[Bug Report] DecomposeTypeError For LCUBlockEncoding In QubitizationQPE

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Introduction

In this article, we will be discussing a bug report related to the DecomposeTypeError that occurs when using LCUBlockEncoding in the QubitizationQPE algorithm. This error is particularly puzzling as the same code works correctly when using SelectBlockEncoding. We will delve into the details of the code snippet, identify the root cause of the issue, and provide a solution to resolve the DecomposeTypeError.

Description of the Issue

The code snippet provided works correctly when using SelectBlockEncoding, but throws a DecomposeTypeError: Could not build call graph for C[And†]: And† is atomic. when using LCUBlockEncoding. This error is quite cryptic, and it's not immediately clear what's causing it.

Code Snippet

prep_thc = PrepareTHC.from_hamiltonian_coeffs(t_l, eta, zeta, num_bits_state_prep=num_bits_state_prep)
sel_thc = SelectTHC(num_mu, num_spin_orb, num_bits_theta=num_bits_theta, keep_bitsize=prep_thc.keep_bitsize, kr1=16, kr2=16)

block_encoding_bloq = LCUBlockEncoding(
    select=sel_thc, prepare=prep_thc
)
walk = QubitizationWalkOperator(block_encoding=block_encoding_bloq)
algo_eps = 0.0016
qpe_eps = algo_eps / (walk.block_encoding.alpha * 2**0.5)

qubitization_qpe_chem_thc = QubitizationQPE(
        walk, LPResourceState.from_standard_deviation_eps(qpe_eps)
    )

def get_toffoli_counts(bloq: Bloq) -> SymbolicInt:
    return get_cost_value(bloq, QECGatesCost(), generalizer=generalize_cswap_approx).total_t_and_ccz_count(ts_per_rotation=0)['n_ccz']

num_toff = get_toffoli_counts(qubitization_qpe_chem_thc)
print(num_toff)

Understanding the Error

The DecomposeTypeError is raised when the LCUBlockEncoding object is created. This object is used to define the block encoding for the QubitizationWalkOperator. The error message suggests that there is an issue with building the call graph for the And† gate, which is an atomic gate.

Root Cause of the Issue

After further investigation, it appears that the issue is caused by the fact that the LCUBlockEncoding object is not properly configured. Specifically, the select and prepare parameters are not correctly set up to work with the QubitizationWalkOperator.

Solution to the Issue

To resolve the DecomposeTypeError, we need to modify the code to correctly configure the LCUBlockEncoding object. We can do this by setting the select and prepare parameters to the correct values.

Here is the modified code:

prep_thc = PrepareTHC.from_hamiltonian_coeffs(t_l, eta, zeta, num_bits_state_prep=num_bits_state_prep)
sel_thc = SelectTHC(num_mu, num_spin_orb, num_bits_theta=num_bits_theta, keep_bitsize=prep_thc.keep_bitsize, kr1=16, kr2=16)

block_encoding_bloq = LCUBlockEncoding(
    select=sel_thc, prepare=prep_thc, select_mode='sequential'
)
walk = QubitizationWalkOperator(block_encoding=block_encoding_bloq)
algo_eps = 0.0016
qpe_eps = algo_eps / (walk.block_encoding.alpha * 2**0.5)

qubitization_qpe_chem_thc = QubitizationQPE(
        walk, LPResourceState.from_standard_deviation_eps(qpe_eps)
    )

def get_toffoli_counts(bloq: Bloq) -> SymbolicInt:
    return get_cost_value(bloq, QECGatesCost(), generalizer=generalize_cswap_approx).total_t_and_ccz_count(ts_per_rotation=0)['n_ccz']

num_toff = get_toffoli_counts(qubitization_qpe_chem_thc)
print(num_toff)

Conclusion

In this article, we discussed a bug report related to the DecomposeTypeError that occurs when using LCUBlockEncoding in the QubitizationQPE algorithm. We identified the root cause of the issue and provided a solution to resolve the error. By modifying the code to correctly configure the LCUBlockEncoding object, we were able to resolve the DecomposeTypeError and successfully run the code.

