Exploring the Observer Effect
Learn how we can explore the observer effect and draw a circuit.
A qubit is a two-level quantum system in a superposition of the quantum states and , unless we observe it. Once we observe it, there are distinct probabilities of measuring 0 or 1. In physics, this is known as the observer effect. The observer effect states that the mere observation of a phenomenon inevitably changes that phenomenon itself. For instance, if you measure the temperature in your room, you’re taking away a little bit of the energy to heat the mercury in the thermometer. This loss of energy cools down the rest of your room. In the world we experience, the effects of observation are often negligible.
However, in the sub-atomic world of quantum mechanics, these effects matter. The mere observation of a quantum bit changes its state from a superposition of the states and to either one value. Therefore, even an observation manipulates the system that we need to consider when developing a quantum circuit.
A circuit without measurement
Let’s revisit the quantum circuit from the lesson Exploring the Quantum States. Here’s the code and the result if we run it:
from qiskit import QuantumCircuit, execute, Aerfrom qiskit.visualization import plot_histogramfrom math import sqrt# Create a quantum circuit with one qubitqc = QuantumCircuit(1)# Define state |Psi>initial_state = [1/sqrt(2), 1/sqrt(2)]# Apply initialization operation to the qubit at position 0qc.initialize(initial_state, 0)# Tell Qiskit how to simulate our circuitbackend = Aer.get_backend('statevector_simulator')# Do the simulation, returning the resultresult = execute(qc,backend).result()# Get the data and display histogramcounts = result.get_counts()plot_histogram(counts)
Our circuit consists of a single qubit in line 6. It has the initial state [1/sqrt(2), 1/sqrt(2)] in line 9 that we initialize our quantum circuit with in line 12.
Here is the Dirac and the vector notation of this state:
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