An experimental test of the nonlocal energy alteration between two quantum memories

Quantum technologies operate by leveraging various quantum mechanical effects, including entanglement. Entanglement occurs when two or more particles share correlated states even if they are distant.

When two particles are spin entangled, the intrinsic angular momentum (i.e., spin) of one particle can influence that of its entangled partner. This would suggest that the energy of the second particle can be altered via a nonlocal correlation, without enabling faster-than-light communication.

Researchers at Shanghai Jiao Tong University and Hefei National Laboratory recently carried out a study aimed at testing this theoretical prediction experimentally using two quantum memories.

Their findings, published in Physical Review Letters, appear to confirm the existence of nonlocal energy alterations, thus broadening the present understanding of quantum nonlocality.

“When two particles are in a spin-entangled state, measuring one particle nonlocally influences the spin state of the other,” Xian-Min Jin and Dr. Jian-Peng Dou, co-authors of the paper, told Phys.org.

“This insight led us to a bold conjecture: quantum correlations could enable the nonlocal alteration of energy distribution in space. This seemingly surreal phenomenon was alluded to in the de Broglie-Bohm theory, yet it has neither been formally named nor experimentally tested.”

To probe the existence of the nonlocal energy alteration predicted by earlier theoretical works, Jin, Dr. Dou and their colleagues used two quantum memories, devices that can generate, store, probe and retrieve quantum states.

Using these memories, they created an optical device that can separate and recombine a quantum system’s wavefunctions to measure quantum interference, also known as a Mach-Zehnder interferometer.

“We denote the Stokes photon (S1) generated during the write process of two quantum memories as the first particle, while the simultaneously generated atomic excitation serves as the second particle,” explained Jin and Dr. Dou.

“Since these two particles originate from the same spontaneous Raman scattering process, they naturally possess the quantum correlation required for this study.”

With their experimental setup, the researchers were able to determine the position of the atomic excitation (i.e., serving as the second particle in the system) and its associated measurement. This was attained either through a strong measurement by performing a readout operation on the quantum memories, or through a weak probe-based method known as single-photon Raman scattering.

“The weak probe process can be metaphorically described as follows: imagine an observer with obstructed vision attempting to locate the atomic excitation (i.e., the energy),” said Jin and Dr. Dou.

“Each observation only slightly perturbs the quantum memory, while yielding blurred yet useful information about the energy’s position. Although this information about position is imprecise, it plays a crucial role when combined with post-selection, allowing the verification of quantum correlations between past and future events.”

Jin, Dr. Dou and his colleagues were ultimately able to predict the distribution of Bohm trajectories of the Stokes photon in their system, as well as changes in the position of the atomic excitation and associated conditional probabilities.

They then compared the magnitude of the probabilities they measured, to verify the nonlocal nature of the de Broglie-Bohm interpretation, which is the theory predicting the existence of the nonlocal energy alteration they observed.

“Our experimental results are consistent with the predictions of the nonlocal theory,” said Jin and Dr. Dou. “The results imply that, in the framework of the de Broglie-Bohm theory, for two entangled particles, the energy carried by one of them can be changed from one place to another under the non-local influence of the other particle.

“This is exactly the ‘nonlocal energy alteration’ proposed in the study. It is important to emphasize that the term used here is ‘alteration’ rather than ‘transfer,’ meaning that this process does not involve superluminal energy transmission (i.e., it is a nonlocal energy modification induced by quantum correlations).”

The researchers’ experimental exploration of quantum nonlocality from an energy standpoint yielded interesting results, which could inform future studies focusing on nonlocal energy alterations between spin entangled particles.

Other physicists could soon draw inspiration from their study, using similar experimental methods to test the de Broglie-Bohm theory.

“For the time being, we do not reject the probabilistic interpretation of quantum mechanics while supporting Bohm’s theory,” added Jin and Dr. Dou.

“In this study, quantum memory exhibits unique capabilities that could contribute to testing fundamental problems in quantum mechanics. These include in-depth investigations of quantum nonlocality, delayed choice, the empty wave, light-speed oscillations in the interference region, and the intrinsic consistency between quantum mechanics and the principles of relativity.”

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