Verified quantum phase estimation

An apparatus for quantum phase estimation of a first N-qubit unitary operator on a quantum state, the apparatus comprising:

• one or more classical processors;
• and
• quantum computing hardware in data communication with the one or more classical processors, wherein the quantum computing hardware comprises:
• one or more system registers, each system register comprising one or more qubits,
• one or more control qubits, and
• a plurality of control devices configured to operate the one or more system registers and one or more control qubits;
• wherein the apparatus is configured to perform operations comprising:
• initializing a first classical variable and a second classical variable;
• generating a set of measurement data, comprising repeatedly performing a phase estimation experiment, wherein at each repetition a current value of a classical variable is incremented based on a measured outcome of the phase estimation experiment and performing one repetition of the phase estimation experiment comprises:
• preparing a system register comprising N qubits in the quantum state, comprising applying a second unitary operator to the system register, wherein prior to application of the second unitary operator each qubit in the system register is initialized in a target computational basis state;
• applying, conditioned on a state of a control qubit, the first unitary operator a number of times to the system register in the quantum state to generate an evolved quantum state, wherein prior to application of the first unitary operator a number of times the control qubit is initialized in a superposition state;
• applying an inverse of the second unitary operator to the system register in the evolved quantum state and measuring each qubit in the system register to determine an output quantum state of the system register;
• measuring the control qubit to obtain a corresponding measurement result m;
• and
• post-selecting on the target computational basis state, comprising in response to determining that the output quantum state indicates that each qubit was in the target computational basis state prior to measurement, incrementing the first or second classical variable by (−1)m;
• and
• estimating one or more phases, eigenstate amplitudes or expectation values of the first unitary operator or other operators based on the set of measurement data.

A method for quantum phase estimation of a first N-qubit unitary operator on a quantum state, the method comprising:

• initializing a first classical variable and a second classical variable;
• generating a set of measurement data, comprising repeatedly performing a phase estimation experiment, wherein at each repetition a current value of a classical variable is incremented based on a measured outcome of the phase estimation experiment and performing one repetition of the phase estimation experiment comprises:
• preparing a register comprising N qubits in an initial quantum state, comprising preparing N−1 qubits in a target computational basis state and an N-th qubit in a superposition state;
• applying a second N-qubit unitary operator to the register in the initial quantum state to obtain a superposition state, the superposition state comprising a superposition of the quantum state and an eigenstate of the first N-qubit unitary operator;
• applying the first N-qubit unitary operator a number of times to the register in the superposition state to generate an evolved superposition state;
• applying an inverse of the second N-qubit unitary operator to the register in the evolved superposition state and measuring each of the N−1 qubits in the register to determine an output states of the N−1 qubits;
• measuring the N-th qubit to obtain a corresponding measurement result m;
• and
• post-selecting on the target computational basis state, comprising in response to determining that the output states of the N−1 qubits indicate that each of the N−1 qubits were in the target computational basis state prior to measurement, incrementing the first or second classical variable by (−1)m;
• and
• estimating one or more phases, eigenstate amplitudes or expectation values of the first unitary operator based on the set of measurement data.

An apparatus for quantum phase estimation of a first N-qubit unitary operator on a quantum state, the apparatus comprising:

• one or more classical processors;
• and
• quantum computing hardware in data communication with the one or more classical processors, wherein the quantum computing hardware comprises:
• one or more qubit registers, each qubit register comprising one or more qubits,
• a plurality of control devices configured to operate the one or more qubit registers;
• wherein the apparatus is configured to perform operations comprising:
• initializing a first classical variable and a second classical variable;
• generating a set of measurement data, comprising repeatedly performing a phase estimation experiment, wherein at each repetition a current value of a classical variable is incremented based on a measured outcome of the phase estimation experiment and performing one repetition of the phase estimation experiment comprises:
• preparing a register comprising N qubits in an initial quantum state, comprising preparing N−1 qubits in a target computational basis state and an N-th qubit in a superposition state;
• applying a second N-qubit unitary operator to the register in the initial quantum state to obtain a superposition state, the superposition state comprising a superposition of the quantum state and an eigenstate of the first N-qubit unitary operator;
• applying the first N-qubit unitary operator a number of times to the register in the superposition state to generate an evolved superposition state;
• applying an inverse of the second N-qubit unitary operator to the register in the evolved superposition state and measuring each of the N−1 qubits in the register to determine an output states of the N−1 qubits;
• measuring the N-th qubit to obtain a corresponding measurement result m;
• and
• post-selecting on the target computational basis state, comprising in response to determining that the output states of the N−1 qubits indicate that each of the N−1 qubits were in the target computational basis state prior to measurement, incrementing the first or second classical variable by (−1)m;
• and
• estimating one or more phases, eigenstate amplitudes or expectation values of the first unitary operator based on the set of measurement data.

