Tag Archives: Alexei Kitaev

Creating time crystals with a quantum computer

This November 30, 2021 news item on phys.org about time crystals caught my attention,

There is a huge global effort to engineer a computer capable of harnessing the power of quantum physics to carry out computations of unprecedented complexity. While formidable technological obstacles still stand in the way of creating such a quantum computer, today’s early prototypes are still capable of remarkable feats.

For example, the creation of a new phase of matter called a “time crystal.” Just as a crystal’s structure repeats in space, a time crystal repeats in time and, importantly, does so infinitely and without any further input of energy—like a clock that runs forever without any batteries. The quest to realize this phase of matter has been a longstanding challenge in theory and experiment—one that has now finally come to fruition.

In research published Nov. 30 [2021] in Nature, a team of scientists from Stanford University, Google Quantum AI, the Max Planck Institute for Physics of Complex Systems and Oxford University detail their creation of a time crystal using Google’s Sycamore quantum computing hardware.

The Google Sycamore chip used in the creation of a time crystal. Credit: Google Quantum AI [downloaded from https://phys.org/news/2021-11-physicists-crystals-quantum.html]

A November 30, 2021 Stanford University news release (also on EurekAlert) by Taylor Kubota, which originated the news item, delves further into the work and into the nature of time crystals,

“The big picture is that we are taking the devices that are meant to be the quantum computers of the future and thinking of them as complex quantum systems in their own right,” said Matteo Ippoliti, a postdoctoral scholar at Stanford and co-lead author of the work. “Instead of computation, we’re putting the computer to work as a new experimental platform to realize and detect new phases of matter.”

For the team, the excitement of their achievement lies not only in creating a new phase of matter but in opening up opportunities to explore new regimes in their field of condensed matter physics, which studies the novel phenomena and properties brought about by the collective interactions of many objects in a system. (Such interactions can be far richer than the properties of the individual objects.)

“Time-crystals are a striking example of a new type of non-equilibrium quantum phase of matter,” said Vedika Khemani, assistant professor of physics at Stanford and a senior author of the paper. “While much of our understanding of condensed matter physics is based on equilibrium systems, these new quantum devices are providing us a fascinating window into new non-equilibrium regimes in many-body physics.”

What a time crystal is and isn’t

The basic ingredients to make this time crystal are as follows: The physics equivalent of a fruit fly and something to give it a kick. The fruit fly of physics is the Ising model, a longstanding tool for understanding various physical phenomena – including phase transitions and magnetism – which consists of a lattice where each site is occupied by a particle that can be in two states, represented as a spin up or down.

During her graduate school years, Khemani, her doctoral advisor Shivaji Sondhi, then at Princeton University, and Achilleas Lazarides and Roderich Moessner at the Max Planck Institute for Physics of Complex Systems stumbled upon this recipe for making time crystals unintentionally. They were studying non-equilibrium many-body localized systems – systems where the particles get “stuck” in the state in which they started and can never relax to an equilibrium state. They were interested in exploring phases that might develop in such systems when they are periodically “kicked” by a laser. Not only did they manage to find stable non-equilibrium phases, they found one where the spins of the particles flipped between patterns that repeat in time forever, at a period twice that of the driving period of the laser, thus making a time crystal.

The periodic kick of the laser establishes a specific rhythm to the dynamics. Normally the “dance” of the spins should sync up with this rhythm, but in a time crystal it doesn’t. Instead, the spins flip between two states, completing a cycle only after being kicked by the laser twice. This means that the system’s “time translation symmetry” is broken. Symmetries play a fundamental role in physics, and they are often broken – explaining the origins of regular crystals, magnets and many other phenomena; however, time translation symmetry stands out because unlike other symmetries, it can’t be broken in equilibrium. The periodic kick is a loophole that makes time crystals possible.

The doubling of the oscillation period is unusual, but not unprecedented. And long-lived oscillations are also very common in the quantum dynamics of few-particle systems. What makes a time crystal unique is that it’s a system of millions of things that are showing this kind of concerted behavior without any energy coming in or leaking out.

“It’s a completely robust phase of matter, where you’re not fine-tuning parameters or states but your system is still quantum,” said Sondhi, professor of physics at Oxford and co-author of the paper. “There’s no feed of energy, there’s no drain of energy, and it keeps going forever and it involves many strongly interacting particles.”

While this may sound suspiciously close to a “perpetual motion machine,” a closer look reveals that time crystals don’t break any laws of physics. Entropy – a measure of disorder in the system – remains stationary over time, marginally satisfying the second law of thermodynamics by not decreasing.

Between the development of this plan for a time crystal and the quantum computer experiment that brought it to reality, many experiments by many different teams of researchers achieved various almost-time-crystal milestones. However, providing all the ingredients in the recipe for “many-body localization” (the phenomenon that enables an infinitely stable time crystal) had remained an outstanding challenge.

