Quantum Weirdness Research Articles

On measurement and quantum nondemolition

In his letter, Christopher Monroe recommends that the term “quantum nondemolition” be abandoned (PHYSICS TODAY, January 2011, page 8). I agree, but for somewhat different reasons. Quantum measurement theory has always used the concept of ideal measurements, or “measurements of the first kind,” in which the state of the measured system does not change—that is, it is not “demolished”—if it is already in an eigenstate of the observable. Although an ideal measurement is often a good approximation when dealing with conserved particles, its realization requires special effort in the case of photons, which are usually absorbed while being measured. The concept of ideal measurements is certainly meaningful, but I do not agree with the suggestion that the repeated preparation of the same photon state should be regarded as being equivalent to its nondemolition during a measurement. The term nondemolition may be quite useful in photon experiments or in the construction of gravitational-wave antennae, where it was first used, as far as I know. But in a general context, the traditional terminology would be more to the point, since nonabsorption of the measured photon does not necessarily define an ideal measurement.Much of the confusion originates in the abridged description of a measurement as a stochastic jump from a superposition a∣0〉 + b∣1〉 of a microscopic system into ∣0〉 or ∣1〉 with probabilities ∣a∣2 or ∣b∣2, respectively. That description is a relic from the times when quantum theory was expected to apply only to isolated atoms, electrons, and so forth. (It does not seem to be a genuine part of the Copenhagen interpretation, which describes measurement outcomes in classical terms and often assumes that the wavefunction loses its meaning thereafter.) In most situations, though, one has to include other interacting systems in the unitary description. In fact, all of the much-discussed weirdness of quantum theory has been derived by using entangled quantum states for all involved systems. Therefore, most of the inflationary confusing vocabulary of quantum weirdness—including spooky action, quantum teleportation, quantum eraser, quantum Darwinism, sudden death, quantum information, and even quantum jumps—could be abandoned if one used instead a consistent description in terms of entangled wavefunctions, at least as far as this entanglement remains relevant.11. See, for example, H. D. Zeh, Foundations of Physics 40 9-10 1476 (2010). https://doi.org/10.1007/s10701-009-9383-9 Usually this is so until irreversible decoherence, which was itself derived from unitary interaction with the environment, has occurred in the chain of interactions leading to observation. However, I think it is definitely wrong to say that decoherence theory allows us to “think of any measurement as a [quantum nondemolition] measurement plus a possible interaction with a reservoir.” Although I personally prefer the assumption that unitary dynamics is universal, I agree that some stochastic dynamics has to be used somewhere along the chain of interactions that ends at the observer. It does not matter whether you describe this observed indeterminism in terms of a collapse or a branching of a universal wavefunction into dynamically autonomous components (unlike the collapse, the branching does not require any new dynamical postulate; it is a consequence of the Schrödinger equation with local interactions). Such a real or apparent collapse is an important part of the effective dynamics of “our quantum world”—our “branch,” if you like—that should be carefully analyzed rather than being used ad hoc in a pragmatic and confusing manner. In neither case would it describe a mere increase of information about an initially incompletely known quantum state of the involved systems; the indeterminism is described by a change of a state, which is known to be different from an ensemble of potential states.REFERENCESSection:ChooseTop of pageREFERENCES <<1. See, for example, H. D. Zeh, Foundations of Physics 40 9-10 1476 (2010). https://doi.org/10.1007/s10701-009-9383-9 , Google ScholarCrossref, ISI© 2011 American Institute of Physics.

A user's guide to the universe: surviving the perils of black holes, time paradoxes, and quantum uncertainty

Acknowledgments. Introduction. So, what do you do? 1 Special Relativity. happens if I'm traveling at the speed of light, and I try to look at myself in a mirror? can't you tell how fast a ship is moving through fog? How fast does a light beam go if you're running beside it? you head off in a spaceship traveling at nearly the speed of light, what horrors await you when you return? you reach the speed of light (and look at yourself in a mirror)? Isn't relativity supposed to be about turning atoms into limitless power? 2 Quantum Weirdness. Schrodinger's Cat Dead or Alive? Is light made of tiny particles, or a big wave? you change reality just by looking at it? you look at them closely enough, what are electrons, really? Is there some way I can blame quantum mechanics for all those times I lose things? I build a transporter, like on Star Trek ? a tree falls in the forest and no one hears it, does it make a sound? 3 Randomness. God play dice with the the physical world is so unpredictable, why doesn't it always seem that way? How does carbon dating work? Does God play dice with the universe? 4 The Standard Model. Why didn't the Large Hadron Collider destroy Earth? What do we need a multibillion-dollar accelerator for, anyway? How do we discover subatomic particles? are there so many different rules for different particles? Where do the forces really come from? can't I lose weight (or mass)-all of it? How could little ol' LHC possibly destroy the great big world? we discover the Higgs, can physicists just call it a day? 5 Time Travel. Can I build a time I build a perpetual motion machine? Are black holes real, or are they just made up by bored physicists? What happens if you fall into a black hole? you go back in time and buy stock in Microsoft? Who does time travel right? How can I build a practical time machine? What are my prospects for changing the past? 6 The Expanding Universe. If the universe is expanding, what's it expanding Where is the center of the universe? What's at the edge of the universe? What is empty space made of? How empty is space? Where's all of the stuff? is the universe accelerating? What is the shape of the universe? What's the universe expanding into? 7 The Big Bang. happened before the Big can't we see all the way back to the Big Bang? Shouldn't the universe be (half) fi lled with antimatter? Where do atoms come from? How did particles gain all that weight? Is there an exact duplicate of you somewhere else in time and space? is there matter? What happened at the very beginning of time? What was before the beginning? 8 Extraterrestrials. there life on other planets? Where is everybody? How many habitable planets are there? How long do intelligent civilizations last? What are the odds against our own existence? 9 The Future. don't we know? What is Dark Matter? How long do protons last? How massive or nuetinos? What won't we know anytime soon? Further Reading. Technical Reading. Index.

