# US 11,308,416 B2 · Adaptive basis selection for encoded fusion measurements

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- Number: US 11,308,416 B2
- Document ID: US11308416B2
- Status: Granted
- Kind: B2
- Application number: 17/216,450
- Filed: 2021-03-29
- Published: 2021-09-30
- Granted: 2022-04-19
- Earliest claimed: 2020-03-30
- Assignees: Psiquantum, Corp.
- Applicants: PsiQuantum, Corp.
- Inventors: Mihir Pant; Naomi Nickerson; Hector Bombin Palomo
- Web page: https://patentgrep.com/patent/US11308416B2

## CPC

- G06F 11/25: Testing of logic operation, e.g. by logic analysers
- G06F 11/1004: {to protect a block of data words, e.g. CRC or checksum (G06F11/1076 takes precedence; security arrangements for protecting computers or computer systems against unauthorized activity G06F21/00)}
- G06N 3/08: Learning methods
- G06N 10/00: Quantum computing, i.e. information processing based on quantum-mechanical phenomena
- G06N 10/40: Physical realisations or architectures of quantum processors or components for manipulating qubits, e.g. qubit coupling or qubit control
- G06N 10/70: Quantum error correction, detection or prevention, e.g. surface codes or magic state distillation
- G06F 3/0604: {Improving or facilitating administration, e.g. storage management}
- G06F 3/0659: {Command handling arrangements, e.g. command buffers, queues, command scheduling}
- G06F 3/0679: {Non-volatile semiconductor memory device, e.g. flash memory, one time programmable memory [OTP]}
- G06N 10/20: Models of quantum computing, e.g. quantum circuits or universal quantum computers
- G06N 10/60: Quantum algorithms, e.g. based on quantum optimisation, quantum Fourier or Hadamard transforms
- G06N 10/80: Quantum programming, e.g. interfaces, languages or software-development kits for creating or handling programs capable of running on quantum computers; Platforms for simulating or accessing quantum computers, e.g. cloud-based quantum computing
- H10N 99/05: {Devices based on quantum mechanical effects, e.g. quantum interference devices or metal single-electron transistors}
- G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N)
- G06N: COMPUTING ARRANGEMENTS BASED ON SPECIFIC COMPUTATIONAL MODELS
- H10N: ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR

## Abstract

A quantum computing system and methods for performing fusion based quantum computing on encoded qubits. A fusion controller sequentially performs a series of fusion measurements on respective physical qubits of first and second encoded qubits to obtain a respective series of classical measurement results. For respective fusion measurements of the series of fusion measurements, a basis for performing the respective fusion measurement is selected based on classical measurement results of previous fusion measurements. An encoded fusion measurement result is determined based on the classical measurement results, and the encoded fusion measurement result is stored in a memory medium.

## Claims

### Claim 1

A method for performing quantum computing, the method comprising:
performing a first fusion measurement on a first physical qubit of a first encoded qubit and a second physical qubit of a second encoded qubit;
obtaining a first classical measurement result of the first fusion measurement;
selecting a basis for performing a second fusion measurement based at least in part on the first classical measurement result, wherein the selected basis determines a failure basis for the second fusion measurement based at least in part on whether the first classical measurement result comprises a success, failure or loss outcome;
performing the second fusion measurement on a third physical qubit of the first encoded qubit and a fourth physical qubit of the second encoded qubit according to the selected basis;
obtaining a second classical measurement result of the second fusion measurement;
determining an encoded fusion measurement result of a fusion of the first encoded qubit and the second encoded qubit based at least in part on the first and second classical measurement results; and
storing the encoded fusion measurement result in a non-transitory memory medium, wherein selecting the basis for performing the second fusion measurement based at least in part on the first classical measurement result comprises:
determining that, when the second fusion measurement fails in a first basis, the encoded qubit measurement result may still be determined based at least in part on the first and the second classical measurement results; and
selecting the first basis as the selected basis for performing the second fusion measurement based at least in part on determining that, when the second fusion measurement fails in the first basis, the encoded qubit measurement result may still be determined based at least in part on the first and second classical measurement results.

### Claim 2 (depends on 1)

The method of claim 1,
wherein selecting the basis for performing the second fusion measurement comprises:
determining, based on stabilizers of the first and second encoded qubits, a plurality of effective measurement results that correspond to the encoded fusion measurement result;
determining that a first effective measurement result of the plurality of effective measurement results comprises the first classical measurement result and the second classical measurement result when the second fusion measurement fails in the selected basis.

### Claim 3 (depends on 1)

The method of claim 1, the method further comprising:
determining, based at least in part on the first classical measurement result, whether to employ boosting for the second fusion measurement.