For Khemani and her collaborators, the final step to time crystal success was working with a team at Google Quantum AI. Together, this group used Google’s Sycamore quantum computing hardware to program 20 “spins” using the quantum version of a classical computer’s bits of information, known as qubits.

Revealing just how intense the interest in time crystals currently is, another time crystal was published in Science this month [November 2021]. That crystal was created using qubits within a diamond by researchers at Delft University of Technology in the Netherlands.

Quantum opportunities

The researchers were able to confirm their claim of a true time crystal thanks to special capabilities of the quantum computer. Although the finite size and coherence time of the (imperfect) quantum device meant that their experiment was limited in size and duration – so that the time crystal oscillations could only be observed for a few hundred cycles rather than indefinitely – the researchers devised various protocols for assessing the stability of their creation. These included running the simulation forward and backward in time and scaling its size.

“We managed to use the versatility of the quantum computer to help us analyze its own limitations,” said Moessner, co-author of the paper and director at the Max Planck Institute for Physics of Complex Systems. “It essentially told us how to correct for its own errors, so that the fingerprint of ideal time-crystalline behavior could be ascertained from finite time observations.”

A key signature of an ideal time crystal is that it shows indefinite oscillations from all states. Verifying this robustness to choice of states was a key experimental challenge, and the researchers devised a protocol to probe over a million states of their time crystal in just a single run of the machine, requiring mere milliseconds of runtime. This is like viewing a physical crystal from many angles to verify its repetitive structure.

“A unique feature of our quantum processor is its ability to create highly complex quantum states,” said Xiao Mi, a researcher at Google and co-lead author of the paper. “These states allow the phase structures of matter to be effectively verified without needing to investigate the entire computational space – an otherwise intractable task.”

Creating a new phase of matter is unquestionably exciting on a fundamental level. In addition, the fact that these researchers were able to do so points to the increasing usefulness of quantum computers for applications other than computing. “I am optimistic that with more and better qubits, our approach can become a main method in studying non-equilibrium dynamics,” said Pedram Roushan, researcher at Google and senior author of the paper.

“We think that the most exciting use for quantum computers right now is as platforms for fundamental quantum physics,” said Ippoliti. “With the unique capabilities of these systems, there’s hope that you might discover some new phenomenon that you hadn’t predicted.”

A view of the Google dilution refrigerator, which houses the Sycamore chip. Credit: Google Quantum AI [downloaded from https://scitechdaily.com/stanford-and-google-team-up-to-create-time-crystals-with-quantum-computers/]

Here’s a link to and a citation for the paper,

Time-Crystalline Eigenstate Order on a Quantum Processor by Xiao Mi, Matteo Ippoliti, Chris Quintana, Ami Greene, Zijun Chen, Jonathan Gross, Frank Arute, Kunal Arya, Juan Atalaya, Ryan Babbush, Joseph C. Bardin, Joao Basso, Andreas Bengtsson, Alexander Bilmes, Alexandre Bourassa, Leon Brill, Michael Broughton, Bob B. Buckley, David A. Buell, Brian Burkett, Nicholas Bushnell, Benjamin Chiaro, Roberto Collins, William Courtney, Dripto Debroy, Sean Demura, Alan R. Derk, Andrew Dunsworth, Daniel Eppens, Catherine Erickson, Edward Farhi, Austin G. Fowler, Brooks Foxen, Craig Gidney, Marissa Giustina, Matthew P. Harrigan, Sean D. Harrington, Jeremy Hilton, Alan Ho, Sabrina Hong, Trent Huang, Ashley Huff, William J. Huggins, L. B. Ioffe, Sergei V. Isakov, Justin Iveland, Evan Jeffrey, Zhang Jiang, Cody Jones, Dvir Kafri, Tanuj Khattar, Seon Kim, Alexei Kitaev, Paul V. Klimov, Alexander N. Korotkov, Fedor Kostritsa, David Landhuis, Pavel Laptev, Joonho Lee, Kenny Lee, Aditya Locharla, Erik Lucero, Orion Martin, Jarrod R. McClean, Trevor McCourt, Matt McEwen, Kevin C. Miao, Masoud Mohseni, Shirin Montazeri, Wojciech Mruczkiewicz, Ofer Naaman, Matthew Neeley, Charles Neill, Michael Newman, Murphy Yuezhen Niu, Thomas E. O’Brien, Alex Opremcak, Eric Ostby, Balint Pato, Andre Petukhov, Nicholas C. Rubin, Daniel Sank, Kevin J. Satzinger, Vladimir Shvarts, Yuan Su, Doug Strain, Marco Szalay, Matthew D. Trevithick, Benjamin Villalonga, Theodore White, Z. Jamie Yao, Ping Yeh, Juhwan Yoo, Adam Zalcman, Hartmut Neven, Sergio Boixo, Vadim Smelyanskiy, Anthony Megrant, Julian Kelly, Yu Chen, S. L. Sondhi, Roderich Moessner, Kostyantyn Kechedzhi, Vedika Khemani & Pedram Roushan. Nature (2021) DOI: https://doi.org/10.1038/s41586-021-04257-w Published 30 November 2021

This is a preview of the unedited paper being provided by Nature. Click on the Download PDF button (to the right of the title) to get access.

