Setting the Stage for Quantum Computing

No one has made a quantum computer, but it is not hard to understand their appeal to physicists like Olivier Pfister. Today's computers reflect the logic at the heart of classical physics.  Things are either on or off, all or nothing, true or false.  A bit, the binary building block of computer logic, is represented by a 1 or 0, and the triumph of software engineers is their ability to write sophisticated instructions from this basic element, to work in a language based on just two integers.

In a binary quantum computer, a quantum bit, or qubit, can be both 1 and 0 as well as the superposition of those two states at the same time-in which case a measurement of the qubit's value could produce either result randomly.  Because of the principle of quantum superposition, a series of just 40 binary qubits has the potential to represent every single binary number between zero and a trillion simultaneously, vastly reducing the amount of code needed to specify an instruction.

Furthermore, quantum systems can become entangled.  In these cases, measuring one system defines the value of the other entangled system instantaneously.  In a quantum computer, entangled qubits can be used to run computing processes in parallel, in particular for cases where classical computing can only run them sequentially.  The implications of this are stunning.  “A quantum computer could break the codes that are at the heart of modern encryption systems in a matter of minutes,” Pfister exclaims.

But there is an enormous amount of work that must be accomplished before the promise of quantum computing is realized, which is another reason why the field is so exciting to Pfister.  “We are not really far along,” he says.  “There are new models for quantum computing appearing all the time.  The great thing about this is that the field is so open-ended and challenging.”  For instance, qubits could be a free electron whose spin points up or down, an electron in an atom that is excited or not excited, or a photon of two different colors.  Whatever the choice, scientists must be able to control the microscopic qubit without it changing states under the influence of the physical environment, a daunting challenge.

Pfister is a quantum optics expert and is developing quantum systems based on light to implement qubits and home in on a consequence of entanglement called teleportation.  Quantum teleportation is the only known way to transmit quantum information perfectly between two physical systems.  Unlike the kind of teleportation practiced by Captain Kirk on “Star Trek,” information about a qubit is transferred rather than the qubit itself.  Quantum teleportation is also a building block for quantum computing architectures.  Pfister has created limited entanglement between two beams of light in his lab and has recently proposed a new method for entangling four beams, and potentially a lot more.

The key, he says, is patience.  “This work is based on proven mathematical theories,” he says.  “We know it is incredibly powerful and will enable us to do extraordinary things, but we are still mapping the territory.”  Pfister has received funding for his research from the National Science Foundation and the Army Research Office.