Wormholes on Earth?
According to a group of mathematicians, it may be possible to create devices with internal tunnels that are invisible to detection by electromagnetic waves—wormholes, in a sense. The group discusses the idea in a paper published in the October 29 online edition of Physical Review Letters.
The scientists say that by custom designing the values of two parameters that describe electromagnetic (EM) materials, the electrical permittivity and magnetic permeability, around and inside a cylinder, a novel optical device could be produced. Essentially, most of the device would be invisible to detection by external EM radiation of a certain frequency, with only the ends of the cylinder being visible and accessible to the EM waves.
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“The chosen values for the permittivity and permeability would cause the coating to manipulate EM waves in a way that is not seen in nature,” explained University of Rochester mathematician Allan Greenleaf, one of the paper’s authors.
Permittivity is a measure of a material’s readiness to become electrically polarized in response to an applied electric field (how well it “permits” the field). Permeability describes how magnetized a material becomes when a magnetic field is applied. Modern EM materials known as metamaterials allow theoretical designs, such as a wormhole, to be physically constructed, at least in principle.
Greenleaf and his colleagues, Yaroslav Kurylev of University College in London, Matti Lassas of the Helsinki University of Technology, and Gunther Uhlmann of the University of Washington, use the word “wormhole” in more of a mathematical sense than physical. That is, the devices would act as wormholes from the viewpoint of Maxwell’s equations, the four fundamental equations that describe the relationship between electric fields, magnetic fields, electric charge, and electric current.
For any other frequencies than those for which the permittivity and permeability were designed, the tunnel region would look roughly like a solid cylinder. But for the right frequencies, says Greenleaf, “the tunnel has the effect of changing the topology of space. The electromagnetic waves behave as though they are propagating through a space to which a handle has been attached, in the same way that ants crawling on the door of your refrigerator have two ways to get from one end of the handle to the other: by traveling over the handle or on the flat surface underneath.”
That is, any object within the tunnel is only visible to EM waves that enter at one of the tunnel’s ends. Conversely, any EM waves emitted by an object in the tunnel can only leave through one of the ends. However, Greenleaf says that it’s important to note that the shape of space has not actually been changed, as does happen for Einstein-Rosen wormholes in general relativity.
This effect could have interesting applications. For example, a magnetic dipole (such as a bar magnet) placed near one of the ends would, at the other end, appear to approximate a magnetic monopole, a theoretical particle with only one magnetic pole, i.e. that has magnetic charge. True magnetic monopoles have never been discovered, and the work by Greenleaf and his colleagues does not claim otherwise.
The scientists propose other possible applications, such as in magnetic resonance imaging (MRI), where a wormhole device could be used to allow doctors to operate on a patient while simultaneously imaging the patient. Doctors could insert metal surgical tools into the tunnel area without disturbing the MRI machine’s magnetic field.
Another example is an optical computer, where active components could be placed inside wormholes such as to not interact with each other and cause malfunctions.
Metamaterials for invisibility, while still in the very early stages of development, are already being researched. Last year, scientists from Duke University created a device that renders a copper disc invisible to observation by microwaves.
Citation: Allan Greenleaf, Yaroslav Kurylev, Matti Lassas, and Gunther Uhlmann “Electromagnetic Wormholes and Virtual Magnetic Monopoles from Metamaterials” Phys. Rev. Lett. 99, 183901 (2007)
Breakthrough toward industrial-scale production of nanodevices
Scientists in Maryland are reporting an important advance toward the long-sought goal of industrial-scale fabrication of nanowire-based devices like ultra-sensitive sensors, light emitting diodes, and transistors for inexpensive, high-performance electronics products. The study is scheduled for the current issue of ACS’ Chemistry of Materials.
In the report, Babak Nikoobakht points out that existing state-of-the-art assembly methods for nanowire-based devices require complicated, multi-step treatments, painstaking alignments steps, and other processing for nanowires , which are thousands of times smaller than the diameter of a human hair.
The goal is to electrically address the coordinates of millions of nanowires on a surface in order to produce the components of electronic circuits. The study describes a new method in which zinc oxide nanowires are grown in the exact positions where nanodevices later will be fabricated, in a way that involves a minimum number of fabrication steps and is suitable for industrial-scale applications.