A method for quantum error mitigation in a quantum computing system, the method comprising:

• generating a set of classical control data comprising at least one classical variable by repeating a plurality of iterations, wherein each iteration comprises:
• preparing a system register comprising multiple qubits in an initial quantum state;
• applying a unitary operator to the system register in the initial quantum state to obtain a first evolved quantum state, wherein the unitary operator is dependent on a target quantum computation;
• performing, conditioned on a state of a control qubit initialized in a superposition state, the target quantum computation on the system register in the first evolved quantum state to obtain a second evolved quantum state;
• applying an inverse of the unitary operator to the system register in the second evolved quantum state to obtain a third evolved quantum state;
• and
• measuring i) each qubit in the system register in the third evolved quantum state to determine an output quantum state of the system register and ii) the control qubit to determine an output quantum state of the control qubit;
• updating the at least one classical variable using the output quantum state of the control qubit unless the output quantum state of the system register indicates that the system register was not in the initial quantum state prior to measurement;
• and
• after the plurality of iterations have been completed, changing an operating parameter of the quantum computing system or adjusting a measured value based on the set of classical control data.

An apparatus comprising:

• one or more classical processors;
• and
• quantum computing hardware in data communication with the one or more classical processors, wherein the quantum computing hardware comprises:
• one or more system registers, each system register comprising one or more qubits,
• one or more control qubits, and
• a plurality of control devices configured to operate the one or more system registers and one or more control qubits;
• wherein the apparatus is configured to perform operations comprising:
• generating a set of classical control data comprising at least one classical variable by repeating a plurality of iterations, wherein each iteration comprises:
• preparing a system register comprising multiple qubits in an initial quantum state;
• applying a unitary operator to the system register in the initial quantum state to obtain a first evolved quantum state, wherein the unitary operator is dependent on a target quantum computation;
• performing, conditioned on a state of a control qubit initialized in a superposition state, the target quantum computation on the system register in the first evolved quantum state to obtain a second evolved quantum state;
• applying an inverse of the unitary operator to the system register in the second evolved quantum state to obtain a third evolved quantum state;
• and
• measuring i) each qubit in the system register in the third evolved quantum state to determine an output quantum state of the system register and ii) the control qubit to determine an output quantum state of the control qubit;
• updating the at least one classical variable using the output quantum state of the control qubit unless the output quantum state of the system register indicates that the system register was not in the initial quantum state prior to measurement;
• and
• after the plurality of iterations have been completed, changing an operating parameter of the quantum computing system or adjusting a measured value based on the set of classical control data.

A method for quantum error mitigation in a quantum computing system, the method comprising:

• generating a set of classical control data comprising at least one classical variable by repeating a plurality of iterations, wherein each iteration comprises:
• preparing a system register comprising multiple qubits in an initial quantum state;
• applying a unitary operator to the system register in the initial quantum state to obtain a first evolved quantum state, wherein the unitary operator is dependent on a target quantum computation;
• performing the target quantum computation on the system register in the first evolved quantum state to obtain a second evolved quantum state;
• applying an inverse of the unitary operator to the system register in the second evolved quantum state and measuring each qubit in the system register to determine an output quantum state of the system register;
• and
• after applying the inverse of the unitary operator, updating the at least one classical variable using the output quantum state of the system register unless the output quantum state indicates that the system register was not in the initial quantum state prior to measurement;
• and
• after the plurality of iterations have been completed, changing an operating parameter of the quantum computing system or adjusting a measured value based on the set of classical control data.

An apparatus comprising:

• one or more classical processors;
• and
• quantum computing hardware in data communication with the one or more classical processors, wherein the quantum computing hardware comprises:
• one or more qubit registers, each qubit register comprising one or more qubits,
• a plurality of control devices configured to operate the one or more qubit registers;
• wherein the apparatus is configured to perform operations comprising:
• generating a set of classical control data comprising at least one classical variable by repeating a plurality of iterations, wherein each iteration comprises:
• preparing a system register comprising multiple qubits in an initial quantum state;
• applying a unitary operator to the system register in the initial quantum state to obtain a first evolved quantum state, wherein the unitary operator is dependent on a target quantum computation;
• performing the target quantum computation on the system register in the first evolved quantum state to obtain a second evolved quantum state;
• applying an inverse of the unitary operator to the system register in the second evolved quantum state and measuring each qubit in the system register to determine an output quantum state of the system register;
• and
• after applying the inverse of the unitary operator, updating the at least one classical variable using the output quantum state of the system register unless the output quantum state indicates that the system register was not in the initial quantum state prior to measurement;
• and
• after the plurality of iterations have been completed, changing an operating parameter of the quantum computing system or adjusting a measured value based on the set of classical control data.