The Code Book: The Secret History of Codes and Code-breaking

Just as Whit Diffie predicted in the early 1970s, we are now entering the Information Age, a post-industrial era in which information is the most valuable commodity.

The uncertainty principle is another weird consequence of quantum theory.
Wiesner’s quantum mo

It is this ability to block certain photons that explains how Polaroid sunglasses work. In fact, you can demonstrate the effect of Polaroid filters by experimenting with a pair of Polaroid sunglasses. First remove one lens, and close that eye so that you are looking with just the other eye through the remaining lens. Not surprisingly, the world looks quite dark because the lens blocks many of the photons that would otherwise have reached your eye. At this point, all the photons reaching your eye have the same polarisation. Next, hold the other lens in front of the lens you are looking through, and rotate it slowly. At one point in the rotation, the loose lens will have no effect on the amount of light reaching your eye because its orientation is the same as the fixed lens – all the photons that get through the loose lens also pass through the fixed lens. If you now rotate the loose lens through 90°, it will turn completely black. In this configuration, the polarisation of the loose lens is perpendicular to the polarisation of the fixed lens, so that any photons that get through the loose lens are blocked by the fixed lens. If you now rotate the loose lens by 45°, then you reach an intermediate stage in which the lenses are partially misaligned, and half of the photons that pass through the loose lens manage to get through the fixed lens.

The angle of vibration is known as the polarisation of the photon, and a light bulb generates photons of all polarisations,

The angle of vibration is known as the polarisation of the photon,

The angle of vibration is known as the polarisation of the photon, and a light bulb generates photons of all polarisations, which means that some photons will vibrate up and down, some from side to side, and others at all angles in between.

Wiesner was proposing the bizarre concept of quantum money, which had the great advantage of being impossible to counterfeit.

Whether we adopt superposition or the many-worlds interpretation, quantum theory is a perplexing philosophy. Nevertheless, it has shown itself to be the most successful and practical scientific theory ever conceived. Besides its unique capacity to explain the result of Young’s experiment, quantum theory successfully explains many other phenomena. Only quantum theory allows physicists to calculate the consequences of nuclear reactions in power stations; only quantum theory can explain the wonders of DNA; only quantum theory explains how the sun shines; only quantum theory can be used to design the laser that reads the CDs in your stereo. Thus, like it or not, we live in a quantum world.

Anyone who can contemplate quantum mechanics without getting dizzy hasn’t understood it.’ In other words, prepare to meet some rather bizarre ideas.

tempest attack, which aims to detect the electromagnetic signals emitted by the electronics in a computer’s display unit. If Eve parks a van outside Alice’s house, she can use sensitive tempest equipment to identify each individual keystroke that Alice makes on her computer. This would allow Eve to intercept the message as it is typed into the computer, before it is encrypted