# US 12,225,116 B1 · Quantum key distribution network with centralized optical pulse generation

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- Number: US 12,225,116 B1
- Document ID: US12225116B1
- Status: Granted
- Kind: B1
- Application number: 18/073,240
- Filed: 2022-12-01
- Granted: 2025-02-11
- Earliest claimed: 2021-09-23
- Assignees: Cable Television Laboratories, Inc.
- Applicants: Cable Television Laboratories, Inc.
- Inventors: Jing Wang; Bernardo Huberman
- Web page: https://patentgrep.com/patent/US12225116B1

## CPC

- H04L 9/0852: {Quantum cryptography (transmission systems employing electromagnetic waves other than radio waves, e.g. light, infrared H04B10/00; wavelength-division multiplex systems H04J14/02; WDM arrangements H04J14/03)}
- H04B 10/70: Photonic quantum communication
- H04L 9/0858: {Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding}
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)
- H04B: TRANSMISSION

## Abstract

A method performed by a node of a quantum key distribution (QKD) network includes receiving, from a hub of the QKD network, a user-node pulse train of optical-pulse pairs, each of the optical-pulse pairs comprising a first pulse and a second pulse having an optical phase shift relative to the first pulse. The method further includes splitting the user-node pulse train into first and second pulse trains, calibrating an asymmetric Mach-Zehnder interferometer with the first pulse train, blocking the second pulse of each of the optical-pulse pairs of the second pulse train to generate a filtered pulse train, splitting the filtered pulse train into a timing pulse train and a pre-qubit pulse train, delaying the pre-qubit pulse train into a delayed pulse train, and encoding the delayed pulse train into a photonic-qubit pulse train and transmitting the photonic-qubit pulse train to the hub.

## Claims

### Claim 1

A method performed by a node of a quantum key distribution network, comprising:
receiving, from a hub of the quantum key distribution network, a user-node pulse train of optical-pulse pairs, each of the optical-pulse pairs comprising a first pulse and second pulse that is temporally delayed relative to the first pulse, the second pulse having an optical phase shift relative to the first pulse;
splitting the user-node pulse train into first and second pulse trains;
calibrating an asymmetric Mach-Zehnder interferometer with the first pulse train such that an interferometer phase shift between first and second arms of the asymmetric Mach-Zehnder interferometer matches the optical phase shift;
blocking the second pulse of each of the optical-pulse pairs of the second pulse train to generate a filtered pulse train;
splitting the filtered pulse train into a timing pulse train and a pre-qubit pulse train;
detecting the timing pulse train to generate an electronic timing signal;
delaying the pre-qubit pulse train into a delayed pulse train;
encoding, based on the electronic timing signal and using the asymmetric Mach-Zehnder interferometer, the delayed pulse train into a photonic-qubit pulse train; and
transmitting the photonic-qubit pulse train to the hub.

### Claim 2 (depends on 1)

The method of claim 1, wherein said encoding includes one or both of time-bin encoding and phase encoding.

### Claim 3 (depends on 1)

The method of claim 1, wherein said blocking occurs synchronously with the electronic timing signal.

### Claim 4 (depends on 1)

The method of claim 1, wherein said delaying is based on a temporal delay transmitted to the node by the hub via a classical communication channel.

### Claim 5 (depends on 4)

The method of claim 4, wherein the temporal delay is no greater than one-half of a repetition period of the user-node pulse train.

### Claim 6 (depends on 1)

The method of claim 1, wherein said receiving includes receiving the user-node pulse train via a fiber optic channel.

### Claim 7

A node for a quantum key distribution network, comprising:
an optical input operable to receive, from a hub of the quantum key distribution network, a user-node pulse train of optical-pulse pairs, each of the optical-pulse pairs comprising a first pulse and a second pulse that is temporally delayed relative to the first pulse, the second pulse having an optical phase shift relative to the first pulse;
a first beamsplitter configured to split the user-node pulse train into first and second pulse trains;
a modulator operable to block the second pulse of each of the optical-pulse pairs of the second pulse train to generate a filtered pulse train;
a second beamsplitter configured to split the filtered pulse train into an timing pulse train and a pre-qubit pulse train;
a photodetector operable to detect the timing pulse train to generate an electronic timing signal;
an optical delay operable to delay the pre-qubit pulse train into a delayed pulse train;
a qubit encoder including an asymmetric Mach-Zehnder interferometer, the qubit encoder being operable to encode, based on the electronic timing signal, the delayed pulse train into a photonic-qubit pulse train;
a phase calibrator operable to calibrate the asymmetric Mach-Zehnder interferometer with the first pulse train such that an interferometer phase shift between first and second arms of the asymmetric Mach-Zehnder interferometer matches the optical phase shift; and
an optical output operable to transmit the photonic-qubit pulse train to the hub.

