System Bits: Sept. 3

Maintaining an unstable quantum system
In an experiment that could have implications for quantum computers and quantum simulators, researchers at the Georgia Institute of Technology have demonstrated a way to maintain an unstable quantum system by applying bursts of microwave radiation – a quantum analog to vibrating the inverted pendulum.

While a simple pendulum has two equilibrium points: hanging in the “down” position and perfectly inverted in the “up” position. While the “down” position is a stable equilibrium, the inverted position is definitely not stable. Any infinitesimal deviation from perfectly inverted is enough to cause the pendulum to eventually swing down. It has been known for more than 100 years that an inverted pendulum can be stabilized by vibrating the pivot point, which is known as dynamic stabilization, and it has led to a broad range of applications including charged particle traps, mass spectrometers and high-energy particle accelerators.

Many-body quantum systems can also be placed into unstable non-equilibrium states, and like the inverted pendulum of classical physics, they typically evolve away from these states but in the Georgia Tech experiment, the researchers used microwave pulses of varying amplitudes and frequencies to control a quantum system composed of a cloud of approximately 40,000 rubidium atoms cooled nearly to absolute zero.

The research experimentally demonstrated dynamical stabilization of a non-equilibrium many-body quantum system.

In this work, they have demonstrated that the quantum dynamics of a cloud of atoms can be controlled to maintain them in a non-equilibrium configuration analogous to the inverted pendulum by controlling the internal spins of the atoms that give each atom a small magnetic moment. The spins are oriented in an external magnetic field against their will such that they would prefer to flip their orientation to the equilibrium position.

Controlling and manipulating single-particle quantum systems or simple collections of atoms, electrons and photons has been a focus of the physics community over recent decades. These capabilities have formed the foundation for technologies such as lasers, magnetic resonance imaging, atomic clocks and new atomic sensors for magnetic fields and inertial guidance.

Researchers are now studying more complex systems that involve many additional interacting particles, perhaps thousands of them which could lead to developments in quantum computing, quantum simulations and improved measurements.

Making secure communications superhighways
While the needed spectrum to carry additional communications traffic exists, government-regulated allocations has some of this space off-limits to wireless providers so researchers at Virginia Tech, Duke University and Vassar College suggest that the existing communications spectrum superhighways in the sky be shared.

A critical challenges that needs to be addressed for spectrum sharing is the problem of spectrum security and enforcement, with the primary concern the interference experienced by primary users due to rogue transmissions by maliciously intended secondary users when the two groups operate in the same band.

To counter this problem, the researchers are the principal investigators on a new $1.2 million award from the National Science Foundation’s Secure and Trustworthy Cyberspace program. Their goal is to make trustworthy spectrum sharing technically and economically viable. The primary users of these crowded, yet invisible superhighways are the license holders or in the case of federal government users, incumbents of a given spectrum. There are also secondary, “opportunistic” users.

The threat from the opportunistic users is serious for two reasons, the researchers said. First, interference caused by rogue transmitters will undermine the advantages of spectrum sharing and seriously hinder its world-wide adoption. Second, cognitive radios that make spectrum sharing possible can be used to launch very destructive jamming attacks. Counter measures against these threats can be classified into two enforcement approaches: ex-ante or preventive and ex-post or punitive enforcement. The research is focused on studying the key mechanisms in both of these enforcement approaches.

~Ann Steffora Mutschler