12/13/2023 0 Comments Quantum coherence![]() The transition then couples the states |g,0› and |e,1› where we are now specifying both the electronic state (|g› or |e›) and the motional state ( n=0 or 1). We can also drive Rabi oscillations between the motional ground state ( n=0) and the first excited state ( n=1) by tuning the laser to the first blue sideband of the optical transition. The Rabi frequency is Ω 0/2π ≈ 28 kHz and the overall visibility decays with a coherence time of τ ≈ 0.7 ms. Figure 1. Rabi oscillation on the carrier transition after ground-state cooling. In our calcium system the optical coherence time is of the order of 1 ms and is probably limited by the linewidth of the highly-stabilised laser at 729 nm that we use to drive the Rabi oscillations. electrical or magnetic field noise) or heating of the ion. The number of oscillations seen is limited only by decoherence effects in the system such as laser intensity or frequency noise, environmental perturbations (e.g. The ion stays in the motional ground state n=0 throughout. This is effectively a two-level atom and the system displays “Rabi oscillations” as it moves coherently from the ground state to the excited state and back in a series of oscillations, as shown in Figure 1. A laser tuned exactly to the frequency corresponding to a transition from the electronic ground state of the ion (|g›) to a long-lived excited state (|e›) will drive the ion coherently between these two states. with motional quantum number n=0, as described in the sideband cooling page. 127, 190604 (2021).In order to demonstrate the coherence properties of our system, we need to first prepare the ion in its ground state of motion, i.e. Funo, “Superconducting-like heat current: Effective cancellation of current-dissipation trade-off by quantum coherence,” Phys. Katherine Wright is the Deputy Editor of Physics Magazine. This “quantum lubrication” leads to “superconducting,” dissipationless flow of heat during fast engine cycles, allowing the engine to achieve its maximum efficiency under these operating conditions-something that isn’t possible for classical engines. Here, the quantum equivalent of friction-an entropy increase associated with the absorption and release of heat-can be completely eliminated if the engine contains enough quantum coherence. Tajima and Funo predict that the prospects are better for a quantum heat engine, however. A classical engine could achieve the maximum efficiency only if its cycles were infinitely slow. The engine’s efficiency depends on the rate of this process, with faster cycles dissipating more energy through friction. First, the engine connects to the hot reservoir, absorbing heat, then it connects to the cold reservoir, releasing that heat. These quantum engines might be used to cool down quantum systems or to transport energy in nanoscale devices.Ī heat engine generates power from the heat flow between hot and cold reservoirs. Tajima and Funo say that the results could help researchers build future quantum heat engines that are more powerful and efficient than current classical engines. ![]() Now, Hiroyasu Tajima of the University of Electro-Communications and Ken Funo of the RIKEN Cluster for Pioneering Research, both in Japan, have taken a step toward answering that question by looking at how quantum coherence influences “friction” in a quantum heat engine. But while researchers know that this quantum coherence is key to the operation of quantum sensors and quantum information systems, for example, it is less clear how it might affect thermodynamic devices, such as quantum heat engines. Without the ability of quantum states to maintain their entanglement and superposition under external forcing, quantum technologies that outperform their classical counterparts would be a pipe dream.
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