Inflation, the brief period that occurred less than a second after the Big Bang, is nearly as difficult to fathom as the Big Bang itself. Physicists calculate that inflation lasted for just a tiny fraction of a second, yet during this time the Universe grew in size by a factor of 10^78. Also during this time, a very important thing occurred: fluctuations in the quantum vacuum appeared, which later resulted in the temperature fluctuations in the cosmic microwave background (CMB) that in turn produced large-scale structures such as galaxies. But in a new study, physicists now think that their understanding of the features of primordial quantum fluctuations – also called the inflationary power spectrum – may require a few small corrections due to currently unknown physics. These new corrections could allow scientists to search for experimental evidence to test a variety of quantum gravity theories, including string theory. 

Theoretical physicists Mark G. Jackson of the University of Paris-7 Diderot in Paris, France, and Koenraad Schalm of the University of Leiden in Leiden, The Netherlands, have published their study on these possible signatures of new physics in the inflationary power spectrum in a recent issue of Physical Review Letters.

Primordial fluctuations
The physicists’ work focuses on the Planck scale, the ultra-high-energy conditions at the time of the Big Bang. Although the universe at this point was almost completely homogeneous, the violent dynamics of inflation produced tiny inhomogeneities from the quantum vacuum. Virtual pairs of particles from the quantum vacuum began popping in and out of existence, some of which could absorb energy and become real. Physicists think that all matter today, from galaxies to living things, originated from these primordial quantum fluctuations. But physicists are even more interested in this era for what it may reveal about quantum gravity.

“The Planck scale is the energy at which the two major theories in physics – gravity and quantum field theory – necessarily combine,” Jackson and Schalm told PhysOrg.com. “The resultant theory of quantum gravity is one of the major open problems in physics, though by now there is a lot of evidence that string theory is the answer. In an ideal world one would wish to test this experimentally. Unfortunately, this Planck energy scale is laughably beyond the reach of standard experiments such as particle accelerators: it would be like reaching out your hand to touch the Moon. Fortunately, Nature did once perform an ultra-high-energy experiment possibly capable of probing the Planck scale: the Big Bang. Now while we can’t re-do the Big Bang, we can witness its consequences.” 

 (via Physicists search for new physics in primordial quantum fluctuations)

I don’t know about you, but when I think of lasers, I think of boxes on heavy, stabilized tables. Inside the boxes, the optical elements are mounted on stabilized mounts and everything is generally held as solidly in place as possible. The one thing that you generally don’t do is give a laser a good shaking. Unless it has already stopped working, in which case, have at it… preferably with a hammer.

Finding a paper that demonstrated a laser with better performance when it was being shaken compared to when it was held came as a bit of a shock.

The reason why the idea of shaking a laser is so shocking to me is that the lasers I am used to working with have optical elements that need to maintain a precise alignment with respect to one another. Temperature changes, vibrations, shaking, and “thumping the box to fix it” are all really bad ideas. But of course not all lasers are like this.

The laser in your laser pointer, CD, DVD, and Blu-Ray players are all monolithic devices. That is, they are made from a single piece of material, or materials, that are deposited on one another. You can obviously shake a laser pointer (much to the delight of cats), but this capability doesn’t scale. If you were to shake a laser pointer with acoustic waves that had a wavelength about the same size as the device (in the GHz range), then I would expect that things would probably go wrong rather quickly.

In most cases, at least. A group of researchers from Germany and Russia have now made a laser that works better when it is shaken. The reason why this occurs lies in the peculiar nature of the laser used by the researchers.

The laser that the researchers worked with was made from quantum dots (see side bar) embedded in a semiconductor material that had mirrors deposited on either side of it. This means that the distance between the two mirrors was extremely short. The researchers don’t state how big the distance was, but from the figures, I estimate that it wasn’t much longer than eight micrometers. For comparison, the wavelength emitted by the quantum dots was around 900nm.

(via A laser that works better shaken, not stirred)

As a quantum theory of gravity, loop quantum gravity could potentially solve one of the biggest problems in physics: reconciling general relativity and quantum mechanics. But like all tentative theories of quantum gravity, loop quantum gravity has never been experimentally tested. Now in a new study, scientists have found that, when black holes evaporate, the radiation they emit could potentially reveal “footprints” of loop quantum gravity, distinct from the usual Hawking radiation that black holes are expected to emit.

In this way, evaporating black holes could enable the first ever experimental test for any theory of quantum gravity. However, the proposed test would not be easy, since scientists have not yet been able to detect any kind of radiation from an evaporating black hole.

The scientists, from institutions in France and the US, have published their study called “Probing Loop Quantum Gravity with Evaporating Black Holes” in a recent issue of Physical Review Letters.

