# US 7,609,382 B2 · System and method of detecting entangled photons

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- Number: US 7,609,382 B2
- Document ID: US7609382B2
- Status: Granted
- Kind: B2
- Application number: 10/850,394
- Filed: 2004-05-21
- Published: 2005-01-13
- Granted: 2009-10-27
- Earliest claimed: 2003-05-23
- Assignees: General Dynamics Advanced Information System, Inc,
- Inventors: Keith Kastella; Kenneth Augustyn; Ralph Conti; Richard Freeling
- Web page: https://patentgrep.com/patent/US7609382B2

## CPC

- H04B 10/70: Photonic quantum communication
- H04B: TRANSMISSION

## Abstract

A system and method of detecting entangled photon pairs, each pair including a signal photon and an idler photon, is disclosed. Entangled photon pairs are provided having an entanglement time and an entanglement area selected to substantially increase an associated entangled two-photon cross-section of an associated target medium. The entangled photon pairs are also selected to have an energy distribution between the signal photon and the idler photon to substantially decrease an associated random two-photon absorption cross section of the target medium. The entangled photon pairs are directed to the target medium, and at least one entangled-photon pair being absorbed by the target medium is detected.

## Claims

### Claim 1

A method of detecting entangled-photon pairs configured to have a predetermined quantum state relationship with a prepared target medium, each entangled-photon pair comprising a signal photon and an idler photon, the method comprising:
supplying the target medium;
configuring a quantum state of the target medium relative to the entangled-photon pairs thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled two-photon cross-section for the entangled-photon pairs and decrease a random two-photon absorption cross-section for the entangled-photon pairs;
providing entangled-photon pairs, at least a first portion of the entangled-photon pairs having an entanglement time and an entanglement area to substantially increase an associated entangled two-photon cross-section of the prepared target medium;
ensuring that at least a second portion of the entangled-photon pairs have an energy distribution between the signal photon and the idler photon to substantially decrease an associated random two-photon absorption cross section of the target medium;
directing photons included in the first portion and the second portion to the target medium;
detecting at least one entangled-photon pair being absorbed by the target medium; and
outputting an electrical signal representing said detecting.

### Claim 2 (depends on 1)

The method of claim 1 wherein said detecting comprises detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization.

### Claim 3 (depends on 1)

The method of claim 1 wherein said providing comprises providing a coherent light source directed at a nonlinear crystal.

### Claim 4 (depends on 1)

The method of claim 1 wherein the target medium comprises one of rubidium, cesium, and an alkali.

### Claim 5 (depends on 1)

The method of claim 1 where the target medium comprises a quantum dot.

### Claim 6

A method of detecting entangled-photon pairs configured to have a predetermined quantum state relationship with a prepared target medium, each entangled-photon pair comprising a signal photon and an idler photon, the method comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled-photon pairs thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled two-photon cross-section for the entangled-photon pairs and decrease a random two-photon absorption cross-section for the entangled-photon pairs;
receiving entangled-photon pairs, at least a portion of the entangled-photon pairs configured to have at least one of: an entanglement time to substantially maximize an associated entangled photon cross-section of the prepared target medium, and an entanglement area to substantially maximize an associated entangled photon cross-section of the prepared target medium;
the entangled photon pairs having an energy distribution among the signal photon and the idler photon in each pair to substantially minimize an associated random two-photon absorption cross-section of the prepared target medium;
detecting at least one entangled-photon pair being absorbed by the prepared target medium; and
outputting an electrical signal representing said detecting.

### Claim 7 (depends on 6)

The method of claim 6 wherein said detecting comprises at least one of detecting fluorescence, detecting phosphorescence, detecting direct electron transfer, and detecting ionization.

### Claim 8 (depends on 6)

The method of claim 6 where the target medium comprises one of rubidium, cesium, and an alkali.

### Claim 9 (depends on 6)

The method of claim 6 where the target medium comprises a quantum dot.

### Claim 10

A method of providing entangled-photon pairs configured to have a large entangled-photon pair cross section and a small random photon pair cross section relative to a prepared target medium, the method comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled-photon pairs thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled two-photon cross-section for the entangled-photon pairs and decrease a random two-photon absorption cross-section for the entangled-photon pairs;
providing entangled photon pairs having at least one of an entanglement time to substantially maximize an associated entangled photon cross-section for the prepared target medium, and an entanglement area to substantially maximize an associated entangled photon cross-section for the prepared target medium;
selecting entangled-photon pairs having an energy distribution among a signal photon and an idler photon to substantially minimize an associated random photon pair absorption cross section of the prepared target medium; and
sending the entangled photons to a receiver, the receiver configured to cause at least a portion of the entangled photons to come into contact with the prepared target medium.

