A Quantum Critique

Many reputable thinkers have had issues with quantum mechanics, notably Einstein, who said "God does not play dice with the universe", referring to the probabilistic outcomes predicted by standard quantum theory. The EPR Paradox (the 'E' stands for Einstein) and Bell's Theorem are two well-known examples of the somewhat peculiar predictions of quantum mechanics. John Bell is quoted saying about quantum mechanics

I did not dare to think that it was false, but I knew it was rotten!

The Fate of the Quantum by Nobel prize winner Gerard 't Hooft is a sophisticated critique of quantum mechanics which also touches upon philosophical issues such as Free Will. The standard interpretations of quantum theory often invoke the free will of experimenters to make independent decisions at the last second. 't Hooft complains that alternative interpretations are sometimes dismissed, on grounds more philosophical than scientific:

This explanation is usually also dismissed. It is called a ‘conspiracy theory’, and that is considered to be disgusting. But are ‘disgusting’, or ‘ridiculous’, valid arguments in a mathematical proof? We have reasons to doubt that.

More concretely the paper mentions the concept of Superdeterminism, which probably eliminates the possibility of free will and also evades the assumptions behind Bell's Theorem.
For what it's worth - probably not that much haha - I'm with 't Hooft on this one. Over the years I've bored my friends and even written an outline of a paper along these lines, but haven't polished my arguments into publishable form, so it's nice to see that someone as distinguished as 't Hooft has basically saved me the trouble. 't Hooft doesn't, at the moment, have a completely fleshed out proposal to compete with quantum mechanics and, alas I don't either. He finds Quantum cellular automata interesting in this context and so did I at one point, but he also mentions the same problem that caused me to give up on that - issues of compatibility with relativity.

“On the quantum theory of radiation” by Albert Einstein

In Einstein's 1917 paper “On the quantum theory of radiation” he introduced the concepts of stimulated and spontaneous emission of radiation, the effects that make possible lasers and many other fascinating devices. He accomplished this in masterful fashion by starting with the simplest of assumptions, all but one of which were traditional classical physics. Using just one basic quantum notion - Bohr's idea of quantized molecular energy levels - his amazing powers of deduction led him to hypothesize new observable physical phenomenon as well as rederiving Planck's radiation law in a very neat way. This was apparently also the first time that anyone realized that photons should carry momentum as well as energy. This English translation of the original paper is beautifully written. Here are two nice retrospectives: Einstein as armchair detective: The case of stimulated radiation by Vasant Natarajan; Rereading Einstein on Radiation by Daniel Kleppner.
A dim recollection of freshman physics is probably enough to follow much of Einstein's train of logic.
"A Theory Should be as Simple as Possible - but not Simpler"

Nonlocal Correlations without Alignment or Calibration

Nonlocal correlations are one of the spookiest things about quantum mechanics. Einstein didn't much care for quantum mechanics, he tried to show that quantum mechanics was inconsistent with his special theory of relativity with the EPR paradox. While thought provoking, the EPR paradox did not end up being viewed as a fatal flaw for quantum mechanics.
Experimental schemes for actually testing quantum nonlocality have required quite a bit of alignment and calibration of different parts of the experiment which might make a skeptic wonder if such experiments really are an ironclad test of nonlocality. The preprint Guaranteed violation of a Bell inequality without aligned reference frames or calibrated devices is a clever test of quantum nonlocality which doesn't require a lot of the alignment and calibration steps used in other approaches.

Ghost Imaging

Quantum effects are sometimes very strange and occasionally even useful. Ghost imaging is a particularly bizarre example. It's possible to create an image using pairs of photons. One photon is sent to the subject and the other is sent to the camera. An image of the subject is created using the photon which is received by the camera - even though that photon was never anywhere near the subject! This is possible because of the (weird) quantum nature of photons. Here's a blog post that discusses recent research in which the photon source is classical instead of quantum - supposedly you can even use sunlight. Making ghost images by getting sunlight to act quantum

Entanglement at High Temperature

Entanglement is one of the most characteristic attributes of "quantum weirdness". Entangled systems are usually microscopic and low temperature. But when the system is not in thermal equilibrium, it should be possible to create entanglement at higher temperatures. Applications may include photosynthesis and perhaps even practical devices some day. See Quantum physics: Hot entanglement in Nature.

