Ex nihilo: Dynamical Casimir effect in metamaterial converts vacuum
fluctuations into real photons
March 8, 2013 by Stuart Mason Dambrot
Copyright © PNAS, doi:10.1073/pnas.1212705110 (Phys.org)
—In the strange world of quantum mechanics, the vacuum state (sometimes
referred to as the quantum vacuum, simply as the vacuum) is a quantum
system's lowest possible energy state. While not containing physical
particles, neither is it an empty void: Rather, the quantum vacuum contains
fluctuating electromagnetic waves and so-called virtual particles, the
latter being known to transition into and out of existence. In addition,
the vacuum state has zero-point energy – the lowest quantized energy level
of a quantum mechanical system – that manifests itself as the static
Casimir effect, an attractive interaction between the opposite walls of an
electromagnetic cavity. Recently, scientists at Aalto University in Finland
and VTT Technical Research Centre of Finland demonstrated the dynamical
Casimir effect using a Josephson metamaterial embedded in a microwave
cavity. They showed that under certain conditions, real photons are
generated in pairs, and concluded that their creation was consistent with
quantum field theory predictions.
Researcher Pasi Lähteenmäki discussed the challenges he and his colleagues
– G. S. Paraoanu, Juha Hassel and Pertti J. Hakonen – encountered in their
study. Regarding their demonstration of the dynamical Casimir effect using
a Josephson metamaterial embedded in a microwave cavity at 5.4 GHz,
Lähteenmäki tells Phys.org that the main challenge in general is to get
high-quality samples. In addition, Lähteenmäki adds, they had to ensure
that the origin of the noise is quantum and not some unaccounted source of
excess noise, such as thermal imbalance between the environment and the
sample, or possibly leakage of external noise.
Modulating the effective length of the cavity by flux-biasing the SQUID
(superconducting quantum interference device) metamaterial had its
challenges as well. "The pump signal needs to be rather strong, yet at the
same time one wants to be sure that no excess noise enters the system
through the pump line, Lähteenmäki notes, "and good filtering means high
attenuation, which is a requirement contradictory to a strong signal.
Also," Lähteenmäki continues, "at 10.8 GHz the pump frequency is rather
high – and at that frequency range both the sample and the setup is rather
prone to electrical resonances which can limit the usable frequencies."
In short, the flux profile needs to be such that the pumping doesn't
counteract itself. In addition, trapping flux in SQUID loops can also
become a problem, limiting the range of optimal operating points and
causing excess loss. The researchers also showed that photons at
frequencies symmetric with respect to half the modulation frequency of the
cavity are generated in pairs. "In general, with frequency locked signal
analyzers today the extraction of this correlation is not particularly
problematic – especially since the low noise amplifier noise is not
correlated at different frequencies," Lähteenmäki explains.
That said, issues related to data collection and averaging include
amplifier gain drift and phase randomization of the pump signal (relative
to the detection phase) if the state of the generator is changed. "The
noise temperature of the low noise amplifier sets some limits to the amount
of data that needs to be collected, especially in the case where one is
operating in the regime of low parametric gain."
Lastly, the team also found that at large detunings of the cavity from half
the modulation frequency, they found power spectra that clearly showed the
theoretically-predicted hallmark of the dynamical Casimir effect. "Large
detunings imply low intensity of generated radiation," notes Lähteenmäki.
"This means long averaging times, so the system should be kept stable for a
long period.
Also, the system needs to be fairly resonance-free over a large range of
frequencies to get decent data – and/or one needs to know the
characteristics of these resonances and noise temperature of the low noise
amplifier rather well." Lähteenmäki points out that addressing these issues
required a number of insights and innovations. "We combated amplifier drift
by continuously switching the pump on and off, and recording the difference
in the observed output power, suitable operating points were searched for
using trial and error, and trapping the photon flux was eliminated by
applying a heat pulse to the sample and letting it cool down again.
The researchers also magnetically shielded the sample with a
superconductive shield, and minimized the effect of losses by changing the
coupling of the existing samples by making different valued vacuum coupling
capacitors with focused ion beam (FIB) cuts. "However," Lähteenmäki
stresses, "our biggest issue – ruling out the source of classical noise as
opposed to quantum noise – was accomplished primarily by characterizing the
sample and the environment well" Thermal imbalance was ruled out by the
symmetry of the sparrow-tail shape of the noise spectrum.
It was essential for the scientists to clearly demonstrate that the
observed substantial photon flux could not be assigned to parametric
amplification of thermal fluctuations. "By characterizing the parametric
gain with a network analyzer," Lähteenmäki notes, "we found that in order
to explain the amount of noise one gets, the device would need to have
significantly higher gain than is observed if the only source of noise was
thermal." Moreover, confirming that photon pair creation is a direct
consequence of the noncommutativity structure of quantum field theory was
equally important. "Basically the experimental results fit the theoretical
predictions rather well – and in the absence of other sources of noise, the
theory implies that we should get no output from this sort of device. Since
we see output consistent with the theoretical predictions, the conclusion
was logical."
Moving forward, Lähteenmäki describes next steps in their research.
"Instead of a continuous wave pump, we could have a straight flux line and
feed it with a step-like flux pulse," Lähteenmäki says. "This would allow
the creation of an analogue to a black hole event horizon. In fact," he
adds, "we're hoping to create an artificial event horizon in a metamaterial
similar to the one used in our current research and study Hawking radiation
originating from it. Also, it would be nice to be able to run experiments
on Bell's inequalities."
His personal interests, Lähteenmäki says, are fundamental quantum
mechanics, quantum information and properties of the vacuum itself. "The
obvious applications for these devices," he notes, "come from quantum
computation, and in general they may serve as components for multitude of
sensitive measurements. I believe the interest towards low loss
metamaterials is high and the field is just getting started. Our results
show that these devices have potential and can offer a fruitful platform
for many experiments and perhaps practical devices as well. Improving such
devices – especially eliminating the losses and making them function more
robustly – would allow them to create a general purpose component suitable
for creating entangled microwave photon pairs, low noise amplification,
squeezed vacuum, and other functions that can be very useful for quantum
computation and general experiments in the quantum mechanics and in
studying the vacuum."
Another possibility, Lähteenmäki adds, is to create a metamaterial which
would allow them to stop signal propagation in the material entirely and
allow them to resume it later. "This would act as a kind of slow light
memory that would store the photon for later use." Other areas of research
might benefit from their study as well, Lähteenmäki says. "There are some
connections to cosmology, the big bang, cosmic inflation, and other areas.
These metamaterials could possibly offer an analogy to such events and
serve as a platform to simulate the evolution of such conditions. Who
knows," he ponders, concluding that "perhaps we'd find clues to the
mysteries of dark matter and dark energy or other fundamental questions
from such systems."
More information: Dynamical Casimir effect
in a Josephson metamaterial, PNAS Published online before print February
12, 2013, doi:10.1073/pnas.1212705110Journal reference: Proceedings of the
National Academy of Sciences Copyright 2013 Phys.org All rights reserved.
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