Portland, Ore. - In the search for a physical system that could encode quantum states and thus form the basis for a practical quantum computer, researchers at the University of Michigan and the University of Rochester are turning to photonics.
Photons, like electrons, are quantum particles and can be manipulated with optical devices. By making use of semiconductor structures such as acousto-optic modulators or quantum wells, photons can modify the quantum states of electrons.
In a recent experiment at the University of Michigan, researchers used a magnetic semiconductor material that confined electrons in a quantum well. Subsequently lasing the well with ultrafast pulses entangled the electrons' spin states. Entanglement is the fundamental basis for quantum computing.
"After studying the results of others who have tried all kinds of different approaches to controlling qubits [quantum bits], we found a method based on semiconductor technology that, when combined with advances in nanotechnology, we think holds great promise for practical implementations," said professor Roberto Merlin, a physicist on the project at the university's Optical Physics Interdisciplinary Laboratory.
Another project, at the University of Rochester's Center for Quantum Information, is using methods based on nonlinear optical waveguides to investigate both quantum entanglement between photons and more conventional physics based on photon interference. The work, led by Ian Walmsley, a physicist specializing in ultrafast phenomena, has seen some success on both fronts.
Though not a pure quantum-state operation, photon interference has turned out to be useful in decoding quantum states and might serve as a practical I/O method for a quantum pro-cessor, the Rochester team reports. In addition, the optical interference techniques developed at the lab could be applied to quantum communications over optical fibers, an area that has recently spawned an actual prototype of a secure communications system based on quantum principles.
The Rochester team has developed a new type of high-brightness optical source that achieves tight control of a photon's wavefunction inside of an optical waveguide. The physical technique is to use phase matching to control two-photon interactions. Confining the photons in the waveguide cavity has allowed the researchers to first entangle and then disentangle photon states.
While these experiments have been successful in generating two pairs of entangled photons, the problem facing the researchers is how to generate a large number of pairs in order to achieve some practical information-encoding ability. The probability of generating stable pairs decreases exponentially with the number of pairs.
Researchers worldwide are searching for semiconductors that can house quantum states due to the computational boost that quantum information processing could achieve. Today, experimental single-electron transistors can represent only a digital "1" or "0," depending upon whether the charge is present or absent. However, quantum states encode bits in what is known as a "superposition of states," which means that a single electron or photon can represent both logical values simultaneously.
A quantum parameter such as an electron's spin state can be used as the representation of a qubit. As long as the spin of an electron is undisturbed, the qubit represents both a 1 and a 0 simultaneously. When the spin of one electron interacts with another, the result can perform parallel computations on all the values encoded into their wavefunction.
Unfortunately, the very thing that makes quantum systems useful-their ability to superpose values-makes them even more prone to errors than classical systems. The nebulous state of qubits can be destroyed by a wide variety of factors, all of which boil down to an inadvertent coupling to the environment, resulting in decoherence of the superposed values.
To solve this problem, quantum error-correction methods were proposed as early as 1995 and first demonstrated in 1998. Since then, many groups have refined quantum error-correction encoding techniques, which basically replicate a nebulous qubit's value onto separate physical systems that are "entangled"-that is, their nebulous values are synchronized over time despite different physical locations.
Entanglement enables observers to subsequently "compare" the resultant qubits after a calculation, without "observing" their nebulous values, to see if any differences arose between the copies. Such differences indicate an error, which usually resets the system to try that calculation over again. Entanglement also aids in cryptography by being able to detect eavesdropping.
In the University of Michigan work, Merlin's group achieved entanglement of three noninteracting electrons, by virtue of a 5-watt, 532-nanometer laser producing 130-femtosecond pulses at 82 MHz, focused down to a dot with a diameter of 400 microns. Each laser pulse supplied the energy to create what physicists call an exciton-a bound electron-hole pair-with a diameter of about 5 nm in a cadmium-tellurium quantum well. Electrons within that radius from donor manganese impurities in the quantum well became entangled. In the experiment, three such noninteracting electrons were entangled.
"The source of our qubits is electrons bound to donors-here, manganese impurities in a cadmium-tellurium quantum well," said Merlin. "In principle we could entangle thousands of electrons, making our method very scalable."
The formation of excitons from an electron-hole pair is a coulomb interaction, here resulting from the optical energy added by the laser to confined paramagnetic manganese impurities in the presence of a magnetic field. The distance between the electron and hole within the exciton is called the Bohr radius-in this case, it's 5 nm.
Excitons typically move freely within a bulk semiconductor, but when the exciton is trapped in a well, thin wire or quantum dot with dimensions of the same order as the exciton, a confinement effect occurs. A quantum well confines the exciton in only one dimension, leaving it free in the other two, while a quantum wire confines it in two dimensions, leaving it only one dimension in which to move. A quantum dot confines the exciton in all three dimensions.
"We have shown that electrons can be optically excited to generate many-spin Raman coherences in nonoverlapping ex-citons," Merlin said. "Our procedure is potentially set-specific and scalable for quantumcomputing applications."
In the experiment, the manganese electrons within the radius of the exciton became entangled after three laser bursts. With repeated laser bursts, Merlin proposes to entangle an arbitrary number of electrons using his semiconductor-based method. The entanglement was attributed to resonant transitions between Zeeman split spin states-which can be sensed by detecting a harmonic of the fundamental Zeeman frequency that corresponds to the number of entangled electrons. In the experiment, three electrons were entangled, shown by detecting the third harmonic of the Zeeman frequency.
"Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle an arbitrarily large number of spins," said Merlin.
Next Merlin intends to use a masking method to make it possible to aim the laser beam at specific regions of the semiconductor, so that the semiconductor device can be addressed randomly. "Reading and writing we have demonstrated here, but only for an ensemble of electrons. Right now it's 'almost' like having a quantum computer, except that we are turning on and off all the bits at the same time. Next we want to use masking to selectively address individual qubits," said Merlin.
Also on Merlin's drawing board is a more refined laser pulse that in addition to forming arbitrary excitons also assists in performing specific quantum calculations. "We want to use pulse shaping to put a little bump here or a spike there," he said. "We think that by shaping the pulse we can control the entire wavefunction of the electron, which you will need to do to perform quantum computations."
Merlin's research was funded by ACS Petroleum Research Fund, the National Science Foundation and the Air Force Office of Scientific Research. The lab is part of Michigan's Frontiers in Optical Coherent and Ultrafast Science Center.
- Chappell Brown contributed to this report