“This method, due to its scalability and ease of device fabrication, goes beyond the current state-of-the-art assembly of nanowire-based devices,” the report states. “It is believed to be an attractive approach for mass fabrication of nanowire-based transistors and sensors and is expected to impact nanotechnology in fabrication of nonconventional nanodevices.”
Nonlocality of a Single Particle Demonstrated Without Objections
Usually when physicists talk about nonlocality in quantum mechanics, they’re referring to the fact that two particles can have immediate effects on each other, even when separated by large distances. Einstein famously called the phenomena “spooky interaction at a distance” because information about a particle seems to be traveling faster than the speed of light, violating the laws of causality.
Although the idea is counterintuitive, nonlocality is now widely accepted by physicists, albeit almost exclusively for two-particle systems. So far, no experiment has sufficiently demonstrated the nonlocality of a single particle, although explanations have been proposed since 1991 (starting with Tan, Walls, and Collett).
Since then, the issue has been strongly debated by physicists. In 1994, Lucien Hardy proposed a modified scheme of Tan, Walls, and Collett’s claim. However, others (notably Greenberger, Horne, and Zeilinger) objected to Hardy’s scheme, claiming that it was really a multi-particle effect in disguise, and could not be demonstrated experimentally.Now, Jacob Dunningham from the University of Leeds and Vlatko Vedral from the University of Leeds and the National University of Singapore have modified Hardy’s scheme, publishing their results in a recent issue of Physical Review Letters. By eliminating all unphysical inputs, their scheme allows for a real experiment, and ensures that only a single particle exhibits nonlocality. Plus, Dunningham and Vedral’s scheme not only applies to single photons, but to atoms and single massive particles, as well.
“The greatest significance of this work is that it shows how superposition and entanglement are the same ‘mystery,’” Dunningham explained to PhysOrg.com. “Feynman famously said that superposition is the only mystery in quantum mechanics, but more recently entanglement has been widely considered as an additional fundamental feature of quantum physics. Here we show that they are one and the same.”
In Hardy’s original scheme, one photon and a vacuum state arrive at a beam splitter, a glass prism that splits a beam of light in two. Two observers, Alice and Bob, have the option to either measure one of the beams, or to combine their beam with a coherent light beam, split the resulting beam with another beam splitter, and then measure the two outputs (also known as a “homodyne detection”).
Alice and Bob’s decisions could result in four possible combinations. First, if they both measure their beam from the original beam splitter, only one will detect a photon. Second, if Alice adds a coherent state to her beam while Bob measures his original split beam, Alice has two chances of detecting a photon, at the two outputs (c1, d1) of her beam splitter. Hardy showed that, if Alice detected a photon at c1, Bob would not detect a photon; but if Alice detected a photon at d1, Bob must detect a photon. In the third possibility, the roles of Alice and Bob are simply switched, with the same results.
In the fourth possibility, both Alice and Bob make homodyne detections. If they both detect particles at their d detectors (d1 and d2, respectively), then they both infer that the other must detect a photon from the original source. This is a problem, because they cannot both be right—there is only one original photon.
Hardy argued that this scheme demonstrates the nonlocality of a single particle when one eliminates the implicit local assumption that Alice’s result is independent of Bob’s measurement (and vice versa). Rather, one observer’s result does depend on the other’s measurement, so that, due to a nonlocal influence, the second observer’s measurement is determined by the first observer’s measurement.
“If we try and interpret this experimental scheme using only classical physics, it turns out that it is not possible for the outcomes of all four of the proposed experiments to be consistent,” Vedral explained. “The outcome of experiment four is not consistent with the others. Classical physics assumes that the particle exists independent of our observing (or measuring) it, and also that one measurement cannot influence a particle at a distance.
“For example, what Alice does cannot affect Bob’s particle,” he continued. “Since the outcomes of this scheme are not consistent with classical physics, we must drop one of the assumptions. This means that if we wish to maintain the view that reality exists independent of our measurements (e.g. the moon is there even if we don’t look at it), we are forced to accept that the world is nonlocal. This is how Hardy based his argument for nonlocality on the contradictory outcomes.”