### Claim 11 (depends on 10)

The method of claim 10 further comprising detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization.

### Claim 12 (depends on 10)

The method of claim 10 wherein said entangled photon source comprises a nonlinear crystal.

### Claim 13 (depends on 10)

The method of claim 10 wherein the target medium comprises one of rubidium, cesium, and an alkali.

### Claim 14

A method of detecting entangled photons, the entangled photons configured to have a predetermined quantum relationship with a prepared target medium, the method comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled multi-photon cross-section for the entangled-photon pairs and decrease a random multi-photon absorption cross-section for the entangled photons;
producing entangled photons tailored for entangled photon absorption by the prepared target medium;
conditioning the entangled photons to reduce random multi-photon absorption by the target medium;
directing the entangled photons to the prepared target medium;
detecting at least one entangled-photon absorption by the prepared target medium; and
outputting an electrical signal representing said detecting.

### Claim 15 (depends on 14)

The method of claim 14 wherein the entangled photons comprise entangled photon pairs.

### Claim 16 (depends on 15)

The method of claim 15 further comprising delaying one photon of each entangled-photon pair by a predetermined amount of time.

### Claim 17 (depends on 14)

The method of claim 14 wherein said conditioning comprises selecting entangled photons having a predetermined energy distribution among the photons comprising each pair.

### Claim 18 (depends on 14)

The method of claim 14 wherein said detecting comprises detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization.

### Claim 19 (depends on 14)

The method of claim 14 wherein:
said producing comprises producing single photons having a pump frequency consistent with a predetermined energy level transition of the target medium; and
said producing further comprises splitting the single photons into entangled-photon pairs.

### Claim 20 (depends on 14)

The method of claim 14 wherein the target medium comprises one of rubidium, cesium, and an alkali.

### Claim 21 (depends on 14)

The method of claim 14 where the target medium comprises a quantum dot.

### Claim 22 (depends on 14)

The method of claim 14 wherein said producing comprises producing according to an entanglement time and an entanglement area, at least one of the entangled photon time and the entangled photon area selected to increase entangled-photon absorption by the target medium.

### Claim 23

A method of providing entangled photons configured to have a high entangled-photon absorption and low random multi-photon absorption for a corresponding predetermined and prepared target medium, the method comprising:
supplying a target medium
configuring a quantum state of the target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
producing entangled photons tailored for entangled photon absorption by the prepared target medium;
conditioning the entangled photons to reduce random multi-photon absorption by the prepared target medium;
sending the entangled photons to a receiver, the receiver configured to cause at least a portion of the entangled photons to come into contact with the prepared target medium.

### Claim 24 (depends on 23)

The method of claim 23 wherein the entangled photons comprise entangled-photon pairs.

### Claim 25 (depends on 24)

The method of claim 24 further comprising delaying one photon of each entangled-photon pair by a predetermined amount of time.

### Claim 26 (depends on 24)

The method of claim 24, wherein said conditioning comprises selecting entangled-photon pairs having a predetermined energy distribution among the photons comprising each pair.

### Claim 27 (depends on 23)

The method of claim 23 further comprising detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization.

### Claim 28 (depends on 23)

The method of claim 23 wherein the target medium comprises one of rubidium, cesium, and an alkali.

### Claim 29 (depends on 23)

The method of claim 23 where the target medium comprises a quantum dot.

### Claim 30 (depends on 23)

The method of claim 23 wherein:
said producing comprises producing single photons having a pump frequency consistent with a predetermined energy level transition of the target medium; and
said producing further comprises splitting the single photons into entangled-photon pairs.

### Claim 31 (depends on 23)

The method of claim 23 wherein said producing comprises producing according to an entanglement time and an entanglement area, at least one of the entangled photon time and the entangled photon area selected to increase entangled-photon absorption by the target medium.

### Claim 32 (depends on 23)

The method of claim 23 wherein said producing comprises producing using a nonlinear crystal.

### Claim 33

A method of detecting entangled photons configured to have a predetermined quantum state relationship with a prepared target medium, the method comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
receiving entangled photons, the entangled photons being tailored for entangled-photon absorption by the prepared target medium, the entangled photons being conditioned to reduce random multi-photon absorption by the prepared target medium;
causing the entangled photons to come into contact with the prepared target medium;
detecting at least one entangled-photon absorption by the prepared target medium; and
outputting an electrical signal representing said detecting.

### Claim 34 (depends on 33)

The method of claim 33 wherein the entangled photons comprise entangled-photon pairs.

### Claim 35 (depends on 34)

The method of claim 34 wherein the entangled photons comprise entangled-photon pairs selected to have a predetermined energy distribution among the photons comprising each pair.

