# In landmark experiment, researchers test superconducting qubits to pass a Bell test

In a groundbreaking experiment, Simon Storz at the Swiss Federal Institute of Technology in Zurich and his colleagues performed a superconducting circuit that passed a Bell test. The Bell test is the all-important test in physics for confirming the quantum behavior of a system. The superconducting circuits are utilized in quantum computers, which helps to prove that their quantum bits are entangled.

For bits or particles to be entangled, the measurement of one’s characteristics affects the measured characteristics of the other particle immediately in a process called a non-local correlation. When this happens, it simply means the effects of the entanglement must travel at a speed faster than light. This test for the quantum effect is called Bell’s inequality.

## Bell inequality

The test for this quantum effect, Bell’s inequality, sets a limit on the number of occurrences in which particles end up in the same state by chance without an entanglement. By violating Bell’s inequality, this is proof that a pair of particles are actually entangled.

In the past, Bell tests have been carried out in several systems. However, this is the first on a superconducting circuit. For this test, the two entangled systems will be far enough apart so that a signal could not have traveled between them at the speed of light at the same time it takes to measure the systems. This is a daunting task in a superconducting circuit, as the systems must be kept at a temperature close to zero.

Simon Storz and his colleagues managed this by connecting two entangled parts of the circuits called qubits or quantum bits using microwaves sent through a chilled 30-meter-long aluminum tube. They did this while also keeping each qubit in its own refrigerator.

Furthermore, they proceeded to use a random number generator to decide the measurement to make on the qubits to eliminate any human bias. Simon Storz and his colleagues made over 4 million measurements at a rate of 12,500 measurements per second. This speed is necessary to ensure that each pair of measurements occurs faster than light could travel down the tube between the two quantum bits. By analyzing these data points, the researchers found that Bell’s inequality was violated, and the quantum bits underwent what Albert Einstein called “spooky action at a distance.”