BRIGHT ENTANGLED PHOTON SOURCES

Abstract

The generation of entangled photons is provided by two-photon emission by an emission center immersed in an optical microcavity (MC). The MC is designed to reduce or to suppress the emission of single photons at the fundamental emission wavelength (λ.sub.g) of the emitter and increase the emission for the two-photon emission wavelength (2λ.sub.g). A reflector is added only to reflect single photons and will not reflect the biphotons.

Claims

1. A device for increasing biphoton emission from a photon source, comprising: one or more quantum emitters cladded by a wider band gap quantum barrier; and a Distributed Bragg Reflector, configured for high reflection and high suppression of emission at λ.sub.g while maintaining high transmission at 2λ.sub.g.

2. The device according to claim 1, wherein the Distributed Bragg Reflector is composed of alternating layers of low index material and a high index material.

3. The device according to claim 1, wherein the quantum emitters comprise quantum wells, and quantum barrier and the wells are made of Hg.sub.0.6Cd.sub.0.4Te and HgTe respectively.

4. The device according to claim 1, wherein the quantum emitters comprise quantum wells, wherein the barrier can be 1200 nm-thick Hg.sub.0.6Cd.sub.0.4Te and the quantum wells can be 1.5 nm thick of HgTe.

5. The device according to claim 1, wherein the quantum emitters comprise quantum wells, wherein the quantum wells are composed of Ga.sub.1-xAl.sub.xAs (x=0.10), the quantum barrier is composed of Ga.sub.1-xAl.sub.xAs (x=0.30), and Distributed Bragg Reflector are composed of AlAs/Ga.sub.1-xAl.sub.xAs(x=0.5) alloys.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1(a) is a schematic sectional view of an exemplary embodiment of the invention;

[0026] FIG. 1(b) is a schematic energy level diagram of the embodiment of FIG. 1(a); and

[0027] FIG. 2 is a schematic sectional view of an alternate embodiment of the invention.

DETAILED DESCRIPTION

[0028] While this invention is susceptible of embodiment in many different forms, there are shown in the drawings, and will be described herein in detail, specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated.

[0029] An exemplary embodiment of the invention uses a QE in the form of quantum well (QW) in a device concept to increase the biphoton flux further by removing Coulomb blockade restriction and increase efficiency further by suppressing the one-photon emission across the bandgap while simultaneously increasing the biphoton extraction. The exemplary embodiment uses a special high Q cavity engineered to reduce the photon density of states at the one-photon wavelength (λ.sub.g) and to enhance the photon density of states at biphoton wavelength (2λ.sub.g). The design for the cavity uses a Distributed Bragg Reflector (DBR)—a stack of alternating high and low refractive index dielectrics design for high reflection around λ.sub.g and high transmission around 2λ.sub.g

[0030] FIGS. 1(a)-1(b) illustrate a schematic design of an optically injected device 10. FIG. 1(a) illustrates the schematic sectional view of the device 10. Bars denote a series of QW 1, barrier 2 and DBRs 3. Pump photons propagate through the DBR and are absorbed in the barrier located beneath the DBR.

[0031] FIG. 1(b) illustrates an energy level line up of this device. The QW 1 is embedded in between the barriers 2 which is a wider band gap dielectric and sandwiched between the two DBRs 3 to form a microcavity resonant at 2λ.sub.g. In both figures, any emitted photon 12 of wavelength λ.sub.g is reflected by the DBR. The emission of biphotons 14 of wavelength 2λ.sub.g is enhanced via the Purcell effect induced by the microcavity. These biphotons are transmitted through the DBRs.

[0032] The QW is designed to have a band gap energy smaller than that of the barrier 2.

[0033] Because the carriers in the QW structure are constrained, the states are quantized in conduction and valence bands to yield states to be larger than the bulk band gap of the QW material, as shown by lines 22, 24. The QW is designed to have fundamental emission at the wavelength of λ.sub.g. The pump photons of wavelength λ.sub.P, are launched into the device normal to the barrier and normal to the DBR, and into the cavity, the region between two DBRs. The pump photons are absorbed in the barrier 2 creating electron-hole (e-h) pairs (filled and unfilled circles in FIG. 1b) which are transferred to the QW.