“For decades, Planck-scale physics has been thought to be untestable,” coauthor Aurélien Barrau of the French National Institute of Nuclear and Particle Physics (IN2P3) told PhysOrg.com. “Nowadays, it seems that it might enter the realm of experimental physics! This is very exciting, especially in the appealing framework of loop quantum gravity.”

In their study, the scientists have used algorithms to show that primordial black holes are expected to reveal two distinct loop quantum gravity signatures, while larger black holes are expected to reveal one distinct signature. These signatures refer to features in the black hole’s energy spectrum, such as broad peaks at certain energy levels.

Using Monte Carlo simulations, the scientists estimated the circumstances under which they could discriminate the predicted signatures of loop quantum gravity and those of the Hawking radiation that black holes are expected to emit with or without loop quantum gravity. They found that a discrimination is possible as long as there are enough black holes or a relatively small error on the energy reconstruction.

While the scientists have shown that an analysis of black hole evaporation could possibly serve as a probe for loop quantum gravity, they note that one of the biggest challenges will be simply detecting evaporating black holes.

“We should be honest: this detection will be difficult,” Barrau said. “But it is far from being impossible.”

(via Physicists propose test for loop quantum gravity)

Quantum mechanics has a concept called a “wave function.” It’s incredibly important because it holds all the measurable information about a particle (or group of particles) within it. In practice, the wave function describes a set of probabilities that change in time. When we make a measurement, we are really poking at the wave function, causing these probabilities to collapse and take on a definite value. The value that the wave function predicts is determined by the relative probabilities of all the possible measurement results.

But physically, the wave function is problematic. It is often possible to figure out the physical meaning of a symbol in an equation by looking at the physical units you would use to measure it. A quick examination of the wave function shows that the units of the wave function don’t make a great deal of sense. To avoid a mental hernia, physicists tell each other that the wave function is a useful calculation tool, but only has physical relevance in terms of statistics, rather than having some concrete existence. In other words, it’s not really “real.”

Until now, we have taken comfort from the idea that, real or not, the results from the wave function would be the same. So no worries, right? Quite possibly wrong. In a paper posted on the arXiv, a trio of researchers has shown that you can’t have it both ways; a purely statistical wave function will not always give the same results as a wave function with real physical significance.

(via The insanely weird quantum wave function might be “real” after all)

Hang on, we didn’t know how molecules conduct electricity?

Sometimes the intersection of physics, engineering, and “we want the shiny” can be a bit weird. In the drive to smaller and more efficient electronic devices, some are trying to shrink existing approaches, while others are heading straight to the ultimate end point: using molecules to do everything. The basic idea is that electronic conduction through a molecule can be controlled by using electrons to modify the electronic or physical configuration of the molecule. Since it may only take a few femtoseconds (10^-15s) to change this state, chemists paint pictures of high-speed electronic nirvana. The automatic response is: “Let’s build it NOOOOW.”

As any good scientist would do, when these ideas were suggested, they didn’t think too hard about whether it would work; instead, they just tried it. It wasn’t easy, but examples of molecular conductors are littered throughout the scientific record. In real life, these molecules worked, but nowhere near well enough to make devices. With some time to think about things, scientists were faced with a pressing question: why the hell do these things work at all? Handwaving explanations have abounded, but now, a good robust explanation has been put forward.

(via Ars Technica)

Quantum optical link sets new time records

Quantum communication could be an option for the absolutely secure transfer of data. The key component in quantum communication over long distances is the special phenomenon called entanglement between two atomic systems. Entanglement between two atomic systems is very fragile and up until now researchers have only been able to maintain the entanglement for a fraction of a second. But in new experiments at the Niels Bohr Institute researchers have succeeded in setting new records and maintaining the entanglement for up to an hour. The results are published in the scientific journal Physical Review Letters.

Entanglement is a curious phenomenon in quantum mechanics which Albert Einstein called ”spukhafte Fernwirkung” (spooky action at a distance). Two separate entangled systems have a ghostlike connection even when they are placed at a large distance without being directly connected to each other. It is said that their states are correlated. This means that if you read out the one system, the other system will ‘know’ about it. In the experiments at the Niels Bohr Institute, the spins of two gas clouds of caesium atoms are entangled.

Control of a spontaneous process

To create the entangled state of the two atomic clouds the researchers use light. Light consists of photons, which are the smallest parts (a quantum) of a light pulse. When you shine a laser beam on atoms the photons are absorbed and subsequently re-emitted spontaneously. This process has been an impediment to the experiments because it is uncontrolled.

“Now we have managed to control this ‘spontaneous’ process and use it”, explains Eugene Polzik, Professor and Director of the Danish National Research Foundation Center, Quantop at the Niels Bohr Institute at the University of Copenhagen.

