ion trap

Large-scale quantum computers, which are an active pursuit of many university labs and tech giants, remain years away. But that hasn’t stopped some scientists from thinking ahead, to a time when quantum computers might be linked together in a network or a single quantum computer might be split up across many interconnected nodes.A group of physicists at the University of Maryland, working with JQI Fellow Christopher Monroe, are pursuing the second goal, attempting to wire up isolated modules of trapped atomic ions with light. They imagine many modules, each with a hundred or so ions, linked together to form a quantum computer that is inherently scalable: If you want a bigger computer, simply add more modules to the mix.In a paper published recently in Physical Review Letters, Monroe and his collaborators reported on putting together many of the pieces needed to create such a module. It includes two different species of ions: an ytterbium ion for storing information and a barium ion for generating the light that communicates with other nodes.This dual-species approach isolates the storage and communication tasks of a network node. With a single species, manipulating the communication ion with a laser could easily corrupt the storage ion. In several experiments, the researchers demonstrated that they could successfully isolate the two ions from each other, transfer information between them and capture light generated by both ions. 

Nature doesn’t have the best memory. If you fill a box with air and divide it in half with a barrier, it’s easy to tell molecules on the left from molecules on the right. But after removing the barrier and waiting a short while, the molecules get mixed together, and it becomes impossible to tell where a given molecule started. The air-in-a-box system loses any memory of its initial conditions.The universe has been forgetting its own initial state since the Big Bang, a fact linked to the unrelenting forward march of time. Systems that forget where they started are said to have thermalized, since it is often—but not always—an exchange of heat and energy with some other system that causes the memory loss. For example, a melting ice cube forgets its orderly arrangement of water molecules when heat from its surroundings splits the cube’s crystal bonds. In some sense, the initial information about the ice cube—the structure of the crystal, the distance between molecules, etc.—leaks away.The opposite case is localization, where information about the initial arrangement sticks around. Such a situation is rare, like an ice cube that never melts, but one example is Anderson localization, in which particles or waves in a crystal are trapped near impurities. They tend to bounce off defects in the crystal and scatter in random directions, yielding no net movement. If there are enough impurities in a region, the particles or waves never escape.Since the discovery of Anderson localization in 1958, it has been an open question whether interacting collections of quantum particles can also localize, a phenomenon known as many-body localization. Now, researchers working with JQI and QuICS Fellow Christopher Monroe have directly observed this localization in a system of 10 interacting ions, trapped and zapped by electric fields and lasers. Their findings are one of the first direct observations of many-body localization in a quantum system, and they open up the possibility of studying the phenomenon with more ions. The results were published June 6 in Nature Physics.

Harnessing quantum systems for information processing will require controlling large numbers of basic building blocks called qubits. The qubits must be isolated, and in most cases cooled such that, among other things, errors in qubit operations do not overwhelm the system, rendering it useless. Led by JQI Fellow Christopher Monroe, physicists have recently demonstrated important steps towards implementing a proposed type of gate, which does not rely on super-cooling their ion qubits. This work, published as an Editor’s Suggestion in Physical Review Letters, implements ultrafast sensing and control of an ion's motion, which is required to realize these hot gates. Notably, this experiment demonstrates thermometry over an unprecedented range of temperatures--from zero-point to room temperature.

A team of researchers led by Duke University and the University of Maryland has been tapped by the nation’s “Q Branch” to take quantum computing efforts to the next level using one of the field’s leading technologies—ion traps.

In quantum mechanics, symmetry describes more than just the patterns that matter takes — it is used to classify the nature of quantum states. These states can be entangled, exhibiting peculiar connections that cannot be explained without the use of quantum physics. For some entangled states, the symmetry of these connections can offer a kind of protection against disruptions. Physicists are interested in exploring these classes of protected states because building a useful quantum device requires its building blocks to be robust against outside disturbances that may interfere with device operations.
Recently, JQI researchers under the direction of Christopher Monroe have used trapped atomic ions to construct a system that could potentially support a type of symmetry-protected quantum state. 

JQI researchers, under the direction of Christopher Monroe have demonstrated modular entanglement between two atomic systems, separated by one meter. Here, photons are the long distance information carriers entangling multiple qubit modules. Inside of a single module, quantized collective vibrations called phonons connect individual qubits. In the latest result, one module contains two qubits and a second module houses a single qubit. This work was published in the journal Nature Physics, along with two related results that appeared in the Physical Review journals.
The two-by-one qubit entanglement is an experimental result that follows the theoretical design by Monroe and collaborators on building a modular universal quantum computer, published earlier in 2014.

Magnetic resonance imaging (MRI), which is the medical application of nuclear magnetic resonance spectroscopy, is a powerful diagnostic tool. MRI works by resonantly exciting hydrogen atoms and measuring the relaxation time -- different materials return to equilibrium at different rates; this is how contrast develops (i.e. between soft and hard tissue). By comparing the measurements to a known spectrum of relaxation times, medical professionals can determine whether the imaged tissue is muscle, bone, or even a cancerous growth.

In quantum mechanics, interactions between particles can give rise to entanglement, which is a strange type of connection that could never be described by a non-quantum, classical theory. These connections, called quantum correlations, are present in entangled systems even if the objects are not physically linked (with wires, for example). Entanglement is at the heart of what distinguishes purely quantum systems from classical ones; it is why they are potentially useful, but it sometimes makes them very difficult to understand.

How do you build a universal quantum computer? Turns out, this question was addressed by theoretical physicists about 15 years ago. The answer was laid out in a research paper and has become known as the DiVincenzo criteria [See Gallery Sidebar for information on this criteria]. The prescription is pretty clear at a glance; yet in practice the physical implementation of a full-scale universal quantum computer remains an extraordinary challenge.

JQI researchers under the direction of Chris Monroe have produced quantum entanglement between a single atom’s motion and its spin state thousands of times faster than previously reported, demonstrating unprecedented control of atomic motion. This work, which may lead to faster and better quantum computer logic gates, is described a recent issue of Physical Review Letters.