Researchers have developed an experimental method to determine whether the functions performed by quantum computers are the result of quantum mechanics or just a clever twist on classical physics.
In a groundbreaking study published on April 22, 2025 in the journal Physical Review
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The scientists achieved this by creating a programmable 73-qubit “honeycomb” quantum processor and training it using a hybrid quantum-classical technique called variable quantum circuits (VQC). This is a machine learning loop in which a classical computer iteratively helps a quantum computer perform tasks with greater accuracy.
In this case, the computer’s task was to reach a lower energy state than classical physics could achieve. By identifying this energy state, the researchers demonstrated quantum mechanics.
Use 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 phones, laptops, PCs, servers, and supercomputers are constrained by the fundamental laws of classical physics.
Classical computing bits use 1s and 0s to perform complex calculations, but they can only process calculations in sequence. After all, there are limits to what you can accomplish in a feasible amount of time.
Quantum computers, on the other hand, use qubits (qubits, the classical equivalent of bits) to perform complex calculations in parallel, taking advantage of the strange laws of quantum mechanics, such as quantum entanglement. If the state of a bit can be represented as on or off (1 or 0), then the qubit occupies a superposition of both on and off states until it is measured (that is, it can be in either state or any combination of states).
Quantum entanglement occurs when two qubits become correlated over distance. Measuring the states reveals the entangled states of the associated qubits. According to classical physics, this is the same as tossing a coin in London to determine the outcome of flipping a coin simultaneously in New York. As more entangled qubits are added to the system, the computational space grows exponentially.
Once large enough, the theoretical computational space of a quantum computer becomes mathematically intractable for a binary computer system. This is called “quantum supremacy” or “quantum supremacy.”
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Although quantum phenomena can be demonstrated using experiments such as the double-slit experiment, it is difficult to prove that multi-qubit systems truly exploit quantum mechanics. It also becomes exponentially more difficult as the number of qubits in a quantum system increases.
Bell test and spooky action from afar
Physicists like Albert Einstein have long wondered about the threshold at which quantum phenomena break Newton’s laws of physics. Essentially, the question becomes: Is there no classical explanation for quantum operations, or is there simply no explanation found?
For example, when shown entanglement, Einstein famously called it “spooky movement seen from a distance.” His worldview was based on local realism, which held that objects are influenced only by their immediate environment (locality) and that their properties exist determinately before we can measure them (realism).
Entanglement destroys this relativity. When two particles become entangled, they exist in a state of nonlocality. To prove this, scientists perform the Bell test, named after Irish physicist John Stewart Bell. This involves measuring entangled particles in multiple randomly selected ways and checking the statistical results.
If the correlation between the measured outcomes is stronger than classical theory allows (a limit known as Bell’s inequality), the system is said to be nonlocal.
This proves that “spooky behavior in the distance” is not just a coincidence, a mathematical trick, or the result of a classic simulation, but is real.
Brute force simulation
One of the main hurdles in determining whether quantum computation is actually quantum in nature is the fact that classical computers can simulate quantum states to some extent using brute-force mathematics. This makes it difficult to determine exactly what is going on “under the hood.”
Since there are no red flags or sirens to indicate that the laws of physics have been broken when a quantum operation is performed, scientists must find a way to demonstrate the fundamental quantum mechanics behind it.
To accomplish this, the researchers performed an experiment by setting a 73-qubit quantum computer to the lowest possible energy state and measuring the energy in the system.
In classical physics, the lowest ground state that can be achieved is zero. A ball rolling down a hill has a high excited energy state. In its lowest energy state, the ground state, the ball has no energy and is at rest.
However, the same ball operating according to the laws of quantum mechanics can have energy states lower than zero. This is possible through entanglement. If one ball becomes entangled with another and both are correlated through functionally opposite energy states, one or both can be placed in a negative energy state.
Since this is not possible with the laws of classical physics, confirmation of this negative state would by definition prove that the physics driving the system is indeed quantum.
The confirmed results were very low in energy, 48 standard deviations below the absolute minimum energy level that a classical system can have.
The researchers demonstrated these nonlocal correlations in groups of up to 24 qubits within a larger system, the largest number ever demonstrated at once using this method, the scientists say in the study.
They added that the study establishes a pioneering method to examine quantum activity.
Further development of these techniques could allow engineers to demonstrate performance on different quantum architectures, understand when quantum states “decoheal” to classical states, and provide the foundation for building even larger and more powerful quantum computers.
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