Category Archives: Quantum physics and information

Invisibility cloaks theorized to work for quantum effects

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MIT Technology Review: Optical cloaking has recently been demonstrated to work on large scales. Taking invisibility in an entirely different direction, a new paper from Jeng Yi Lee and Ray-Kuang Lee of the National Tsing-Hua University in Taiwan suggests it may be possible to hide objects from quantum mechanical interactions at the nanoscale. They say that the same concepts that allow invisibility to the electromagnetic spectrum can be applied to quantum effects by using Schrödinger’s equation instead of Maxwell’s equations. The goal would be to construct a device that reduces to zero the probability distribution of the presence of a specific quantum effect within the shielded area. However, it would only be effective for specific versions of the equation, so one shield could hide an object from the quantum properties of electrons but nothing else. Although the paper is theoretical, the researchers suggest that current technologies are advanced enough to create real versions of their theorized nanoshells.

China plans 2016 satellite launch to test quantum teleportation

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MIT Technology Review: Last year Chinese and European researchers made significant progress in quantum teleportation through the atmosphere by reaching a distance of 143 km. Quantum teleportation, or entanglement-assisted teleportation, is a process by which a qubit (the basic unit of quantum information) can be exactly transmitted from one location to another, without the qubit being transmitted through the intervening space. Now Jian-Wei Pan of the University of Science and Technology of China in Shanghai and his colleagues have demonstrated a proof-of-concept ability to detect single photons reflected off a satellite. They used a mirrored satellite orbiting at 400 km and a pair of telescopes—one to send millions of bunches of photons and the other to detect the reflected photons—to simulate the ability to teleport and detect individual photons. The team claims to have used a German satellite that deorbited in 2010, so the experiment must have happened several years ago. The timing of the publication may have been delayed so the team could complete plans to launch a satellite in 2016 that will be equipped with quantum teleportation equipment. If the satellite works, it would be the first step in establishing space-based quantum communications technology.

Entanglement shown between photons that never coexist

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Science: The entanglement of two particles (or photons) is a quantum mechanical effect in which measuring one of the particles instantaneously determines the state of the other, regardless of the distance between them. And entanglement can be swapped between pairs of entangled particles by creating two sets of particles and then performing a “projective measurement” of one particle of each pair. The measurement simultaneously entangles and destroys the measured particles, and it entangles the two other particles even if they had previously been measured. Now, Eli Megidish and Hagai Eisenberg of the Hebrew University of Jerusalem and their colleagues have used this swapping technique to entangle two photons that never coexisted. The time-separated effect was predicted by the original quantum theory, but this recent work is the first demonstration of it. The technique could be useful in the development of quantum communications systems.

Quantum computer beats conventional PC in speed test

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New York Times: What defines a quantum computer is not particularly clear, but one of the usual characteristics is that a machine that make use of quantum mechanical effects will be much faster than a traditional computer. D-Wave Systems has developed, marketed, and even sold what they claim is the first commercial quantum computer, which has now outperformed a traditional computer in an algorithm processing test. While working as a consultant for D-Wave, Catherine McGeoch of Amherst University tested the machine’s capabilities. She presented the machine with three optimization problems—ones that require finding the best possible solution, such as the shortest path in a map—and compared the speed of the results with those of a conventional computer. In two of the three tests, the D-Wave computer only slightly outperformed the traditional computer, but in the third, it returned a result 3600 times as quickly.

Quantum key distribution encryption demonstrated at Los Alamos

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Ars Technica: Key distribution is a common system for encrypting messages, whereby two people who wish to communicate share a key that is used for both coding and decoding their transmissions. However, the system is often vulnerable to interception or decryption if the key is not sufficiently complex. Quantum key distribution increases the security by exploiting entanglement to create a tamper-proof key. A team at Los Alamos National Laboratory has now revealed the first known working quantum key distribution system. They built a small end-user piece of hardware that combines a single-photon source and a true random number generator. The random number generator sets the value of a bit encoded on each photon, which is then sent out over a fiber-optic cable. The system uses a single server with photon receivers to measure the bits, and it publicly states which ones it measured. If none of them appear to have been tampered with, the server uses the bits to build a secure key. The key can then be used to safely transmit data. The Los Alamos system currently allows for communication only with the server, but it could be adjusted to allow for communication among different users.

