pgptag writes “This talk by Dr. Suzanne Gilbert (video) explains why quantum computers are useful, and also dispels some of the myths about what they can and cannot do. It addresses some of the practical ways in which we can build quantum computers and gives realistic timescales for how far away commercially useful systems might be.”

Source: Separating Hope From Hype In Quantum Computing

Physicists have designed the world’s smallest refrigerator, small enough that it can’t hold any of your food. The fridge consists of three qubits–quantum particles that act as on-off switches. These quantum particles could be ions, atoms, or subatomic particles.

Other small systems have been created, but this is the first that doesn’t rely on external mechanisms, such as sophisticated lasers. “The whole guts of the fridge, it’s all accounted for and not hidden in some macroscopic object which is really doing the work,” [coauthor Noah] Linden says. [Science News]

Kitchen refrigerators work by shuttling heat away from one area (where you store your food) and dumping it somewhere else (the coils behind). This transfer isn’t news. Fans of thermodynamics have built devices to wick away heat from one source and dump it somewhere else since the nineteenth century. The device proposed in a paper to appear in Physical Review Letters uses the same basic technique but at a much smaller scale–on the size of three qubits, connected to two “baths,” one cold (or around room temperature) and one hot.

You could represent an excited qubit as a 1, and an unexcited qubit as a 0. The trio might start in a state of “101″  which means “excited unexcited excited” or “010″ which means “unexcited excited unexcited.” Since the three qubits are linked, the system could flip between the two setups.  You might think of the excited qubits passing their excitement, or transferring energy, to the unexcited qubits–until the excited qubits become unexcited and the unexcited qubits become excited. If they are all the same temperature, they can flip back and forth from 101 to 010 to 101… all the live long day.

But this equal flipping changes when you add the hot and cold baths:

The trick is to put the first two qubits in contact with a cold bath and the third one in contact with a hot bath. The higher temperature makes it more likely that the third qubit will be in its excited state—and thus that the trio will be in the (101) state instead of the (010) state. But that means the system is more likely to flip out of (101) and into (010) than the other way around. So on average the flipping takes the first qubit from its excited state to its ground state and draws energy out of the first qubit. After a flip, the qubits essentially reset by interacting with the baths, allowing the cycle to start again. [ScienceNOW]

The researchers say they can just leave the system be and it will continue drawing energy out of that first qubit as long as the hot bath remains hot. Besides serving as an interesting look at thermodynamics on a quantum scale, the researchers believe the setup, when built, may find use in quantum computing.

Though one possibility for actually constructing the very mini-fridge involves capturing ions to act as qubits, ScienceNOW reports, an even smaller hypothetical fridge uses a particle with three levels instead of two.

Linden and his team also propose an even smaller system, in which a single particle with three distinct levels of quantum information, called a qutrit, acts as the refrigerator. “We believe this is the smallest possible thing you can call a fridge,” Linden says. [Science News]

Though quantum theorists may now hold the title for smallest refrigerator, particle physicists’ Large Hadron Collider with its superfluid helium “cryogenic distribution system” can cool a 16 mile accelerator ring, arguably making the LHC the planet’s largest fridge.

Image: flickr /Tweek

Source: Quantum Fridge: The Quest to Build the World’s Smallest Refrigerator

Martin Hellman writes “According to an article in Nature magazine, quantum hackers have performed the first ‘invisible’ attack on two commercial quantum cryptographic systems. By using lasers on the systems — which use quantum states of light to encrypt information for transmission —’ they have fully cracked their encryption keys, yet left no trace of the hack.’”

Source: Hackers Eavesdrop On Quantum Crypto With Lasers

KentuckyFC writes “In 1978, the CalTech mathematician Robert McEliece developed a cryptosystem based on the (then) new idea of using asymmetric mathematical functions to create different keys for encrypting and decrypting information. The security of these systems relies on mathematical steps that are easy to make in one direction but hard to do in the other. Today, popular encryption systems such as the RSA algorithm use exactly this idea. But in 1994, the mathematician Peter Shor dreamt up a quantum algorithm that could factorise much faster than any classical counterpart and so can break these codes. As soon as the first decent-sized quantum computer is switched on, these codes will become breakable. Since then, cryptographers have been hunting for encryption systems that will be safe in the post quantum world. Now a group of mathematicians have shown that the McEliece encryption system is safe against attack by Shor’s algorithm and all other known quantum algorithms. That’s because it does not depend on factorisation but gets its security from another asymmetric conundrum known as the hidden subgroup problem which they show is immune to all known quantum attacks.”

