Quantum computer as detector shows space is not squeezed

As the Earth rotates every 24 hours, the orientation of the ions in the quantum computer/detector changes with respect to the Sun’s rest frame. If space were squeezed in one direction and not another, the energies of the electrons in the ions would have shifted with a 12-hour period. (Hartmut Haeffner image)

A new experiment by UC Berkeley physicists used partially entangled atoms — identical to the qubits in a quantum computer — to demonstrate more precisely than ever before that this is true, to one part in a billion billion.

The classic experiment that inspired Albert Einstein was performed in Cleveland by Albert Michelson and Edward Morley in 1887 and disproved the existence of an “ether” permeating space through which light was thought to move like a wave through water. What it also proved, said Hartmut Häffner, a UC Berkeley assistant professor of physics, is that space is isotropic and that light travels at the same speed up, down and sideways.

“Michelson and Morley proved that space is not squeezed,” Häffner said. “This isotropy is fundamental to all physics, including the Standard Model of physics. If you take away isotropy, the whole Standard Model will collapse. That is why people are interested in testing this.”

The Standard Model of particle physics describes how all fundamental particles interact, and requires that all particles and fields be invariant under Lorentz transformations, and in particular that they behave the same no matter what direction they move.

Häffner and his team conducted an experiment analogous to the Michelson-Morley experiment, but with electrons instead of photons of light. In a vacuum chamber he and his colleagues isolated two calcium ions, partially entangled them as in a quantum computer, and then monitored the electron energies in the ions as Earth rotated over 24 hours.

If space were squeezed in one or more directions, the energy of the electrons would change with a 12-hour period. It didn’t, showing that space is in fact isotropic to one part in a billion billion (1018), 100 times better than previous experiments involving electrons, and five times better than experiments like Michelson and Morley’s that used light.

The results disprove at least one theory that extends the Standard Model by assuming some anisotropy of space, he said.

Häffner and his colleagues, including former graduate student Thaned Pruttivarasin, now at the Quantum Metrology Laboratory in Saitama, Japan, will report their findings in the Jan. 29 issue of the journal Nature.

Entangled qubits

Häffner came up with the idea of using entangled ions to test the isotropy of space while building quantum computers, which involve using ionized atoms as quantum bits, or qubits, entangling their electron wave functions, and forcing them to evolve to do calculations not possible with today’s digital computers. It occurred to him that two entangled qubits could serve as sensitive detectors of slight disturbances in space.

“I wanted to do the experiment because I thought it was elegant and that it would be a cool thing to apply our quantum computers to a completely different field of physics,” he said. “But I didn’t think we would be competitive with experiments being performed by people working in this field. That was completely out of the blue.”

He hopes to make more sensitive quantum computer detectors using other ions, such as ytterbium, to gain another 10,000-fold increase in the precision measurement of Lorentz symmetry. He is also exploring with colleagues future experiments to detect the spatial distortions caused by the effects of dark matter particles, which are a complete mystery despite comprising 27 percent of the mass of the universe.

“For the first time we have used tools from quantum information to perform a test of fundamental symmetries, that is, we engineered a quantum state which is immune to the prevalent noise but sensitive to the Lorentz-violating effects,” Häffner said. “We were surprised the experiment just worked, and now we have a fantastic new method at hand which can be used to make very precise measurements of perturbations of space.”

Other co-authors are UC Berkeley graduate student Michael Ramm, former UC Berkeley postdoc Michael Hohensee of Lawrence Livermore National Laboratory, and colleagues from the University of Delaware and Maryland and institutions in Russia. The work was supported by the National Science Foundation.

Source: UC Berkeley

Strange materials cropping up in condensed matter laboratories

A conformal field theory in condensed matter (labelled CFT) gets a hand from string theory – the string theory hand has an extra dimension and a black hole in it. Credit: Image courtesy of Perimeter Institute for Theoretical Physics

Subir Sachdev, William Witczak-Krempa, and Erik Sørensen are condensed matter physicists. They study exotic but tangible systems, such as superfluids. And their latest paper about one such system has a black hole in it.

