Tag Archives: superposition

A cartoon illustration of a levitated drop of superfluid helium. A single photon circulating inside the drop (red arrow) will be used to produce the superposition. The drop's gravitational field (illustrated schematically in the background) may play a role in limiting the lifetime of such a superposition.

Credit: Yale News

Opening a window on quantum gravity

Yale University has received a grant from the W. M. Keck Foundation to fund experiments that researchers hope will provide new insights into quantum gravity. Jack Harris, associate professor of physics, will lead a Yale team that aims to address a long-standing question in physics — how the classical behavior of macroscopic objects emerges from microscopic constituents that obey the laws of quantum mechanics.

Very small objects like photons and electrons are known for their odd behavior. Thanks to the laws of quantum mechanics, they can act as particles or waves, appear in multiple places at once, and mysteriously interact over great distances. The question is why these behaviors are not observed in larger objects.

A cartoon illustration of a levitated drop of superfluid helium. A single photon circulating inside the drop (red arrow) will be used to produce the superposition. The drop's gravitational field (illustrated schematically in the background) may play a role in limiting the lifetime of such a superposition. Credit: Yale News
A cartoon illustration of a levitated drop of superfluid helium. A single photon circulating inside the drop (red arrow) will be used to produce the superposition. The drop’s gravitational field (illustrated schematically in the background) may play a role in limiting the lifetime of such a superposition.
Credit: Yale News

Scientists know that friction plays an important part in producing classical behavior in macroscopic objects, but many suspect that gravity also suppresses quantum effects. Unfortunately, there has been no practical way to test this possibility, and in the absence of a full quantum theory of gravity, it is difficult even to make any quantitative predictions.

To address this problem, Harris will create a novel instrument that will enable a drop of liquid helium to exhibit quantum mechanical effects. “A millimeter across,” Harris said, “our droplet will be five orders of magnitude more massive than any other object in which quantum effects have been observed. It will enable us to explore quantum behavior on unprecedentedly macroscopic scales and to provide the first experimental tests of leading models of gravity at the quantum level.”

Game-changing research

The W.M. Keck Foundation grant will fund five years of activity at the Harris lab, which is part of Yale’s Department of Physics. In the first year, Harris and his team will construct their apparatus, and in subsequent years they will use it to perform increasingly sophisticated experiments.

“We are extremely grateful to the W.M. Keck Foundation for this generous support,” said Steven Girvin, the Eugene Higgins Professor of Physics and deputy provost for research. “This is a forward-looking grant that will advance truly ground-breaking research.”

Girvin, whose own research interests include quantum computing, described the Harris project as a possible game-changer. “Truly quantum mechanical behaviors have been observed in the flight of molecules through a vacuum and in the flow of electrons through superconductive circuits, but nothing has been accomplished on this scale. If Jack succeeds, this would be the first time that an object visible to the naked eye has bulk motion that exhibits genuine quantum mechanical effects.”

Into the whispering gallery

To explain his project, Harris invokes an architectural quirk of St. Paul’s cathedral, a London landmark with a famous “whispering gallery.” High up in its main dome, a whisper uttered against one wall is easily audible at great distances, as the sound waves skim along the dome’s interior. Harris plans to create his own whispering gallery, albeit on a smaller scale, using a droplet of liquid helium suspended in a powerful magnetic field. Rather than sound waves, Harris’ gallery will bounce a single photon.

This approach is closely related to an idea proposed by Albert Einstein in the 1920s, but until now, it has remained beyond the technical capabilities of experimentalists. To complete the experiment, Harris will need to combine recent advances in three different areas of physics: the study of optical cavities (objects that can capture photons), magnetic levitation, and the strange, frictionless world of superfluid helium. “Superfluid liquid helium has particular properties, like absence of viscosity and near-absence of optical absorption,” Harris explained. “In our device, a drop of liquid helium will be made to capture a single photon, which will bounce around inside. We expect to see the drop respond to the photon. “A photon always behaves quantum mechanically,” he added. “If you have a macroscopic object — our helium drop — that responds appreciably to a photon, the quantum mechanical behavior can be transferred to the large object. Our device will be ideally suited to studying quantum effects in the drop’s motion.” Potential applications for Harris’ research include new approaches to computing, cryptography, and communications. But Harris is most excited about the implications for fundamental physics: “Finding a theory of quantum gravity has been an outstanding challenge in physics for several decades, and it has proceeded largely without input from experiments. We hope that our research can provide some empirical data in this arena.”

