# Quantum laser turns electron wave into memory

ANN ARBOR, Mich. — How many electrons does it take to remember the entire contents of the Library of Congress? Only one, according to University of Michigan professor Philip Bucksbaum. Since electrons, like all elementary particles, are actually waves, Bucksbaum has found a way to phase-encode any number of ones and zeros along a single electron's continuously oscillating waveform.

"Our work in quantum-phase registers is highly experimental, but theoretically there is really no limit to how long a string of 1s and 0s you can store in one," said Bucksbaum.

In practice, Bucksbaum's team is a long way from quantum-phase memory devices. They are sticking with byte-wide vectors encoded with an ultrahigh-speed laser on a single cesium atom. With that setup, the team can store information in quantum-phase bytes instead of on quantum bits, as with most quantum computer designs. So far, all work with quantum computing has used the binary property of electron spin, which is either up or down.

"Most other researchers are using the spin of a quantum particle as a storage medium. Quantum-phase data storage is much more flexible but also very new. It may turn out to be a step toward quantum computers, or it could be a complete dead end," said Bucksbaum.

Quantum phase has long been of interest to researchers, because theoretically atoms could take on unfamiliar characteristics by selectively altering the phase of their waveform. For example, with a polymer, changing its constituent atoms' phase could mimic the wave-function phase of a metal, thereby imbuing plastic with the strength of steel.

Quantum phase for data storage was first proposed by Lov Grover at Lucent Technologies Bell Laboratories (Murray Hill, N.J.) in 1997. Bucksbaum's group was the first to test the theory experimentally, and so far it works like a charm.

**Waves in bathtub**

"Grover speculated that quantum data registers could store and retrieve data by allowing you to search many locations simultaneously; we tested one of his algorithms and confirmed it," said Bucksbaum.

Quantum mechanics holds that electrons behave like the waves sloshing in a bathtub — they exist simultaneously in an infinite number of locations or quantum states within the single wave. In the bathtub example, the surface waves define sets of points, any one of which has a certain probability of being in the wave's location at any one moment.

By sculpting designer wave packets and injecting them into an electron's waveform, Bucksbaum encoded strings of 1s and 0s with laser blasts that reverse the phase from the natural state of the waveform. "Our wave packets enable us to engineer atoms by adjusting the amounts and quantum phases of an atom's electrons," he said.

Usually an electron is bound to an atom but can nevertheless exist in many states simultaneously — like all the points along a wave stretching from one side of a bathtub to the other. All the points are there at once, and all the points along the wave are constantly changing, enabling a specific sequence of waves to encode information.

Bucksbaum used a laser to encode parallel phase reversals along the waveform of an atom's electrons — a pulsating stream of 8-bit phase reversals. A second reference stream enabled the researchers to read back out the original bits by decoding the phase reversals, thereby recovering the stored information like a data register.

"There are an infinite number of individually addressable states — the Coulomb potentials — where quantum bits can be stored," said Bucksbaum. An electron's wave is called a "probability wave," because it is in each possible quantum state simultaneously, each with a certain probability. For instance, the probability that a wave is at the highest point is proportional to the number of places where the waveform is currently hitting its highest point.

For atoms, the infinite number of quantum states comes from the different bound states of the electrons in their orbits. These orbits come in an infinite variety of "quantum jumps" between states — the quantum states are conveniently numbered 1, 2, 3 and so forth. Bucksbaum's laser excited his cesium atom to states 25 through 32 to encode his 8-bit quantum bytes.

Each of these high-energy states has an associated amplitude that can be modulated with ultrashort laser bursts, essentially storing an 8-bit vector into a quantum register. When needed later, the register's value can be read out from the single atom by hitting it again with the laser, this time with a reference encoding.

After the second burst, the atom has been driven to two different states by the two laser pulse streams, causing the two "alternate realities" inside the atom to interfere with each other — like dropping two pebbles into a pond: Both states exist simultaneously along with all the other miscellaneous states (ripples), albeit with a smaller probability.

The interference pattern between the two wave packets enables the 1s and 0s of the original register's value to be decoded, since interference occurs only on positions in the original value that differ from the reference beam.

"The way a data register works is that the electron is simultaneously in many states, meaning the electron is simultaneously occupying many parts of the register, so you can store the whole stream in a single atom," said Bucksbaum.

Bucksbaum has verified the function of his single-atom quantum register with all types of bit patterns, similar to the test patterns used to verify the proper function of a CPU after manufacture. So far he has not found any values that can't be stored and retrieved from a single atom.

"Now we want to find out how long information can be stored," he said.