Ions Clear Another Hurdle Toward Scaled-up Quantum Computing

parallel gates

Scientists at the Joint Quantum Institute (JQI) have been steadily improving the performance of ion trap systems, a leading platform for future quantum computers. Now, a team of researchers led by JQI Fellows Norbert Linke and Christopher Monroe has performed a key experiment on five ion-based quantum bits, or qubits. They used laser pulses to simultaneously create quantum connections between different pairs of qubits—the first time these kinds of parallel operations have been executed in an ion trap. The new study, which is a critical step toward large-scale quantum computation, was published on July 24 in the journal Nature.  

“When it comes to the scaling requirements for a quantum computer, trapped ions check all of the boxes,” says Monroe, who is also the Bice-Sechi Zorn professor in the UMD Department of Physics and co-founder of the quantum computing startup IonQ. “Getting these parallel operations to work further illustrates that advancing ion trap quantum processors is not limited by the physics of qubits and is instead tied to engineering their controllers.” 

Ion traps are devices for capturing charged atoms and molecules, and they are commonly deployed for chemical analysis. In recent decades, physicists and engineers have combined ion traps with sophisticated laser systems to exert control over single atomic ions. Today, this type of hardware is one of the most promising for building a universal quantum computer.

The JQI ion trap used in this study is made from gold-coated electrodes, which carry the electric fields that confine ytterbium ions. The ions are caught in the middle of the trap where they form a line, each one separated from its neighbor by a few microns. This setup enables researchers to have fine control over individual ions and configure them as qubits.

Each ion has internal energy levels or quantum states that are naturally isolated from outside influences. This feature makes them ideal for storing and controlling quantum information, which is notoriously delicate. In this experiment, the research team uses two of these states, called “0” and “1”, as the qubit.

The researchers aim laser pulses at a string of qubits to execute programs on this small-scale quantum computer. The programs, also called circuits, are broken down into a set of single- and two-qubit gates. A single-qubit gate can, for instance, flip the state of an ion from 1 to 0. This is a straightforward task for a laser pulse. A two-qubit gate requires more sophisticated pulses because it involves tailoring the interactions between qubits. Certain two-qubit operations can create entanglement—a quantum connection necessary for quantum computation—between two qubits. 

Until now, circuits in ion trap quantum computers have been limited to a sequence of individual gates, one after another. With this new demonstration, researchers can now do two-qubit gates in parallel, creating entanglement between different pairs of ions simultaneously. The research team achieved this by optimizing the laser pulse sequences used to perform operations, making sure to cancel out unwanted laser-qubit interactions. In this way, they were able to successfully implement simultaneous entangling gates on two separate ion pairs.

According to the authors, parallel entangling gates will enable programs to correct errors during a quantum computation—a near-certain requirement in quantum computers with many more qubits. In addition, a quantum computer that factors large numbers or simulates quantum physics will likely need parallel entangling operations to achieve a speed advantage over conventional computers. 

Story by E. Edwards

In addition to Monroe and Linke, Caroline Figgatt, former JQI graduate student and scientist at Honeywell, was lead author on this research paper and provided background material for this news story. The research paper was published simultaneous to similar work done by former JQI postdoctoral researcher and Tsinghua University professor Kihwan Kim. 

University of Maryland Launches Quantum Technology Center

Date: 
Thursday, August 22, 2019

Newfound Superconductor Material Could Be the ‘Silicon of Quantum Computers’

 We have already found lots of superconductors, but this whimsical illustration shows why one superconductor's newfound properties may make it especially useful. Most known superconductors are spin singlets, found on the island to the left. Uranium ditelluride, however, is a rare spin triplet, found on the island to the right, and also exists at the top of a mountain representing its unusually high resistance to magnetic fields. These properties may make it a good material for making qubits, which could maintain coherence in a quantum computer despite interference from the surrounding environment. Credit: N. Hanacek/NIST We have already found lots of superconductors, but this whimsical illustration shows why one superconductor's newfound properties may make it especially useful. Most known superconductors are spin singlets, found on the island to the left. Uranium ditelluride, however, is a rare spin triplet, found on the island to the right, and also exists at the top of a mountain representing its unusually high resistance to magnetic fields. These properties may make it a good material for making qubits, which could maintain coherence in a quantum computer despite interference from the surrounding environment. Credit: N. Hanacek/NIST

 A collaboration of the NIST Center for Neutron Research, the UMD's Center for Nanophysics and Advanced Materials and the Ames Laboratory has yielded a new superconductor with properties highly advantageous for the development of quantum computers. Uranium ditelluride, or UTe2, described in Science magazine, resists magnetism and could maintain coherence in qubits.  Read more at NIST.gov. 

