Cryptography – Key management – Key distribution
Reexamination Certificate
1998-05-20
2001-11-06
Hayes, Gail (Department: 2131)
Cryptography
Key management
Key distribution
C380S059000, C713S150000
Reexamination Certificate
active
06314189
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for a quantum communication, and more particularly to a method and an apparatus adapted for use in transmitting a quantum mechanical state or qubit from a sender to a receiver.
2. Description of the Prior Art
Present information processing concerns transmissions conveying classical Boolean states 0 and 1 (bits), in which security of the communication becomes of greatest importance with the development of the network of communication. Therefore, cryptography schemes with a security against undetected eavesdroppers have been proposed.
In contrast to this, recent quantum information processing involves quantum states (qubits; practically, the polarized states of a photon). Quantum information theory concerns the transmission of quantum states from a sender to a receiver. The classical Boolean states 0 and 1 can be represented (encoding) by a pair of different states of a qubit. The qubit can exist in an arbitrary complex linear combination (superposition) of classical Boolean states. In quantum information processing, these arbitrary unknown states can be transmitted by quantum teleportation, which guarantees near-perfect security of the communication.
The original scheme of the teleportation was proposed by C. H. Bennett et al. (Physical Review Letters, vol.70 (1993),1895). According to Bennett, in this scheme, the sender and receiver must prearrange the sharing of an EPR-correlated pair of particles. Sender (Alice) makes a joint measurement on her EPR particle and the unknown quantum system (encoded qubit), and sends to Receiver (Bob) the classical results of four measurement outcomes with equal probabilities via a classical message line. Knowing this, Bob can convert the state of his EPR particle into an exact replica of the unknown state which Alice destroyed. Bob can get the transmitted information through the decoding of this replica.
This is a process that disembodies the exact quantum state of a particle [
1
] into classical data and Einstein-Podolsky-Rosen (EPR) correlations (an entangled state of particles [
2
] and [
3
]), and uses these ingredients to reincarnate the state in another particle [
3
] which has never been anywhere near the first particle. This disembodiment of the exact quantum state of the particle [
1
] evades eavesdropping. A quantum teleportation scheme of the type proposed by Bennett et al. is of great interest for quantum communication free from any eavesdropping.
From the measurement theoretical point of view, however, their intact scheme of teleportation would be difficult for its realization, since this original teleportation scheme of an unknown state is based on the existence of non-local long range correlations between the EPR pair of particles relevant to the notion of wave function collapse which is nothing but the almost direct paraphrase of the von Neumann projection postulate. This projection postulate necessarily leads to the EPR paradox. As a result of this paradox, the epistemological discussion on the nature of physical reality cannot be avoided. Such philosophical arguments concerning the conceptual aspects of the EPR problem are beyond the realm of physics. Furthermore, if one follows the von Neumann projection postulate, it is difficult to understand the negative-results measurement and the results of some of the neutron interferometry experiments. A rational theory of measurement and a correct understanding of the EPR problem are necessary to realize quantum teleportation.
It is, therefore, an object of the present invention to provide a new method and apparatus that can realize quantum communication free from any eavesdropping using teleportation without resorting to the von Neumann projection postulate.
SUMMARY OF THE INVENTION
In accordance with the present invention, this object can be attained by a new method for a quantum communication adapted for use in transmitting a quantum state or qubit from a sender to a receiver, comprising the steps of preparing a first particle having a quantum state corresponding to encoded information to be sent, and mutually correlated second and third particles having a quantum-mechanical entanglement, the first and second particles being held by the sender, while the third particle by the receiver; generating a mixture with a predetermined mixing ratio consisting of products of a quantum state derived from the first particle and of its orthogonal state by a quantum state derived from a composite system of the second and third particles, the mixture being shared by the sender and receiver; transmitting outcomes of measurement made by the sender for a composite system of the first and second particles to the receiver via classical communication; and measuring the third particle on the receiver's side to decode the transmitted information encoded on the first particle whose quantum state is recovered on the third particle based on the measurement outcomes transmitted by the sender.
The above-mentioned object can also be preferably attained by a method for a quantum communication adapted for use in transmitting a quantum state or qubit from a sender to a receiver, comprising the steps of generating photons successively; generating from the successively generated photons a first photon with its polarization modified according to encoded information to be sent; generating from the successively generated photons second and third photons having polarization quantum-mechanically correlated; measuring the polarization of the coincidentally appearing first and second photons successively on the sender's side to transmit measurement outcomes therefrom to the receiver via classical communication; and successively measuring on the receiver's side the polarization of the third photon appearing in quantum-mechanically correlated relationship with the second photon, the polarization of the first photon being recovered on the third photon based on the measurement outcomes transmitted by the sender.
The above object can also be attained by a new apparatus for a quantum communication adapted for use in transmitting a quantum state or qubit from a sender to receiver, comprising a light source for generating photons successively; means for successively producing from the photons a first photon with its polarization modified according to encoded information to be sent to produce a qubit having a superposition of horizontally and vertically polarized states with predetermined coefficients; means for successively producing from the photons second and third photons having horizontally or vertically polarized states which are mutually quantum-mechanically correlated; means for successively measuring on the sender's side the polarized states of the coincidentally appearing first and second photons; means for transmitting outcomes of measurement from the sender to receiver via classical communication; and means for measuring on the receiver's side the polarized state of the third photon appearing in quantum-mechanically correlated relationship with the second photon, the superposition of the horizontally and vertically polarized states of the first photon being recovered on the third photon based on the measurement outcomes transmitted by the sender.
In the present invention, the well-known components of spin are measured by following the law of conservation of spin angular momentum to realize quantum teleportation through a mixture as an ancilla. This allows the receiver to recover the sender's chosen qubit on his own particle by teleportation. For practical quantum teleportation, photons we are preferably used instead of spin. In this case, the sender performs coincidence counting on the polarized states of the first and second photons successively and sends its results to the receiver via a classical message. The receiver takes the correlations between the measurement outcomes of the polarized states on the third photon and the
Matsuoka Masahiro
Motoyoshi Akio
Ogura Tetsuya
Yamaguchi Koichi
Yoneda Tetsuya
Adams & Wilks
Hayes Gail
Leaning Jeffrey S.
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