# US 11,309,684 B2 · Coherent single photon source

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- Number: US 11,309,684 B2
- Document ID: US11309684B2
- Status: Granted
- Kind: B2
- Application number: 16/614,578
- Filed: 2018-05-18
- Published: 2020-04-09
- Granted: 2022-04-19
- Earliest claimed: 2017-05-19
- Assignees: Københavns Universitet
- Applicants: Københavns Universitet
- Inventors: Peter Lodahl; Chris Lennart Dreessen; Petru Tighineanu; Anders Søndberg Sørensen; Leonardo Midolo
- Web page: https://patentgrep.com/patent/US11309684B2

## CPC

- H01S 5/1014: {Tapered waveguide, e.g. spotsize converter (H01S5/1064 takes precedence)}
- B82Y 20/00: Nanooptics, e.g. quantum optics or photonic crystals
- G02B 6/107: {Subwavelength-diameter waveguides, e.g. nanowires}
- H01S 5/1042: {Optical microcavities, e.g. cavity dimensions comparable to the wavelength}
- H01S 5/11: Comprising a photonic bandgap structure
- H01S 5/3412: {quantum box or quantum dash}
- H01S 5/34313: {with a well layer having only As as V-compound, e.g. AlGaAs, InGaAs}
- H01S 5/021: {Silicon based substrates}
- H01S 5/0218: {Substrates comprising semiconducting materials from other groups of the Periodic Table than the materials of the active layer}
- H01S 2301/16: Semiconductor lasers with special structural design to influence the modes, e.g. specific multimode
- H10H 20/821: of the light-emitting regions, e.g. non-planar junctions
- H01S: DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- B82Y: SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- H10H: INORGANIC LIGHT-EMITTING SEMICONDUCTOR DEVICES HAVING POTENTIAL BARRIERS

## Abstract

The invention relates to coherent single photon sources that provide photons with a high degree of indistinguishability. It is a disadvantage of single photon sources based on QDs in nanophotonic structures that, even at low temperatures, acoustic vibrations interact with the QDs to reduce the coherence of the emitted spectrum. The invention uses mechanical clamping of the nanostructure to damp vibrations leading to a weaker QD—phonon coupling and a higher degree of indistinguishability between successively emitted photons. The clamp is mechanically connected to the length of the photonic nanostructure and has a stiffness and a size sufficient to suppress low frequency vibrations (ω≤10 GHz) in a combined structure of the clamp and the nanostructure.

## Claims

### Claim 1

A coherent single photon source with wavelength λ, comprising an elongated, semiconductor photonic nanostructure and a self-assembled quantum dot positioned inside the photonic nanostructure, wherein the photonic nanostructure has a cross-sectional area smaller than 2 μm2, wherein the single photon source further comprises an elongated clamp formed in a material that is transparent and has a refractive index smaller than 2 at the wavelength λ, and, wherein the clamp is mechanically connected to the length of the photonic nanostructure and has a stiffness and a size sufficient to suppress low frequency vibrations (ω≤10 GHz) in a combined structure of the clamp and the nanostructure.

### Claim 2 (depends on 1)

The coherent single photon source according to claim 1, wherein the stiffness of the clamp is expressed by an averaged Young's modulus for the combined structure, Eav, and a cross-sectional area of the combined structure, A, and, wherein Eav and A are selected in accordance with:
for Eavϵ[0 GPa; 3 GPa[, A≥10.5 μm2;
for Eavϵ[3 GPa; 5.5 GPa[, A≥7 μm2;
for Eavϵ[5.5 GPa; 70 GPa[, A≥1 μm2; and
for Eav≥70 GPa, A≥0.5 μm2.

### Claim 3 (depends on 1)

The coherent single photon source according to claim 1, wherein the clamp at least partly surrounds the photonic nanostructure in transverse directions.

### Claim 4 (depends on 3)

The coherent single photon source according to claim 3, wherein the photonic nanostructure is embedded in the clamp in transverse directions.

### Claim 5 (depends on 1)

The coherent single photon source according to claim 1, further comprising a means for coupling out photons emitted by the quantum dot in the photonic nanostructure.

### Claim 6 (depends on 5)

The coherent single photon source according to claim 5, wherein the means for coupling out photons is a single mode nanophotonic waveguide section, which is an extension of the photonic nanostructure and, which is mechanically connected to the clamp along its length and at least partly surrounded by the clamp in transverse directions, the waveguide section being tapered in a direction away from the nanostructure to evanescently couple photons from the single mode to a guided mode in the clamp.

### Claim 7 (depends on 1)

The coherent single photon source according to claim 1, further comprising a means for coupling in photons to excite the quantum dot in the photonic nanostructure.

### Claim 8 (depends on 1)

The coherent single photon source according to claim 1, wherein the photonic nanostructure is a cavity in that it comprises reflectors at both ends of the nanostructure.

### Claim 9 (depends on 1)

The coherent single photon source according to claim 1, wherein the photonic nanostructure is a nanophotonic waveguide that is single-mode at the wavelength λ.

### Claim 10

A method for improving the indistinguishability of a single photon source having an elongated, semiconductor photonic nanostructure and a self-assembled quantum dot positioned inside the photonic nanostructure, wherein the photonic nanostructure has a cross-sectional area smaller than 2 μm2, the method comprising:
exciting the quantum dot to emit photons; and
increasing the indistinguishability of photons successively emitted into the photonic nanostructure with a clamp formed in a clamping material that is transparent and has a refractive index smaller than 2 at the wavelength λ, the clamp being mechanically connected to the length of the photonic nanostructure and having a stiffness and a size sufficient to suppress low frequency vibrations (ω≤10 GHz) in a combined structure of the clamp and the nanostructure.