Quantum Computing Glossary

What is Measurement ?

Related Terms

A measurement is an operation which happens during or in the final step of a quantum computation. Usually, measurements are used in the end to average through the possible quantum circuit computations. However, mid-circuits measurements can also occur and can be used for feedforward outcome or adaptive quantum computation. 

In a quantum circuit, single or multi-qubit measurements happen. A single-qubit measurement collapses the state of the specific qubit and corresponds to an expectation value of an operator measure. However, in the case of multi-qubit measurements we measure simultaneously expectation values of two or more qubits. This will induce the collapse of two or more qubits in a state.  

 Research topics regarding measurements in quantum computation 

  1. The measurement-based quantum computation (MBQC) approach is a type of computation where an initial large resource state is prepared, and the computation is performed only through single-qubit measurements and corrections, instead of the usual gates in a quantum circuit. This approach has also been called one-way quantum computer. Research has been done in the generation of optimal initial resource states for MBQC.  
  1. Measurements can be used to entangle states too. For example, multi-qubit heralded measurements are used in fusion quantum computing to entangle photonic qubits. In addition, counterintuitively, single-qubit measurements can be used in an entangled state to entangle qubits that are not directly correlated to each other, with applications in quantum communication protocols.  
  1. Measurements are also important in quantum error correction, where ancilla qubits are measured and used to correct a noisy state.  

Frequently asked question about measurements: 

  1. How fast do the measurements happen?  

It depends on the platform, but the readout of qubits can be slow and can take up a substantial amount of time of the overall computation. For photonic platforms, the measurement process is fast and can be done, for instance, with superconducting nanowire single-photon detectors.  

  1. Is a single measurement sufficient?  

No, in most cases, the goal is to estimate an expectation value and thus thousands of rounds of measurements should be performed to extract information about a circuit, before averaging over all the realizations. Additionally, the outcomes can be used to reconstruct the quantum state (density matrix) through quantum state tomography.