And the logistics to make it work are overwhelming.

• For 75 years, scientists have consistently pondered the Fermi Paradox, which asks why we don’t hear from other civilizations when there are so many Earth-like worlds in the galaxy.
• A recent study analyzes whether these civilizations might be using quantum communication technologies beyond our own, which could explain why we don’t ‘hear’ them.
• Although interstellar quantum communication is possible, the technology to detect such communications is still far from our reach.

In 1950, Enrico Fermi asked the question that all of us have likely pondered at some point in our lives: Where are all of the aliens? He wasn’t the first to consider this question—Soviet sci-fi legend Konstantin Tsiolkovsky, for one, asked a similar query in some of his unpublished manuscripts—and he certainly wouldn’t be the last. If anything, the question has accumulated ever greater urgency as astronomers have slowly realized that there are likely billions of Earth-like planets in our galaxy alone, and we’re discovering more tantalizing, potentially-life-supporting planetary candidates all the time.

This ‘Fermi Paradox’ has spawned dozens of theories, ideas, and hypotheses in the 75 years since. Maybe a “Great Filter” lies in our distant past—the unlikely development of eukaryotic cells is a compelling candidate—or maybe (and this is the real bummer) it still lies ahead in our future. Are the aliens just not interested? A galaxy-spanning intelligence scoring a solid “III” on the Kardashev Scale would likely be indifferent about a sub-I species intent on poisoning its own atmosphere. In other words, maybe we’re an ant among giants.

Or, maybe more simply, aliens are reaching out to us, but we’re just not listening—not in the right way, at least.

In a study published back in 2020 in the journal Physical Review D, University of Edinburgh physicist Arjun Berera determined that quantum communication—that is, communication that leverages photon qubits rather than the more classical radio waves we use today—could maintain what’s known as coherence over interstellar distances. This idea got Berera’s colleague Lantham Boyle, a fellow theoretical physicist at the University of Edinburgh, to start pondering if aliens throughout our galaxy (and beyond) could be using communication technologies outside of the classical realm (specifically quantum communication) that we simply can’t hear.

“It’s interesting that our galaxy (and the sea of cosmic background radiation in which it’s embedded) ‘does’ permit interstellar quantum communication in certain frequency bands,” Boyle told Phys.org back in September.

This curiosity eventually led to the writing of a paper, which has been uploaded to the pre-print server arXiv, titled “On Interstellar Quantum Communication and the Fermi Paradox.” In the paper, Boyle sets out to determine if an institute like the Search for Extraterrestrial Intelligence (SETI) could somehow incorporate quantum communication detection as part of their never-ending search for interstellar beings. While the answer to that question is technically yes, it’s practically a very strong, no-bones-about-it “no.”

The problem is the size of the dish we’d need to construct in order to hear this quantum convo. For example, Boyle calculated that interstellar quantum communication would need to use wavelengths of at least 26.5 centimeters in order to avoid quantum depolarization due to the cosmic microwave background (CMB). That’s all well and good, but that means that to communicate quantumly with Alpha Centauri—the nearest star to our own—we’d need a diffraction-limited telescope with a diameter of roughly 100 kilometers (60 miles), which is an area larger than the city of London. To put it mildly, SETI doesn’t have that kind of budget.

“We have seen that the sender must place nearly all of their photons into our receiving telescope, which implies that the signal must be so highly directed that only the intended receiving telescope can hope to detect any sign of the communication,” Boyle wrote. “This is in sharp contrast to classical communication, where one can broadcast photons indiscriminately into space, and an observer in any direction who detects a small fraction of those photons can still receive the message.”

Of course, if such an advanced civilization is capable of overcoming these engineering challenges, it’s also likely that they could just glimpse our little corner of the cosmos and know we’re not technologically equipped to hear what they’re sending.

So, who knows? Maybe some silicon-based lifeforms orbiting a M-type star in the Large Magellanic Cloud have a regular quantum correspondence with the reigning Kardashev III civilization in Andromeda all about the peculiar apes on one particular spiral arm of the Milky Way that won’t return their calls.