System Bits: April 8

Caltech researchers have made a discovery that supports the idea that waveguides coupled with the surface plasmon particle could become an important piece of the elusive quantum computing puzzle; a team of physicists from the University of Vienna, the Institute of Photonic Sciences in Barcelona and the Swiss Federal Institute of Technology in Zürich have discovered the second law of thermodynamics is not absolutely strict at the nanoscale.


Quantum photon properties revealed in plasmon particle
For years, researchers have been interested in developing quantum computers—the theoretical next generation of technology that will outperform conventional computers that involves storing information in qubits rather than in bits used by computers today. One approach for computing with qubits relies on the creation of two single photons that interfere with one another in a device called a waveguide and results from a recent applied science study at Caltech support the idea that waveguides coupled with another quantum particle—the surface plasmon—could also become an important piece of the quantum computing puzzle.

As their name suggests, surface plasmons exist on a surface—in this case the surface of a metal, at the point where the metal meets the air. Metals are conductive materials, which means that electrons within the metal are free to move around. On the surface of the metal, these free electrons move together, in a collective motion, creating waves of electrons. Plasmons—the quantum particles of these coordinated waves—are akin to photons, the quantum particles of light (and all other forms of electromagnetic radiation).

As the surface of a metal is like a sea of electrons, then surface plasmons are the ripples or waves on this sea, the researchers explained. These waves are especially interesting because they oscillate at optical frequencies. Therefore, if light is shone at the metal surface, one of these plasmon waves can be launched, pushing the ripples of electrons across the surface of the metal. Because these plasmons directly couple with light, researchers have used them in photovoltaic cells and other applications for solar energy.

In the future, they may also hold promise for applications in quantum computing. However, the plasmon’s odd behavior, which falls somewhere between that of an electron and that of a photon, makes it difficult to characterize. According to quantum theory, it should be possible to analyze these plasmonic waves using quantum mechanics—the physics that governs the behavior of matter and light at the atomic and subatomic scale—in the same way it can be used to study electromagnetic waves, like light.

However, in the past, researchers were lacking the experimental evidence to support this theory. To find that evidence, the researchers looked at one particular phenomenon observed of photons—quantum interference—to see if plasmons also exhibit this effect.

The experiment confirmed that two indistinguishable photons can be converted into two indistinguishable surface plasmons that, like photons, display quantum interference and this could be important for the development of quantum computing.

Plasmons are coherent enough to exhibit quantum interference in waveguides that can then be integrated in compact chip-based devices and circuits, which may one day enable computation and measurement schemes based on quantum interference.

Nanoscale surprises
In nature there are many processes that can never be reversed and the physical law that captures this behavior is the second law of thermodynamics, which posits that the entropy of a system – a measure for the disorder of a system – never decreases spontaneously, thus favoring disorder (high entropy) over order (low entropy). However, in the microscopic world of atoms and molecules, this law softens up and looses its absolute strictness, according to a team of physicists of the University of Vienna, the Institute of Photonic Sciences in Barcelona and the Swiss Federal Institute of Technology in Zürich.

In fact, at the nanoscale the second law can be fleetingly violated. On rare occasions, one may observe events that never happen on the macroscopic scale such as, for example heat transfer from cold to hot — unheard of in our daily lives. Although on average the second law of thermodynamics remains valid even in nanoscale systems, scientists are intrigued by these rare events and are investigating the meaning of irreversibility at the nanoscale.

Recently, the physicists accurately predicting the likelihood of events transiently violating the second law of thermodynamics.

The experimental and theoretical framework presented by research team has a wide range of applications. Objects with sizes in the nanometer range, such as the molecular building blocks of living cells or nanotechnological devices, are continuously exposed to a random buffeting due to the thermal motion of the molecules around them. As miniaturization proceeds to smaller and smaller scales nanomachines will experience increasingly random conditions. Further studies will be carried out to illuminate the fundamental physics of nanoscale systems out of equilibrium. The planned research will be fundamental to help us understand how nanomachines perform under these fluctuating conditions.