UCSB is at the cutting edge of a new field of technology called quantum computing, which may revolutionize the way people think of computers and information.
Physics professor David Awschalom and a team of researchers are rapidly advancing the understanding of quantum computing by putting theory into practice. The team has built a device that manipulates the quantum properties of matter – a first step toward building a computer that operates on these properties. Such a quantum computer would perform certain calculations much faster than current computers, allowing scientists to predict the weather, simulate chemical reactions and defeat even the most complex encryption schemes.
Conventional computers rely on handling all data in binary form. Regardless of whether a computer is processing a graphic from the Internet or composing a letter to John, the information is broken down into a long string of 1’s and 0’s. This binary form is preferred because the 1’s and 0’s correspond to on and off electrical signals on the computer’s circuit board. Binary information can be sent and received very quickly.
Quantum computing does not use a binary system of sending data. Instead, it stores data by changing a quantum property of the system. This property can have an almost infinite number of values.
“In arithmetic, you have something that’s a zero or a one. In quantum physics, you can be a zero or a one or any combination of the two,” Awschalom said. “The idea is that one can do calculations in a fundamentally different way.”
One way to think about this would be to recognize that a drawing made only from black lines on a white background does not carry as much information as a black-and-white photograph, which can have any of an infinite number of grays.
Quantum Mechanics
Quantum mechanics is a branch of physics that describes a group of unintuitive properties of matter. One of these properties is that an atom cannot have any value of energy; it must have one of a number of discrete values.
In a fluorescent light, electricity energizes gas atoms, raising them to a higher energy state. The energy level of the atoms does not rise gradually; rather it suddenly jumps to the next available level. This process is known as quantization, and each energy level is called a quantum.
Quantum mechanics also dictates that subatomic particles have a property called spin. Spin is quantized and is characterized as “up” or “down” for an electron. However, every time a spin measurement is made, a different value may be recorded. It is almost as if the spin oscillates randomly from up to down and back over time.
In actuality, the spin is not really “up” or “down,” but is undefined until a measurement is made. If out of 100 measurements, “up” is measured 30 times, and “down” is measured 70 times, then the particle is said to be 30 percent “up.” This percentage is a convenient way to assign a value to a phenomenon that is difficult to measure and describe.
Florian Meier, a post-doc working with Awschalom, said that this is how quantum computing takes advantage of handling data.
“That’s the magic of quantum mechanics – it doesn’t have to be ‘up’ or ‘down,’ but somewhere in between,” Meier said.
Awschalom said the ability for a piece of data to have a very large number of values gives a computer operating on it far more flexibility and power than conventional systems that only use 0 or 1. A computer that operates on the quantum principles would produce more complex solutions more quickly than conventional binary computers could.
Spinning Electrons
One way to store data using quantum mechanics is to control the spin of an electron. This is similar to the way in which a conventional computer controls the electricity in its circuits, but different in that a quantum device can take on any of a large number of values, not just 0 or 1 as in a conventional computer.
Awschalom has built a device that controls the spin of electrons in order to store and process data. By controlling spin, the percentage of “up” measurements can be changed in relation to the total number of measurements.
By using a magnetic field, the average value of an electron’s spin can be translated into an angle, which can have any number of directions. This happens because the spin behaves like a little magnet itself, responding by rotating in reaction to the imposed magnetic field.
“What we do here is actually control the [electron angle],” Awschalom said. “That’s just been done in the last year with electrical gates by a group of graduate students in the Physics Dept. with students in electrical engineering and materials science.”
Awschalom said that this is the first part in the plan to build a quantum computer. Currently, Awschalom’s device controls the spin of many electrons at once in a piece of semiconductive material. Ideally each electron could be controlled individually, giving more flexibility to the whole system.
“Part two is, ‘Can you make little containers to hold an individual electrons?’ That’s being done here with quantum dots,” Awschalom said. “The Materials Dept. here is one of the strongest departments in the world to make quantum dots.”
A quantum dot is usually made from a very small piece of semiconductor material. The dot contains a small number of electrons, which can be controlled by an external device. It can be used to absorb and emit light, but since it holds a small number of electrons, it is also an ideal candidate for devices that manipulate electron spin.
“The third step, which is the really tricky one, is to take these two quantum states and bring them together in a controllable way,” Awschalom said.
This third step encompasses the computational aspect of quantum computing. The first two steps are concerned only with building an infrastructure that will allow interactions between quantum devices.
Computing
The computation done in a quantum computer relies on the interaction between quantum states. One way this could be accomplished is bringing together two electrons in which their spin has already been assigned. When the electrons are brought in close proximity, they will interact with each other, and change each other’s spin.
“In some sense, one spin acts like a magnetic field on the other one,” Meier said. “The difficult part will be to ensure [that] spins interact for a certain amount of time.”
The spins will not produce a repeatable result unless the amount of time can be accurately controlled. After the interaction occurs, the computational result will be encoded in the state of the resultant spins.
Theoretically, it can already be determined how a quantum computer will excel over a conventional computer, even though one has not been built yet.
“The famous problem for [quantum computing] is factoring numbers,” Meier said. “The number factoring is typically what is used in encoding or encryption schemes.”
Number factoring is the process in which a number is divided into smaller numbers that multiply to the original number. For example, three and five are factors of 15. It is difficult for a conventional computer to find factors without first knowing what the factors are.
“When you send an e-mail with a standard encryption scheme, it will be encoded on a scheme which is based on the fact that you can much more easily multiply two factors than find the prime factor of a number,” Meier said.
Quantum computers would change this because they could calculate factors far more quickly, instantly making conventional encryption schemes obsolete.
“If it does happen, it will have an enormous change on the way people think of information processing,” Awschalom said. “It’s very clear that some calculations will go astronomically faster with a quantum computer. We’re not talking factors of two or three or ten, but factors of a million.”
Awschalom said with this additional computing power, goals as diverse as predicting the weather and determining chemical reactions will be closer to realization.
“In a sense, quantum computing is the killer app for quantum mechanics,” Awschalom said.
Quantum Communication
Another branch of technology that makes use of quantum mechanics is quantum communication. Currently there are a small handful of companies worldwide that are already selling devices that guarantee private communication.
“If someone tries to eavesdrop by reading it – this is where you can use quantum mechanics to your advantage,” Awshalom said. “Looking at it destroys it. You can build a 100 percent tamper-proof scheme like that.”
By reading a quantum value – electron spin, for example – the state of the system is inherently changed when a measurement is made. Current commercial systems use photons instead of electrons and send the data via fiber optics.
“If it does happen, [quantum technology] will have an enormous change on the way people think of information processing,” Awschalom said.
