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Leap Toward Quantum Computers
June 17, 2010   
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Researchers at the Physics Department of the University of Warsaw have devised a new way of encoding and retrieving quantum information using manganese atoms. Their experiment promises to be an important step on the road to building quantum computers with enormous processing power.

Today’s computers are becoming increasingly fast thanks in large measure to miniaturization: the number of transistors in a processor more or less doubles every 18 months. However, miniaturization cannot continue indefinitely, experts say. The granular, atomic nature of matter stands squarely in its way.

Quantum bits instead of just bits
Current computers use bits. Each bit can be in one of only two states, usually noted as 0 and 1. A quantum computer will function with quantum bits, or qubits, which besides these two states, can also be in a mixture or superposition of the two.

“In order to store information in a manganese atom, we use its spin, that is the quantum property dealing with a particle’s rotational characteristics,” says Prof. Jan Gaj at the Physics Department of the University of Warsaw. Since spin in an atom of manganese can have as many as six values, this atom can hold more than two bits of information. Similarly to a normal qubit, a manganese atom can find itself in a superposition of its possible states. If this state could be extended to a group of manganese atoms, each subsequent atom would exponentially increase the computational capacity of the quantum computer. As an example, a quantum computer made up of 10 manganese atoms could process over 60 million states in each step, whereas one built out of normal qubits could only handle just over 1,000. Meanwhile, a classical computer would only process one state out of a possible 1,024.

Manganese makes the difference
The Warsaw scientists started off by creating quantum droplets, that is special, self-organizing, semiconducting structures. Made from cadmium telluride they are on the order of a billionth of a meter in size, and surrounded by zinc telluride. Quantum droplets are sometimes called “synthetic atoms,” since the electrons trapped inside them emit light in the same way as those within atoms, that is through photons with strictly defined energies.

These quantum droplets were created on a semiconducting plate by Piotr Wojnar, Ph.D., from a research team led by Prof. Jacek Kossut at the Physics Institute of the Polish Academy of Sciences. Wojnar directed tiny bundles of manganese atoms at the droplets, which were growing in a vacuum, in such a manner so as to deposit a single manganese atom on the greatest possible number of droplets. The plate prepared in this manner was sent off to the Physics Department at the University of Warsaw, where scientists placed it in an optical measuring system, thanks to which it was possible to find the droplets containing single manganese atoms within a few hours.

A single semiconductor plate contains lots of quantum droplets. Each droplet is composed of thousands of atoms—in each case placed slightly differently. In consequence, each droplet emits photons with characteristic energies. This effect is extremely significant because it allows the physicists to target one particular droplet and establish contact with it. The photons the droplet emits contain information about the state of the electrons trapped within. If the electrons inside the quantum droplet have reacted with the manganese atom, the emitted light will contain six characteristic peaks corresponding to the six possible spin states of the manganese atom. If one of the peaks dominates, this means that the manganese atom is most often in the corresponding spin state.

Switching the manganese atom to the chosen spin state requires subtler methods, which explains why researchers at the Physics Department look for two droplets that are close together and form a pair. Using laser light, it is possible to “throw” an electron with a specific spin into one droplet, from which it will tunnel into the other droplet containing the manganese atom and start to react with it. By repeatedly doing this, the physicists are able to put the manganese atom into the chosen spin state. The atom then spends approximately one thousandth of a second in this new state.

“A millisecond may not seem much, yet we must remember that during this interval we change the state of the atom several hundred thousand times. This is enough to carry out a whole series of operations,” says Gaj.

Gaining momentum
Further research will be conducted with even greater precision thanks to new equipment from the National Laboratory for Quantum Technologies, Gaj says. This includes new femtosecond impulse lasers, a superconducting magnet capable of creating stronger magnetic fields than those in the tunnels at the Large Hadron Collider in Switzerland, as well as a tuned optical oscillator, thanks to which it is possible to match the frequency of the laser beam to that of the oscillations of electrons within each quantum droplet.

“Thanks to this extremely modern equipment we will be able to continue our research at the highest possible level and run experiments that no one in the world has done before us,” Gaj said.
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