Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-09
2004-04-27
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C257S009000, C257S013000, C257S025000
Reexamination Certificate
active
06728281
ABSTRACT:
STATEMENT REGARDING JST SPONSORED RESEARCH OR DEVELOPMENT
This invention was supported in part by grant number 93J006 from Japan Science and Technology Corporation (JST). The Japan Science and Technology Corporation has certain rights in the invention.
FIELD OF THE INVENTION
The present invention relates to the field of generation of quantum-mechanical states of light. In particular, it relates to the development of devices to produce a stream of regulated and directed single pairs of photons.
BACKGROUND ART
Recent progress in the field of quantum optics has enabled scientists to perform experiments that test the fundamental principles of quantum mechanics, which were previously only possible as thought experiments. Furthermore, scientists have come to realize that those fundamental principles can be exploited technologically. For example, there is growing interest in the new fields of quantum cryptography, quantum teleportation and quantum computing. These experiments require that a quantum system be prepared in a well-defined state. Out of the many candidate systems, single photons or pairs of photons have been most widely used.
The recently demonstrated scheme of quantum cryptography involves encoding information on the polarization of a single photon or pairs of polarization-entangled photons. Protection against eavesdropping is provided by the quantum-mechanical fact that measurement of the information will inevitably modify the state of the photon. The single-photon version of quantum cryptography (BB84 protocol) is vulnerable if more than one photon is sent by mistake, and therefore a stream of regulated single photon is needed. The entangled-photon-pair-version (Ekert protocol) does not have this vulnerability, but nevertheless, a compact source of regulated pairs of polarization-entangled photons would make this scheme more attractive as a method of rapid and secure communication.
Other technological applications are possible for a device that can generate regulatediphoton streams. For example, the regulated photon stream will have very stable intensity, with fluctuations well below those of standard light sources. The device could thus potentially see use as a high-precision light standard. It could also be applied in classical low-power optical communications networks. A stream of single photons at regular time intervals provides a rapid stream of bits, which can potentially be used to store information. This represents the lowest possible power consumption in optical communication: one photon per bit.
The principle sources of single photons in use are highly attenuated lasers, or light-emitting diodes (LED's). Optical pulses from the sources are reduced in intensity by absorption or reflection until each pulse contains, on average, less than one photon. Since the deletion of photons during attenuation is a random process, the number of photons in each pulse is also random. Many pulses will contain no photons, and some will contain more than one. This will limit the rate at which measurements can be done, and will also lead to errors in the experiments. It would be preferable to have a regulated source, where the number of photons in each pulse is well known.
Progress toward this goal was achieved by using single atoms, single trapped ions, or single molecules. An atom, or ion, or molecule was excited using a laser beam, and the resulting emitted light was observed. Because there is a certain amount of dead time between emissions of photons, the output photon flux is better regulated than for a laser or LED. In fact, if the exciting laser was pulsed in the right way, it would in principle be possible to obtain exactly one output photon per input pulse. However, these methods of producing regulated photons require complex and delicate experimental setups, and are thus not easily reproduced or used. Another difficulty in these methods is that the direction of photon emission is random. In other words, the photons fly in all directions, and are thus not easily collected and used in a subsequent experiment or system.
Another proposed source that overcomes the problem of random emission direction involves the use of strongly interacting photons in a nonlinear cavity. An optical cavity is used to enclose an atomic medium, which is exposed to a coupling laser beam that allows for strong non-linearity in the absence of loss. A pulsed laser beam is directed towards one end of the cavity. If the pulses have the correct shape, the output pulses from the other end of the cavity will each contain one, and only one, photon. This stream of regulated single photons will be directed in a well-known direction. However, the experiment setup is again quite complex and difficult to operate. It is thus difficult to incorporate into a large experiment or to use in a technological application.
One more proposed source involves pumping a quantum dot with a surface acoustic wave (SAW) (“Photon Trains and Lasing: The Periodically Pumped Quantum Dot” by C. Wiele et al., published in October 1998 in Physical Review A, vol. 58). The quantum dot is a small region of semiconductor material that can contain only one electron and one hole. The SAW is a periodic deformation that travels along the semiconductor surface. The wave can trap electrons and holes and move them along the surface. It may be possible to make the wave such that only one electron and one hole will be transported in each period. If the wave then passes over an appropriate quantum dot, the electron and hole will be trapped by the dot. They will then recombine to produce a photon. The photons will not be emitted in any particular direction. As well, it is not yet evident whether it will actually be possible to create a SAW such that each period contains exactly one electron and one hole. Finally, there will be errors in the output photon stream when the dot fails to trap both carriers.
A single photon turnstile device was realized in a mesoscopic double barrier p-i-n junction (“A Single-Photon Turnstile Device” by J. Kim et al., published in Feb. 11, 1999 in Letters to Nature, vol. 397, and “Turnstile Device for Heralded Single Photons: Coulomb Blockade of Electron and Hole Tunneling in Quantum Confined p-i-n Heterojunctions” by A. Imamoglu, published in Jan. 10, 1994 in Physical Review Letters, vol. 72). Regulated single photons were produced using a combination of simultaneous Coulomb blockade effect for electrons and holes and resonant tunneling in a mesoscopic p-n junction. The structure generally comprises of an intrinsic central quantum well (QW) in the middle of a p-n junction, and n-type and p-type side quantum wells (QWs) isolated from the central QW by tunnel barriers (
FIG. 1
a
). The lateral size of the device is reduced to increase the single-particle charging energy e
2
/2C
i
where C
i
(i=n or p) is the capacitance between the central QW and the side QWs. The device is designed such that the electron and hole tunneling conditions are separated in applied bias voltage, and thus can be controlled independently. The electron resonant tunneling condition into an electron sub-band in the central QW is satisfied at a certain bias voltage V
0
. When an electron tunnels, the Coulomb blockade effect shifts the electron sub-band energy off of resonance, so that the subsequent electron tunneling is inhibited (
FIG. 1
b
). Then the bias is increased to V
0
+&Dgr;V to satisfy the hole resonant tunneling condition. If a single hole tunnels into the hole sub-band of the central QW, the subsequent hole tunneling is inhibited due to the Coulomb blockade effect for holes. By modulating the bias voltage between the electron and the hole resonant tunneling conditions periodically, it is possible to inject a single electron and a single hole into the central QW periodically, if the tunneling time is much shorter than the pulse duration. If the radiative recombination time of an electron-hole pair is also much shorter than the pulse duration, one (and only one) photon is emitted per modulation period.
A GaAs/AlGaAs thre
Benson Oliver
Kim Jungsang
Pelton Matthew
Santori Charles
Ip Paul
Lumen Intellectual Property Services Inc.
Menefee James
The Board of Trustees of the Leland Stanford Junior University
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