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A hybrid quantum-classical (HQC) computer prepares a quantum Boltzmann machine (QBM) in a pure state. The state is evolved in time according to a chaotic, tunable quantum Hamiltonian. The pure state locally approximates a (potentially highly correlated) quantum thermal state at a known temperature. With the chaotic quantum Hamiltonian, a quantum quench can be performed to locally sample observables in quantum thermal states. With the samples, an inverse temperature of the QBM can be approximated, as needed for determining the correct sign and magnitude of the gradient of a loss function of the QBM.

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**Independent Claim 1.**A hybrid quantum-classical computer, comprising: a classical computing componenta quantum computing component comprising: a first plurality of m qubits and a second plurality of n qubits prepared in a first quantum state, wherein n is less than mthe first plurality of qubits interacting with each other according to a Hamiltonian specified by the classical computing componentthe second plurality of qubits interacting with each other according to the Hamiltonianand the second plurality of qubits weakly interacting with the first plurality of qubits according to the Hamiltonianand a measurement unit that measures: (1) a first set of expectation values of observables on the first plurality of qubitsand (2) a second set of expectation values of observables on the second plurality of qubitsand the classical computing component comprising a processor that receives the first and second sets of expectation values from the measurement unit and prepares a second quantum state based on the first quantum state and the first and second sets of expectation values.**Independent Claim 12.**A method for preparing, by a hybrid quantum-classical computer, a state of a quantum Boltzmann machine that follows a probability distribution which locally approximates a Boltzmann distribution at a known temperature, the hybrid quantum-classical computer comprising: a classical computing componenta quantum computing component comprising: a measurement unita first plurality of m qubits and a second plurality of n qubits prepared in a first quantum state, wherein n is less than mthe first plurality of qubits interacting with each other according to a Hamiltonian specified by the classical computing componentthe second plurality of qubits interacting with each other according to the Hamiltonianand the second plurality of qubits weakly interacting with the first plurality of qubits according to the Hamiltonianand the classical computing component including a processor, a non-transitory computer-readable medium, and computer-program instructions stored in the non-transitory computer-readable mediumthe method comprising: at the measurement unit of the quantum computing component: (1) measuring a first set of expectation values of observables on the first plurality of qubitsand (2) measuring a second set of expectation values of observables on the second plurality of qubitsand at the classical computing component: (3) receiving the first and second sets of expectation values from the measurement unitand (4) preparing a second quantum state based on the first quantum state and the first and second sets of expectation values.