### Claim 4 (depends on 3)

The method of claim 3,
wherein determining whether to employ boosting comprises:
determining whether the encoded fusion measurement result is determinable when the second fusion measurement fails in the selected basis, wherein it is determined not to employ boosting when the encoded fusion measurement result is determinable when the second fusion measurement fails in the selected basis.

### Claim 5 (depends on 3)

The method of claim 3,
wherein determining whether to employ boosting comprises:
determining a photon loss rate of the second fusion measurement;
determining loss probabilities for performing the second fusion measurement with and without boosting based on the photon loss rate;
determining success probabilities for performing the second fusion measurement with and without boosting based on the loss probabilities and based on failure probabilities for performing the second fusion measurement with and without boosting; and
determining whether to employ boosting based on a comparison of the success probabilities for performing the second fusion measurement with and without boosting.

### Claim 6 (depends on 1)

The method of claim 1,
wherein selecting the basis for performing the second fusion measurement comprises determining whether to apply a Hadamard gate at a fusion site while performing the second fusion measurement.

### Claim 7 (depends on 1)

The method of claim 1, the method further comprising:
computing an output of a quantum computational algorithm based at least in part on the encoded fusion measurement result; and
storing the output in the non-transitory memory medium.

### Claim 8

A system, comprising:
a non-transitory computer-readable memory medium;
a first encoded qubit comprising a first plurality of physical qubits;
a second encoded qubit comprising a second plurality of physical qubits;
a fusion controller; and
a plurality of fusion sites coupled to the fusion controller, wherein the system is configured to:
perform a first fusion measurement at a first fusion site of the plurality of fusion sites on a first physical qubit of the first plurality of physical qubits and a second physical qubit of the second plurality of physical qubits;
obtain a first classical measurement result of the first fusion measurement;
select a basis for performing a second fusion measurement based at least in part on the first classical measurement result, wherein the selected basis determines a failure basis for the second fusion measurement based at least in part on whether the first classical measurement result comprises a success, failure or loss outcome;
perform the second fusion measurement at a second fusion site of the plurality of fusion sites on a third physical qubit of the first plurality of physical qubits and a fourth physical qubit of the second plurality of physical qubits according to the selected basis;
obtain a second classical measurement result of the second fusion measurement;
determine an encoded fusion measurement result of a fusion of the first encoded qubit and the second encoded qubit based at least in part on the first and second classical measurement results; and
store the encoded fusion measurement result in a non-transitory memory medium,
wherein, in selecting the basis for performing the second fusion measurement based at least in part on the first classical measurement result, the system is further configured to:
determine that, when the second fusion measurement fails in a first basis, the encoded qubit measurement result may still be determined based at least in part on the first and the second classical measurement results; and
select the first basis as the selected basis for performing the second fusion measurement based at least in part on determining that, when the second fusion measurement fails in the first basis, the encoded qubit measurement result may still be determined based at least in part on the first and second classical measurement results.

### Claim 9 (depends on 8)

The system of claim 8, wherein the system is further configured to:
select one or more bases for performing one or more subsequent fusion measurements on physical qubits from the first and second plurality of physical qubits, wherein the one or more bases are selected based at least in part on classical measurement results of previous fusion measurements;
perform the one or more subsequent fusion measurements; and
obtain one or more subsequent classical measurement results,
wherein determining the encoded fusion measurement result is performed further based at least in part on the one or more subsequent classical measurement results.

### Claim 10 (depends on 8)

The system of claim 8,
wherein, in selecting the basis for performing the second fusion measurement based at least in part on the first classical measurement result, the system is further configured to:
determine, based on stabilizers of the first and second encoded qubits, a plurality of effective measurement results that correspond to the encoded fusion measurement result;
determine that a first effective measurement result of the plurality of effective measurement results comprises the first classical measurement result and the second classical measurement result when the second fusion measurement fails in the selected basis.

### Claim 11 (depends on 8)

The system of claim 8, wherein the system is further configured to:
determine whether the encoded fusion measurement result is determinable when the second fusion measurement fails in the selected basis;
employ boosting for the second fusion measurement when the encoded fusion measurement result is not determinable when the second fusion measurement fails in the selected basis; and
refrain from employing boosting for the second fusion measurement when the encoded fusion measurement result is determinable when the second fusion measurement fails in the selected basis.

### Claim 12 (depends on 8)

The system of claim 8,
wherein selecting the basis for performing the second fusion measurement comprises determining whether to apply a Hadamard gate at the second fusion site while performing the second fusion measurement.

