Quantum Computers for Judy

This post introduces a new feature of PalosVerdesBlog, the song request. Talking today with Zone Bridge friend Texas-Judy Floyd, I asked what she would like me to write about. It was a rhetorical question. Judy shot right back: Tell us about how you became you. Oh my dear, she wants an existential expose’ about moi. I begged off that one but she quickly came up with another topic: something about recent advances in computer technology. Judy’s wish is my command.

I no longer find computer technology to be riveting. After so many generations of Moore's Law, it seems like the future will be much like the past, doubling processor speed every 18 months due to doubling the number of transistors per square inch on integrated circuits every 18 months.

But perhaps there is a limit out there when shrinking the size of circuitry packed onto silicon chips eventually reaches a point where individual elements are no larger than a few atoms. Here a problem could arise since at atomic scales the physical laws that govern the behavior and properties of the circuit are inherently quantum mechanical in nature. Atoms behave differently than marbles.

Ordinary computers utilize bits of magnetic material that can be magnetized in either of two states, say up or down, and the magnetic state can be described in binary arithmetic by a “bit” that can take on one of the values 0 or 1. The magnetization is a macroscopic effect obeying the rules of classical physics.

Atoms, however, follow the more elusive rules of quantum physics. A simple 2-level atom is superficially similar to the magnetic bit with up and down states. But when you measure the atom’s condition, you may find it up, you may find it down. Experiments are not deterministic, not repeatable. In fact the state of the atom is not described by classical bits; rather it is described by a “superposition” of the up and down states.

Quantum pioneer Erwin Schroedinger explained this odd situation with his famous dead cat thought experiment. Imagine a box containing a cat, a vial of cyanide and an atom. If the atom were to decay and emit a photon, that would trigger the release of cyanide and the cat would be no more. The quantum description of the system is a superposition of one state where the atom is still excited and the cat alive and another state in which the atom has decayed and the cat is dead. Poor cat.

These more complicated superposition states are described by quantum bits or “qubits” that can have more than 2 values. For example a quantum system of 5 qubits (atoms) can exist in any one of 32 states (2 raised to the 5th power) and any one state is represented by a list of 5 zeros and ones (eg. 01100).

The reason why this is so important in computers is that a physical operation on an n-qubit system simultaneously operates on all n states in one fell swoop. This is massive parallelism when compared to normal computers where one clock cycle yields one binary operation. Thus our tiny 5 qubit system executes 32 operations in one clock cycle. Imagine the speed of a larger quantum computer.

Quantum computers are a hot research topic at major universities, government labs and companies such as Intel. Two researchers who graduated with me from the University of Rochester quantum optics group are actively engaged in this work, Dan James at Los Alamos and Jeff Kimble at Cal Tech.