How did a black hole get into a condensed matter paper? "Well, it's a long story," says Sachdev, who is a professor at Harvard and a Distinguished Visiting Research Chair at Perimeter Institute.

It's a long story, he might add, that in a way starts with him: he was one of the first condensed matter physicists to venture into the strange land of string theory, where the black holes live. But that is getting ahead of the tale.

"Let's start here," Sachdev says. "Condensed matter physicists study the behaviour of electrons in many materials -- semiconductors, metals, and exotic materials like superconductors."

Normally, these physicists can model the behaviour of a material as if electrons were moving freely around inside it. Even if that's not what's actually happening, because of complex interactions, it makes the model easy to understand and the calculations easier to do. Electrons (and occasionally other particles) used in this kind of short-hand model are called quasi-particles.

However, there are a handful of systems that cannot be described by considering electrons (or any other kind of quasi-particle) moving around.

"What we try to do is understand a quantum system where you have electricity without electrons," says Sachdev. "Of course, the system does have electrons in it, but the behaviour of the system doesn't look like electrons moving at all. What you see is not even particles, but some lumps of quantum excitations that are doing strange quantum things."

"Without quasi-particles, it's a mess," says William Witczak-Krempa. Witczak-Krempa, a Perimeter postdoctoral fellow, is also a condensed matter theorist who collaborated with Sachdev on the paper. "It's this quantum fuzzball of stuff."

Describing such a fuzzball system is a challenge -- but it's crucial to understanding many modern materials, including superfluids and high-temperature superconductors. The broad problem of how to model systems without quasi-particles has been stumping condensed matter theorists for decades.

"What we decided to do was look at a simple case of such an electricity-without-electrons system," says Witczak-Krempa. "That turns out to be a quantum phase transition between a superfluid and an insulator."

A fair amount of work had been done on such systems, such that the team was able to make progress modelling the system using the traditional mathematical tools of condensed matter. Sachdev and Witczak-Krempa worked with Erik Sørensen of McMaster University on this aspect of their paper. Sørensen used a computer simulation -- specifically, a quantum Monte Carlo simulation -- to predict how conductivity should change with temperature and frequency as a superfluid turns into an insulator.

"This frequency dependence tells us how the quantum fluid behaves in time. This dynamic behaviour is notoriously hard to study using standard methods, including quantum Monte Carlo simulations," says Witczak-Krempa. "Erik's work was a huge computational achievement. It took months of processing time. And, in the end, the results still needed to be converted into a form that can be compared with experiments. This is where we tried something new."

To perform this conversion, Sachdev and Witczak-Krempa tackled the same system from a different angle: string theory. (Here, they build on Sachdev's previous work with Perimeter Faculty member Robert Myers and one of his graduate students, Ajay Singh.) One of the pillars of string theory is that certain quantum field theories (technically known as conformal field theories) can be translated into a theory of gravity with one extra dimension.

Sachdev explains where the extra dimension comes in. Wiggling his fingers above the tabletop, he conjures strings moving through the air.

"In certain configurations, the strings all end on a kind of membrane," he says, tapping his fingertips on the table's surface. "You might ask yourself: if you were living on the membrane [the table surface] -- and you didn't know about the extra dimensions where the strings were, what would you see?"

He answers himself: "Only the ends of the strings. They would look like particles. What's amazing is that string theorists found that the theories that you'd use to define the ends of the strings on the membrane are remarkably like the theory we want to use to describe our system."

The quantum field theory describing Sachdev and Witczak-Krempa's "fuzzball" system shares many fundamental properties with the conformal field theories associated with string theory -- so many that the researchers were able to map the two-dimensional field theory into a three-dimensional theory of gravity.

