Can quantum mechanics fully describe macroscopic reality? Everyday objects are typically well-described by classical mechanics, whereas atomic-scale objects are governed by quantum mechanics. Exploring the boundary between the two domains could enable fundamental tests of quantum mechanics and the development of new sensing technologies for gravitational measurements.
Quantum mechanics posits that even large objects behave as waves. However, the spatial extent of this wave-like behaviour, known as the “coherence length”, is far smaller than the size of large objects. This renders quantum phenomena effectively unobservable for such systems. “To push quantum physics into the macroscopic domain, we need to increase both [mass and coherence length] simultaneously”, explains lead researcher Massimiliano Rossi. This pursuit motivated the team’s recent study, which is described in Physical Review Letters.
Playing with light
The researchers studied large objects called silica nanoparticles, which are 100 nm in diameter. The nanoparticles were held and levitated in vacuum using a tightly-focused laser beam.
Nanoparticles naturally scatter the laser light, and the phase of the scattered photons encodes information about the nanoparticle’s centre-of-mass position. The researchers used this information in a feedback loop, applying electric fields to cool the nanoparticles close to their quantum ground state. The colder sample is in a more “pure” quantum state, such that the quantum wave-like behaviour extends farther in space and the coherence length is longer than in a hot sample. The team measured an initial coherence length of 21 pm (21 × 10-12 m).
Further extending the coherence length required careful manipulation of the laser light. The researchers started with high-power light, which provided a tight harmonic potential for the nanoparticles – like a marble trapped at the bottom of a steep bowl. An advantage of using a light-induced potential is that the curvature of the bowl is easily tuned over a large range by adjusting the laser power.
The researchers lowered the laser power in two pulses, each of which caused the bowl to become shallower, therefore allowing the marble to roll around and explore more of the bowl. In the experiment, this translated to an expansion of the nanoparticle’s coherence length to 73 pm, more than three-fold that of the initial value.
Preserving quantum information
Rossi notes that the main experimental challenge was limiting decoherence, a process that destroys quantum information. He explains that when a nanoparticle interacts with its surroundings, it becomes correlated with a noisy and unmeasurably complex environment. This interaction causes the nanoparticle’s motion to become increasingly random when measured. As a result, the nanoparticle’s quantum mechanical behaviour is washed out and the particle is well described as a classical ball.
It was therefore critical that the researchers expand the coherence length faster than the rate of any decoherence. To achieve this, they meticulously measured, identified, and suppressed all sources of decoherence, with the dominant source being laser light scattering. Scattering was reduced during the expansion pulses because of the lower laser power.
The achieved 73 pm remains orders of magnitude smaller than the size of the nanoparticle, which was 100 nm in diameter. However, Rossi remarks that “we do not know of any fundamental reason why achieving nanometre coherence lengths should be impossible.” One next step could be to use more expansion pulses to increase the coherence length further.
With a longer expansion time, the main challenge would be to outpace decoherence. Researchers propose using hybrid traps that employ both light and electric fields to confine the nanoparticles, since an electric trap would reduce the decoherence from light scattering. Rossi is now pursuing this direction in his new research group at the Delft University of Technology in the Netherlands.
Adding energy to a system usually heats it up, but physicists at the University of Innsbruck in Austria have now discovered a scenario in which this is not the case. Their new platform – a one-dimensional fluid of strongly interacting atoms cooled to just a few nanokelvin above absolute zero and periodically “kicked” using an external force – could be used to study how objects transition from being quantum and ordered to classical and chaotic.
Our everyday world is chaotic and chaos plays a crucial and often useful role in many areas of science – from nonlinear complex systems in mathematics, physics and biology to ecology, meteorology and economics. How a system evolves depends on its initial conditions, but this evolution is, by nature, inherently unpredictable.
While we know how chaos emerges in classical systems, how it does so in quantum materials is still little understood. When this happens, the quantum system reverts to being a classical one.
The quantum kicked rotor
Researchers have traditionally studied chaotic behaviour in driven systems – that is, rotating objects periodically kicked by an external force. The quantum version of these is the quantum kicked rotor (QKR). Here, quantum coherence effects can prevent the system from absorbing external energy, meaning that, in contrast to its classical counterpart, it doesn’t heat up – even if a lot of energy is applied. This “dynamical localization” effect has already been seen in dilute ultracold atomic gases.
The QKR is a highly idealized single-particle model system, explains study lead Hanns-Christoph Nägerl. However, real-world systems contain many particles that interact with each other – something that can destroy dynamical localization. Recent theoretical work has suggested that this localization may persist in some types of interacting, even strongly interacting, many-body quantum systems – for example, in 1D bosonic gases.
In the new work, Nägerl and colleagues made a QKR by subjecting samples of ultracold caesium (Cs) atoms to periodic kicks by means of a “flashed-on lattice potential”. They did this by loading a Bose-Einstein condensate of these atoms into an array of narrow 1D tubes created by a 2D optical lattice formed by laser beams propagating in the x–y plane at right angles to each other. They then increased the power of the beams to heat up the Cs atoms.
Many-body dynamical localization
The researchers expected the atoms to collectively absorb energy over the course of the experiment. Instead, when they recorded how their momentum distribution evolved, they found that it actually stopped spreading and that the system’s energy reached a plateau. “Despite being continually kicked and strongly interacting, it no longer absorbed energy,” says Nägerl. “We say that it had localized in momentum space – a phenomenon known as many-body dynamical localization (MBDL).”
In this state, quantum coherence and many-body interactions prevent the system from heating up, he adds. “The momentum distribution essentially freezes and retains whatever structure it has.”
Nägerl and colleagues repeated the experiment by varying the interaction between the atoms – from zero (non-interacting) to strongly interacting. They found that the system always localizes.
Quantum coherence is crucial for preventing thermalization
“We had already found localization for our interacting QKR in earlier work and set out to reproduce these results in this new study,” Nägerl tells Physics World. “We had not previously realised the significance of our findings and thought that perhaps we were doing something wrong, which turned out not to be the case.”
The MBDL is fragile, however – something the researchers proved by introducing randomness into the laser pulses. A small amount of disorder is enough the destroy the localization effect and restore diffusion, explains Nägerl: the momentum distribution smears out and the kinetic energy of the system rises sharply, meaning that it is absorbing energy.
“This test highlights that quantum coherence is crucial for preventing thermalization in such driven many-body systems,” he says.
Simulating such a system on classical computers is only possible for two or three particles, but the one studied in this work, reported in Science, contains 20 or more. “Our new experiments now provide precious data to which we can compare the QKR model system, which is a paradigmatic one in quantum physics,” adds Nägerl.
Looking ahead, the researchers say they would now like to find out how stable MBDL is to various external perturbations. “In our present work, we report on MBDL in 1D, but would it happen in a 2D or a 3D system?” asks Nägerl. “I would like to do an experiment in which we have a 1D + 1D situation, that is, where the 1D is allowed to communicate with just one neighbouring 1D system (via tunnelling; by lowering the barrier to this system in a controlled way).”
Another way of perturbing the system would be to add a local defect – for example a bump in the potential of a different atom, he says. “Generally speaking, we would like to measure the ‘phase diagram’ for MBDL, where the axes of the graph would quantify the strength of the various perturbations we apply.”
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The image was taken with a long focal-length telescope from the AstroCamp Observatory, Nerpio, Spain.
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