### Claim 13 (depends on 8)

The system of claim 8, wherein the system is further configured to:
compute an output of a quantum computational algorithm based at least in part on the encoded fusion measurement result; and
store the output in the non-transitory computer-readable memory medium.

### Claim 14 (depends on 8)

The system of claim 8, wherein the system is further configured to:
determine, based at least in part on the first classical measurement result, whether to employ boosting for the second fusion measurement, wherein in determining whether to employ boosting, the system is configured to:
determine a photon loss rate of the second fusion measurement;
determine loss probabilities for performing the second fusion measurement with and without boosting based on the photon loss rate;
determine success probabilities for performing the second fusion measurement with and without boosting based on the loss probabilities and based on failure probabilities for performing the second fusion measurement with and without boosting; and
determine whether to employ boosting based on a comparison of the success probabilities for performing the second fusion measurement with and without boosting.

### Claim 15

A non-transitory computer-readable memory medium storing program instructions which, when executed by a processor, cause a fusion controller to:
perform a first fusion measurement on a first physical qubit of a first encoded qubit and a second physical qubit of a second encoded qubit;
obtain a first classical measurement result of the first fusion measurement;
select a first basis for performing a second fusion measurement based at least in part on the first classical measurement result, wherein the selected basis determines a failure basis for the second fusion measurement based at least in part on whether the first classical measurement result comprises a success, failure or loss outcome;
perform the second fusion measurement on a third physical qubit of the second encoded qubit and a fourth physical qubit of a third encoded qubit according to the first basis;
obtain a second classical measurement result of the second fusion measurement;
select a second basis for performing a third fusion measurement based at least in part on the first and second classical measurement results;
perform the third fusion measurement on a fifth physical qubit of the third encoded qubit and a sixth physical qubit of the first encoded qubit according to the second basis;
obtain a third classical measurement result of the third fusion measurement;
determine an encoded fusion measurement result of a fusion of the first, second and third encoded qubits based at least in part on the first, second and third classical measurement results; and
store the encoded fusion measurement result in the non-transitory computer-readable memory medium,
wherein, in selecting the first basis for performing the second fusion measurement based at least in part on the first classical measurement result, the program instructions are further executable to cause the fusion controller to:
determine that, when the second fusion measurement fails in the first basis, the encoded qubit measurement result may still be determined based at least in part on the first and the second classical measurement results.

### Claim 16 (depends on 15)

The non-transitory computer-readable memory medium of claim 15,
wherein, in selecting the second basis for performing the third fusion measurement based at least in part on the first and second classical measurement results, the program instructions are further executable to cause the fusion controller to:
determine that, when the third fusion measurement fails in the second basis, the encoded qubit measurement result may still be determined based at least in part on the first, second and third classical measurement results.

### Claim 17 (depends on 15)

The non-transitory computer-readable memory medium of claim 15, wherein the program instructions are further executable to cause the fusion controller to:
determine whether the encoded fusion measurement result is determinable when the second fusion measurement fails in the first basis;
employ boosting for the second fusion measurement when the encoded fusion measurement result is not determinable when the second fusion measurement fails in the first basis; and
refrain from employing boosting for the second fusion measurement when the encoded fusion measurement result is determinable when the second fusion measurement fails in the first basis.

### Claim 18 (depends on 15)

The non-transitory computer-readable memory medium of claim 15,
wherein selecting the first basis for performing the second fusion measurement comprises determining whether to apply a first Hadamard gate while performing the second fusion measurement, and
wherein selecting the second basis for performing the third fusion measurement comprises determining whether to apply a second Hadamard gate while performing the third fusion measurement.

### Claim 19 (depends on 15)

The non-transitory computer-readable memory medium of claim 15, wherein the program instructions are further executable to cause the fusion controller to:
compute an output of a quantum computational algorithm based at least in part on the encoded fusion measurement result; and
store the output in the non-transitory computer-readable memory medium.

### Claim 20 (depends on 15)

The non-transitory computer-readable memory medium of claim 15, wherein the program instructions are further executable to cause the fusion controller to:
determine, based at least in part on the first classical measurement result, whether to employ boosting for the second fusion measurement, wherein in determining whether to employ boosting, the program instructions are further executable to cause the fusion controller to:
determine a photon loss rate of the second fusion measurement;
determine loss probabilities for performing the second fusion measurement with and without boosting based on the photon loss rate;
determine success probabilities for performing the second fusion measurement with and without boosting based on the loss probabilities and based on failure probabilities for performing the second fusion measurement with and without boosting; and
determine whether to employ boosting based on a comparison of the success probabilities for performing the second fusion measurement with and without boosting.