Is there a problem with Quantum Theory?

Jeremy Bernstein recounts the history of controversies in the interpretation of quantum theory in the preprint Dear Fellow Quantum Mechanics. In particular to we need to take a quantum/probabilistic view of past events as well as future ones? Creepy.

Quantum Theory and Relativity

There are subtle issues with the compatability of the two workhorse theories of contemporary physics:  special relativity and quantum mechanics.  The EPR paradox from the 1930's (the E stands for Einstein)  and Bell's Theorem from the 1960's are two of the classic examples.  The weight of suspicion often tends to fall on the quantum side.  Einstein, notably unenthusiastic about the fundamental role of probability in quantum mechanics,  believed that "God does not throw dice".  John Bell, when asked if quantum mechanics could be wrong replied, "I hesitated to think it might be wrong;  but I knew it was rotten".  Nonetheless quantum mechanics has turned out to be fantastically accurate and the basis for much our current understanding of matter and radiation - likewise for special relativity.  The preprint Can quantum theory and special relativity peacefully coexist? discusses the issues.

A Quantum Tripwire

The quantum tripwire can detect an intruder with a very low probability of being noticed and low probabilities of false negatives and false positives.   It uses the quantum Zeno effect.  It is reported to work in realistic conditions.  Here's the preprint:  An Invisible Quantum Tripwire.

More Quantum Inequalities

Quantum mechanics: The usefulness of uselessness in Nature.
Bell pointed out that quantum mechanics violates certain inequalities that are true in classical physics. This article discusses other inequalities that even quantum mechanics respects but which distinguish it from "super quantum theories" that would be able to violate these inequalities.

Problems with Quantum Mechanics

Might I say immediately … we always have had a great deal of difficulty in understanding the world view that quantum mechanics represents … I cannot define the real problem, therefore I suspect there’s no real problem, but I am not sure there’s no real problem.

Richard Feynman

Jeremy Bernstein in FAPP and Non-FAPP: A Pedagogical Essay discusses the foundations of quantum mechanics. Bernstein is a very good writer and he explains some aspects of this problematic field quite well. In the end, however, it still seems just as problematic as ever.

FAPP - For All Practical Purposes

Once Bernstein happened to visit physicist Erwin Schroedinger in his Vienna apartment.

There was no cat. He did not like cats.

Quantum Interference

Pairs Rule Quantum Interference in Science.

Quantum interference is one of the most mysterious features of quantum mechanics. In fact, Feynman referred to the double-slit interference experiment for single particles as the "only" mystery in quantum mechanics (1). On page 418 of this issue, Sinha et al. (2) describe a recent experiment that shows that quantum interference from a single photon arises only from pairs of possible paths through an interferometer. There is no need to invoke additional interference terms that might arise from interactions of three or more paths.

The Stern Gerlach Experiment

The Stern Gerlach Experiment by Jeremy Bernstein, preprint. A history and analysis of the Stern-Gerlach Experiment, a key piece of evidence for quantum mechanics.

Quantum Lithography

On the efficiency of quantum lithography - preprint.

Quantum lithography promises, in principle, unlimited feature resolution, independent of wavelength. However, in the literature at least two different theoretical descriptions of quantum lithography exist. They differ in to which extent they predict that the photons retain spatial correlation, and while both predict the same feature size, they differ vastly in predicting how efficiently a quantum lithographic pattern can be exposed.
Until recently, essentially all experiments reported have been performed in such a way that it is difficult to distinguish between the two theoretical explanations. However, recently an experiment was performed which gives different outcomes for the two theories. We comment on the experiment and show that the model that fits the data unfortunately indicates that the trade-off between resolution and efficiency in quantum lithography is very unfavorable.