[0034] The pump photons are absorbed in the barriers and are transferred to the QW, creating electrons 30 (filled circles in FIG. 1b) and holes 32 (hollow circles in FIG. 1b). In the QW, those carriers, electrons and holes, will recombine by process: (a) emitting one-photons 12 with wavelength λ.sub.g, (b) non-radiatively (not shown), and (c) emitting biphotons 14 with wavelength 2λ.sub.g. The nonradiative recombination rates mediated by the Shockley-Read-Hall process can be minimized by defect-free growth with a low impurity concentration. Non-radiative Auger processes can be kept to a minimum by keeping the carrier density <10.sup.17 cm.sup.3.

[0035] The direct competition determining the emission efficiency for 2λ.sub.g biphotons is between processes (a) and (c). If left to themselves, process (a) would dominate as the one-photon emission rate is about 10.sup.5 times larger than the two-photon emission rate. However, two features are added to overcome this disadvantage. First, the QWs are placed at the location where the one-photon density of states is near zero, thus reducing the number of emitted photons to be very little. Then, all-angle reflecting DBRs (Region 3) are added to reflect all emitted one-photons back to QW, thus maintaining a near-constant electron-hole (e-h) density in the QW (Region 1). These two features force e-h pairs to decay through two-photon emission at wavelength 2λ.sub.g. The micro cavity (MC) anti-resonates at λ.sub.g and it doesn't affect the emission at 2λ.sub.g. In addition, the DBRs are carefully designed to have over 90% transmission of 2λ.sub.g photons.

[0036] For example, the design and materials can be chosen to produce entangled photons of wavelength 10.6 μm. The DBR materials can be CdTe (low index) and Hg.sub.0.28Cd.sub.0.72Te (high index). The spacers 2 can each be 1200 nm thickness of Hg.sub.0.60Cd.sub.0.40Te and the well region 1 can be 1.5 nm thickness of HgTe for an LWIR design. The calculated optical property of the cavity with the MQWs indicate near zero emission of 5.3 μm photons and over 90% transmission of 10.6 μm photons, leading to an efficiency of about 1.2%, which is 5 orders of magnitude larger the current state of art.

[0037] As another example for generation of biphotons of wavelength 1550 nm, the device can alternately be comprised of Ga.sub.1-xAl.sub.xAs (x=0.3) for the barrier and Ga.sub.1-xAl.sub.xAs (x=0.10) for the QWs, and AlAs/Ga.sub.1-xAl.sub.xAs (x=0.5) for the DBR. The calculated optical property of the cavity with the MQWs indicate near zero emission of 0.775 μm photons and over 90% transmission of 1.55 μm photons, leading to an efficiency of about 66%, which is 6 orders of magnitude larger the current state of art.

[0038] In addition, a higher density of biphoton emission is possible in the device because the carrier density in QW is not limited by Coulomb blockade as in the case of QDs. The electron-hole recombination takes place at the center r of the Brillouin Zone and thus biphotons are emitted at random direction with their momentum adding to zero. In other words, the biphotons are emitted 4π steradian within the cavity, instead of a narrow cone in the SPDC approach. As the biphotons exit the DBRs, the angular distribution will be narrow and determined by cavity design. The larger angular distribution enables the increase in the emitted photon density without adding time-bin errors. Even more importantly, the photons will be entangled in energy, polarization, and space. The hyper entanglement improves the SNR even more. By recycling the single photons, this device is expected to provide orders of magnitude improvement in the efficiency of biphoton generation compared to natural χ(2) crystals. The designs can be grown with molecular-beam epitaxy (MBE) or Metal Organic Chemical Vapor Deposition (MOCVD) and fabricated with standard processing methods and are amenable for monolithic integration thus improving in size, weight, and power (SWaP) performance.

[0039] FIG. 2 illustrates an alternate embodiment wherein each barrier 2 of FIG. 1(a) is replaced by a barrier layer 2a and a separate spacer layer 2b. The barrier layer is chosen appropriately for the chosen pump wavelength and thickness is chosen to stay within diffusion length of photocarrier in that material. The spacer layer does not have this limitation, except it has to be transparent to pump. The spacer layer can be added without affecting any quantum property, only to make sure that the fundamental emission suppressed by designing low density of states for photons. In cases like HgCdTe for LWIR, the diffusion length is in microns and so the barrier thickness can be varied to optimize the field for suppression. Wherein in cases like GaAlAs for SWIR, the diffusion length is 100 nm and additional space is needed to optimize the field.

[0040] From the foregoing, it will be observed that numerous variations and modifications may be incorporated without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred.