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AN INTRODUCTION TO QUANTUM TANTRA
Quantum theory is the most far-ranging and successful attempt to understand the physical world ever devised by human beings. Quantum founding father and firm antirealist Werner Heisenberg declared, “An atom is not a thing”. To construct his vision of quantum reality, Heisenberg took quantum theory's vibratory possibilities literally: the attributes of unobserved objects exist, according to Heisenberg, exactly as represented in the theory-as possibilities, not actualities. The unobserved atom does not really have a definite position, for instance, but only a tendency, an inclination, to be in several possible positions all at the same time. In Heisenberg's view an atom is certainly real, but its attributes dwell in an existential limbo "halfway between an idea and a fact", a quivering state of attenuated existence that Heisenberg called "potentia", a world devoid of single-valued actuality but teeming with billions upon billions of unrealized possibilities. Since quantum theory technically applies to everything, not just to atoms, all objects without exception must exist in this partially unreal state of "objective indefiniteness" until someone (or something) decides to look at them. In the act of observation--called by physicists the "act of measurement"--one of the object's vibratory possibilities is promoted to a condition of full actuality, and all other possibilities vanish without a trace. Which possibility is singled out to become real during a measurement is apparently a matter of "pure chance", that is, its causes (if any) lie completely outside the world of physical law.
By the late Twenties quantum theorists had solved in elaborate detail the most pressing physics problem of that era--how light interacts with atoms. But along with its astonishing power to predict the most subtle light-matter effects, this fledgling theory created a host of philosophical problems, not the least of which was the bizarre notion that the world is in some sense "not real" except during an act of measurement. The quantum reality problem arises primarily because quantum theory describes the world in two ways, not one. Quantum theory represents an object differently depending on whether it is being observed or not being observed. Every physicist without exception uses this twofold quantum description in his or her own work, but physicists hold many divergent opinions about "what is actually going on" during these two stages in an object's existence: being observed and not-being-observed. Whenever an object--bulldog, baseball, or baryon--is not under observation, quantum physicists represent that object as a "wave of probability", called the object's "wave function". Instead of definite values for attributes such as position, velocity and spin, each of the object's attributes takes on--in the mathematics at least--a wide range of possible values, values that oscillate in a wavelike manner at a variety of different frequencies. This way of treating unobserved objects is one of quantum theory's most peculiar features. Physicists treat an unobserved object not as a real thing but as a probability wave, not as an actual happening but only as a bundle of vibratory possibilities.
On the other hand, when an object is observed, it always manifests at one particular place, with one particular spin and velocity, instead of a smeared-out range of physical properties. During the act of measurement, the mathematical description abruptly shifts--from a spread-out range of possible attributes (unmeasured object) to single-valued actual attributes (measured object). This sudden measurement-induced switch of descriptions is called "the collapse of the wave function", or simply "the quantum jump". What actually happens during a quantum jump is the biggest mystery in quantum physics.
CHAPTER 2
QUANTUM JUMPS LEADING TO FLUORESCENCE BLINKING
In physics, a quantum leap or quantum jump is a change of an electron from one quantum state to another within an atom. It is discontinuous; the electron jumps from one energy level to another instantaneously. Electrons keep changing – leaping – vibrating – all the time. But minor changes are insignificant. It is said that the only thing constant is change. Only when the energy level jump is significant, does it really matter as a quantum leap.
Imagine driving your car at night while its headlights display an annoying blinking behavior, switching on and off randomly. To add to the nuisance, the blinking has no definite time scale. In fact, although in most of our nightly journeys our headlights display quite rapid blinking, rendering at least some visibility, occasionally they remain off for almost the entire journey. Ridiculous and impractical as that behavior may seem, such is the situation commonly encountered by scientists- A wide variety of natural and artificial nanoscopic light emitters, from fluorescent proteins to semiconductor nano structures, display a blinking behavior like that described above. The emission (on) and no emission (off) periods have a duration that varies from less than a millisecond to several minutes and more. The probability of occurrence of the on and off times is characterized by a power law, which is a typical sign of high complexity and is fundamentally different from what is expected from the quantum jump mechanism of fluorescence blinking predicted at the dawn of quantum mechanics.
Almost a century ago, Niels Bohr proposed his now famous model in which electrons occupy discrete energy levels, or orbits within an atom. That energy discretization led Bohr to the “Quantum Jump” prediction: Since electrons cannot be between states, they must undertake instantaneous leaps from one state to another. These jumps were detected as interruptions in the fluorescence emission of single ions when a second electronic transition from a common ground or excited state was pumped in parallel. Experiments on single fluorescent molecules again revealed the occurrence of quantum jumps as interruptions of the fluorescence signal.
2.1 QUANTUM JUMP IN THREE LEVEL SYSTEMS
The first observations of quantum jumps were made in the mid-1980s when trapping and optical spectroscopy of individual ions became feasible. Those initial observations used a single ion Ba+ or Hg+ that can undergo two distinct electronic transitions from one common state: a strong, highly probable transition and a weak, much less frequent one. When the ion was illuminated with light resonant with transitions, the strong transition dominated and the ion performed many cycles of excitation and de-excitation, emitting a continuous stream of fluorescence photons. Eventually, the much less probable weak transition took place, with a de-excitation time orders of magnitude longer. Thus quantum jumps to the weak level were easily detected because they momentarily interrupted the strong fluorescence emission.