The well-known thought experiment of Einstein, Rosen and Podolsky is being tested with a new demonstration involving hundreds of entangled atoms. EPR (Einstein, Podolsky, and Rosen) put forward a thesis in 1935 that quantum physics offers only a partial explanation of reality. Two assumptions support the argument. First, there is an "element of reality" which means that if the value of a physical property of a system can be predicted with certainty without causing any disruption to the system, it has a value even when it is not measured. Second, physical processes have effects that act locally, rather than having an instantaneous effect over long distances.
These "local realism" assumptions were later tested experimentally by John Bell, and subsequent Bell tests proved these assumptions wrong for systems consisting of a few tiny particles such as electrons or photons. Paolo Colciaghi and colleagues at the University of Basel in Switzerland have now tested the case of EPR for a larger system with clouds of hundreds of atoms. Their findings cast doubt on the applicability of the local realism of EPR in the context of mesoscopic large systems.
The EPR considered a system consisting of two particles, A and B, separated in space and having non-observable pairs in common, including their positions and momentums. Systems are set up in such a way that the momentum of the particles is not related to each other and their positions are related to each other.
Because of the coupling between observables, an experimenter must be able to accurately measure B to predict the position or momentum of particle A. It is very important to note that the system is configured in such a way that the particles are "space separated", eliminating the possibility of A being disturbed by a measurement at B.
EPR concludes that particle positions and momenta are well defined simultaneously under the assumption of local realism. However, quantum mechanics prohibits the simultaneous and precise definition of momentum and position values. The EPR argued that quantum mechanics was insufficient to explain this contradiction, pointing out that a complete theory should include a possibility currently known as local latent variables, which Bell tests now exclude [2, 3]. Bell tests have since refuted the theory of EPR.
Colciaghi and colleagues use clouds of several hundred rubidium-87 atoms, as opposed to the individual particle pairs used in most Bell experiments. To circulate the condensation atoms, they first prepare a single Bose-Einstein condensate in a trap and devise an interaction to do so (Figure 1). The condensate expands to produce two entangled clouds separated by up to 100 meters when released from the trap. To test the dilemma it is necessary to measure two unentangled observables. Colciaghi and colleagues replace the position and momentum predicted by the EPR with "pseudospins," a pair of quantum states that form a two-level system like spin.
These "spins" are defined by two levels of hyperfine, and the spin of each cloud is determined by the sum of the atomic numbers in the two levels. By directly counting the atoms in each level, the value of the first of the non-common spin observables is obtained. The second, complementary spin observable is determined using a pulse that interacts with the atoms prior to counting. There have been previous EPR tests using atomic ensembles, but this experiment differs significantly in that the measurement conditions for each cloud are selected individually. A true EPR paradox must have this independence; otherwise we cannot ignore a system-to-system effect.
Colciaghi and colleagues investigate EPR correlations by calculating errors in estimating the spin of cloud A from the spin measurements of cloud B, first when the pulses are absent and then when pulses are applied to both cloud A and cloud B. Although non-zero, the sum of these errors is negligibly less than the lower bound of the experiment for the Heisenberg uncertainty product. Thus the paradox is proven, because it is possible to infer spins that are not common to A with a precision that cannot be measured by any local quantum state.
An experiment involving a large number of atoms is interestingly macroscopic if these correlations are the result of a measurement made in B affecting the result in A in non-classical ways.
The researchers then modify their experiments in a highly instructive way. The EPR argument was challenged by Schrödinger in 1935 with his famous illustration of the cat in a superposition state. A lesser known scenario is Schrödinger's suggestion of a scenario where two complementary variables are measured simultaneously “one by direct measurement and the other by indirect measurement” by changing the measurement settings. Schrödinger wondered whether the values of both variables could be determined exactly for this choice of measurement settings and questioned whether this value determination would be consistent with quantum mechanics. Such a situation was produced by Colciaghi et al by changing the pulses that decide which spin to measure: they change the tuning of cloud A while leaving the configuration of cloud B unchanged.
Researchers have shown that while they can measure a cloud A variable directly, they can determine the value of the complementary variable indirectly from a measurement in cloud B. They also show how the correlation with the measurement in B is restored by changing the setting of A once again. This shows that changing the setting of cloud A does not change how well the complementary variable in A is predicted by the measurement in B. Does this result indicate that the measurement result in A has a reality aspect after the setting in B is fixed?
Once the measurement parameters have been established, the system is ready for the counting of atoms at two levels, following any contact of the atoms with the pulses for direct measurement of each variable. Regardless of whether the counting has taken place or not, are the atoms to be counted already at these levels? The mesoscopic nature of the experiment seems to support Schrödinger's theory more strongly because it seems that the observable values will be fixed once the measurement parameters have been determined but before the measurements are terminated by counting the atoms.
The implications of the results are not entirely clear. To change the environment that changes the quantum state, a second interaction is required to validate the implicitly obtained value in A.
Consequently, neither Bell's theorem, which concerns variables defined before the interactions that fix the settings, nor the assertion that the values for both spins are decided before measurement violates the uncertainty principle. System A appears to be defined by a wave function in which, as Schrödinger pointed out after the indirect measurement in B, the indirectly measured value is "completely sharp" in Schrödinger's terms, but the directly measured value is "completely uncertain".
Although x and p are continuous and thus apparently not subject to this constraint, Schrödinger argues that when two observation values are measured simultaneously, x2 + p2 further questioned the validity of simultaneous values for the x-position and p-momentum by showing that its value must be a single integer.
📩 01/06/2023 12:52