### Claim 8 (depends on 7)

The node of claim 7, the qubit encoder being operable to perform one or both of time-bin and phase encoding.

### Claim 9 (depends on 7)

The node of claim 7, the modulator being operable to synchronously block the second pulse based on the electronic timing signal.

### Claim 10 (depends on 7)

The node of claim 7, the optical delay being an electronically tunable optical delay line.

### Claim 11 (depends on 10)

The node of claim 10, further comprising an electronic delay generator operable to control the electronically tunable optical delay line.

### Claim 12 (depends on 7)

The node of claim 7, each of the optical input and the optical output comprising a fiber-optic port.

### Claim 13

A method performed by a hub of a quantum key distribution network, comprising:
transforming, with an asymmetric Mach-Zehnder interferometer, an optical pulse train into a double pulse train of optical-pulse pairs, each of the optical-pulse pairs comprising a first pulse and a second pulse that is temporally delayed relative to the first pulse, the second pulse having an optical phase shift relative to the first pulse;
splitting the double pulse train into a first-node pulse train and a second-node pulse train;
transmitting the first-node pulse train to a first node of the quantum key distribution network, the first node transforming the first-node pulse train into a first photonic-qubit pulse train;
transmitting the second-node pulse train to a second node of the quantum key distribution network, the second node transforming the second-node pulse train into a second photonic-qubit pulse train;
receiving the first photonic-qubit pulse train from the first node;
receiving the second photonic-qubit pulse train from the second node; and
performing a partial Bell-state measurement with a first photonic qubit of the first photonic-qubit pulse train and a second photonic qubit of the second photonic-qubit pulse train, the first and second photonic qubits being at least partially indistinguishable.

### Claim 14 (depends on 13)

The method of claim 13, further comprising publicly announcing a result of the partial Bell-state measurement.

### Claim 15 (depends on 13)

The method of claim 13, further comprising generating the optical pulse train.

### Claim 16 (depends on 15)

The method of claim 15, wherein said generating comprises modulating a laser.

### Claim 17 (depends on 13)

The method of claim 13, further comprising optically switching the first-node pulse train to a third node of the quantum key distribution network.

### Claim 18 (depends on 13)

The method of claim 13, wherein said transmitting the first-node pulse train includes transmitting the first-node pulse train to the first node via a fiber-optic channel.

### Claim 19 (depends on 13)

The method of claim 13, wherein said receiving the first photonic-qubit pulse train includes receiving the first photonic-qubit pulse train via a fiber-optic channel.

### Claim 20

A hub for a quantum key distribution network, comprising:
an asymmetric Mach-Zehnder interferometer operable to transform an optical pulse train into a double pulse train of optical-pulse pairs, each of the optical-pulse pairs comprising a first pulse and a second pulse that is temporally delayed relative to the first pulse, the second pulse having an optical phase shift relative to the first pulse;
a beamsplitter configured to split the double pulse train into a first-node pulse train and a second-node pulse train;
a first optical output operable to transmit the first-node pulse train to a first node of the quantum key distribution network such that the first node transforms the first-node pulse train into a first photonic-qubit pulse train;
a second optical output operable to transmit the second-node pulse train to a second node of the quantum key distribution network such that the second node transforms the second-node pulse train into a second photonic-qubit pulse train;
a first optical input operable to receive the first photonic-qubit pulse train;
a second optical input operable to receive the second photonic-qubit pulse train; and
a Bell-state measurement module operable to perform a partial Bell-state measurement with a first photonic qubit of the first photonic-qubit pulse train and a second photonic qubit of the second photonic-qubit pulse train, the first and second photonic qubits being at least partially indistinguishable.

### Claim 21 (depends on 20)

The hub of claim 20, further comprising a laser system operable to generate the optical pulse train.

### Claim 22 (depends on 21)

The hub of claim 21, the laser system comprising a laser and a modulator operable to modulate an intensity of a continuous-wave laser beam outputted by the laser.

### Claim 23 (depends on 21)

The hub of claim 21, the laser system comprising a laser diode and a modulator circuit operable to modulate an electrical current that drives the laser diode.

### Claim 24 (depends on 20)

The hub of claim 20, further comprising a third optical output and an optical switch, the first-node pulse train being transmitted from the first optical output when the optical switch is in a first position, the first-node pulse train being transmitted from the third optical output when the optical switch is in a second position.

### Claim 25 (depends on 20)

The hub of claim 20, further comprising a signal-processing circuit operable to transmit a result of the partial Bell-state measurement via a classical communication channel.

### Claim 26 (depends on 20)

The hub of claim 20, each of the first and second optical inputs and the first and second optical outputs comprising a fiber-optic port.