An exotic ultracold gas known as a Bose–Einstein condensate has been produced and studied in space. Such gases could be used to build quantum sensors that probe the properties of the Universe with extreme precision.
Many great discoveries in modern physics depend on the invention of sensors based on new principles. For example, in 1887, an optical interferometer — a sensor based on wave interference — was used to disprove the existence of luminiferous aether, a universal medium through which light waves were thought to propagate1. In 1968, radio telescopes were used to discover extreme astronomical objects known as pulsars2. And in 2016, a laser interferometer was used to detect gravitational waves3. Writing in Nature, Becker et al.4 demonstrate how space-borne sensors based on an exotic state of matter called a Bose–Einstein condensate might provide the next big discovery.
A fundamental principle of quantum physics is wave–particle duality, which describes elementary particles in terms of quantum-mechanical waves (de Broglie waves). The higher the velocity of a particle, the shorter the wavelength of the de Broglie wave. For a cloud of hot atoms, the de Broglie wavelengths are so short that each atom can be considered as an individual object (Fig. 1a).
Figure 1 | Production and application of a Bose–Einstein condensate. a, In quantum physics, matter can behave like a wave that has a particular wavelength. For a cloud of hot atoms, these wavelengths are so short that each atom can be regarded as an individual object. If the atoms are cooled, the wavelengths become longer. And if the atoms are cooled to a critical temperature, the wavelengths are large enough to cover the extent of the atomic cloud. Most of the atoms condense into a state known as a Bose–Einstein condensate (BEC), in which they can be regarded as a single matter wave (red). Becker et al.4 have produced and analysed a BEC in space. b, BECs can be used in sensors known as atom interferometers, in which laser beams cause a matter wave to split into two and then recombine to generate an interference pattern that is sensitive to external perturbations.
If these atoms are cooled, the de Broglie wavelengths become longer. And if the atoms are cooled to a critical temperature (typically several hundred nanokelvin), the wavelengths become large enough to cover the whole atomic cloud. In this scenario, most of the atoms condense into a state in which they all behave in the same manner, and can be regarded as a single matter wave. Such a state is known as a Bose–Einstein condensate (BEC).
Producing a BEC is not easy. Even though the concept was proposed5,6 in 1924–1925, a BEC was not realized7,8 until 1995, after two types of cooling (laser and evaporative) had been invented. Since then, the matter waves associated with BECs have been widely used in atom interferometry (Fig. 1b). Atom interferometers use laser beams to split up matter waves and then recombine them to produce interference patterns. These patterns are sensitive to vibrations, changes in temperature and other disturbances.
Sensors based on matter waves differ from those based on light because atoms have a mass and an internal structure. The mass means that matter-wave sensors are extremely sensitive to gravity. They are therefore more suited to work in space, where gravity is extremely weak (a condition known as microgravity), than they are to work on the ground. Moreover, the internal structure of atoms means that there are more ways to control the properties of matter-wave sensors than those of optical sensors.
Becker and colleagues developed a BEC set-up for a rocket, which was launched to a height of 243 kilometres before returning to the ground. The BEC was produced while the rocket was in space, which is a milestone on the path towards building space-borne matter-wave sensors. During the launch phase and the 6 minutes of space flight, an astonishing 110 BEC-related experiments were carried out. The BEC set-up was only slightly bigger than the average human, withstood the vibrations and shocks during the launch of the rocket, and automatically conducted all of the experiments. Such a set-up represents a technical marvel in modern atomic physics.
The authors compared the formation of the BEC in space with that of one on the ground. They found that there were more atoms in the space-based BEC than in the ground-based one, although the fraction of atoms in the atomic cloud that were condensed was lower in space than on the ground. In an atom interferometer, a greater number of condensed atoms can give rise to a stronger interference signal, whereas a larger condensation fraction increases the signal-to-noise ratio. As a result, precision interferometry requires both a large number of condensed atoms and a high condensation fraction. The authors should therefore try to improve the condensation fraction for their space-borne BEC.
Becker et al. demonstrated transport of the BEC away from the surface of the chip on which it was formed — a key step towards realizing more-complex motion. Such motion, combined with further manipulation, would enable the natural expansion of the BEC to be precisely controlled, maximizing the time that the atomic cloud could be used in an interferometer. The transport of the BEC from the chip caused complex oscillations in the shape of the atomic cloud. These oscillations reveal valuable details about the hydrodynamic behaviour of the BEC, but their impact on interferometry performance needs further investigation.
On the ground, microgravity can be achieved for only a few seconds. But in space, it can be supported for essentially an infinite length of time, offering new opportunities for studying cold-atom physics. For example, a BEC in microgravity could reach temperatures as low as picokelvin (equal to 10-12K) or even femtokelvin (10-15K) ranges, compared with nanokelvin on the ground. Gases at such low temperatures are an ideal platform for probing fundamental physics, and the authors’ space-borne BEC is the first step towards this goal.
Becker and colleagues’ work paves the way for quantum sensors in space that could be used to conduct experiments that are not possible on Earth. Examples include detecting gravitational waves in a frequency range that is not usually accessible, sensing possible ultralight dark-matter particles and observing subtle effects associated with Einstein’s general theory of relativity. Who knows what mysteries of the Universe could be revealed by space-borne quantum sensors.
Nature 562, 351-352 (2018)
17 October 2018
By professor Liang Liu,the Key Laboratory of Quantum Optics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences.