Photon source

Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction

Reexamination Certificate

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C257S084000, C359S285000

Reexamination Certificate

active

06657222

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical source. More specifically, the present invention is concerned with an optical source which can be configured to emit a regular stream of single photons or a regular stream of pulses of n photons (where n is an integer).
BACKGROUND OF THE INVENTION
With the emergent fields of quantum communications, cryptography, teleportation and quantum computation, there is a need for a source which can be controlled to produce single photons or pulses of a fixed number of photons on demand. A single photon source is a particularly secure source because any unauthorised attempt to read information from this source can be detected. Also, a single photon source provides a source of strongly sub-Poissonian light which has a very high signal to noise ratio.
Recently, there have been attempts to make a single photon source based on an electron-turnstile device. This work has been reported by Kim et al, Nature, 397, 500 (1999). This device comprises a quantum dot. An AC voltage is applied to the device to drive an electron into the dot during the first half of the voltage cycle, then a hole is driven into the dot in the second half of the cycle. The hole and the electron will combine and a photon is emitted. The regular spacing of photon detection events arises due to regular pumping of electrons and holes into the quantum dot via an AC voltage. However, this device shows poor electron current quantization and, as a result, the photon “current” is noisy. The device can only operate with maximum frequencies in the MHz range which results in a low photon emission rate. Also, the device is only capable of operation at a temperature of around 50 mK.
Single photon states may also be produced using down-conversion or with a highly attenuated laser. The latter method is currently used in secure quantum communication channels. Here, the laser is set up to produce less than 1 photon per pulse such that only some pulses actually contain a photon, other contain none. However, this method is unreliable as, due to the Poissonian nature of laser light, some pulses contain more than one photon.
It has also been suggested to use a surface acoustic wave (SAW) to pump a dot which can trap a single electron and a hole (Wiele et al. Phys Rev. A, 58, R2680 (1998)). A SAW propagating on a piezo electric material is accompanied by a travelling wave of electrostatic potential. Free charge can interact with the electrostatic potential, and an acoustoelectric current can be created. If the potential is sufficiently strong, it can be used to spatially separate photo-excited electron-hole pairs where the electrons are held in the minima of the electrostatic wave, and the holes are held in the maxima Recombination is therefore suppressed, and the charges are carried along by the electrostatic potential. In the device described in the paper, a SAW is used to spatially separate photo-excited electrons and holes into alternate “wires” of electrons and holes which move with the SAW.
A “stressor” dot is provided which is a quantum dot formed by local potential minimum in a buried quantum well caused by a structure on the surface of the device. The stressor dot attempts to trap an electron and a hole from the moving “wires”. The electron and hole then recombine in the stressor dot. This device suffers from the problem that a dot with the postulated properties necessary for operation has never been fabricated, and it is not clear how it would be fabricated. It is also possible that the electrons and holes may just be swept past the dot and not be trapped.
SUMMARY OF THE INVENTION
The present invention addresses the above problems, and in a first aspect provides a photon source, comprising a first semiconductor region having excess carriers with a first conductivity type, and a second semiconductor region having excess carriers with a second conductivity type opposite to that of the first conductivity type, means for creating a surface acoustic wave (SAW) travelling from the first semiconductor region to the second semiconductor region such that excess carriers from the first semiconductor region are carried by the wave to the second region and quantizing means for quantizing the carrier transport caused by the wave such that the number of carriers introduced in the second semiconductor region can be controlled to the accuracy of a single carrier.
The source can be configured such that an integer number of carriers are introduced into the second semiconductor at regular intervals. For a single photon source, the source is configured such that single carriers one at a time are introduced into the second semiconductor region at regular intervals.
The carriers of the first conductivity type can either be electrons or holes. For the purposes of this explanation, the device will be discussed only with electrons as the first carriers with the first conductivity type. However, it will be appreciated by those skilled in the art that the device could be operated with holes as the carrier with the first conductivity type.
The applicants do not wish to be bound by any specific theory as to the operation of the device. However, it is believed that the surface acoustic wave (SAW) is accompanied by a travelling wave of electrostatic potential which modulates the conduction and valence bands. The first region and the second region of the device form a P-N junction. (There may be an insulator between the p and n regions).
Electrons are carried from the n-type (first) region into the p-type (second) region by the SAW potential. The SAW forces the electrons to be carried in the minima of the electro-static potential. When electrons arrive at the p-type region, they recombine with the holes to produce photons.
The source is provided with quantizing means which quantizes the transport of the carriers carried by the SAW potential such that the number of electrons located in each SAW minimum can be controlled to the accuracy of a single carrier. The quantizing means are located such that they quantize the transport of the carriers before they are introduced into the second region. The quantizing means can control the number of carriers in the SAW minima to be 1, 2 or more if required
For the purposes of this explanation, it will be assumed that a single electron is located in each SAW minimum. However, quantizing means could be controlled such that more than one electron is located in each SAW minimum. Therefore, after the SAW has travelled through the quantizing means, a single electron is introduced into the p-type region per cycle of the SAW field. The electron can then recombine with an excess hole.
In the source of the present invention, a stream of single photons or pulses of n-photons are produced by introducing or injecting carriers (an “injection event”) of one type into an environment where they can combine with carriers of the opposite type to emit a photon (a “photon emission event”). To produce an ideal single photon source, there is a need for a strong correlation between the injection events and photon emission events.
A temporal uncertainty arises in the photon emission events as the carriers may not recombine immediately to emit a photon. In addition, missing events occur as the carriers may non-radiatively recombine which does not result in the emission of a photon. To achieve a strong correlation between injection events and photon emission events, the uncertainty in the time of the photon emission events must be smaller than the time between injection events. Preferably, the uncertainty is at most 50%, more preferably at most 10%, even more preferably at most 1% of the time between injection events. The requirements on uncertainty are application-dependent.
The total recombination time primarily dictates the temporal uncertainty in the photon emission events. As the electrons are introduced into the p-type region with the SAW, the SAW period is, in this case, the time difference between injection events. Generally, the radiative recombination time is greater than or eq

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