when it is not in
this state. Then, our procedure selects atoms in the specific state without
changing their state in any way.

It is customary to think that, unlike classical mechanics, quantum
mechanics imposes severe restrictions on the minimal disturbance of the
system due to the measurement procedure. We have, however, presented
here an ultimately delicate quantum measurement that is impossible to
perform classically. We have found that it is possible to obtain certain
information about a region in space without any interaction in that region
either in the past or at present."

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