### Claim 36 (depends on 34)

The method of claim 34 further comprising delaying one photon of each entangled-photon pair by a predetermined amount of time.

### Claim 37 (depends on 33)

The method of claim 33 wherein the entangled photons have an entangled photon time and an entangled photon area, at least one of the entangled photon time and the entangled photon area selected to increase entangled-photon absorption by the prepared target medium.

### Claim 38 (depends on 33)

The method of claim 33 wherein said detecting comprises detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization.

### Claim 39 (depends on 33)

The method of claim 33 wherein the target medium comprises one of rubidium, cesium, and an alkali.

### Claim 40 (depends on 33)

The method of claim 33 where the target medium comprises a quantum dot.

### Claim 41

A system for detecting entangled photons configured to have a predetermined quantum state relationship with a prepared target medium, the system comprising:
a prepared target medium configured to have a quantum state to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
an entangled photon source configured to produce entangled photons tailored for entangled-photon absorption by the prepared target medium;
a screen configured to pass entangled photons having an energy distribution selected to reduce random multi-photon absorption by the prepared target medium; and
a sensor configured to detect entangled-photon absorption by the prepared target medium.

### Claim 42 (depends on 41)

The system of claim 41 wherein the system further comprises a line for delaying at least one photon from each set of entangled photons by an amount of time selected to increase entangled photon absorption by the prepared target medium.

### Claim 43 (depends on 41)

The system of claim 41 wherein said entangled photon source is configured to produce entangled photon having an entanglement area, the entanglement area selected to increase entangled-photon absorption by the prepared target medium.

### Claim 44 (depends on 41)

The system of claim 41 wherein said entangled photon source is configured to produce entangled photon having an entanglement time, the entanglement time selected to increase entangled-photon absorption by the prepared target medium.

### Claim 45 (depends on 41)

The system of claim 41 wherein said sensor is configured to detect at least one of fluorescence, phosphorescence, direct electron transfer, and ionization.

### Claim 46 (depends on 41)

The system of claim 41 wherein said entangled photon source comprises a nonlinear crystal.

### Claim 47 (depends on 41)

The system of claim 41 wherein the target medium comprises one of rubidium, cesium, and an alkali.

### Claim 48 (depends on 41)

The method of claim 41 where the target medium comprises a quantum dot.

### Claim 49

A system for producing entangled photons having high entangled-photon absorption and low random multi-photon absorption for a prepared target medium, the prepared target medium configured to have a predetermined quantum relationship with the entangled photons, the system comprising:
a prepared target medium having a predetermined quantum state relative to the entangled photons, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
an entangled photon source configured for producing entangled photons tailored for entangled-photon absorption by the prepared target medium; and
a screen configured to pass entangled-photon pairs having an energy distribution selected to reduce random photon absorption by the prepared target medium.

### Claim 50 (depends on 49)

The system of claim 49 wherein the entangled photon source is configured to produce entangled-photon pairs.

### Claim 51 (depends on 50)

The system of claim 50, further comprising a delay line configured to delay one photon of each entangled-photon pair by a predetermined amount of time.

### Claim 52 (depends on 49)

The system of claim 49 further comprising a sensor configured to detect entangled-photon absorption by the prepared target medium.

### Claim 53 (depends on 49)

The system of claim 49 wherein said sensor is capable of detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization.

### Claim 54 (depends on 49)

The system of claim 49 wherein said entangled photon source comprises:
a photon source configured to produce photons having a pump frequency consistent with a predetermined energy level transition of the prepared target medium; and
a nonlinear crystal configured to split the photons into entangled photon pairs.

### Claim 55 (depends on 49)

The system of claim 49 wherein the nonlinear crystal comprises a beta barium borate crystal.

### Claim 56 (depends on 49)

The system of claim 49 wherein said entangled photon source is configured to produce entangled photons having an entanglement time, the entanglement time selected to increase entangled-photon absorption by the prepared target medium.

### Claim 57 (depends on 49)

The system of claim 49 wherein said entangled photon source is configured to produce entangled photons having an entanglement area, the entanglement area selected to increase entangled-photon absorption by the prepared target medium.

### Claim 58 (depends on 49)

The system of claim 49 wherein the target medium comprises one of rubidium, cesium, and an alkali.

### Claim 59 (depends on 49)

The method of claim 49 where the target medium comprises a quantum dot.

### Claim 60

A system for detecting entangled photons configured to have a predetermined quantum state relationship with a prepared target medium, the system comprising:
a prepared target medium configured with a quantum state to absorb entangled photons tailored for entangled-photon absorption by the prepared target medium, the entangled photons being conditioned to reduce random photon absorption by the prepared target medium; and
a sensor configured to detect at least one entangled photon absorption by the prepared target medium.

