Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2001-07-31
2002-09-03
Chaudhuri, Olik (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
Reexamination Certificate
active
06444999
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to quantum communication and a quantum computer that employs light, and more particularly to a quantum circuit that effects measurements of quantum states.
The explosive increase in widespread use of the Internet and the practicability of electronic commerce transactions have increased the social needs for encryption technology, which includes maintenance of confidentiality of communications, prevention of forgery, and user authentication. At present, common key methods such as DES (Data Encryption Standard) codes and public key methods including RAS (Rivest, Shamir, Adleman Public Key Encryption) codes are in wide use. However, these methods are based on “safety through computational load,” and current encryption methods are therefore always threatened by progress in computer hardware and code decryption algorithms.
The laws of physics, in contrast, guarantee the safety of “physical codes” such as quantum codes, and these physical codes therefore can guarantee ultimate safety that does not depend on limitations of the capabilities of computers. Putting such an encryption method into practical use would have an extremely powerful social impact, and such encryption methods are expected to become one of the technological foundations of the information industry in the future.
The transmission of quantum information, of which quantum code is representative, is limited to a distance of several tens of kilometers due to loss and disturbance of the quanta (photons) that are used in transmission. In addition, intercommunication is limited to the two ends of the optical fiber that makes the transmission path, and quantum communication with multiple partners therefore requires the establishment of a large number of optical fibers. To solve these problems and realize quantum networking that can implement quantum information communication over a wide range requires technology such as relays and exchanges. Of course, relays and exchanges can be realized by converting quantum information to classical information at a relay station, but such a solution would interrupt the advantageous properties inherent in quantum information communication. In a case in which quantum encryption keys are distributed, for example, a wire-tapper could obtain access to all information by breaking into a relay station, and the safety against wiretapping that is the advantage of quantum information would be lost.
For these reasons, there is a need for quantum relays and quantum exchanges that relay and exchange quantum information as is. Quantum relays and quantum exchanges are also vital for distributed quantum computers. The utilization of quantum teleportation by a quantum relay can realize various effects as described hereinbelow. Quantum information is carried by entangled photon pairs that imply quantum correlation and by classical information separately. When entanglement swapping is used, transmission can be implemented by swapping entangled photon pairs at successive repeaters. The sender provides the swap information to the receiver as classical information.
By means of this entanglement swapping method, the receiver and sender can share entangled photon pairs even when separated by great distances.
Quantum teleportation and entanglement swapping are predicated on the generation of entangled photon pairs and the measurement of Bell states. In Bell-state measurement, two photons are discriminated to be in one of four entangled states, referred to as Bell states. Although the principles of quantum teleportation and entanglement swapping have been confirmed through experimentation, it is not possible with the currently available technology to discriminate all of four Bell states, and transmission by any desired quantum state is therefore yet to be realized.
In Science, 282, 706 (1998), Furusawa, A. et al. reported on quantum teleportation that does not require measurement of Bell states. Although this method enables transmission in any quantum state, it entails 100% squeezing of light. However, due to the insufficient squeezing of the light source that is currently obtainable, the obtainable fidelity of the transmitted quantum states is no higher than 58%. Nevertheless, a fidelity of nearly 100% can be expected if Bell-state measurement can be realized.
For the measurement of Bell states, methods are normally adopted that employ detection circuits composed of photon detectors and linear-optical elements such as semitransparent mirrors and polarization beam splitters. However, such methods can discriminate only two of the four Bell states.
The Bell state of two photons can be represented in the framework of concept of the linear-optics by the following equations:
&PHgr;(±)=(|
x>|x>±|y>|y
>)/2
½
(1)
&PSgr;(±)=(|
x>|y>±|y>|x>)/
2
½
(2)
where |x> and |y> are the state functions for photons polarized in the directions of the x-axis and the y-axis, respectively.
As can be understood from equations (1) and (2), a Bell state is a superposed state of a two-photon state specified by a set of two directions of polarization and the state specified by the exchanged directions of polarization of two photons. &PHgr;(±) are states in which the two photons polarize in the same direction, the state being symmetrical (+) or antisymmetrical (−) with respect to exchange of the directions of polarization; and &PSgr;(±) are states in which the two photons polarize in orthogonal directions, the state being symmetrical (+) or antisymmetrical (−) with respect to the exchange of the direction of polarization.
FIG. 1
shows the configuration of an example of a Bell-state measurement circuit of the prior art that employs linear-optical elements.
The Bell-state measurement circuit is constituted by linear-optical elements including semitransparent mirror
51
and polarization beam splitters
52
and
53
, and photon detectors
54
,
55
,
56
, and
57
. Polarization beam splitters
52
and
53
split incident polarized light into s-polarized light that oscillates in a direction perpendicular to the plane of incidence, and p-polarized light that oscillates within the plane of incidence. Photon detectors
54
and
55
detect s-polarized light and p-polarized light, respectively, emerging from polarization beam splitter
52
. Photon detectors
56
and
57
detect the s-polarized light and p-polarized light, respectively, emerging from polarization beam splitter
53
.
Polarized light of one of the four Bell states is incident on this Bell-state measurement circuit. The possible Bell states obtained from the response of photon detectors
54
-
57
are shown in Table 1.
As can be seen from Table 1, the Bell-state measurement circuit realized by the linear-optical elements shown in
FIG. 1
can discriminate the &PSgr;(±) states but cannot discriminate one of the &PHgr;(±) states. Although it is known that the use of a quantum gate, referred to as a light-controlled NOT gate, enables all four Bell states to be discriminated, a practically realizable device of this type of quantum gate has still not been known.
TABLE 1
detector that detects a photon
possible Bell state
54 and 55
&PSgr; (+)
54 and 57
&PSgr; (−)
55 and 56
&PSgr; (−)
56 and 57
&PSgr; (+)
any one of 54-57
&PHgr; (±)
Scully M. O. et al. have recently proposed in Physical Review Letters 83, 4433 (1999) a method of measuring Bell states that employs two-photon absorption. In this method, the Bell states are specified in terms of circularly polarized light, the light is passed through three atomic cells to cause two-photon absorption, and the fluorescence emitted from the atomic cell caused by two-photon absorption is observed. The absorption of only one specific Bell state can be caused if the atoms in the cells are prepared in advance by a strong electromagnetic field such
Chaudhuri Olik
NEC Corporation
Vesperman William C.
Young & Thompson
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