(via Niels Bohr Institute - University of Copenhagen)

Building a better quantum computer with lasers and (impure) diamonds

If the development of a quantum computer were like motor racing, then we would currently be in the twisty-turny bit that comes before we barrel over the mountain and hit the long, fast straightaway. We know the requirements for quantum computing; we even know systems that kinda-sorta meet these requirements. But no existing quantum computing architecture—that is, how we make quantum bits (qubits) and perform operations on them—is really all that satisfying. If you don’t even know which materials are best for building a quantum computer, it makes progress awfully slow.

As a result, a lot of researchers have moved away from constructing proof-of-principle demonstrations of quantum computing, and are now trying to create clever ways to make qubits better behaved. A pair of papers look into the prospects for using impure diamonds as an architecture for quantum computing

(via Ars Technica)

Reducing the signal-to-noise ratio to get a lock on quantum signals

There is an art to making good measurements. In the ideal case, you want a really sensitive measuring device so that you can measure the smallest of changes to the signal you are interested in. But what you find is that most of the sensitivity is wasted, because in addition to signal, there is always noise. And for most “on the bleeding edge” experiments, the noise is huge compared to the signal.

In classical physics, there is a seemingly magical machine, called a lock-in amplifier, that extracts signal from the deepest and darkest pits of noise. But it seemed that the way quantum mechanics worked would make a quantum lock-in relatively useless. It turns out that, with a bit of clever thinking, you can make a quantum lock-in, and the resulting measurements have even more exquisite sensitivity.

So what is this lock-in amplifier that you speak of? Basically, if you already know something about the signal that you want to extract, then you can use that to get the rest. Imagine that I have a very weak light emitter sitting in a very bright room and I want to know how bright it is. The bright light in the room is the noise and I want to see my very weak emitter. There are several possibilities that are, on the surface, very different, but they’re actually all the same thing.

In this first case, I know what color to look for. Say it emits a very pure form of blue light—I design a filter that only passes that specific color of blue and look into the room through that filter. I will, with any luck, see my signal. That’s because, for any given signal, the total amount of noise is nearly always much greater than the signal, but the amount of noise just in the region of interest around the signal is usually smaller than the signal. So any filter that is specific enough will allow you to see the signal.

How do you filter the signal? Typically, it’s very small and changes slowly, putting it right in the worst of the noise. The first step is to shift the signal to a higher frequency, which we do by multiplying it with some pure oscillating source. Now we are looking for very small changes on a high frequency signal, where it is still drowned in the surrounding noise. 

To extract the signal, we multiply it again, this time with a second oscillator with the same frequency. This has two effects. First, the oscillator can be very strong, so it acts as a sort of amplifier, making the signal stronger. The second is that we can now just look at the part of the signal that’s changing very slowly. All the noise, apart from the tiny component that happened to have the same frequency and phase as the oscillator, gets eliminated.

(via Ars Technica)

Does Quantum Mechanics Flout the Laws of Thermodynamics?

 …In quantum physics, you can erase information, but rather than adding heat to the environment you can actually take it away! This sounds like it contradicts Landauer’s principle. Even more worryingly, since we argued the equivalence with the second law, this would mean that quantum physics contradicts the second law. Quantum physics seems to allow us to have a cake and eat it, in that it allows us to erase information and cool the environment too.

But this, luckily for the second law (though not for would-be inventors of perpetual motion machines), is not the case. Landauer’s insight is still fine, and erasing information adds entropy to the environment. What saves the second law is that, in quantum physics, entropy can actually be negative. Adding negative entropy is the same as taking entropy away. The key phenomenon behind it is the spookiest of all quantum phenomena, entanglement.

To understand the connection between entanglement and negative entropy we have to go back to Schrödinger’s view of entanglement. When two systems are entangled, we have complete information about their joint state, but have no information about their individual states. If we are erasing the state, as a whole we need not generate entropy (since the state has zero entropy), but if we erase subsystems individually, then each will contribute to entropy generation. The difference between the global and local erasing is negative entropy. To rephrase, if we have to erase some information, it helps to know whether this information arises from the entanglement with another system. Then, by invoking the other system in the erasure, we can actually erase and the environment can lose entropy.

Landauer’s erasure therefore acquires a new dimension when entanglement is allowed, but even then it still remains fully compliant with the second law. For better or for worse, the entropy of the whole universe still cannot be decreased even with the full assistance of the quantum magic of entanglement and negative entropies of quantum objects.

(via Scientific American Guest Blog)

Moving mirrors make light from nothing

A team of physicists is claiming to have coaxed sparks from the vacuum of empty space¹. If verified, the finding would be one of the most unusual experimental proofs of quantum mechanics in recent years and “a significant milestone”, says John Pendry, a theoretical physicist at Imperial College London who was not involved in the study.

