Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-11-17
2004-10-05
Bruce, David V. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S207000
Reexamination Certificate
active
06800837
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-328333, filed Nov. 18, 1999; and No. 2000-344273, filed Nov. 10, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a method for quantum information processing using a solid-state element, and more particularly to relates a method for quantum information processing in which operation is optically performed and which can attain high scalability of quantum bits (qubits) and to a quantum information processor.
A new information processing method is proposed for performing information processing in quantum processes in which quantum states of an atom such as a ground state and an excited state are set so as to correspond to “0” and “1” and bits are expressed by using each quantum state |0> or |1> or a superposition state &agr;|0>+&bgr;|1> their of (where &agr; and &bgr; are complex numbers). Quantum computers based on such quantum information processing are proposed and formulated by Bennioff (P. Bennioff, Phys. Rev. Lett., 48, 1581 (1992)), Feynman (R. P. Feynman, Found. Phys., 16, 507 (1986)), and Deutsch (Proc. Roy. Soc. London, Ser. A400, 96 (1985)), and are now popularly studied.
In a conventional computer (a classical computer), a bit carrying information takes a value of “0” or “1”. On the contrary, a bit in the quantum computation can take a value of not only |0> or |1> but also their superposition state &agr;|0>+&bgr;|1>. Such a bit is called a quantum bit (qubit). In the quantum computation, a plurality of (N) qubits is simultaneously dealt with and the whole qubits are subjected to unitary transformation called a gate operation to perform computation. Since the N qubits simultaneously express 2
N
numbers, it becomes possible to make 2
N
parallel computations. Therefore, it is possible to make extremely rapid computations for a certain problem.
Thus, the quantum computer has a potential capacity exceeding that of the classical computer in quality and is expected as the future information processing technology and computing technology. However, it has been considered that it is extremely difficult to realize the quantum computer. This is because it is difficult in practice to retain the superposition quantum states during computations and prevent a change other than the intended change of states by the gate operation from occurring. Further, in the quantum computation, it is necessary to couple the qubits to each other with retaining quantum coherency, but this is also difficult.
However, so far, some physical systems which make it possible to realize the quantum computation are proposed, and recently, some experiments are demonstrated.
One example is a method using ion trap that is theoretically proposed by Cirac and Zoller (J. I. Cirac and P. Zoller, Phys. Rev. Lett., 74, 4091 (1995)). In this method, individual ions are separated from one another by a distance of the order of micrometer or more and held in an electromagnetic trap at extremely low temperatures, and electron excited levels and a collective vibrational level of the ions are used. The collective vibrational level is a vibrational excited state related to the center-of-mass motion of all of the ions and serves to couple individual ions, i.e., qubits. An independent ion in the trap is hard to receive unnecessary interaction from the external world, and can retain the superposition state for a long period of time, which is a major premise for the quantum computation. However, it is necessary to use a large-scale apparatus for the ion trap at extremely low temperatures and thus it is difficult to reduce the size of the element. Further, the qubit is distinguished based on the position of the ion and a spatially converged laser beam is irradiated to aim at the specified ion. Thus, since the processing operation is effected with the individual qubits distinguished from one another by selectively applying the laser beam to the specified ion, it is necessary to separate the ions by a distance of at least approximately the wavelength of light, and therefore the integration of the elements and the scalability of the qubits are restricted.
Proposal of an NMR quantum computer using a nuclear spin of an atom in a molecule as a qubit is known as another physical system which can be experimented (N. A. Gershenfeld, I. Chuang, Science, 275, 350 (1997)). In this method, a magnetic field is applied to molecules in a solution, thereby allowing energy levels of the nuclear spin to cause Zeeman splitting. Then, the computation is executed by operating the quantum state of the nuclear spin, i.e., the qubit by affecting a high-frequency electromagnetic field resonant with the split energy level. The degree of the Zeeman splitting is different depending on the types of atoms and also different depending on the position of the atom in the molecule even if the atoms are of the same type. Therefore, it becomes possible to select a nuclear spin resonant with the frequency of the high-frequency electromagnetic field and to operate a single qubit. In the NMR quantum computer, the computation up to three bits is demonstrated. However, in this method, since each molecule acts as one computer, there occurs a problem that the number of qubits cannot be freely increased.
The above two examples are most advanced researches at present in which experiments for a quantum gate operation and execution of a simple computation algorithm are performed. However, as described above, for practical computation, a problem occurs in the scalability of the qubits. Further, in the above examples, a single ion in a trap or a nuclear spin of a molecule in a solution is used as a qubit. However, it is desired to make quantum computation by use of solid-state qubits that can be easily dealt with and have an advantage in reduction in size and integration.
As a study for realizing the quantum computation using a solid-state element, an experiment of a qubit using a Josephson junction is known (Y. Nakamura, Yu. A. Paskin and J. S. Tsai, Nature, 398, 786 (1999)). Nakamura et al. have succeeded in creating a superposition state of two states different in the number of electrons by use of microelectrodes in superconductive states. However, in this case, an advanced fine fabricating process is required for formation of qubits and coupling between a plurality of qubits. Further, an effective method for coupling coherently a large number of qubits is not known.
In addition, it is proposed a method for executing a quantum computation in which a metal atom or a molecule is held in fullerene and the electron states of a &pgr; electron of the fullerene are utilized as qubits (Fukumi et al., Jpn. Pat. Appln. KOKAI Publication No. 10-254569). In this method, the phenomenon is utilized that light frequencies for exciting the &pgr; electron of respective fullerene molecules are different depending on the number of carbon atoms in the fullerene or the type of the metal atom or the molecule, and fullerene used as a qubit is selected according to the wavelength of irradiated light to perform a processing operation. In this method, the qubits are coupled by bonding the fullerene molecules with an organic cross-linking molecule. In other words, an artificial “molecule”, in which the fullerene serves as an atom and the organic cross-linking molecule serves as the interatomic bond, is used instead of the molecule in the NMR computer. However, since a highly fine fabricating technology or synthesis process is required for coupling qubits in this method, it is considered difficult to attain scalability to a large number of qubits. Further, since two levels of the ground state and the excited state of the &pgr; electron coupled through an allowed transition are utilized for a qubit, decoherence by relaxation is easily caused, and therefore difficulty is expecte
Ichimura Kouichi
Tanamoto Tetsufumi
Bruce David V.
Kao Chih-Cheng Glen
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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