Everyday entanglement: Physicists take quantum weirdness out of the lab

Everyday entanglement: Physicists take quantum weirdness out of the lab

Finite Quantum Measure Spaces

Quantum measure spaces possess a certain “quantum weirdness” and lack some of the simplicity and intuitive nature of their classical counterparts. Much of this unusual behavior is due to a phenomenon called quantum interference, which is a recurrent theme in the present article. Because of this interference, quantum measures need not be additive but satisfy a more general condition called grade-2 additivity. Examples of quantum measure spaces such as “quantum coins” and particle-an tip article pairs are considered. Even more general spaces called super-quantum measure spaces are discussed. You don't need quantum mechanics or measure theory to understand this article.

Time and Spacetime: The Crystallizing Block Universe

The nature of the future is completely different from the nature of the past. When quantum effects are significant, the future shows all the signs of quantum weirdness, including duality, uncertainty, and entanglement. With the passage of time, after the time-irreversible process of state-vector reduction has taken place, the past emerges, with the previous quantum uncertainty replaced by the classical certainty of definite particle identities and states. The present time is where this transition largely takes place, but the process does not take place uniformly: Evidence from delayed choice and related experiments shows that isolated patches of quantum indeterminacy remain, and that their transition from probability to certainty only takes place later. Thus, when quantum effects are significant, the picture of a classical Evolving Block Universe (`EBU') cedes place to one of a Crystallizing Block Universe (`CBU'), which reflects this quantum transition from indeterminacy to certainty, while nevertheless resembling the EBU on large enough scales.

Some quantum weirdness in physiology

Quantum mechanics seems alien to physiology. Alarm bells go off in our heads when we hear even people of such genius as Sir Roger Penrose (1) invoke the weird coherence of quantum mechanical wave functions to explain biological function. Of course, it is only some of the “weirder” parts of quantum mechanics that bother us. Structural biochemistry is founded on the rigid geometrical relationships involved in chemical bonding that arise from quantum mechanics; the α-helix could only have been discovered by Pauling by acknowledging the power of quantum mechanical resonance to flatten the peptide bonding unit (2). Nevertheless, most modern biomolecular scientists view quantum mechanics much as deists view their God; it merely sets the stage for action and then classically understandable, largely deterministic, pictures take over. In this issue of PNAS Ishizaki and Fleming (3), by combining experimental and theoretical investigations, demonstrate that quantum coherence effects play a big role in light energy transport in photosynthetic green sulfur bacteria under physiological conditions. Quantum coherence allows a nonclassical simultaneous exploration of many paths of energy flow through the many chromophores of a light-harvesting complex, thereby significantly increasing the efficiency of the energy capture process, presumably helping the bacteria to survive in low light.

Quantum weirdness and surrealism

Quantum weirdness and surrealism

Quantum weirdness and surrealism

A joint exploration of early modern physics and the surreal art movement shows these twentieth-century revolutions had more in common than we thought, explains Philip Ball.

A quantum less quirky

What physicists want for Christmas is a solution to the philosophical conundrums of quantum mechanics. They will be disappointed, but work that dissolves one aspect of quantum weirdness is some consolation.

Quantum Weirdness in the Lab

The surprising effects of adding or subtracting photons from a light beam may yield tools for information processing.

To Escape From Quantum Weirdness, Put the Pedal to the Metal

Acceleration unravels a weird connection between widely separated particles known as entanglement, physicists calculate.

Recycling Quantum Weirdness

A new scheme allows quantum mechanically entangled pairs of particles to be reused in interactions that would normally destroy their entanglement.

Putting a Quantum Computer to Work in a Cup of Coffee

A quantum computer, tapping into the quantum weirdness of the microscopic world, could in theory collapse calculations that would take billions of years on mundane supercomputers into a few seconds. Experimentally, though, quantum computers have been daunting. Now two groups of researchers have come up with a way to realize at least some of that potential by applying nuclear magnetic resonance—a standard technique in medical imaging and chemical analysis—to a liquid as simple as coffee (see Research Article on p. 350 ). By flipping, coupling, and reading out the spins of individual nuclei in the coffee, the strategy can turn it into a quantum logic gate and even a quantum integrated circuit.

Opening Window on Quantum Weirdness

Opening Window on Quantum Weirdness

Opening Window on Quantum Weirdness

Opening Window on Quantum Weirdness

Opening Window on Quantum Weirdness

Opening Window on Quantum Weirdness