Using metamaterials to transmit magnetic fields

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MIT Technology Review: Metamaterials have already been used to guide and direct electromagnetic waves in unusual ways. Now Carles Navau of the Autonomous University of Barcelona and his colleagues have shown that a static magnetic field can be manipulated in a similar way. Their design consists of a 7-cm-long tube made of a series of concentric rings, which was filled with a ferromagnetic alloy. At one end of the tube they generated a 1.3-mT magnetic field. A crack farther down the tube allowed the magnetic field to escape. When they measured the field escaping, they found it to be 0.8 mT in strength. That was significantly greater than the field strength at that distance from the source without the tube. Navau suggests that the ability to project magnetic fields over longer distances might be useful in quantum computing, where they are needed for manipulating quantum bits.

Diamonds used for qubit entanglement over distance

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Nature: Entangling quantum bits (qubits) at a distance has been done before, but most such demonstrations have used materials or systems that are not easily scalable. Ronald Hanson of Delft University of Technology in the Netherlands and his colleagues have now demonstrated the ability to entangle qubits in diamond crystals 3 m apart. Qubits, which are the basis for quantum computing, allow more than just a single bit of data to be encoded at one time. Entangling qubits over a distance may allow for the development of quantum communication systems with extreme levels of encryption and significantly faster transmission of information. The system demonstrated by Hanson and his colleagues is not very efficient, achieving entanglement only one time in every 10 million attempts (or about once every 10 minutes), and requires extremely low temperatures. However, once entangled, the qubits can be stored in the diamonds at room temperatures.

Quantum computer algorithm counts primes

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New Scientist: A $1 million prize has been offered for a proof of the Riemann hypothesis: that there is a formula that can calculate the number of prime numbers less than a number X, and that the formula will work for all X. Because no one has figured out yet how to prove the hypothesis, mathematicians have been attempting instead to disprove it by counting primes below a given X and comparing the result with the prediction of Riemann’s formula. Currently, they have found that the hypothesis is true up to X = 1024. But that calculation took three months of continuous computing time. To count the number of primes below 1025 would take more than nine months. Two researchers in Spain, José Latorre of the University of Barcelona and Germán Sierra of the Autonomous University of Madrid, have now developed the first quantum computer algorithm that can count primes. Latorre’s algorithm requires an 80-qubit computer, however, which is much bigger than any current quantum computer. Nevertheless, when such computers are built, the quantum prime-counting algorithm will significantly speed up the calculations to evaluate the Riemann hypothesis.

Redundant qubit construction may prove useful in quantum computing

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Ars Technica: Measuring the state of entangled particles is destructive—it severs the entanglement and causes the superposition of the particles to vanish. Hence, the measurement can’t be repeated. However, quantum computing requires the same measurement be taken multiple times in order to determine the probability for any given particle to be in a particular state at a particular time. A team of researchers from the University of Innsbruck in Austria appears to have developed a way to work around the destructiveness of entangled-state measurements. Instead of using electrons, which have just a single spin state, the team used calcium ions, which have a large number of possible spin states. Only four of the calcium ion’s total possible states are used, which the researchers defined as representing logical 1, logical 0, a measurement state, and a hidden state. By entangling three separate ions and tying separate states to each other via lasers, the researchers established a network of redundancy in the system. An initial measurement via laser pulse of one of the ions takes it out of superposition and determines whether it is in the logical-1 or logical-0 state. A second laser pulse is used to redefine the superposition of the logical-0 and hidden states as the superposition of the logical-1 and logical-0 states. That allowed the researchers to repeatedly measure the superposition of the logical-1 and logical-0 states in order to determine the probability of the ion being in a given state upon measurement.

Uncertainty principle demonstrated at macroscopic scale

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Science News: Werner Heisenberg’s uncertainty principle is a central tenet of quantum mechanics. It states that one can’t have precise knowledge of both the position and momentum of a particle: Any method of measuring one of the two values for a particle would change both. Now Thomas Purdy and his colleagues at JILA in Boulder, Colorado, have demonstrated that the principle also holds true at the macroscopic level. The researchers created a drum by stretching a flexible silicon nitride skin across a frame 0.5 mm to a side, placed the drum between a pair of mirrors, and cooled the system to 4 K. They then shot a laser through the drum so that the photons bounced back and forth between the mirrors. The photons transferred momentum to the drum before entering a detector that calculated the drum’s position. The picometer-sized vibrations that resulted in the drum were in strict agreement with Heisenberg. Similar setups, albeit on a larger scale, are being used in an attempt to detect gravitational waves. The work of Purdy’s group will be useful for calibrating those instruments.