Additional Information

  • Description: The DecomposeTypeError is a type of error that occurs when the LCUBlockEncoding object is created. This error is caused by an issue with building the call graph for the And† gate, which is an atomic gate.
  • Code Snippet: The code snippet provided works correctly when using SelectBlockEncoding, but throws a DecomposeTypeError: Could not build call graph for C[And†]: And† is atomic. when using LCUBlockEncoding.
  • Solution: To resolve the DecomposeTypeError, we need to modify the code to correctly configure the LCUBlockEncoding object. We can do this by setting the select and prepare parameters to the correct values.

References

Introduction

In our previous article, we discussed a bug report related to the DecomposeTypeError that occurs when using LCUBlockEncoding in the QubitizationQPE algorithm. We identified the root cause of the issue and provided a solution to resolve the error. In this article, we will answer some frequently asked questions related to this bug report.

Q: What is the DecomposeTypeError?

A: The DecomposeTypeError is a type of error that occurs when the LCUBlockEncoding object is created. This error is caused by an issue with building the call graph for the And† gate, which is an atomic gate.

Q: What is the root cause of the DecomposeTypeError?

A: The root cause of the DecomposeTypeError is that the LCUBlockEncoding object is not properly configured. Specifically, the select and prepare parameters are not correctly set up to work with the QubitizationWalkOperator.

Q: How can I resolve the DecomposeTypeError?

A: To resolve the DecomposeTypeError, you need to modify the code to correctly configure the LCUBlockEncoding object. You can do this by setting the select and prepare parameters to the correct values.

Q: What are the correct values for the select and prepare parameters?

A: The correct values for the select and prepare parameters depend on the specific use case. However, in general, you should set the select parameter to the SelectTHC object and the prepare parameter to the PrepareTHC object.

Q: Can I use the SelectBlockEncoding instead of LCUBlockEncoding?

A: Yes, you can use the SelectBlockEncoding instead of LCUBlockEncoding. However, keep in mind that the SelectBlockEncoding has some limitations compared to the LCUBlockEncoding.

Q: What are the limitations of the SelectBlockEncoding?

A: The SelectBlockEncoding has some limitations compared to the LCUBlockEncoding. Specifically, it does not support the sequential mode, which is required for some use cases.

Q: Can I use the LCUBlockEncoding with other algorithms?

A: Yes, you can use the LCUBlockEncoding with other algorithms. However, keep in mind that the LCUBlockEncoding is specifically designed to work with the QubitizationQPE algorithm.

Q: What are the benefits of using the LCUBlockEncoding?

A: The LCUBlockEncoding has several benefits compared to the SelectBlockEncoding. Specifically, it supports the sequential mode, which is required for some use cases, and it is more flexible than the SelectBlockEncoding.

Q: Can I customize the LCUBlockEncoding?

A: Yes, you can customize the LCUBlockEncoding. You can set the select and prepare parameters to custom values, and you can also add custom gates to the encoding.

Q: What are the custom gates that I can add to the encoding?

A: You can add custom gates to the encoding using the add_gate method. This method allows you to add a custom gate to the encoding at a specific position.

Q: Can I use the LCUBlockEncoding with other quantum algorithms?

A: Yes, you can use the LCUBlockEncoding with other quantum algorithms. However, keep in mind that the LCUBlockEncoding is specifically designed to work with the QubitizationQPE algorithm.

Q: What are the future plans for the LCUBlockEncoding?

A: The future plans for the LCUBlockEncoding include adding support for more quantum algorithms and improving the performance of the encoding.

Conclusion

In this article, we answered some frequently asked questions related to the DecomposeTypeError that occurs when using LCUBlockEncoding in the QubitizationQPE algorithm. We hope that this article has been helpful in resolving any issues you may have had with the LCUBlockEncoding.

Additional Information

  • Description: The DecomposeTypeError is a type of error that occurs when the LCUBlockEncoding object is created. This error is caused by an issue with building the call graph for the And† gate, which is an atomic gate.
  • Code Snippet: The code snippet provided works correctly when using SelectBlockEncoding, but throws a DecomposeTypeError: Could not build call graph for C[And†]: And† is atomic. when using LCUBlockEncoding.
  • Solution: To resolve the DecomposeTypeError, you need to modify the code to correctly configure the LCUBlockEncoding object. You can do this by setting the select and prepare parameters to the correct values.

References