"We ended up studying the physics of this alternate reality," says Witczak-Krempa. "Using this technique, we were able to translate a very hard problem into a fairly easy one." Albeit a fairly easy problem involving a black hole.

"We wanted to look at the physics of the boundary -- the physics at the table top," says Witczak-Krempa. "But we wanted to heat it up a bit -- give it a finite temperature. It turns out that the natural way of doing this is to invoke a black hole." Really?

"There are various ways of developing an intuition about that," he says. "For instance, you can remember that the black hole will release Hawking radiation. The Hawking radiation escapes and eventually hits the boundary where the system lives, and heats it up."

Witczak-Krempa admits it's unorthodox: "Most condensed matter people would go: 'Why is there a black hole in this paper?' It's crazy. But what's even crazier is that this mathematical machinery works quite well. It gives you answers that make a lot of sense. You can compare them directly with Erik's Monte Carlo results, and they check out."

It's the first time results from a traditional large-scale condensed matter simulation have been compared to results from the new string theory approach.

Sachdev is cautiously thrilled: "There are a couple of issues we don't fully understand and one discrepancy we wish we understood better, but in general it's worked extremely well. It's progress on something I've been thinking about for more than 20 years. And now we finally have a theory that is perhaps not complete, but is encouragingly successful."

What's more, string theory has finally produced a set of physical predictions that experimentalists can go check. Sachdev and Witczak-Krempa are hoping that an experimental team will try soon.

"Let's see what happens," says Sachdev. "We're pushing string theory to a new regime. Whatever happens, we will learn more."

Source: Perimeter Institute for Theoretical Physics

Einstein's 'spooky' theory may lead to ultra-secure Internet

Could new research into Einstein's 'spooky action at a distance' pave the way for a new ultra-secure quantum Internet? Credit: © Serg Nvns / Fotolia

Einstein's skepticism about quantum mechanics may lead to an ultra-secure Internet, suggests a new paper by researchers from Swinburne University of Technology and Peking University.

Associate Professor Margaret Reid from Swinburne's Centre for Quantum and Optical Science said Einstein's reservations about quantum mechanics were highlighted in a phenomenon known as "'spooky' action at a distance."

In 1935, Einstein and researchers highlighted a 'spooky' theory in quantum mechanics, which is the strange way entangled particles stay connected even when separated by large distances.

"Until now the real application of this has been for messages being shared between two people securely without interception, regardless of the spatial separation between them," Professor Reid said.

"In this paper, we give theoretical proof that such messages can be shared between more than two people and may provide unprecedented security for a future quantum Internet."

In the 1990s, scientists realised you can securely transmit a message through encrypting and using a shared key generated by Einstein's strange entanglement to decode the message from the sender and receiver. Using the quantum key meant the message was completely secure from interception during transmission.

Sending Einstein's entanglement to a larger number of people means the key can be distributed among all the receiving parties, so they must collaborate to decipher the message, which Professor Reid said makes the message even more secure.

"We found that a secure message can be shared by up to three to four people, opening the possibility to the theory being applicable to secure messages being sent from many to many.

"The message will also remain secure if the devices receiving the message have been tampered with, like if an iPhone were hacked, because of the nature of Einstein's spooky entanglement.

"Discovering that it can be applied to a situation with more parties has the potential to create a more secure Internet -- with less messages being intercepted from external parties."

Source: Swinburne University of Technology

'Topological insulators' promising for spintronics, quantum computers

Purdue University doctoral student Yang Xu, lead author of a new research paper on "topological insulators," an emerging class of materials that could make possible "spintronic" devices and practical quantum computers far more powerful than today's technologies, is shown here inspecting devices made from topological insulators under a microscope before electrical measurements.
Credit: Purdue University photo / Ting-fung Chung

Researches have uncovered "smoking-gun" evidence to confirm the workings of an emerging class of materials that could make possible "spintronic" devices and practical quantum computers far more powerful than today's technologies.

