Quantum Computers Check Each Other’s Work

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Currently, all quantum computers involve only a few qubits "and thus can be easily verified by a classical computer, or on a piece of large numbers of qubits or two entangled quantum computers. But these still lie particular quantum computer and the computation it carries out. Still, the more traps users build into the tasks, the better they can ensure the quantum computer they test is computing accurately. "The test is designed in such a way that the quantum computer cannot distinguish the trap from its normal tasks," Walther says.

The researchers used a 4-qubit quantum computer as the verifier, but any size will do, and the more qubits the better, Walther notes. The technique is scalable, so it could be used even on computers with hundreds of qubits, he says, and it can be applied to any of the many existing quantum computing platforms.

"Like almost all current quantum computing experiments, this currently has the status of a fun demonstration proof of concept, rather than anything that's directly useful yet," says theoretical computer scientist Scott Aaronson at the Massachusetts Institute of Technology in Cambridge. But that doesn’t detract from the importance of these demonstrations, he adds. "I'm very happy that they're done, as they're necessary first steps if we're ever going to have useful quantum computers."

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Improving measurements by reducing quantum noise

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Fermionic Quantum Magnetism

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Quantum probe technique resonates with Caltech/JPL researchers

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$(document).ready(function(){$('.star-rating').rating({callback: function(value, link){ $.post("../../", {fuseaction: 'starrating.submitrating', starratingid:$("#starratingid").val(), objecttype:$("#objecttype").val(), objectid:$("#objectid").val(),starrating:value}, function(obj) { $('.star-rating').rating('readOnly',true); var newobj = eval('(' + obj + ')'); $('#starratingleftcontainer').html('Average Rating: ' + newobj.ratio + ' / 5 ('+ newobj.ratinghits +' ratings)'); $('#starratingid').val(newobj.starratingid); });}});$("#starratinghelpicon").hover(function() {$("#starratinghelptextcontainer").animate({opacity: "show"}, "slow");}, function() {$("#starratinghelptextcontainer").animate({opacity: "hide"}, "fast");});});Average Rating: 5 / 5 (1 ratings) Quantum probe technique resonates with Caltech/JPL researchers

A team from the California Institute of Technology (Caltech) and the Jet Propulsion Laboratory (JPL) has successfully demonstrated a system that will serve as a test-bed for exploring quantum mechanics in new limits and could shed light on fundamental issues such as the division between the classical and quantum worlds.

One of the fundamental issues with making measurements in the quantum regime is that the act of measuring can actually destroy the fragile quantum state of such a system. This occurs despite one’s best attempts to be discreet in the characterization process. Basically, measuring messes things up.

Such sensitive measurements are especially difficult for macroscopic scale objects, and even on micrometer scales the quantum nature of objects is generally lost in the noise of the larger world.

In atomic physics and, more recently, in solid-state physics, researchers have gotten around this conundrum by using other quantum systems as probes. For example, the quantum signature of an oscillating system, like an electromagnetic cavity, can be imprinted on a probe and then inferred through an independent measurement of the probe – without destroying the quantum state of the original system. Such probes include individual atoms and nano-scale electronic quantum bits or "qubits."

A qubit is the quantum computing equivalent of the binary digital bit in classical computing. Like the “1” or “0” of its digital counterpart, the qubit has a state that can be controlled and measured.

In the new technique demonstrated by the Caltech and JPL team, the qubit is a tiny electrode in which electrons can be made to take on a single energy state, i.e., charged or uncharged. The qubit is placed in close proximity to a nanoresonator, which is a tiny beam of stiff material that vibrates at high frequency. In this configuration, the vibration frequency of the resonator is sensitive to the energy state of the quantum bit, and vice versa. The Caltech and JPL scientists used the sensitive dependence of the resonator frequency to probe the state of the qubit. In principle, though, either component can be used as a probe to determine quantum information about the other.

“We’ve demonstrated that we can use the nanomechanical resonator to probe quantum effects in the qubit. Based on these measurements, it looks we should be able to turn the experiment around and start looking for quantum effects in the mechanical system,” said Caltech’s Matt LaHaye. LaHaye is lead author of a paper describing the team’s work, published in the June 18 issue of the journal Nature.

According to quantum mechanics, the energy stored in any vibrating system – including macroscopic scale systems – should take discrete, or quantized, values. This discreteness of energy has been verified beautifully in other vibrating systems, such as atomic and electromagnetic systems. However, observation of this subtle quantum effect in ordinary mechanical structures, like the nanoresonator described above, remains a hotly pursued challenge in quantum measurement. The new nanoscale system demonstrated by the Caltech and JPL team is a very promising approach that could ultimately meet this challenge.

“What’s really exciting is that a lot of people have been thinking about how to use this coupled system for manipulating and measuring quantum states of mechanical objects. There are many possible experiments and directions in which we could head, all with the potential for very rich physics,” said LaHaye.

A tantalizing possibility for the team’s measurement technique is to observe the resonator in a quantum superposition of states, where it is simultaneously oscillating at two distinct frequencies. It may also be possible to examine the quantum imprint of the resonator on the energy state of the qubit to look for “quantum jumps” in the resonator’s energy.

The JPL team members had been developing the qubits for quantum computing applications and contributed to the research with fabrication of the structures and expertise in their measurement.

