Add MPSStateSpace.Sample and its unit test for small case. #460
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| @@ -586,6 +586,51 @@ TEST(MPSStateSpaceTest, InnerProduct4) { | |||
| EXPECT_NEAR(f, 0.5524, 1e-4); | |||
| } | |||
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| TEST(MPSStateSpaceTest, SamplingSmall) { | |||
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Could we maybe add another test with other states, perhaps a simple GHZ or bit flipped state as well ?
| const MPS& state, uint64_t num_samples, unsigned seed) const { | ||
| std::vector<uint64_t> bitstrings; | ||
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| if (num_samples > 0) { | ||
| double norm = 0; | ||
| uint64_t size = MinSize(state.num_qubits()) / 2; | ||
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| const fp_type* p = state.get(); | ||
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| for (uint64_t k = 0; k < size; ++k) { | ||
| auto re = p[2 * k]; | ||
| auto im = p[2 * k + 1]; | ||
| norm += re * re + im * im; | ||
| } | ||
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| auto rs = GenerateRandomValues<DistrRealType>(num_samples, seed, norm); | ||
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| uint64_t m = 0; | ||
| double csum = 0; | ||
| bitstrings.reserve(num_samples); | ||
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| for (uint64_t k = 0; k < size; ++k) { | ||
| auto re = p[2 * k]; | ||
| auto im = p[2 * k + 1]; | ||
| csum += re * re + im * im; | ||
| while (rs[m] < csum && m < num_samples) { | ||
| bitstrings.emplace_back(k); | ||
| ++m; | ||
| } | ||
| } | ||
| } |
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I'm a little worried about this implementation of Sample(). If a user were to make an MPS on 64 qubits or more, I think the uint64_t type might overflow. Also, Is this logic used here correct ? Looking at the implementation done here (https://github.com/quantumlib/Cirq/blob/master/cirq-core/cirq/contrib/quimb/mps_simulator.py#L469) it looks like we should be tracing out qubits as we go and assigning them values for each sample we draw. Are we still doing this here ?
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Yes good catch, thank you for the standard process of MPS measurement. I need to add the contraction process (tracing out). Let me work on it more.
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Going to close this now since it is done in #490 . |
Resolve #441