Source: 1978 Cryptosystem Resists Quantum Attack

eldavojohn writes “As we strive closer and closer to quantum computing, physics may need to be improved. A paper released in Nature Physics suggests that the limit defined by Heisenberg’s Uncertainty Principle can be beaten with quantum memory. From the article, ‘The cadre of scientists behind the current paper realized that, by using the process of entanglement, it would be possible to essentially use two particles to figure out the complete state of one. They might even be able to measure incompatible variables like position and momentum. The measurements might not be perfectly precise, but the process could allow them to beat the limit of the uncertainty principle.’ Will we find out that Heisenberg was shortsighted in limiting the power of quantum physics or will the scientists be surprised to find that such a theoretical scenario — once conducted — performs unexpectedly in Heisenberg’s favor?”

Source: Defeating Heisenberg’s Uncertainty Principle

relliker writes in with word of a paper up on the ArXiv by Seth Lloyd and co-workers, exploring the possibility that “postselection” effects in non-linear quantum mechanics might allow paradox-free time travel. “Lloyd’s time machine gets around [the grandfather paradox] because of the probabilistic nature of quantum mechanics: anything that this time machine allows can also happen with finite probability anyway… Another interesting feature of this machine is that it does not require any of the distortions of spacetime that traditional time machines rely on. In these, the fabric of spacetime has to be ruthlessly twisted in a way that allows the time travel to occur. … Postselection can only occur if quantum mechanics is nonlinear, something that seems possible in theory but has never been observed in practice. All the evidence so far is that quantum mechanics is linear. In fact some theorists propose that the seemingly impossible things that postselection allows is a kind of proof that quantum mechanics must be linear.”

Source: The Possibility of Paradox-Free Time Travel

To test the basics of quantum theory, physicists recently pulled out an antique. In a paper published today in Science, they confirmed a staple of quantum mechanics, using a test derived from a classic nineteenth century light experiment.

In particular, the researchers questioned how particles move through three slits, something previously too difficult to measure. They found that the particles behaved just like quantum theory–or more specifically the Born Rule–would have predicted.

As physicist Chad Orzel describes in his blog, that’s bad news for theorists hoping to tweak this rule to solve Nobel Prize-worthy problems related to quantum gravity or Grand Unifying Theories.

[The study is good news if] you’re the ghost of Max Born, or the author of an introductory quantum book…. This was disappointing news for some theorists, though, as there are a number of ways to approach problems … that would require some modification of the Born rule. [Uncertain Principles]

But how did they do it?

Step 1: Watching Light Waves

Throw a pebble in a pond and it creates waves. Throw two pebbles in a pond and they will create waves that interact. Where the peaks of two waves meet, they will create an even bigger wave. Where the peak of a wave meets the trough of another, they will cancel each other out–as if there is no wave at all.

Thomas Young’s 1800s double-slit experiment involves shining one color of light through two open slits to hit a screen. If light is a particle, Young imagined, then you get two streaks, like spray-paint through a stencil. That’s not what he saw. Invisible ripples created visible effects. On the screen, bright lines appeared where the waves built on one another. Other places the light waves canceled each other out leaving only darkness.

Step 2: Watching Particles Wave, Too

In the 20th century, quantum physicists did a similar experiment with particles, including electrons, firing them through two open slits. Classical physics would predict that the particles would land in two streaks on the other side. Instead, they saw a sight just like Young’s interference pattern. The particles were somehow interfering with each other, and more amazingly, even a particle fired alone created the pattern. It was interfering with itself.

This surprising effect provided one of the first clues to the weird world of quantum mechanics. Now precise measurements have been made on a version with three slits–and they again confirm the predictions of quantum mechanics. [New Scientist]

Why would you even bother trying three slits? That gets into the specifics of quantum mechanics and the Born Rule.

Step 3: Watching Probability Waves

So what type of waves are crashing into one another when a particle passes through a slit? Probability waves.

The value of a probability wave in various experiments is in part calculated by the Born Rule. In a double slit experiment–the probability waves values show that the electron is more likely to appear in one of the “bright” spots of the interference pattern and less likely to appear in one of the dark spots.