The work resulted from a collaboration between Profs. Michael Roukes and Keith Schwab at Caltech and Dr. Pierre Echternach at JPL. Measurements at milli-Kelvin temperatures were performed by Dr. Matt LaHaye and graduate student Junho Suh at Caltech. Electron beam lithography was performed by Richard Muller at JPL.

Reference: M.D. LaHaye, J. Suh, P.M. Echternach, K.C. Schwab, M.L. Roukes. (2009). “Nanomechanical measurements of a superconducting qubit.” Nature 459, 960-964.

Caltech press release: http://media.caltech.edu/press_releases/13271

JPL Microdevices Laboratory Website: http://microdevices.jpl.nasa.gov

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New technique for cooling molecules may be stepping stone to quantum computing

Eric Hudson. Credit: Reed Hutchinson

(Phys.org) —The next generation of computers promises far greater power and faster processing speeds than today's silicon-based based machines. These "quantum computers"—so called because they would harness the unique quantum mechanical properties of atomic particles—could draw their computing power from a collection of super-cooled molecules.

But chilling molecules to a fraction of a degree above absolute zero, the temperature at which they can be manipulated to store and transmit data, has proven to be a difficult challenge for scientists.

Now, UCLA physicists have pioneered a new technique that combines two traditional atomic cooling technologies and brings normally springy molecules to a frozen standstill. Their research is published March 28 in the journal Nature.

"Scientists have been trying to cool molecules for a decade and have succeeded with only a few special molecules," said Eric Hudson, a UCLA assistant professor of physics and the paper's senior author. "Our technique is a completely different approach to the problem—it is a lot easier to implement than the other techniques and should work with hundreds of different molecules."

Previous attempts to create ultracold molecules were only effective with one or two specific kinds. Creating a method that can be used with many different molecules would be a major step forward because it is difficult to say which materials might be used in quantum computers or other future applications, Hudson said.

By immersing charged barium chloride molecules in an ultracold cloud of calcium atoms, Hudson and his colleagues are able to prevent most of the molecules from vibrating and rotating. Halting the molecules is a necessary hurdle to overcome before they can be used to store information like a traditional computer does.

"The goal is to build a computer that doesn't work with zeros and ones, but with quantum mechanical objects," Hudson said. "A quantum computer could crack any code created by a classical computer and transmit information perfectly securely."

Hudson's experiment makes molecules extremely cold under highly controlled conditions to reveal the quantum mechanical properties that are hidden under normal circumstances. At room temperature, molecules rocket around, bouncing into each other and exchanging energy. Any information a scientist attempted to store in such a chaotic system would quickly become gibberish.

"We isolate these molecular systems in a vacuum, effectively levitating them in the middle of nothing," Hudson said. "This removes them from the rest of the world that wants to make them classical."

The quantum mechanical world of subatomic particles deviates from the classical world that we observe with the naked eye because according to quantum mechanics, electrons can only exist at specific energy levels. In a quantum computer made of a collection of single atoms, information might be stored by boosting some atomic electrons to higher energy levels while leaving others at lower energy states. However, these atomic energy states are not stable enough to reliably preserve data, Hudson said.

"One of the challenges with atoms is that their energy states are very easily influenced by the outside world," Hudson said. "You make this beautiful quantum state, but then the outside world tries to destroy that information."

Instead of saving data in easily disrupted atomic energy states, a more robust way to store information is in the rotational energy states of molecules, Hudson said. A spinning molecule in the lowest energy rotational state could represent a binary one, while a stationary molecule could represent a binary zero.

Despite applications for quantum computing and other industries, cooling molecules to extremely low temperatures has proved a challenge. Even the simplest molecule composed of only two atoms is a far more complex system than a single atom. Each molecule vibrates and rotates like a miniature whirling slinky, and all of that movement must be stilled so that the molecule can lose energy and cool down.

A new cooling technique

To solve the ultracold molecule conundrum, Hudson and his group first created a floating cloud of calcium atoms corralled by incoming laser beams from all directions. This magneto-optical trap keeps the atoms stationary as it cools them to nearly absolute zero. They then use specialized rods with high, oscillating voltages as part of an ion trap to confine a cloud of positively-charged barium chloride molecules within the ultracold ball of calcium atoms to complete the cooling process.

For the vibrating, energetic molecules to lose heat, they must spend a significant amount of time in contact with the surrounding ultracold atom cloud. Hudson and his colleagues used barium chloride ions, molecules missing one electron, because charged molecules are easier to trap and cool than their neutral counterparts. The use of molecular ions is an essential innovation because previous efforts have demonstrated that neutral molecules ricochet off ultracold atoms without sufficient heat transfer.

"When a molecular ion and a neutral atom get close together they get in tight and bang off each other a bunch before the ion goes away," Hudson said. "When they collide like that it is very easy for the energy in one to go to the other."

While magneto-optical and ion traps are not new to the world of molecular physics, Hudson and his colleagues became the first group to combine these methods to create a cloud of ultracold molecules. This paper is the result of over four years of work spent designing, building, and testing their experiment.

"These two different technologies earned Nobel prizes for the scientists who developed them, but there wasn't really a body of knowledge about how to put these two procedures together," Hudson said.

More information: The research is funded by the Army Research Office and the National Science Foundation.

Journal reference: Nature

Provided by University of California, Los Angeles

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Advancing secure communications: A better single-photon emitter for quantum cryptography

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