 

Corkscrew Photons May Leave Behind a Spontaneous Twist

A new prediction argues that some materials might experience a torque when they are hotter than their surroundings. (Credit: E. Edwards/JQI)

 

Everything radiates. Whether it's a car door, a pair of shoes or the cover of a book, anything hotter than absolute zero (i.e., pretty much everything) is constantly shedding radiation in the form of photons, the quantum particles of light.

A twin process—absorption—is usually also present. As photons carry away energy, passers-by from the environment can be absorbed to replenish it. When absorption and emission occur at the same rate, scientists say that an object is in equilibrium with its environment. This often means that object and environment share the same temperature.

Far away from equilibrium, new behaviors can emerge. In a paper published August 1, 2019 as an Editors’ Suggestion in the journal Physical Review Letters, scientists at JQI and Michigan State University suggest that certain materials may experience a spontaneous twisting force if they are hotter than their surroundings.

"The fact that a material might feel a torque due to a temperature difference with the environment is very unusual," says lead author Mohammad Maghrebi, a former JQI postdoctoral researcher who is now an assistant professor at Michigan State University.

The effect, which hasn't yet been observed in an experiment, is predicted to arise in a thin ribbon of a material called a topological insulator (TI)—something that allows electrical current to flow on its surface but not through its innards.

In this case, the researchers made two additional assumptions about the TI. One is that it is hotter than its environment. And another is that the TI has some magnetic impurities that affect the behavior of electrons on its surface.

These magnetic impurities interact with a quantum property of the electrons called spin. Spin is part of the basic character of an electron, much like electric charge, and it describes the particle’s intrinsic angular momentum—the tendency of an object to continue rotating. Photons, too, can carry angular momentum.

Although electrons don’t physically rotate, they can still gain and lose angular momentum, albeit only in discrete chunks. Each electron has two spin values—up and down—and the magnetic impurities ensure that one value sits at a higher energy than the other. In the presence of these impurities, electrons can flip their spin from up to down and vice versa by emitting or absorbing a photon that carries the right amount of energy and angular momentum.

Maghrebi and two colleagues, JQI Fellows Jay Deep Sau and Alexey Gorshkov, showed that radiation emanating from this kind of TI carries angular momentum skewed in one rotational direction, like a corkscrew that twists clockwise. The material gets left with a deficit of angular momentum, causing it to feel a torque in the opposite direction (in this example, counterclockwise).

The authors say that TIs are ideal for spotting this effect because they play host to the right kind of interaction between electrons and light. TIs already link electron spin with the momentum of their motion, and it's through this motion that electrons in the material ordinarily absorb and emit light.

If an electron on the surface of this particular kind of TI starts with its spin pointing up, it can shed energy and angular momentum by changing its spin from up to down and emitting a photon. Since the TI is hotter than its environment, electrons will flip from up to down more often than the reverse. That’s because the environment has a lower temperature and lacks the energy to replace the radiation coming from the TI. The result of this imbalance is a torque on the thin TI sample, driven by the random emission of radiation.

Future experiments might observe the effect in one of two ways, the authors say. The most likely method is indirect, requiring experimenters to heat up a TI by running a current through it and collecting the emitted light. By measuring the average angular momentum of the radiation, an experiment might detect the asymmetry and confirm one consequence of the new prediction.

A more direct—and likely more difficult—observation would involve actually measuring the torque on the thin film by looking for tiny rotations. Maghrebi says that he's brought up the idea to several experimentalists. "They were not horrified by having to measure something like a torque, but, at the same time, I think it really depends on the setup," he says. "It certainly didn't sound like it was impossible."

Story by Chris Cesare: https://jqi.umd.edu/news/corkscrew-photons-may-leave-behind-spontaneous-twist

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