Justin Dressel, Chapman University – Grainy Digital Photographs

Dressel_JWhy are your digital pictures so grainy?

Justin Dressel, Assistant Professor of Physics at Chapman University, explains how quantum physics has a lot to do with what shows up on your screen.

Dr. Dressel researches the foundations of quantum physics, which is a natural intersection point between physics, mathematics, and computer science. His recent research has focused on algebraic approaches to generalized quantum measurements, quantum computation with superconducting transmon quantum bits using circuit quantum electrodynamics, and Clifford algebraic approaches to relativistic field theory. Though the bulk of his work is theoretical in nature, he works closely with experimental teams at U Rochester, UC Berkeley, and UC Santa Barbara.

Modern experimental techniques have enabled the unprecedented control of quantum systems, which has fueled Dr. Dressel’s interest in experimentally-grounded foundational questions. A typical laboratory measuring device couples indirectly to quantum systems of interest, and thus collects only imperfectly correlated information. Such a noisy measurement collapses the quantum state only partially. Partitioning the averages of such generalized measurements into conditioned sub-averages can reveal intriguing behavior that is hidden in the total average. Sufficiently “weak” coupling even permits individual measurements to leave the quantum state nearly unperturbed while still extracting useful information on average after many repeated trials. If one then monitors a quantum system continuously using such a weakly coupled meter, one obtains stochastic quantum state trajectories that exhibit competition between the natural dynamics and the measurement collapse. Recently, these quantum trajectories have been experimentally verified as part of the ongoing effort to build a working quantum computer, where they arise naturally during the measurement of superconducting transmon quantum bits using microwave fields.

Grainy Digital Photographs

AMico

During a candle-lit dinner a camera clicks: an intimate moment has been captured. The lighting is dim, the exposure brief. Little light sneaks past the shutter. On inspection, the photo is grainy, like static on an old television set.

But why?

If light were like waves at the beach or sound waves in the air, it could have any brightness, or, more precisely, dimness. A candle-lit photo would still be dim, but it would not be so grainy. The light level would vary smoothly, like gradually lowering a dimmer switch on the wall from brightness to total darkness, including every dimness in between. The dim light would cover the whole photo smoothly, keeping detail intact, even if difficult to see.

But light is not a simple wave. The dimmer switch for light is not smooth, but jumps in small steps from one brightness to the next. Each click of Nature’s dimmer switch for light has a name: one quantum of light, also called a photon. A quantum is one of a quantity, one discrete unit, and lies at the heart of quantum physics.

How do these quanta affect your camera? A digital camera image is gridded into millions of pixels. In bright light these megapixels collect billions of photons, making the discreteness of light nearly invisible, much like the grains of sand composing a sand dune. However, in dim light the pixels collect relatively few photons, which arrive at random times and at random locations. Detail is lost and the image becomes grainy, with spots of bright and dark where photons have and have not yet appeared.

So, you can thank quantum physics for making your digital photos grainy.

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