However, Greenberger, Horne, and Zeilinger took issue with Hardy’s argument, pointing out that combining a photon and a vacuum does not result in an observable state, and therefore could not be performed in a real experiment. They even attempted a scheme that didn’t use these so-called “partlycle” superpositions, but found that the entire system then demonstrated nonlocality, making it impossible to attribute nonlocality to a single particle.
Dunningham and Vedral’s proposal makes a few key changes to Hardy’s scheme. First, instead of using coherent states of a photon and vacuum, they use mixed states—a mixture of coherent states averaged over all phases of the particles. In this way, they don’t violate superselection rules and so avoid objections that have been raised before.
Then, for the homodyne detections, they ensure that the coherent light beam combining with the original beam has the same phase. Having the same phase is key, as it ensures that Alice and Bob can consistently compare their measurement results. The coherent states are only classically correlated with the single particle state. This means that, when Alice and Bob perform their homodyne detections, and one detection influences the other, the nonlocality must stem from the original single-particle state.
Because the main importance is maintaining a common average phase—but not a specific phase—Dunningham and Vedral’s scheme could, in principle, be carried out in the laboratory. Also, the researchers suggest that, by using beam splitters for atoms and atom detectors, their scheme could conceivably verify the nonlocality of a single massive particle, in addition to a massless photon.
“An important feature of this work is that it shows how this experiment could be carried out without violating the number conservation superselection rule,” Dunningham said. “This is important because people are often happy to accept such violations for massless particles (e.g. photons) but not for massive particles such as atoms. By avoiding this violation altogether, we show that the outcomes of this proposed experiment should be the same for both massive and massless particles.”
The scientists note an interesting comparison of their result to a principle of Leibniz’s metaphysics, the identity of indiscernibles. According to the principle, a pair of entangled quantum particles must be indiscernible from a single particle, since both objects have in common all the same properties—this is the only stipulation of the principle, number being irrelevant. The single-state nonlocality demonstrated here reinforces the equivalence of a single state and an entangled state—giving more credence to the position that quantum field theory, where fields are fundamental and particles secondary, is a close representation of reality.
The world’s smallest double slit experiment
The big world of classical physics mostly seems sensible: waves are waves and particles are particles, and the moon rises whether anyone watches or not. The tiny quantum world is different: particles are waves (and vice versa), and quantum systems remain in a state of multiple possibilities until they are measured — which amounts to an intrusion by an observer from the big world — and forced to choose: the exact position or momentum of an electron, say.
On what scale do the quantum world and the classical world begin to cross into each other? How big does an “observer” have to be? It’s a long-argued question of fundamental scientific interest and practical importance as well, with significant implications for attempts to build solid-state quantum computers.
Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory and their collaborators at the University of Frankfurt, Germany; Kansas State University; and Auburn University have now established that quantum particles start behaving in a classical way on a scale as small as a single hydrogen molecule. They reached this conclusion after performing what they call the world’s simplest — and certainly its smallest — double slit experiment, using as their two “slits” the two proton nuclei of a hydrogen molecule, only 1.4 atomic units apart (a few ten-billionths of a meter). Their results appear in the November 9, 2007 issue of Science.
The double slit experiment
“One of the most powerful ways to explore the quantum world is the double slit experiment,” says Ali Belkacem of Berkeley Lab’s Chemical Sciences Division, one of the research leaders. In its familiar form, the double slit experiment uses a single light source shining through two slits, side by side in an opaque screen; the light that passes through falls on a screen.
If either of the two slits is closed, the light going through the other slit forms a bright bar on the screen, striking the screen like a stream of BBs or Ping-Pong balls or other solid particles. But if both slits are open, the beams overlap to form interference fringes, just as waves in water do, with bright bands where the wavecrests reinforce one another and dark bands where they cancel.
So is light particles or waves? The ambiguous results of early double slit experiments (the first on record was in 1801) were not resolved until well into the 20th century, when it became clear from both experiment and the theory of quantum mechanics that light is both waves and particles — moreover, that particles, including electrons, also have a wave nature.
