May 07, 2010

Ultrafast Fourier Transform with a Femtosecond-Laser-Driven Molecule

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Physical Review Letters - Ultrafast Fourier Transform with a Femtosecond-Laser-Driven Molecule 4 page pdf)

Vibrations of the atoms in a molecule are used to implement a Fourier transform orders of magnitude faster than possible with devices based on conventional electronics.

Abstract :
Wave functions of electrically neutral systems can be used as information carriers to replace real charges in the present Si-based circuit, whose further integration will result in a possible disaster where current leakage is unavoidable with insulators thinned to atomic levels. We have experimentally demonstrated a new logic gate based on the temporal evolution of a wave function. An optically tailored vibrational wave packet in the iodine molecule implements four- and eight-element discrete Fourier transform with arbitrary real and imaginary inputs. The evolution time is 145 fs, which is shorter than the typical clock period of the current fastest Si-based computers by 3 orders of magnitudes.

This ultrafast phase evolution can be applied to other logic gates. We have already proposed the controlled NOT (CNOT) gate with the evolution time to be 12 Tvib for the same sets of vibrational eigenstates. In one of few pioneering experiments regarding implementation of a MEIP (molecular eigenstates information processing) logic gate, Amitay and co-workers assigned the ‘‘yes’’ output of a classical AND gate to a specific temporal evolution of a WP, employing their local definition and one particular input available only within that experiment. In our present study, however, we have demonstrated a conceptually new MEIP logic gate, where we have utilized the temporal evolutions of a WP as gate operations according to their universal definitions with any arbitrary input.

Another promising approach to logic gates in MEIP has been proposed, based on population transfer with shaped intense laser pulses in the nonperturbative regime. This includes four-element DFT with a 6-ps shaped pulse proposed theoretically. Compared with this strong field approach, our free propagation approach may be less universal, but has better energy efficiency and fidelity. These two approaches could complement each other.

In the present demonstration of MEIP, read and write operations have been executed with an ensemble of jetcooled molecules. Addressing of each molecule will be necessary for the future scalability. Rapid progress in the preparation of cold molecules in magnetic traps, in optical dipole traps, or in quantum solids will hopefully be useful for this single-molecule addressing to be combined with our ultrafast MEIP approach.

Viewpoint letter on the laser driven molecular calculations

The authors point out the important feature that the molecular motion executes the Fourier transform in a mere 145 fs. This is several orders of magnitude faster than devices based on silicon electronics are likely to be able to achieve. This observation provokes an enticing proposition—the idea of high-speed, nondissipative logic operations and algorithms would make for a revolution in physical instantiations of computational devices.

However, there are a number of important barriers that will need to be overcome if such devices are to displace current high-speed electronics. First, the programming and readout of the device necessarily require significant resources, both in actual implementation and in principle. This overhead would need to be reduced dramatically for such an approach to be feasible in practice. Second, the connectivity required for implementing large-scale processors is missing. It is not yet possible to string together a sequence of these Fourier transformers with other operations in between in a way that enables a real programmable device to be constructed. Third, this approach does not embrace the favorable scaling of the device that is the hallmark of quantum information processors. This is because the protocol makes use of single-particle interference—a quantum effect to be sure, but one which has analogs in classical wave physics. The use of a molecular valence electron to control and to readout the vibrational state means that the approach lacks the multiparticle entanglement needed to realize exponential improvements on current computational capacity. Recognizing the key element involved as single-particle quantum interference raises the question as to whether a process based on wave interference might well be simulated entirely optically in an equally efficient manner.

Nonetheless, the notion of a classical processor with such a dramatic speed-up suggests that it is worth continuing to explore new ways to use physical systems to encode and manipulate information, and that this connection may reveal new insights into both physics and into information processing.

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