Electronic devices have traditionally relied on charge to encode, store, process and transmit information. A well-known example is the celebrated Field Effect Transistor, which is the workhorse of all modern digital electronic chips. When a transistor channel is filled with charge, the transistor is “on” and encodes the binary bit 1. When the channel is depleted of charge, the transistor is “off” and encodes the binary bit 0. Switching between bits is therefore accomplished by moving charge within the transistor which causes current flow and associated power dissipation. This is a fundamental shortcoming of all charge based devices. Since charge is a scalar, and has only a magnitude, logic bits 0 and 1 must be demarcated by a difference in the magnitude of the stored charge. As a result, switching between bits always involves changing the magnitude of charge and therefore causing a current flow and I2R dissipation. This cannot be avoided.
An electron’s spin, on the other hand, is a pseudo vector with a polarization. If spin polarization is made “bistable” by placing the electron in a static magnetic field, then the two allowed polarizations (parallel and anti-parallel to the field) can encode bits 0 and 1. Switching would require simply flipping the spin without moving charge in space and causing a current flow (I2R = 0). This can result in tremendous energy saving which is currently the most important goal in electronics.
The Single Spin Logic (SSL) idea1 is based on this paradigm. Spins of single electrons in quantum dots encode digital bits. Logic gates are configured by placing the dots in suitable layouts to engineer the spin-spin interactions between them. Inputs are provided by aligning the spins in input dots along desired orientations using locally generated magnetic fields. The arrival of a new input string takes the system to an excited state. When the spins ultimately relax to the many body ground state, the spin polarizations in “output” dots represent the result of a computation in response to the input bits. I will show how the universal NAND gate is configured in this way. With the NAND gate, any arbitrary circuit can be built.
Detailed quantum mechanical calculations show that switching in these circuits dissipate the minimum energy allowed by thermodynamics (the Landauer-Shannon limit), which is kTln(1/p) where p is the bit error probability. With p = 10-9, the energy dissipated is ~ 21 kT, whereas modern transistors dissipate 40,000 – 50,000 kT. The SSL is incomparably superior to spin based transistors (Spin Field Effect Transistors, Spin Junction Transistors, etc.) which do not compare well with traditional transistors. The reason is that they still rely on charge for encoding information and do not exploit the advantage of spin.
I will conclude by showing that SSL type constructs are best realized with organic nanostructures where the spin relaxation time can be extremely long. We have measured a spin relaxation time of 1 second at 100 K in a nanostructure of the -conjugated organic tris(8-hydroxyquinolinolato aluminum) popularly known as Alq3 2.