About the W.M. Keck Foundation

The W.M. Keck Foundation was established in 1954 by William Myron Keck, founder of the Superior Oil Company. The foundation supports pioneering research in science, engineering, and medicine and has provided generous funding for numerous research initiatives at Yale University. In 2014, the Keck Foundation awarded a separate grant to a team of scientists led by Corey O’Hern, associate professor of mechanical engineering at Yale, to explore the physics of systems composed of macro-sized particles. Source : Yale News

Illustration by Michael S. Helfenbein

Yale physicists find a new form of quantum friction


Physicists at Yale University have observed a new form of quantum friction that could serve as a basis for robust information storage in quantum computers in the future. The researchers are building upon decades of research, experimentally demonstrating a procedure theorized nearly 30 years ago.

The results appear in the journal Science and are based on work in the lab of Michel Devoret, the F.W. Beinecke Professor of Applied Physics.

Quantum computers, a technology still in development, would rely on the laws of quantum mechanics to solve certain problems exponentially faster than classical computers. They would store information in quantum systems, such as the spin of an electron or the energy levels of an artificial atom. Called “qubits,” these storage units are the quantum equivalent of classical “bits.” But while bits can be in states like 0 or 1, qubits can simultaneously be in the 0 and 1 state. This property is called quantum superposition; it is a powerful resource, but also very fragile. Ensuring the integrity of quantum information is a major challenge of the field.

 Illustration by Michael S. Helfenbein
Illustration by Michael S. Helfenbein

Zaki Leghtas, first author on the paper and a postdoctoral researcher at Yale, offered the following metaphor to explain this new form of quantum friction:

Imagine a hill surrounded by two basins. If you put a ball at the top of the hill, it will roll down the sides and settle in one of the basins. As it rolls, it loses energy due to the friction between the ball and the ground, and it slows down. This is why it stops at the bottom of the basin. But friction also causes the ball to leave a path in its wake. By looking at either side of the hill and seeing where grass is flattened and stones are pushed aside, you can tell whether the ball rolled into the right or left basin.

This figure depicts the position of a quantum particle over a time of 19 micro-seconds. Dark colors indicate high probability of the particle existing at the specified position. It is a plot of the time-evolution of the Winger function W (⍺) of the quantum system, with black corresponding to 1.0, white to 0, and blue to –0.05.
This figure depicts the position of a quantum particle over a time of 19 micro-seconds. Dark colors indicate high probability of the particle existing at the specified position. It is a plot of the time-evolution of the Winger function W (⍺) of the quantum system, with black corresponding to 1.0, white to 0, and blue to –0.05.

If you replace the ball with a quantum particle, however, you run into a problem. Quantum particles can exist in many states at the same time, so in theory, the particle could occupy both basins simultaneously. But as the particle is rolling down, the friction between the particle and the hill leaves an impact on the environment, which can be measured. The same friction that stops the particle at the bottom also carves the path. This destroys the superposition and forces the particle to exist in only one basin.

Previously, researchers had been able to take advantage of this friction to trap quantum particles in particular basins. But now, Devoret’s lab demonstrates a new type of friction — one that slows the particle as it rolls, but does not carve a path that tells which side it is choosing. This allows the particle to simultaneously exist in both the left and right basins at the same time.

Each of these “basin” states is both stable and steady. While the quantum particle might move around in the basins, small perturbations won’t kick it out of the basins. Furthermore, any superpositions of these two basin states are also stable and steady. This means they could be used as a basis for storing quantum information.

Technically, this is called a two-dimensional quantum steady-state manifold. Devoret and Leghtas point out that the next step is expanding this two-dimensional manifold to four dimensions — adding two more basins to the landscape. This will allow scientists to redundantly encode quantum information and to do error correction within the manifold. Error correction is one of the key components that must be developed in order to make a practical quantum computer feasible.

Additional authors are Steven Touzard, Ioan Pop, Angela Kou, Brian Vlastakis, Andrei Petrenko, Katrina Sliwa, Anirudh Narla, Shyam Shankar, Michael Hatridge, Matthew Reagor, Luigi Frunzio, Robert Schoelkopf, and Mazyar Mirrahimi of Yale. Mirrahimi also has an appointment at the Institut National de Recherche en Informatique et en Automatique Paris-Rocquencourt.

(Main illustration by Michael S. Helfenbein)

Source: Yale News