Quantum Computers

Physics: Quantum computing in Nature.

The race is on to build a computer that exploits quantum mechanics. Such a machine could solve problems in physics, mathematics and cryptography that were once thought intractable, revolutionizing information technology and illuminating the foundations of physics. But when?

Experimental Simulation of Zitterbewegung

Quantum physics: Trapped ion set to quiver

The peculiar ultra-fast trembling motion of a free electron — the Zitterbewegung predicted by Erwin Schrödinger in 1930 when he scrutinized the Dirac equation — has been simulated using a single trapped ion.

When does a pair of Fermions act like a Boson?

Entanglement and Composite Bosons

Under what circumstances can a pair of fermions be treated as an elementary boson? Many authors have done detailed studies of this question, as it applies, for example, to atomic Bose-Einstein condensates, excitons, and Cooper pairs in superconductors. In a 2005 paper, C. K. Law presented evidence that the question can be answered in general in terms of entanglement: two fermions can be treated as an elementary boson if they are sufficiently entangled. Consider, for example, a single hydrogen atom in a harmonic trap. Within the atom, the proton and electron are strongly entangled with respect to their position variables; for example, wherever the proton might be found—it could be anywhere in the trap—the electron is sure to be nearby. Law suggests that this entanglement is the essential property underlying the (approximate) bosonic behavior of the composite particle, allowing, for example, a collection of many hydrogen atoms to form a Bose-Einstein condensate.

Quantum Leaps

I just read the book Quantum Leaps by Jeremy Bernstein. He is an entertaining writer. The book discusses the history of the "Quantum Measurement" problem and also his expereiences as a physics student and journalist - he met many of the key historical figures over the years.
However, he did not actually meet the personalities in this ancedote (my paraphrase follows).
Beria was put in charge of the Soviet nuclear program. He complained to Stalin that the scientists were using quantum techniques, which were held to be in conflict with Marxism. Stalin allegedly told Beria "Leave my physicists alone. We can always shoot them later."

Quantum Measurements and General Relativity

There are sticky issues when trying to understand how quantum measurement and general relativity might work together. Relativistic Model for Gravity-Induced Quantum State Reduction

A Lorentz invariant model for gravity-induced quantum state reduction is presented, which is mainly developed from the physical argument that the time translation operator in a superposition of macroscopic states is ill-defined. The model leads to a new approach how to overcome the basic problem of relativistic reduction models, the conflict between relativistic covariance and the assumption that state reduction leads to an abrupt change of the wave-function on a space-like hyperplane. Reductions are understood in the model as events on whole space-time regions instead on hyperplanes only. This view enforces a radical change for the formulation of the system's dynamics. A stochastic time flow running quasi orthogonal to the deterministic time evolution inside the four-dimensional space-time is proposed. It is shown that it is possible to formulate on the basis of this new view a meaningful physical model. The model is also checked for possible higher order effects, which provide new starting points for experimental research.

Quantum Effects in Biology

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.

Measurement-based quantum computation

Measurement-based quantum computation

Quantum computation offers a promising new kind of information processing, where the non-classical features of quantum mechanics can be harnessed and exploited. A number of models of quantum computation exist, including the now well-studied quantum circuit model. Although these models have been shown to be formally equivalent, their underlying elementary concepts and the requirements for their practical realization can differ significantly. The new paradigm of measurement-based quantum computation, where the processing of quantum information takes place by rounds of simple measurements on qubits prepared in a highly entangled state, is particularly exciting in this regard. In this article we discuss a number of recent developments in measurement-based quantum computation in both fundamental and practical issues, in particular regarding the power of quantum computation, the protection against noise (fault tolerance) and steps toward experimental realization. Moreover, we highlight a number of surprising connections between this field and other branches of physics and mathematics.