Stimulating a charged atomic particle (i.e. an ion) to take a quantum walk
Qubit: Just like a bit is the basic unit of computer information a quantum bit or qubit is a basic unit of quantum information i.e. a qubit is a quantum analogue of the classical bit. A classical computer has a memory made up of bits, whereas a quantum computer maintains a sequence of qubits. Each bit in a classical computer represents either a one or a zero where as a single qubit can represent a one, a zero, or any quantum superposition of these. For example any two-level quantum system can form a qubit, and there are two ways to form a qubit using the electronic states of an ion: 1) Two ground state hyperfine levels (these are called “hyperfine qubits”) 2) A ground state level and an excited level (these are called the “optical qubits”).
For more information on qubits please refer to the post: Programmable quantum information processor using two qubits of information.
Ion traps: Ions, or charged atomic particles, can be confined and suspended in free space using electromagnetic fields. Qubits are stored in stable electronic states of each ion, and quantum information can be processed and transferred through the collective quantized motion of the ions in the trap (interacting through the Coulomb force). An ion trap is based on the idea that since a charged particle cannot be confined in three dimensional space using static electric fields, an electric field oscillating at radio frequency (RF) is applied instead, forming a potential with the shape of a saddle spinning at the RF frequency. The ions, interacting with this oscillating potential over time, end up trapped in the middle of the saddle potential.
Source: http://www.quantiki.org/wiki/index.php/Ion_traps
Stimulating a charged atomic particle (i.e. an ion) to take a quantum walk
A team of physicists from the Institute of Quantum Optics and Quantum Information have been able to stimulate a charged atomic particle (i.e. an ion), which is a two-level quantum system, to perform a quantum walk. The ion was trapped in an electromagnetic ion trap.
In order to make one understand what a quantum walk is, the physicists use the analogy of a random walk.
Random walk: When a hiker comes to a junction s/he has to decide which way to take. All of these decisions, eventually, lead the hiker to the intended destination. When the hiker forgot the map, s/he has to make a decision randomly and gets to the destination with more or less detours. In science this is called a random walk. Examples of random walk are random motion of water molecules – a phenomenon known as Brownian motion and the Galton board where in balls are dropped from the top and they repeatedly bounce either left or right in a random way as they hit pins stuck in the board.
The Innsbruck scientists have now transferred this principle of random walk to quantum systems and stimulated an ion to take a quantum walk. Christian Roos from the Institute of Quantum Optics and Quantum Information (IQOQI) says, “We trap a single atom in an electromagnetic ion trap and cool it to prepare it in the ground state. We then create a quantum mechanical superposition of two inner states and send the atom on a walk.”
Roos explains, “Depending on the internal state, we shift the ion to the right or to the left. Thereby, the motional and internal state of the ion are entangled.” After each step the experimental physicists modify the superposition of the inner states by a laser pulse and again shift the ion to the left or right. The physicists can repeat this randomly controlled process up to 23 times, while collecting data about how quantum walks work. By using a second ion, the scientists extend the experiment, giving the walking ion the additional possibility to stay instead of moving to the right or left.
The statistic analysis of these numerous steps confirms that quantum walks differ from classical (random) walks. While, for example, the balls of a Galton board move away from the starting point statistically very slowly, quantum particles spread much faster on their walk.
These experiments, which have also been realized in a similar way in Bonn, Munich and Erlangen with atoms, ions and photons, can be applied to studying natural phenomena. For example, researchers suspect that the energy transport in plants works more efficiently because of quantum walks than would be the case with classical walks. In addition, a regime of quantum walk is of importance for developing a quantum computer model, which could solve ubiquitous problems. For example, applying quantum walks in such a model would help in finding ‘search quantum algorithms’ that outperform their classical counterparts as different directions could be chosen simultaneously.
The team of physicists was headed by Christian Roos and Rainer Blatt.
Source: http://iqoqi.at/news&newsid=126
March 11, 2010