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13.7: Cosmos And Culture
Wed October 17, 2012
One Step Closer To The Quantum Future
This year's Nobel Prize for physics was given to Serge Haroche of Collège de France and Ecole Normale Supérieure in Paris, France, and to David Wineland from the National Institute of Standards and Technology and the University of Colorado at Boulder. Both have pioneered methods to manipulate quantum systems, that is, entities living in the world of atoms, electrons and other particles. Their discoveries not only have a deep impact on our understanding of the truly bizarre effects that happen in the world of the very small but also have a slew of future practical applications, from ultra-precise clocks to quantum computers.
Adam wrote an excellent op-ed this past Sunday for The New York Times where he dealt with the importance of these discoveries and the weirdness of the quantum world. Today, I'd like to explore the physics of one of these experiments in more detail.
The quantum world is extremely fragile. So much so that one of the greatest challenges in quantum physics is to measure without interfering.
In our reality this is easy: you see a fly, know where it is and, in principle, can measure its velocity. All you have to do is divide the distance it travelled between two points by the time of flight.
But if this fly were an electron, things get much harder. The more precise the measurement of the electron's position, the more vague the measurement of its velocity. This is because the act of measuring interferes with what is being measured.
Say you send particles of light (photons) to bump against an electron and then travel back to a detector to tell you where the electron is. The better you want to pin down the electron's position, the higher the energy of the photon needs to be. (Higher energy means smaller wavelength and hence better precision.) However, a photon with higher energy ends up pushing the electron away as it bumps into it, messing up its position measurement.
With the fly this also happens, but the photons don't have enough energy to push the fly away. That's the crucial difference between the classical world — our world — and the quantum world. The challenge, then, is to measure without interfering, or at least to interfere so little as to preserve the quantum nature of what is being observed.
Haroche managed to "imprison" photons between two mirrors (or better, a cavity) making them ricochet countless times, in effect traveling some 30 thousand miles between the mirrors before being lost. This seesaw motion of the photons between the mirrors creates what is called a standing wave, a coherent superposition of the photons, a typical quantum state. (You can create a classical standing wave by shaking a rope that is tied up at one end. By shaking it with different frequencies you can form standing-wave patterns with different numbers of crests.) To do this, Haroche's experimental set-up needed to be incredibly precise and stable: any external interference would destroy the coherence of the photons. The "mirrors" were made of superconducting material and maintained at very low temperatures.
Information about the state of the photons was obtained by firing individual atoms of the element rubidium, an amazing feat. Haroche was trying to reproduce in the lab the famous Schrödinger's Cat effect, exploring the transition from quantum to classical. Schrödinger imagined that if a cat was imprisoned in a black box with a vial that would release poison conditioned by the decay of radioactive atoms (hence a random quantum event), an external observer couldn't tell if the cat was dead or alive inside the box. In fact, before the observation, the cat should be described as a quantum superposition of two states, cat-dead and cat-alive. Only by observing the contents of the box would the observer solve the puzzle and determine the fate of the unfortunate cat.
(Judging from my cat, she would be dead of a heart attack just for being imprisoned inside the box.)
The photons imprisoned between the mirrors work in a similar way, becoming a quantum superposition of states that is slowly interfered with by the passing rubidium atoms. In a sense, the rubidium atoms chip away at the coherent photon state, robing it of its "quantumness." With this, Haroche was able to investigate, in a controlled way, the slow degradation of a quantum coherent state for the first time.
It's possible, in principle, to reverse the effect and restore the quantum coherence of the photons using collisions with atoms. This will provide an essential control mechanism as scientists and engineers attempt to design quantum computers which don't suffer memory loss. We are not there yet, but everything indicates that within a couple of decades our computers will look and feel very differently from today's machines.