How can we overcome the decoherence problem when matter meets surroundings? This is the question that has stopped high temp. superconduction. Now it may be solved. Universal Dynamical Decoupling of a Single Solid-State Spin from a Spin Bath, by G. de Lange in Science today, may have an answer to the problem, reported by ScienceDaily.
By applying a specially designed sequence of high-precision electromagnetic pulses, the scientists were able to protect the arbitrary quantum state of a single spin, and they made the spin evolve as if it was completely decoupled from the outside world. In this way, scientists achieved a 25-fold increase in the lifetime of the quantum spin state at room temperature. This is the first demonstration of a universal dynamical decoupling realized on a single solid state quantum spin.
The researchers developed and implemented a special kind of quantum control over a single quantum magnetic moment (spin) of an atomic-sized impurity in diamond. These impurities, called nitrogen-vacancy (or N-V) centers, have attracted much attention due to their unusual magnetic and optical properties.
DOE/Ames Laboratory "Implementing dynamical decoupling on a single quantum spin in solid state at room temperature has been an appealing but distant goal for quite a while," said Viatcheslav Dobrovitski.
"Uncontrolled interactions of the spins with the environment have been the major hurdle for implementing quantum technologies. Our results demonstrate that this hurdle can be overcome by advanced control of the spin itself," said Ronald Hanson from Kavli Institute of Nanoscience at Delft.
Besides its importance to fundamental understanding of quantum mechanics, the team's achievement opens a way to using the impurity centers in diamond as highly sensitive nanoscale magnetic sensors, and potentially, as qubits for larger-scale quantum information processing.
Earlier have optical techniques been used. An electron spin localized in a quantum dot is the quantum bit. The spin replaces a classical digital bit, which can take on two values, usually labeled 0 and 1. The electron spin can also take on two values. However, since it is a quantum object, it can also take all values in between. Obviously, such a quantum unit can hold much more information than a classical one. That is why, scientists around the world are trying to find an efficient way to control and manipulate the electron spin in a quantum dot in order to enable new quantum devises using magnetic and electric fields. The electron spin precession frequencies in an external magnetic field are different from each other due to small variations of the quantum dot shape and size. In addition, the electron spin precession frequency has a contribution of a random hyperfine field of the nuclear spins in the quantum dot volume. This makes a coherent control and manipulation of electron spins in an ensemble of quantum dots 'impossible'.
In a Science publication (Science, vol. 313, 341 (2006)),was demonstrated a method, whereby a tailored periodic illumination with a pulsed laser can drive a large fraction of electron spins (up to 30%) in an ensemble of quantum dots into a synchronized motion. almost the whole ensemble of electron spins (90%) precesses coherently under periodic resonant excitation. It turns out that the nuclear contribution to the electron spin precession acts constructively by focusing the electron spin precession in different quantum dots to a few precession modes controlled by the laser excitation protocol, instead of acting as a random perturbation of electron spins, as it was thought previously.
In optical entanglement experiments, a pair of entangled photons may be separated via a beam splitter. Despite their physical separation, the entangled photons continue to act as a single quantum object. They uses electrons in a superconductor in place of photons in an optical system. The electrons they conduct entangle to form what are known as Cooper pairs. In the new experiment, Cooper pairs flow through a superconducting bridge until they reach a carbon nanotube that acts as the electronic equivalent of a beam splitter. Occasionally, the electrons part ways and are directed to separate quantum dots - but remain entangled.
Quantum networks use entangled qubits. Solid-state quantum bits, or "qubits," can communicate with one another over long distances. This would require the nodes that process and store quantum data in qubits to be connected to one another by entanglement, also at long distance.
"Demonstration of quantum entanglement between a solid-state material and photons is an important advance toward linking qubits together into a quantum network."
One can engineer and control the interaction between individual photons and matter in a solid-state material, and the photons can be imprinted with the information stored in a qubit. Builds upon earlier work by Lukin's group to use single atom impurities in diamonds as qubits. Lukin and colleagues have previously shown that these impurities can be controlled by focusing laser light on a diamond lattice flaw where nitrogen replaces an atom of carbon. That previous work showed that the so-called spin degrees of freedom of these impurities make excellent quantum memory.
Lukin and his co-authors now say that these impurities are also remarkable because, when excited with a sequence of finely tuned microwave and laser pulses, they can emit photons one at a time, such that photons are entangled with quantum memory. Such a stream of single photons can be used for secure transmission of information.
"Since photons are the fastest carriers of quantum information, and spin memory can robustly store quantum information for relatively long periods of time, entangled spin-photon pairs are ideal for the realization of quantum networks," Lukin says. "Such a network, a quantum analog to the conventional internet, could allow for absolutely secure communication over long distances."
Rådmark, oct -09, and his team proved experimentally that their six photon qubits are robust and should be able to reliably carry information over long distances.
A new method for combining six photons together results in a highly robust qubit capable of transporting quantum information over long distances.
Driving a qubit along a longer quantum path (routes 2 and 3) dramatically improves the signal quality over that achieved by following the shorter path (route 1). The research applies to information stored in qubits that consisted of Nitrogen-based defects in diamond, as schematically shown on the right.
In most arrangements that rely on Nitrogen atoms in diamond to store data, reading the information also resets the qubit, which means there is only one opportunity to measure the state of the qubit. By developing a technique that involves the spin of the Nitrogen nucleus in the process as well, a team of physicists at the University of Stuttgart in Germany has turned the single step read-out into a multi-step process.
Rather than simply resetting the electron-based qubit when the information is read, the researchers discovered that they can force the state of the Nitrogen nucleus to change state twice before the information in the qubit is finally erased.
A quantum network – in which memory devices that store quantum states are interconnected with quantum information processing devices – is a prototype for designing a quantum internet. The atomic-ensemble memory can receive an arbitrary polarization state of an incoming photon, called a polarization qubit, announce successful storage of the qubit, and later regenerate another photon with the same polarization state.
So now maybe quantum biology also has come a step closer? Synergy and coherence are cornerstones. Earlier we saw that DNA can be made to act as 'transistors' for this world web of information. Is that the essence of the quantum antenna?
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