Fiber loop makes quantum memory

By Eric Smalley, Technology Research News

A relatively simple device that sends individual photons cycling through a fiber-optic loop could provide the memory needed to make ultra powerful computers that use the quantum states of light as bits.

Quantum computers are potentially powerful enough to solve problems that are beyond the most powerful classical computers, including cracking the strongest secret codes and quickly searching huge databases.

Several research teams have shown that it is possible to carry out logic operations using the traits of individual photons -- the fleeting particles of light -- as quantum bits that represent the 1s and 0s of computing. Computers must also be able to briefly store the outcomes of logic operations.

Scientists at Johns Hopkins University have come up with a method for capturing photonic qubits for tiny fractions of a second, which enables them to briefly store information about the state of a quantum particle. The memory device consists of a storage loop and a switch that directs photons into and out of the loop.

The memory device stores a qubit by switching a photon into the loop, where it flies around at the speed of light, said James D. Franson, a physicist at Johns Hopkins University's Applied Physics Laboratory. A short time later, the state of the qubit can be read by switching the photon back out of the loop, he said.

The memory stores binary information that is based on the polarization of photons. A photon is polarized when its electric field vibrates in one of four directions: horizontal, vertical and the two diagonals. The directions are paired, and one of each pair can represent 1 and the other 0.

The researchers used a polarizing beam splitter, which is transparent to one polarization and acts like a mirror to the other, to shunt photons into and out of the loop. The beam splitter separates the two polarization components of the photon, causing one to loop in one direction and the other to loop in the opposite direction. "You can envision these components as traveling in counterpropagating directions through the device," said Franson.

It is only possible to split the polarization components of a photon when the photon is in the weird state of superposition, meaning it is in some mix of the two polarizations at the same time. Quantum particles like photons enter superposition when they are unobserved and otherwise isolated from their environments.

When the photon in the loop passes the opening, it goes through a switch. When the switch is closed, it continuously flips the values of the photon's polarization components, turning horizontal polarization to vertical and vice versa. This causes both parts of the photon to hit the mirror portion of the beam splitter, which keeps the photon inside the loop. When the switch is opened, it no longer changes the polarizations and the photon passes through the beam splitter and exits the loop in the same superposition state as when it entered.

A photon takes 13 nanoseconds, or billionths of a second, to make one round-trip through the memory device, said Franson.

Optical quantum computers are likely to employ laser pulse trains, or pulses of laser light fired at regular intervals. "These pulse trains provide a natural clock cycle for the various quantum logic operations [and] memory readouts," said Franson. The cyclical nature of the memory device fits well with this type of architecture, he said.

In principle, the researchers' device is resistant to errors caused by light-phase shifts, said Franson. As a photon makes multiple passes through the storage device, its wave can gradually stretch or compress at different rates depending on polarization. These changes are neutralized, however, because the storage device repeatedly flips the polarizations, said Franson. "These phase shifts essentially factor out of the final state and may, in some applications, not affect subsequent computations using the stored qubits," he said.

Although researchers have known for a long time that optical fibers can store photons, "this might be the first demonstration," said Eli Yablonovitch, a professor of electrical engineering at the University of California at Los Angeles.

The researchers' device "is a very cute way to provide a limited amount memory" for linear optical quantum computing, said Jonathan Dowling, a principal scientist and supervisor of the quantum computing technologies group of at NASA's Jet Propulsion Laboratory. Its potential uses are limited because "it likely cannot robustly hold the qubits for very long periods of time required for... quantum communication applications such as quantum optical repeaters," he said. Repeaters boost fading signals along communications lines.

The researchers' current prototype cannot store information long because it suffers from photon loss, said Franson. "We estimated about 19 percent loss per cycle, which means we really couldn't store the qubits for very long," he said. In principle, the loss can be overcome by a better design, custom optics and possibly new types of fiber optic components, he said.

Scientists are exploring other means of storing optical qubits, including trapping photons in special semiconductor devices and transferring quantum information from photons to groups of atoms. "Many of these techniques rely on very clever manipulations of fascinating physics," said Franson. The researchers' method is less interesting for basic physics, "but may have some technical advantages for certain applications in the near term," he said. The devices are relatively simple and their timing corresponds to the repetition rate of commercially available lasers commonly used in optical quantum computing experiments, he said.

The researchers are now working on storing a pair of entangled qubits in a pair of synchronized cyclical memory devices, said Franson. Controlling entangled qubits is key to unleashing the power of quantum computing.

If two particles in superposition come into contact, one or more of their properties, like polarization, can become linked, or entangled. If two photons have their polarizations entangled, when one of the photons is measured and leaves superposition, the other photon leaves superposition in the same instant and assumes the opposite polarization regardless of the distance between them.

A sufficiently long string of qubits in superposition can represent every possible solution to a particular problem. Entanglement allows a quantum computer to check all possible solutions with one set of operations. Ordinary computers are much slower because they have to check answers one at a time.

The cyclical memory device could be used in practical applications in five to ten years, said Franson. Researchers generally agree that full-scale quantum computers are 20 years away.

Franson's research colleague was Todd B. Pittman. The research appeared in the December 5, 2002 issue of Physical Review A. The research was funded by the Office of Naval Research (ONR), the Army Research Office (ARO), the National Security Agency (NSA), the Advanced Research Development Activity (ARDA), and the Department of Defense's Independent Research & Development (IR & D) program.

Timeline:   5-10 years
Funding:   Government
TRN Categories:  Physics; Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical paper, "Cyclical Quantum Memory for Photonic Qubits," Physical Review A, December 5, 2002.


April 9/16, 2003

Page One

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