Clas Blomberg - Physics of life-Elsevier Science (2007)

52 Part II. The physics basis
the atoms then appear as the same, all are together in the same states. The entanglement has
a lot of interesting features, and it is also something one tries to exploit for “quantum computers”.
The entangled particles (electrons or atoms) are all in the same state and when they
interact with each other or some external source, each of them represents a number of different
states and possibilities, which would lead to the possibility to perform a number of
different tasks at the same time. In theory this gives the possibility to perform a number of
complicated tasks in parallel, in short time. To achieve this, it is needed that the relevant particles
be kept in the entangled state which means essentially at well-defined quantum states
close to the lowest possible (ground) states. They shall be sheltered from irrelevant interactions
from any environment that would destroy the entanglement. In theory this provides a great
possibility, the difficulty is to keep the system sufficiently isolated and free from intervening
interactions.
There is another concept related to this, coherence. This is most apparent for photons,
radiation quanta. Photons, also apprehended as particles, albeit with zero mass, are another
type of particles than the electrons. There is in that case no Pauli principle, instead there can
well be many photons in the same quantum state, for a photon represented by the frequency
and polarisation direction. Again, the photons are the same. If there are many photons in the
same basic state, then they behave as a unity, there is no distinction between any individual
photons. This is what corresponds to a “coherent” state and such coherent states are in fact
familiar objects, the laser beams. They are formed by strong interaction between the radiation
and some kind of atom or molecule electron states, which gives rise to a uniform radiation
with all photons in principle the same. This uniformity, this coherence is used in various
applications. One can also have photons in entangled states, corresponding to the electronentangled
states. An electromagnetic wave, as we describe in the electromagnetic section, has
an electric and a magnetic component and the directions of the components provide two
polarisation possibilities, either pointing in either of two directions or rotations in either of two
directions. This polarisation corresponds to the electron spin. One can then get entangled states
of photons, in the simplest case with one photon of one polarisation direction, another with the
opposite polarisation. This is not the quantum picture, according to which both photons are
in both states at the same time. This gives rise to the much discussed experiments where one
has had entangled photons that go away in different directions which is discussed later.
We shall also point out here that the wave functions at molecules are also influenced by electrostatic
forces from other ions, atoms and molecules. Forces from neighbouring charges
influence the wave functions and in that way enhance dipole moments. The electron wave
function that connects a hydrogen to an oxygen or a nitrogen can be influenced by a neighbouring
oxygen or nitrogen with a surplus or electrons, thus negatively charged. The electron
will then be moved from the hydrogen nucleus, enhancing a dipole bond. This is what is
called “hydrogen bond”, which plays an important part in forming bonds between various
units in a cell.
Even if there are units that appear essentially neutral, there are quantum mechanical
influences of electron wave functions of neighbouring atoms and molecules. The wave functions
will be correlated, in a sense moving away from each other, and by that reducing direct
Coulomb interaction and instead leading to relatively weak attraction. This is called dispersion
interaction, and can be described by quantum mechanical formalism. The force goes
down with the sixth power of the inverse distance to (1/r 6 ). More commonly, they are called