I intend to post a series of blogs introducing the beginner to quantum physics. This is the second such blog. I ‘ll eventually move on to more contentious issues and ask the Brains Trust out there for feedback.
The first “eureka” moment for in the journey to make sense of quantum physics is described in the following.
Anyone who has seen a supercomputer is immediately struck by the enormous amount of cooling equipment these computers require. That raises a question: why does the process of computing generate so much heat? The answer reaches into fundamental physics, a proper understanding of which will eventually deliver computers that require no power to process information; the ultimate energy conservation device.
Imagine a black box with an input connection on the left had side and an output connection on the right hand side. The input is connected to a device that generates an electrical pulse. The electronic circuits inside the box interpret this pulse as a ‘1’ and take that and flip it so that a ‘0’ is delivered at the output. Alternatively, if a ‘0’ is sent into the box, a ‘1’ is delivered at the output. In summary: Input 0 â†’ Output 1; Input 1 â†’ Output 0.
An opaque partition screen is placed across the middle of the box so if you sit on the input side you cannot see the output or me sitting at the output side. If we both know that the internals are designed to flip the input signal then if I see a “1” appear at my side (the output), I can infer that a “0” must have entered the input, on your side. This is called a reversible operation because purely by examining the output value, the input value can be deduced and vice-versa.
The black box is replaced with a different one. It has two input connections on the left hand side and one output connection on the right hand side. With the opaque partition in place, we run the following experiment. The circuitry inside this new box is configured such that if both of the two inputs is a “1”, then and only then, will the sole output deliver a “1”. This means that if I (sitting at the output) see a “1”, I can immediately infer that a “1” must have been sent into each of the two input lines on your side of the partition. So far so good. However if a “0” appears at the output then I cannot correctly infer what the individual inputs must have been because three separate combinations of input can deliver a “0” at the output: these are (1 and 0) or (0 and 1) or (0 and 0). It is impossible for the person at the output to infer by seeing the “0” which of the three possible combinations were actually input. This is called an irreversible operation because the input cannot be inferred by only examining the output.
The interesting feature of this irreversible operation is that heat is given off when it is performed. However, no heat is given off when reversible information processing is performed. The reason is that information was lost during the processing of the irreversible operation. We know that is true: the input had two pieces of information and that information could not be correctly inferred purely by examining the output; information was lost.
There is a fundamental connection between information loss and heat loss. Energy is lost if information is erased. This phenomenon was first identified by Rolf Landauer, an engineer with IBM in the 1960s. The two operations in the black boxes referred to earlier are used in computer logic (that is, how computers perform calculations). The exciting feature of this is that if you made a computer whose information processing was based entirely on reversible systems (like the first black box noted above), then no heat would be lost, hence no power consumed, by the calculation process. Reversible computing is energy free information processing.
Reversible computer logic is currently being used but there are technical difficulties with its universal use. For example, if a computer processes billions of bits of information and must not erase any, then all that information has to be stored somewhere, so the sheer size of these computers would make them uneconomic.
However, the fact that reversible information processing is energy free is being used to at least limit the power consumption of some computers. In gadgets that need to process information but be miserly with their power consumption, then reversible computing is very useful. Dramatically improved computing efficiency with the extensive use of reversible information processing could be the next big revolution in computing. More importantly to me, it provides an insight between energy loss and information processing, because reversible information processing is the world of quantum physics.