Proposed nanodevice made possible by particle that is its own antiparticle (Majorana particle)

I’m not sure how much the mystery of Ettore Majorana’s disappearance in 1938 has to do with the latest research from Brazil on Majorana particles but it’s definitely fascinating,. From an April 6, 2018 news item on ScienceDaily,

In March 1938, the young Italian physicist Ettore Majorana disappeared mysteriously, leaving his country’s scientific community shaken. The episode remains unexplained, despite Leonardo Scascia’s attempt to unravel the enigma in his book The Disappearance of Majorana (1975).

Majorana, whom Enrico Fermi called a genius of Isaac Newton’s stature, vanished a year after making his main contribution to science. In 1937, when he was only 30, Majorana hypothesized a particle that is its own anti-particle and suggested that it might be the neutrino, whose existence had recently been predicted by Fermi and Wolfgang Pauli.

Eight decades later, Majorana fermions, or simply majoranas, are among the objects most studied by physicists. In addition to neutrinos — whose nature, whether or not they are majoranas, is one of the investigative goals of the mega-experiment Dune — another class not of fundamental particles but of quasi-particles or apparent particles has been investigated in the field of condensed matter. These Majorana quasi-particles can emerge as excitations in topological superconductors.

An April 6, 2018 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) press release on EurekAlert, which originated the news item,  reveals more about the Brazilian research (Note: Links have been removed),

A new study by PhD student Luciano Henrique Siliano Ricco with a scholarship from the São Paulo Research Foundation – FAPESP, in collaboration with his supervisor Antonio Carlos Ferreira Seridonio and others, was conducted on the Ilha Solteira campus of São Paulo State University (UNESP) in Brazil and described in an article in Scientific Reports.

“We propose a theoretical device that acts as a thermoelectric tuner – a tuner of heat and charge – assisted by Majorana fermions,” Seridonio said.

The device consists of a quantum dot (QD), represented in the Figure A by the symbol ε1. QDs are often called “artificial atoms.” In this case, the QD is located between two metallic leads at different temperatures.

The temperature difference is fundamental to allowing thermal energy to flow across the QD. A quasi-one-dimensional superconducting wire – called a Kitaev wire after its proponent, Russian physicist Alexei Kitaev, currently a professor at the California Institute of Technology (Caltech) in the US – is connected to the QD.

In this study, the Kitaev wire was ring- or U-shaped and had two majoranas (η1 and η2) at its edges. The majoranas emerge as excitations characterized by zero-energy modes.

“When the QD is coupled to only one side of the wire, the system behaves resonantly with regard to electrical and thermal conductance. In other words, it behaves like a thermoelectric filter,” said the principal investigator for the FAPESP fellowship.

“I should stress that this behavior as a filter for thermal and electrical energy occurs when the two majoranas ‘see’ each other via the wire, but only one of them ‘sees’ the QD in the connection.”

Another possibility investigated by the researchers involved making the QD “see” the two majoranas at the same time by connecting it to both ends of the Kitaev wire.

“By making the QD ‘see’ more of η1 or η2, i.e., by varying the system’s asymmetry, we can use the artificial atom as a tuner, where the thermal or electrical energy that flows through it is redshifted or blueshifted,” Seridonio said (see Figure B for illustrative explanation).

This theoretical paper, he added, is expected to contribute to the development of thermoelectric devices based on Majorana fermions.

Here’s a link to and a citation for the paper,

Tuning of heat and charge transport by Majorana fermions by L. S. Ricco, F. A. Dessotti, I. A. Shelykh, M. S. Figueira & A. C. Seridonio. Scientific Reportsvolume 8, Article number: 2790 (2018) doi:10.1038/s41598-018-21180-9 Published online: 12 February 2018

This paper is open access.

As I prepared to publish this piece I stumbled across a sad Sept. 3, 2018 article about Brazil and its overnight loss of heritage in a fire by Henry Grabar for slate.com (Note: Links have been removed),

On Sunday night, a fire ripped through Brazil’s National Museum in Rio de Janeiro, destroying the country’s most valuable storehouse of natural and anthropological history within hours.

Most of the 20 million items housed inside—including the skull of Luzia, the oldest human remains ever found in the Americas; one of the world’s largest archives of South America’s indigenous cultures; more than 26,000 fossils, 55,000 stuffed birds, and 5 million insect specimens; and a library of more than 500,000 books—are thought to have been destroyed.

The loss is a symptom of a larger problem as Grabar notes in his article.