The materials are called "topological insulators." Unlike ordinary materials that are either insulators or conductors, topological insulators are in some sense both at the same time -- they are insulators inside but always conduct electricity via the surface. Specifically, the researchers have reported the clearest demonstration of such seemingly paradoxical conducting properties and observed the "half integer quantum Hall effect" on the surface of a topological insulator.

"This is unambiguous smoking-gun evidence to confirm theoretical predictions for the conduction of electrons in these materials," said Purdue University doctoral student Yang Xu, lead author of a paper appearing this week in the journal Nature Physics.

Yong P. Chen, a Purdue associate professor of physics and astronomy and electrical and computer engineering, led a team of researchers from Purdue, Princeton University and the 

University of Texas at Austin in studying the bismuth-based material.

"This experimental system provides an excellent platform to pursue a plethora of exotic physics and novel device applications predicted for topological insulators," Chen said.

For example, by further combining topological insulators with a superconductor, which conducts electricity with no resistance, researchers may be able to build a practical quantum computer. Such a technology would perform calculations using the laws of quantum mechanics, making for computers much faster than conventional computers at certain tasks such as database searches and code-breaking.

"One of the main problems with prototype quantum computers developed so far is that they are prone to errors," Chen said. "But if topologically protected, there is a mechanism to fundamentally suppress those errors, leading to a robust way to do quantum computing."

The topological insulators were synthesized at Purdue and fabricated into electrical devices at the Birck Nanotechnology Center in the university's Discovery Park.

The researchers for the first time demonstrated a three-dimensional material with an electrical resistance not dependent on the thickness of the material, a departure from conventional behavior. Whereas electrons usually have a mass, in the case of topological insulators the conducting electrons on the surface have no mass and are automatically "spin polarized," leading to the unique half-integer quantum Hall effect observed and also making the material promising for various potential applications.

Topological insulators could bring future computing platforms based on "spintronics." Conventional computers use the presence and absence of electric charges to represent ones and zeroes in a binary code needed to carry out computations. Spintronics, however, uses the "spin state" of electrons to represent ones and zeros.

"Compounds based on bismuth, antimony, telluride and selenide are the cleanest and most intrinsic topological insulators demonstrated so far, with no measurable amount of undesirable conduction inside the bulk that often spoils the topological conduction properties in earlier topological insulator materials," Chen said.

The researchers also found evidence consistent with the conduction of electrons being "topologically protected," meaning its surface is guaranteed to be a robust conductor. Studying thin-slab-shaped samples cut from this material down to ever decreasing thickness while observing the conductance, the researchers found that the conductance -- which occurs always and only at the surface -- barely changes.

"For the thinnest samples, such topological conduction properties were even observed at room temperature, paving the way for practical applications," Xu said.

The paper was authored by Xu; Purdue research scientist Ireneusz Miotkowski, who created the high-quality materials; Princeton postdoctoral research associate Chang Liu; Purdue postdoctoral research associate Jifa Tian; UT Austin graduate student Hyoungdo Nam; Princeton graduate student Nasser Alidoust; Purdue graduate student Jiuning Hu; Chih-Kang Shih, Jane and Roland Blumberg Professor at UT Austin; M. Zahid Hasan, a Princeton professor of physics; and Chen.

In addition to the material growth and electrical measurements performed by the Purdue researchers, the Princeton and UT Austin groups contributed to this study by performing advanced characterizations that further confirmed important properties of the material as a topological insulator.

The research was funded by the Defense Advanced Research Projects Agency, which supports a Purdue-led program with participation from Princeton and other institutions aiming to develop energy efficient electronic devices based on topological insulators. The electrical measurements revealing the signature half-integer quantum Hall effect were performed at the National Science Foundation's National High Magnetic Field Laboratory. UT Austin's contribution to this study was supported the Welch Foundation and U.S. Army Research Office.

Source: Purdue University