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arXiv:2012.07605 · Nature

Continuous Bose–Einstein condensation

Chun-Chia Chen, Rodrigo González Escudero, Jiří Minář, Benjamin Pasquiou, Shayne Bennetts, Florian Schreck

Published Jun 8, 2022• 87 citations

Abstract

Abstract Bose–Einstein condensates (BECs) are macroscopic coherent matter waves that have revolutionized quantum science and atomic physics. They are important to quantum simulation1 and sensing2,3, for example, underlying atom interferometers in space4 and ambitious tests of Einstein’s equivalence principle5,6. A long-standing constraint for quantum gas devices has been the need to execute cooling stages time-sequentially, restricting these devices to pulsed operation. Here we demonstrate continuous Bose–Einstein condensation by creating a continuous-wave (CW) condensate of strontium atoms that lasts indefinitely. The coherent matter wave is sustained by amplification through Bose-stimulated gain of atoms from a thermal bath. By steadily replenishing this bath while achieving 1,000 times higher phase-space densities than previous works7,8, we maintain the conditions for condensation. Our experiment is the matter wave analogue of a CW optical laser with fully reflective cavity mirrors. This proof-of-principle demonstration provides a new, hitherto missing piece of atom optics, enabling the construction of continuous coherent-matter-wave devices.

DOI: 10.1038/s41586-022-04731-z

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Abstract Bose–Einstein condensates (BECs) are macroscopic coherent matter waves that have revolutionized quantum science and atomic physics. They are important to quantum simulation1 and sensing2,3, for example, underlying atom interferometers in space4 and ambitious tests of Einstein’s equivalence principle5,6. A long-standing constraint for quantum gas devices has been the need to execute cooling stages time-sequentially, restricting these devices to pulsed operation. Here we demonstrate continuous Bose–Einstein condensation by creating a continuous-wave (CW) condensate of strontium atoms that lasts indefinitely. The coherent matter wave is sustained by amplification through Bose-stimulated gain of atoms from a thermal bath. By steadily replenishing this bath while achieving 1,000 times higher phase-space densities than previous works7,8, we maintain the conditions for condensation. Our experiment is the matter wave analogue of a CW optical laser with fully reflective cavity mirrors. This proof-of-principle demonstration provides a new, hitherto missing piece of atom optics, enabling the construction of continuous coherent-matter-wave devices.

The observation of Bose-Einstein condensation (BEC) in dilute atomic gases was the realization of many long-standing goals: (1) to cool neutral atoms into the ground state of the system, thus exerting ultimate control over the motion and position of atoms limited only by Heisenberg's uncertainty relation; (2) to generate a coherent sample of atoms all occupying the same quantum state (this was subsequently used to realize an atom laser, a device which generates coherent matter waves); and (3) to create a quantum fluid with properties quite different from the quantum liquids He-3 and He-4. This provides a test-ground for many-body theories of the dilute Bose gas which were developed many decades ago, but never tested experimentally.

The experimental observation of Bose–Einstein condensation (BEC) in rubidium in 1995 demonstrated that a macroscopic number of bosons could be produced in a single quantum state of trapped atoms. The occupation of a single quantum state by a large number of identical bosons is the matter-wave analog of the storage of photons in a single mode of a laser cavity. In a conventional laser, one extracts a coherent beam of photons from a cavity by using a partially transmitting mirror as an output coupler. In 1997, Wolfgang Ketterle and his collaborators at MIT built a pulsed output coupler that extracted matter waves from a condensate, and they observed interference between atoms from two separate condensates, thereby demonstrating an atom laser for the first time (see PHYSICS TODAY, March 1997, page 17).

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Quantum degenerate atomic gases are prominent platforms for quantum simulation and high precision sensing. A key challenge for these applications is intrinsic atom loss, which limits quantum gas lifetimes to typically tens of seconds. Here we create a Bose-Einstein condensate of

^{84}\mathrm{Sr}

atoms that is sustained in steady state for minutes. We achieve this by guiding a continuous stream of Sr gas through a sequence of spatially separated laser cooling stages, until it reaches the condensate's location, which we protect against harmful laser cooling photons. This steady-state quantum degenerate gas paves the way to design and study novel driven-dissipative, non-equilibrium quantum systems, and overcomes a key bottleneck in the realization of a continuous-wave atom laser.