The Born Rule says that that we need to look at the interactions of probability waves only from two slits at a time–as opposed to looking at how ripples from all three slits interact at once. If the probability could include an extra value from interactions including all three slits at once, then interference pattern would change.

There was no experimental verification of this proposition until now…. “The existence of third-order interference terms would have tremendous theoretical repercussions–it would shake quantum mechanics to the core,” says [coauthor Gregor] Weihs. [ScienceDaily]

Step 4: Adding and Subtracting Slits

Urbasi Sinha of the University of Waterloo in Ontario, Canada and his team made a comparison. First they looked at the probability values formed by all three slits. Then, by covering up each of the slits in turn, they looked at the pattern formed from two slits at at time.

Adding up the values from each of the two slits, they got the overall pattern formed by three–meaning the Born Rule was right for as close as they could measure.

[T]he three-path interference term came to more or less zero. Co-author Ray Laflamme of the University of Waterloo in Ontario, Canada, “always hoped for three-path interference”, says Weihs. “But then he’s more of a theoretician. If there was three-path interference, there would be a Nobel prize waiting.” [Nature News]

Image: Wikimedia / Copyright © Armedblowfish, all rights reserved.

Source: Another Win for Quantum Mechanics: Passing the Triple-Slit Test

eldavojohn writes “An Australian National University project has completed a proof-of-concept storage unit that relies on bringing light to a standstill inside a crystal and then releasing it later for a read-once storage device. There are a few complexities to work out, such as the -270 degrees Celsius requirement to stop the light. And there is an interesting side effect noted by the team lead: ‘We could entangle the quantum state of two memories, that is, two crystals. According to quantum mechanics, reading out one memory will instantly alter what is stored in the other, no matter how large the distance between them. According to relativity, the way time passes for one memory is affected by how it moves. With a good quantum memory, an experiment to measure how these fundamental effects interact could be as simple as putting one crystal in the back of my car and going for a drive.’ Hopefully this will lead to a better understanding and simple testing of quantum entanglement.”

Source: A Quantum Memory Storage Prototype

A new type of solar cell using “quantum dots” may double the theoretical efficiency of current solar cells–allowing a panel to convert around 60 percent of the sun’s energy that it laps up into electricity. The research on these new cells appeared Friday in Science.

Current silicon-based solar cells lose about 80 percent of the sun’s energy they take in. It’s an inherent flaw: even working at their theoretical ideal, these cells would still lose 70 percent.

We can blame the sun’s diversely energized photons for this inefficiency. Silicon cells can only purposefully harvest photons with just the right amount energy. When they strike the cell, photons with just enough juice will prod an electron into motion (and create an electric current). An overly energized photon will excite the electrons to no purpose; the electrons will just quickly give off that photon’s energy as heat.

In two steps, this project, funded in part by the Department of Energy, salvages these “hot electrons.”

“There are a few steps needed to create what I call this ‘ultimate solar cell,’” says [Xiaoyang] Zhu, professor of chemistry and director of the Center for Materials Chemistry. “First, the cooling rate of hot electrons needs to be slowed down. Second, we need to be able to grab those hot electrons and use them quickly before they lose all of their energy.” [University of Texas at Austin]

Step 1 — Keep Hot Electrons Hot

The researchers from the University of Texas at Austin kept the hot electrons from shedding their energy–by hindering them with quantum dots, nanoscale structures with quantum behaviors:

The group used nanoscale (less than 100 nanometers, or 10-9 meters) crystals of a compound called lead selenide. Like silicon, lead selenide is a semiconductor, meaning it absorbs light energy within a certain bandgap, or range of energies. But semiconducting nanocrystals, also known as quantum dots, exhibit very different properties than their larger counterparts. For one thing, they can hold on to a hot electron for a longer period of time, stretching out the amount of time it takes for the electron to cool. In fact, previous research has shown that quantum dots can increase the lifetime of hot electrons by as much as 1000 times. [Popular Mechanics]

Step 2 — Forcing the Flow

The team next spurred these energetic electrons by pushing them into a conducting material where they could more easily move.

