Quantum computing ‘lie detector’ finally proves these machines tap into Einstein’s spooky action at a distance rather than just faking it
Researchers have developed an experimental method to determine whether the functions performed by a quantum computer are the result of quantum mechanics or simply a clever variation of classical physics.
In a historical study published April 22, 2025, in the newspaper Physical examinationThe researchers describe an experimental test that demonstrates and certifies computational activity that can only be achieved through quantum mechanics.
Scientists achieved this by creating a programmable “honeycomb” of 73 qubits. quantum processor and train it using a hybrid quantum-classical technique called variational quantum circuit (VQC). It is a machine learning loop in which a classical computer iteratively helps a quantum computer perform a task with greater precision.
In this case, the computer’s task was to achieve an energy state so low that it could not be achieved by classical physics. By confirming this energy state, the researchers demonstrated quantum mechanics.
Exploiting the laws of quantum mechanics
One of the ultimate goals of quantum computing is to push the limits of what computers can do beyond what the laws of classical physics allow. Binary computers, such as our phones, laptops, PCs, servers and supercomputers are limited by the fundamental laws of classical physics.
Bits in classical computing use 1s and 0s to perform complex calculations, but they can only process calculations in sequence. Ultimately, there is a limit to what they can accomplish in a reasonable amount of time.
Quantum computers, on the other hand, use qubits – the quantum equivalent of a classical bit – to exploit the strange laws of quantum mechanics, such as quantum entanglementto perform complex calculations in parallel. Where the state of a bit can be represented as on or off (with a 1 or a 0), a qubit occupies a superposition of on and off states (meaning it can be one state or any combination of states) until it is measured.
Quantum entanglement occurs when two qubits become remotely correlated. Measuring the state of one of them reveals the states of all associated entangled qubits. According to the laws of classical physics, this would be equivalent to flipping a coin in London to determine the results of a simultaneous toss in New York. As more entangled qubits are added to a system, the computational space increases exponentially.
At a sufficient size, the theoretical computational space of a quantum computer becomes mathematically intractable for a binary computing system — this is described as “quantum advantage” or “quantum supremacy“.
Although quantum phenomena can be demonstrated using experiments such as the double slit experiencecertifying that a multi-qubit system truly exploits quantum mechanics is a challenge. It also becomes exponentially more difficult as the number of qubits in a quantum system increases.
The Bell Test and Scary Action at a Distance
Physicists like Albert Einstein long considered the threshold from which quantum phenomena violate the laws of Newtonian physics. Essentially, the problem boils down to whether there is no classical explanation for a quantum operation, or whether we simply haven’t found one.
For example, when faced with entanglement, Einstein called it “spooky action at a distance.” His worldview, based on local realism, insisted that objects are affected only by their immediate environment (locality) and that their properties definitely exist before we measure them (realism).
Entanglement breaks this relativity. When two particles intertwine, they exist in a state of nonlocality. To prove this, scientists are carrying out a Bell testnamed after the Irish physicist John Stewart Bell. This involves measuring entangled particles in several randomly chosen ways and verifying the statistical results.
If the correlations between measured outcomes are stronger than any classical theory could ever allow – a limit known as Bell’s inequality – then the system is said to be nonlocal.
This proves that “scary action at a distance” is real and not just the result of chance, mathematical trickery, or classic simulation.
Brute force simulations
One of the main obstacles in determining whether quantum calculations are truly quantum in nature is the fact that classical computers can simulate quantum states, to a certain extent, using brute force mathematics. This makes it difficult to determine exactly what is going on “under the hood”.
Since there are no warning bells or sirens indicating that the laws of physics have been broken when performing a quantum operation, scientists must find ways to demonstrate the quantum mechanics underlying these operations.
To achieve this, the researchers conducted an experiment using a 73-qubit quantum computer by tuning it to its lowest possible energy state and then measuring the energy in the system.
In classical physics, the lowest ground state achievable is zero. A ball rolling down a hill has a high, excited energy state. At its lowest energy state, its ground state, the ball is at rest without energy.
The same ball, operating according to the laws of quantum mechanics, could however have an energy state less than zero. This is possible thanks to entanglement. If a ball is entangled with another ball and the two are correlated through functionally diametrical energy states, one or both can be placed in a negative energy state.
Because this is not possible according to the laws of classical physics, confirmation of this negative state is, by definition, a certification that the physics that governs the system is indeed quantum.
The confirmed result was an energy so low that it fell below the absolute minimum energy level a classical system could ever possess, at 48 standard deviations.
The researchers certified these nonlocal correlations in groups of up to 24 qubits within a larger system, the largest number ever certified at once in this way, the scientists wrote in the study.
This work establishes a pioneering method for verifying quantum activity, they added.
With further development, these techniques could help engineers certify the performance of various quantum architectures, understand when quantum states “decohere” into classical states, and provide the foundation for building even larger and more powerful quantum computers.
