There's a revolution underway in astronomy. In fact, you might say there are
several. In the past 10 years, exoplanet studies have advanced considerably,
gravitational wave astronomy has emerged as a new field, and the first
images of supermassive black holes (SMBHs) have been captured. A related
field, interferometry, has also advanced incredibly thanks to highly
sensitive instruments and the ability to share and combine data from
observatories worldwide. In particular, the science of very-long baseline
interferometry (VLBI) is opening entirely new realms of possibility.
According to a recent study by researchers from Australia and Singapore, a
new quantum technique could enhance optical VLBI. It's known as Stimulated
Raman Adiabatic Passage (STIRAP), which allows quantum information to be
transferred without losses. When imprinted into a quantum error correction
code, this technique could allow for VLBI observations into previously
inaccessible wavelengths. Once integrated with next-generation instruments,
this technique could allow for more detailed studies of black holes,
exoplanets, the Solar System, and the surfaces of distant stars.
The research was led by Zixin Huang, a postdoctoral research fellow with the
Center for Engineered Quantum Systems (EQuS) at Macquarie University in
Sydney, Australia. She was joined by Gavin Brennan, a professor of
theoretical physics with the Department of Electrical and Computer
Engineering and the Center of Quantum Technologies at the National
University of Singapore (NUS), and Yingkai Ouyang, a senior research fellow
with the Center of Quantum Technologies at NUS.
To put it plainly, the interferometry technique consists of combining light
from multiple telescopes to create images of an object that would otherwise
be too difficult to resolve. Very Long Baseline Interferometry refers to a
specific technique used in radio astronomy where signals from an
astronomical radio source (black holes, quasars, pulsars, star-forming
nebulae, etc.) are combined to create detailed images of their structure and
activity. In recent years, VLBI has yielded the most detailed images of the
stars that orbit Sagitarrius A* (Sgr A*), the SMBH at the center of our
galaxy (see above).
It also allowed astronomers with the Event Horizon Telescope (EHT)
Collaboration to capture the first image of a black hole (M87*) and Sgr A*
itself. But as they indicated in their study, classical interferometry is
still hindered by several physical limitations, including information loss,
noise, and the fact that the light obtained is generally quantum in nature
(where photons are entangled). By addressing these limitations, VLBI could
be used for much finer astronomical surveys. Said Dr. Huang to universe
Today via email:
"Current state-of-the-art large baseline imaging systems operate in the
microwave band of the electromagnetic spectrum. To realize optical
interferometry, you need all parts of the interferometer to be stable to
within a fraction of a wavelength of light, so the light can interfere. This
is very hard to do over large distances: sources of noise can come from the
instrument itself, thermal expansion and contraction, vibration and etc.;
and on top of that, there are losses associated with the optical elements."
"The idea of this line of research is to allow us to move into the optical
frequencies from microwaves; these techniques equally apply to infrared. We
can already do large-baseline interferometry in the microwave. However, this
task becomes very difficult in optical frequencies, because even the fastest
electronics cannot directly measure the oscillations of the electric field
at these frequencies."
The key to overcoming these limitations, says Dr. Huang and her colleagues,
is to employ quantum communication techniques like Stimulated Raman
Adiabatic Passage. STIRAP consists of using two coherent light pulses to
transfer optical information between two applicable quantum states. When
applied to VLBI, said Huang, it will allow for efficient and selective
population transfers between quantum states without suffering from the usual
issues of noise or loss.
As they describe in their paper, "Imaging stars with quantum error
correction," the process they envision would involve coherently coupling the
starlight into "dark" atomic states that do not radiate. The next step, said
Huang, is to couple the light with quantum error correction (QEC), a
technique used in quantum computing to protect quantum information from
errors due to decoherence and other "quantum noise." But as Huang indicates,
this same technique could allow for more detailed and accurate
interferometry:
"To mimic a large optical interferometer, the light must be collected and
processed coherently, and we propose to use quantum error correction to
mitigate errors due to loss and noise in this process. Quantum error
correction is a rapidly developing area mainly focused on enabling scalable
quantum computing in the presence of errors. In combination with
pre-distributed entanglement, we can perform the operations that extract the
information we need from starlight while suppressing noise."
To test their theory, the team considered a scenario where two facilities
(Alice and Bob) separated by long distances collect astronomical light. Each
share pre-distributed entanglement and contain "quantum memories" into which
the light is captured, and each prepare its own set of quantum data (qubits)
into some QEC code. The received quantum states are then imprinted onto a
shared QEC code by a decoder, which protects the data from subsequent noisy
operations.
In the "encoder" stage, the signal is captured into the quantum memories via
the STIRAP technique, which allows the incoming light to be coherently
coupled into a non-radiative state of an atom. The ability to capture light
from astronomical sources that account for quantum states (and eliminates
quantum noise and information loss) would be a game-changer for
interferometry. Moreover, these improvements would have significant
implications for other fields of astronomy that are also being
revolutionized today.
"By moving into optical frequencies, such a quantum imaging network will
improve imaging resolution by three to five orders of magnitude," said
Huang. "It would be powerful enough to image small planets around nearby
stars, details of solar systems, kinematics of stellar surfaces, accretion
disks, and potentially details around the event horizons of black holes—none
of which currently planned projects can resolve."
In the near future, the James Webb Space Telescope (JWST) will use its
advanced suite of infrared imaging instruments to characterize exoplanet
atmospheres like never before. The same is true of ground-based
observatories like the Extremely Large Telescope (ELT), Giant Magellan
Telescope (GMT), and Thirty Meter Telescope (TMT). Between their large
primary mirrors, adaptive optics, coronagraphs, and spectrometers, these
observatories will enable Direct Imaging studies of exoplanets, yielding
valuable information about their surfaces and atmospheres.
By taking advantage of new quantum techniques and integrating them with
VLBI, observatories will have another way to capture images of some of the
most inaccessible and hard-to-see objects in our universe.
Reference:
Zixin Huang, Gavin K. Brennen, Yingkai Ouyang, Imaging stars with quantum
error correction. arXiv:2204.06044v1 [quant-ph],
arxiv.org/abs/2204.06044