### Claim 61 (depends on 60)

The system of claim 60 wherein said prepared target medium is selected to absorb entangled-photon pairs.

### Claim 62 (depends on 60)

The system of claim 60 wherein said prepared target medium is selected to pass random photon pairs, where the random photon pairs are formed from entangled-photon pairs having a predetermined energy distribution among the photons comprising each entangled-photon pair.

### Claim 63 (depends on 60)

The system of claim 60 wherein said prepared target medium is selected to absorb entangled-photon pairs having one photon of each entangled-photon pair delayed by a predetermined amount of time.

### Claim 64 (depends on 60)

The system of claim 60 wherein said prepared target medium is selected to absorb entangled photons having a predetermined entanglement time.

### Claim 65 (depends on 60)

The system of claim 60 wherein said prepared target medium is selected to absorb entangled photons having a predetermined entanglement area.

### Claim 66 (depends on 60)

The system of claim 60 wherein said sensor is capable of detecting at least one of fluorescence, phosphorescence, direct electron transfer, and ionization.

### Claim 67 (depends on 60)

The system of claim 60 wherein the prepared target medium comprises one of rubidium, cesium, and an alkali.

### Claim 68

A method of detecting entangled photons configured to have a predetermined relationship with a prepared target medium, the method comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
producing entangled photons having properties for entangled-photon absorption by the prepared target medium;
preparing the entangled photons to reduce random photon absorption by the prepared target medium;
directing the entangled photons to the prepared target medium;
detecting at least one entangled-photon absorption by the prepared target medium; and
outputting an electrical signal representing said detecting.

### Claim 69

A method of providing entangled photons having a high entangled-photon absorption rate for a prepared target medium, the entangled photons configured to have a predetermined quantum state relationship with the prepared target medium, the method comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
producing entangled photons having properties for entangled-photon absorption the prepared target medium;
preparing the entangled photons to reduce random photon absorption by the prepared target medium; and
sending the entangled photons to a receiver, the receiver configured to cause at least a portion of the entangled photons to come into contact with the prepared target medium.

### Claim 70

A method of detecting entangled photons using a prepared target medium, the entangled photons configured to have a predetermined quantum state relationship with the prepared target medium, the method comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
receiving entangled photons, the entangled photons having properties for entangled-photon absorption by the prepared target medium;
preparing the entangled photons to reduce random photon absorption by the prepared target medium;
causing the entangled photons to come into contact with the prepared target medium;
detecting at least one entangled-photon absorption by the prepared target medium; and
outputting an electrical signal representing said detecting.

### Claim 71

A system for detecting entangled photons using a prepared target medium, the entangled photons configured to have a predetermined quantum state relationship with the prepared target medium the method comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
an entangled photon source configured to produce entangled photons having properties for entangled-photon absorption by the prepared target medium;
a screen configured to pass entangled photon having an energy distribution selected to reduce random photon absorption by the prepared target medium; and
a sensor configured to detect entangled-photon absorption by the prepared target medium.

### Claim 72

A system for producing entangled photons having high entangled-photon absorption using a prepared target medium, the entangled photons configured to have a predetermined quantum state relationship with the prepared target medium, the method, the system comprising:
supplying a target medium;
configuring a quantum state of the target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
an entangled photons source configured for producing entangled photons having properties for entangled-photon absorption by the prepared target medium; and
a screen configured to pass entangled photons having an energy distribution selected to reduce random photon absorption by the prepared target medium.

### Claim 73

A system for detecting entangled photons using a prepared target medium, the entangled photons configured to have a predetermined quantum state relationship with the prepared target medium, the system comprising:
a prepared target medium configured to have a quantum state to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
a screen configured to select entangled photons having reduced random photon absorption by the prepared target medium; and
a sensor configured to detect entangled photon absorption by the prepared target medium.

### Claim 74

A system for detecting entangled photons using a prepared target medium, the entangled photons configured to have a predetermined quantum state relationship with the prepared target medium, the system comprising:
means for configuring a quantum state of a target medium relative to the entangled photons thereby producing the prepared target medium, wherein the prepared target medium has a quantum state configured to substantially increase an entangled photon cross-section for the entangled photons and decrease a random multi-photon absorption cross-section for the entangled photons;
means for producing entangled photons having properties for entangled-photon absorption by the prepared target medium;
means for selecting entangled photons having reduced probability of random photon absorption by the prepared target medium;
means for to absorbing entangled photons having properties suitable for entangled-photon absorption by the prepared target medium; and
means for detecting entangled photon absorption by the target medium.