Atomical Logic Computations

-Introduction

The success of the way towards miniaturization and reduced power consumption of logic
machines is driven by Moore’s law, a prediction made by Intel co-founder Gordon E Moore. This is the observation that the number of transistors on an integrated circuit (IC)
doubles about every two years. The processing 
speed of a computer increases as the distance between transistors on the IC decreases. Thereby Moore’s law explains how products can simultaneously drop in price while improving their performance. The
correlation provided by Moore’s law is used as a benchmark, by companies that produce
transistors, for the progress of the industry. 
The future of the computing industry depends on whether the growth rate estimated by
Moore’s law could be continued in the future.  At this point, the current technology and advancement that is mainly based on miniaturization fails to proceed.
Consequently, we need a paradigm shift in the way we manufacture and do computing
once we reach the atomic scale. 

One way, currently being pursued, is to build via the
bottom-up approach, as advocated by Feynman in his famous lecture; 
there is plenty of room at the bottom.
In this approach, one can manipulate the atoms/molecules individually and build up the desired machine.
The result is ongoing, research all over the world trying to address the problem: Quantum
Computing, Neural Networks based computing, DNA-based computing, Quantum Cellular
Automata (QCA), Single Electron Transistor (SET).

I will introduce you to an alternative approach that is set to tackle the aforementioned challenge. This route is usually referred to as molecular logic. But, of course, even within this category there are several branches depending on the input/output.  You see we can interact with molecules chemically (where we use chemical reactions as input/output for our logic), optically (where laser pulses are shone onto the quantum system), and/or electrically (such as electrical voltage in quantum dots). 

In all these cases, what is being proposed is to use quantum systems, their discrete energy levels to be specific, to serve as a switch, logic gates, transistors, and/or integrated circuits that perform logic operations. Optically addressed systems are going to be preferable if the demand is speed, but if market readiness comes into play electrical addressing is the one we are familiar with.

Another variation that can be noted on these routes is the physical system in consideration of each branch.
Depending on the experimental setup the quantum system takes either a solid or solution state. While it is tempting to favor solids to be advantageous, as they can readily set into production using the current technological practice--thus market ready. Yet it is worth mentioning that the logic operation is going to be wired in a different setup than the usual chips. On the contrary, solutions not only require a new technological innovation but also are going to be messy with several (un)controlling parameters in play. 

For an optically addressed quantum system. The physical system we consider is a quantum system and the proposed logic will be a classical input/output. By classical logic, we mean the Boolean and Non-Boolean logic operations. This is distinct from Quantum computers as we are not using qubits.

Because of the discreteness of quantum systems, with the appropriate experimental setup, one gets access to multistates in nano-devices. This multistate system can be addressed either optically or electrically, to do logics. In a nutshell, this can be done by taking advantage of the discreteness of the internal states of a physical system, along with the ability to control the form of the perturbation. For example, by applying perturbations sequentially scientists have proposed several forms of Finite State Machines: simple Set-retet latches, Optical Flip Flops, Full adder, and Full subtraction.  Moreover going beyond binary it was demonstrated how one could do multivalued logic using electrical addressing. On top of this, it has been shown how we can perform complicated logic operations, like making decisions.

The implication here is that, the more measurable quantities you are able to access, and control, the more complicated logic operations you are going to design and implement. In the optical case, this requires an appropriate choice of pulse profile, with which we manage to disturb and control the dynamics of quantum states and, thereby able to perform logic operations. Having more observables (i.e. measurable quantities), consequently, is advantageous if one wants to perform parallel logic. This is so because one can make use of each of the observables to perform logic in parallel. That means each output line performs one logic function.

But just like other kinds of routes molecular logic is also still in its infant stage, meaning it is not market-ready and will take some time to reach that level. Apart from this, another challenge is how to preserve the stored information. This question is not limited to molecular logic but also is an issue in quantum computers. The problem is once you store information in a quantum system it will be lost after a while. This happens because of the interaction with the environment. Protecting the stored information is shown to be challenging.