“It’s the wave nature of electrons that allows them to act in a correlated way in a hydrogen molecule,” says Thorsten Weber of the Chemical Sciences Division, another of the experiment’s leading researchers. “When two particles are part of the same quantum system, their interactions are not restricted to electromagnetism, for example, or gravity. They also possess quantum coherence — they share information about their states nonlocally, even when separated by arbitrary distances.”
Correlation between its two electrons is actually what makes double photoionization possible with a hydrogen molecule. Photoionization means that an energetic photon, in this case an x-ray, knocks an electron out of an atom or molecule, leaving the system with net charge (ionized); in double photoionization a single photon triggers the emission of two electrons.
“The photon hits only one electron, but because they are correlated, because they cohere in the quantum sense, the electron that’s hit flies off in one direction with a certain momentum, and the other electron also flies off at a specific angle to it with a different momentum,” Weber explains.
The experimental set-up used by Belkacem and Weber and their colleagues, being movable, was employed on both beamlines 4.0 and 11.0 of Berkeley Lab’s Advanced Light Source (ALS). In the apparatus a stream of hydrogen gas is sent through an interaction region, where some of the molecules are struck by an x-ray beam from the ALS. When the two negatively charged electrons are knocked out of a molecule, the two positively charged protons (the nuclei of the hydrogen atoms) blow themselves apart by mutual repulsion. An electric field in the experiment’s interaction region separates the positively and negatively charged particles, sending the protons to one detector and the electrons to a detector in the opposite direction.
“It’s what’s called a kinematically complete experiment,” Belkacem says, “one in which every particle is accounted for. We can determine the momentum of all the particles, the initial orientation and distance between the protons, and the momentum of the electrons.”
What the simplest double slit experiment reveals
“At the high photon energies we used for photoionization, most of the time we observed one fast electron and one slow electron,” says Weber. “What we were interested in was the interference patterns.”
Considered as particles, the electrons fly off at an angle to one another that depends on their energy and how they scatter from the two hydrogen nuclei (the “double slit”). Considered as waves, an electron makes an interference pattern that can be seen by calculating the probability that the electron will be found at a given position relative to the orientation of the two nuclei.
The wave nature of the electron means that in a double slit experiment even a single electron is capable of interfering with itself. Double slit experiments with photoionized hydrogen molecules at first showed only the self-interference patterns of the fast electrons, their waves bouncing off both protons, with little action from the slow electrons.
“From these patterns, it might look like the slow electron is not important, that double photoionization is pretty unspectacular,” says Weber. The fast electrons’ energies were 185 to 190 eV (electron volts), while the slow electrons had energies of 5 eV or less. But what happens if the slow electron is given just a bit more energy, say somewhere between 5 and 25 eV? As Weber puts it, “What if we make the slow electron a little more active? What if we turn it into an ‘observer?’”
As long as both electrons are isolated from their surroundings, quantum coherence prevails, as revealed by the fast electron’s wavelike interference pattern. But this interference pattern disappears when the slow electron is made into an observer of the fast one, a stand-in for the larger environment: the quantum system of the fast electron now interacts with the wider world (e.g., its next neighboring particle, the slow electron) and begins to decohere. The system has entered the realm of classical physics.
Not completely, however. And here is what Belkacem calls “the meat of the experiment”: “Even when the interference pattern has disappeared, we can see that coherence is still there, hidden in the entanglement between the two electrons.”
Although one electron has become entangled with its environment, the two electrons are still entangled with each other in a way that allows interference between them to be reconstructed, simply by graphing their correlated momenta from the angles at which the electrons were ejected. Two waveforms appear in the graph, either of which can be projected to show an interference pattern. But the two waveforms are out of phase with each other: viewed simultaneously, interference vanishes.
If the two-electron system is split into its subsytems and one (the “observer”) is thought of as the environment of the other, it becomes evident that classical properties such as loss of coherence can emerge even when only four particles (two electrons, two protons) are involved. Yet because the two electron subsystems are entangled in a tractable way, their quantum coherence can be reconstructed. What Weber calls “the which-way information exchanged between the particles” persists.
Says Belkacem, “For researchers who are trying to build solid-state quantum computers this is both good news and bad news. The bad news is that decoherence and loss of information occur on the very tiny scale of a single hydrogen molecule. The good news is that, theoretically, the information isn’t necessarily lost or at least not completely.”