…At the heart of the experiment is one of the weirdest, and most important, tenets of quantum mechanics: the principle that empty space is anything but. Quantum theory predicts that a vacuum is actually a writhing foam of particles flitting in and out of existence.

The existence of these particles is so fleeting that they are often described as virtual, yet they can have tangible effects. For example, if two mirrors are placed extremely close together, the kinds of virtual light particles, or photons, that can exist between them can be limited. The limit means that more virtual photons exist outside the mirrors than between them, creating a force that pushes the plates together. This ‘Casimir force’ is strong enough at short distances for scientists to physically measure it.

…For decades, theorists have predicted that a similar effect can be produced in a single mirror that is moving very quickly. According to theory, a mirror can absorb energy from virtual photons onto its surface and then re-emit that energy as real photons. The effect only works when the mirror is moving through a vacuum at nearly the speed of light — which is almost impossible for everyday mechanical devices.

Per Delsing, a physicist at the Chalmers University of Technology, and his colleagues circumvented this problem using a piece of quantum electronics known as a superconducting quantum interference device (SQUID), which is extraordinarily sensitive to magnetic fields.

The team fashioned a superconducting circuit in which the SQUID effectively acted as a mirror. Passing a magnetic field through the SQUID moved the mirror slightly, and switching the direction of magnetic field several billion times per second caused it to ‘wiggle’ at around 5% the speed of light, a speed great enough to see the effect.

The result was a shower of microwave photons shaken loose from the vacuum, the team claims. The group’s analysis shows that the frequency of the photons was roughly half the frequency at which they wiggled the mirror — as was predicted by quantum theory.

(via Nature News h/t to fuckyeahquantummechanics)

Multiverse = Many Worlds, Say Physicists

The many worlds interpretation of quantum mechanics is the idea that all possible alternate histories of the universe actually exist. At every point in time, the universe splits into a multitude of existences in which every possible outcome of each quantum process actually happens.

So in this universe you are sitting in front of your computer reading this story, in another you are reading a different story, in yet another you are about to be run over by a truck. In many, you don’t exist at all.

This implies that there are an infinite number of universes, or at least a very large number of them.

That’s weird but it is a small price to pay, say quantum physicists, for the sanity the many worlds interpretation brings to the otherwise crazy notion of quantum mechanics. The reason many physicists love the many worlds idea is that it explains away all the strange paradoxes of quantum mechanics.

For example, the paradox of Schrodinger’s cat—trapped in a box in which a quantum process may or may not have killed it— is that an observer can only tell whether the cat is alive or dead by opening the box.

But before this, the quantum process that may or may not kill it is in a superposition of states, so the cat must be in a superposition too: both alive and dead at the same time.

That’s clearly bizarre but in the many worlds interpretation, the paradox disappears: the cat dies in one universe and lives in another.

Let’s put the many world interpretation aside for a moment and look at another strange idea in modern physics. This is the idea that our universe was born along with a large, possibly infinite, number of other universes. So our cosmos is just one tiny corner of a much larger multiverse.

Today, Leonard Susskind at Stanford University in Palo Alto and Raphael Bousso at the University of California, Berkeley, put forward the idea that the multiverse and the many worlds interpretation of quantum mechanics are formally equivalent.

But there is a caveat. The equivalence only holds if both quantum mechanics and the multiverse take special forms.

(via Technology Review)

First Observation Of 8 Entangled Photons Smashes Entanglement Record

Entanglement is the strange quantum phenomenon in which objects become so closely linked that they share the same existence. In the language of physics, they are described by the same wavefunction.

Entangling things isn’t so difficult really. Most interactions involve entanglement of one sort or another.

The trouble is pinning it down. Entanglement is a fragile and fleeting phenomenon. Blink and it leaks into the environment. That’s why it’s so difficult to preserve, to observe and ultimately so difficult for physicists to play with.

In recent years, physicists have learnt how to entangle all kinds of objects in pairs—photons, electrons, atoms and so on. In 1999, they created a qutrit by entangling three photons. Last year, they even entangled 6 photons.

Today, however, Xing-Can Yao and buddies at the University of Science and Technology of China in Hefei, say they’ve smashed this record by entangling 8 photons, then manipulating and observing them all simultaneously.

That’s no easy feat. Getting eight photons exactly where you want them at the same time is the quantum mechanical equivalent of herding cats (clearly of the Schrodinger variety).

(via Technology Review, h/t to outofcontextscience)

Tracking photons through the classic double-slit experiment

…An exciting new experiment has been published in Science. Funnily enough, it repeats an experiment that is over 200 years old, and I am not certain that it teaches us anything new about the world. But it puts the weirdness of quantum mechanics on display for all to see. 

(via Ars Technica)