Zhu’s team has now figured out the next critical step: how to take those electrons out. They discovered that hot electrons can be transferred from photo-excited lead selenide nanocrystals to an electron conductor made of widely used titanium dioxide. “If we take the hot electrons out, we can do work with them,” says Zhu. “The demonstration of this hot electron transfer establishes that a highly efficient hot carrier solar cell is not just a theoretical concept, but an experimental possibility.” [Science Daily]

There’s just one problem keeping these more efficient cells from competing with their silicon predecessors–hooking them up to a wire to use all that electric current. The hot electrons, it seems, are too hot to handle:

“If we take out electrons from the solar cell that are this fast, or hot, we also lose energy in the wire as heat,” says Zhu. “Our next goal is to adjust the chemistry at the interface to the conducting wire so that we can minimize this additional energy loss.” [University of Texas at Austin]

But quantum dots are not the only solar cell solution. DISCOVER reporter Andrew Moseman describes other front runners on page 14 of our July/August magazine issue, which is on newsstands now.

Image: The University of Texas at Austin

Source: Making Super-Powered Solar Panels Via Quantum Dots

It’s a physics cliche: quantum mechanics looks at the really small, and general relativity looks at the really big, and never the twain shall meet.

In a study published yesterday in Science, physicists describe their attempts to study the overlap between these two theories–by dropping really cold rubidium (only billionths of a degree warmer than absolute zero) from a great height (480 feet). The cold rubidium behaves as an observable, quantum mechanical system and since gravity is a main driver in general relativity, watching gravity’s pull on that system might give researchers glimpses into how to tie the two theories together.

“Both theories cannot be combined,” said researcher [and coauthor of the paper] Ernst Rasel of the University of Hannover in Germany. “In that sense we are looking for a new theory to bring both together.” [Live Science]

Here’s what they did:

Step 1 — Cool it

Physicists first made super-cold Bose-Einstein condensates of rubidium. Since heat is really the random jostling of molecules, to cool things down, experimenters had to make those molecules sit still. They used an elaborate system of lasers to hold the molecules steady.

When rubidium atoms get that cold, they exhibit quantum mechanical behaviors that researchers can observe, acting like one giant particle-wave.

The idea is to chill a cluster of atoms to a temperature that is within a fraction of absolute zero. At that extreme, the atoms all assume the same quantum-mechanical state and begin to behave collectively as a sort of super-atom, known as a Bose-Einstein condensate (BEC). [Nature News]

In this study, researchers contained that complicated system in a two-foot diameter and seven-foot tall cylinder.

Step 2 — Drop it

To test the effects of gravity on that cold glob of atoms, researchers wanted to watch them as they experienced the weightlessness of free fall. That’s why they dropped the experiment in a tower at the Center of Applied Space Technology and Microgravity in Bremen, Germany.

The drop shaft, located at the Center of Applied Space Technology and Microgravity in Bremen, is pictured . . . in all its phallic glory. The sample area is magnetically shielded and can have the air evacuated. Samples dropped from the top will experience nearly five seconds at 10^-6g before experiencing a cushy landing in an eight meter deep pool of loose polystyrene packing foam. [Ars Technica]

Because the fall time is fairly short, researchers repeated the drop 180 times. During the tests they systematically eliminated other effects on the cold atoms, like magnetic fields in the laboratory, to make sure the atoms only felt gravity’s sway.

The idea was to see whether quantum objects break the rule that says that gravity works on all objects in the same way:

It explains why a pebble and a piano fall at the same speed if dropped from the same roof, despite their different masses. It’s also a necessary first step toward describing the effects of gravity as curvature in spacetime. “It’s a very important cornerstone,” said physicist Ernst Rasel of the Leibniz University of Hannover in Germany. But, he added, the equivalence principle “is just a postulate — it’s not coming out of a law.” So of course, physicists have spent the past century trying to break it. [Wired]

Step 3 — Send it into Space?

The experiment didn’t find evidence that gravity acted differently on a quantum scale–but Rasel and his colleagues are justly proud of creating the experimental conditions that can test such a thing. Because this research created a robust little setup of these very special quantum mechanically behaving atoms, one possible next step would be to watch the atoms during an even longer amount of time in free fall, for example, in orbit around the Earth on the International Space Station.

Rasel is just happy that the experiment survived the first drop:

“I was very worried,” Rasel says of the moments before his team first dropped their experiment. “It was coming towards the end of a PhD thesis of a student,” he adds, explaining that it would have caused serious problems if anything went wrong. [Nature News]

Wired has a video of the experiment, here.

Image: flickr / sludgegulper

Source: How (and Why) to Chuck a Quantum Physics Experiment Down a Drop Shaft