Six decades of SETI have yet to produce a detection. Are there strategies we have missed? In today’s essay, Michael Hippke takes us into the realm of quantum communication, explaining how phenomena like ‘squeezed light’ can flag an artificial signal with no ambiguity. Quantum coherence, he argues, can be maintained over interstellar distances, and quantum methods offer advantages in efficiency and security that are compelling. Moreover, techniques exist with commercially available equipment to search for such communications. Hippke is a familiar face on Centauri Dreams, having explored topics from the unusual dimming of Boyajian’s Star to the detection of exomoons using what is known as the orbital sampling effect. He is best known for his Transit Least Squares (TLS) exoplanet detection method, which is now in wide use and has accounted for the discovery of ~ 100 new worlds. An astrophysics researcher at Sonneberg Observatory and visiting scholar for Breakthrough Listen at UC-Berkeley, Michael now introduces Quantum SETI.
by Michael Hippke
Almost all of today’s searches for extraterrestrial intelligence (SETI) are focused on radio waves. It would be possible to extend our search to include interstellar quantum communications.
Quite possibly, our Neanderthal ancestors around the bonfires of the Stone Age marveled at the night sky and scratched their heads. What are all these stars about? Are there other worlds out there which have equally delicious woolly mammoths? Much later, about 200 years ago, the great mathematician Carl Friedrich Gauß proposed to cut down large areas of Siberian forest, in the form of a triangle, to send a message to the inhabitants of the Moon. At the end of the 19th Century, many canals were built, including the Suez and Panama canals. Inspired by these engineering masterpieces, astronomers searched for similar signs of technology on other planets. The logic was clear: What the great human civilization can build must reflect what other civilizations will inevitably build.
Clearly, Martians must equally be in need of canals. Indeed, the Italian astronomer Giovanni Schiaparelli discovered “canali” on Mars in 1877. Other observers joined the effort, and Percival Lowell asserted that the canals exist and must be artificial in origin.
Something similar happened again a short time later when Guglielmo Marconi put the first radio into operation in December 1894. Just a few years later, Nikola Tesla searched for radio waves from Mars, and believed he had made a detection. It turned out to be a mistake, but the search for radio signals from space continued. The “Search for Extraterrestrial Intelligence,” or SETI for short, received a boost in 1960 from two publications in the prestigious journal Nature. For the first time, precise scientific descriptions were given for the frequencies and limits of interstellar communication using radio waves [https://www.nature.com/articles/184844a0] and optical light [https://www.nature.com/articles/190205a0]. Between 1960 and 2018, the SETI Institute recorded at least 104 experiments with radio telescopes [https://technosearch.seti.org/]. All unsuccessful so far, which is also true for searches in the optical domain, for X-rays, or infrared signatures.
Photons? Neutrinos? Higgs bosons?
Particle physics radically changed our view of the world in the 20th century: It was only through the understanding of elementary particles that discoveries such as nuclear fission (atomic weapons, nuclear power plants) became possible. Of the 37 elementary particles known today in the Standard Model, several are suitable for an interstellar communication link. I examined the pros and cons of all relevant particles in a 2018 research paper [https://arxiv.org/abs/1711.07962]. The known photons (light particles) were the “winners”, because they are massless and therefore energetically favorable. In addition, they travel at light speed, can be focused very well, and can carry several bits of information per particle.
Photons are not only known as light particles – they are also present in the electromagnetic spectrum as radio waves, and with higher particle energies than X-rays or gamma rays. In addition, there are other particles that can be more or less reasonably used for communication. For example, it has been demonstrated that neutrinos can be used to transmit data [https://arxiv.org/abs/1203.2847]. Neutrinos have the advantage that they effortlessly penetrate kilometer-thick rock. However, this is also one of their disadvantages: they are extremely difficult to detect, because they also penetrate (almost) every detector.
Incidentally, the particle that is the least suitable of all for long-distance communication is the Higgs boson. It was predicted by Peter Higgs in 1964, but was not observed for the first time until 2012 at the Large Hadron Collider (LHC) at CERN – it also won a Nobel Prize.
The Higgs boson decays after only 10-22 seconds. To keep it alive long enough to travel to the next star, it would have to be accelerated very strongly. Due to the Lorentz factor, its subjective time would then pass more slowly. In practice, however, this is impossible to achieve, because one would have to pump so much energy into the Higgs particle that it would become a black hole. It thus disqualifies itself as a data carrier.
Photons and quanta
Quanta, simply put, are discrete particles in a system that all have the same energy. For example, in 1905 Albert Einstein postulated that particles of light (photons) always have multiples of a smallest amount of energy. This gives rise to the field of quantum mechanics, which describes effects at the smallest level. The transition to the macroscopic, classical world is a grey area – quantum effects have also been demonstrated in fullerenes, which are spheres of 60 carbon atoms. So although quantum effects occur in all particles, it makes sense to focus on photons for interstellar communication because they are superior to other particles for this purpose.
Four advantages of quantum communication
1. Information efficiency
Classical communication with photons, over interstellar distances, can be well illustrated in the particle model. The transmitter generates a pulse of particles, and focuses them through a parabolic mirror into a beam whose minimum diameter is limited by diffraction. This means that the light beam expands over large distances.
For example, if an optical laser beam is focused through a telescope measuring one meter and sent across the 4 light years to Alpha Centauri, the light cone there is already as wide as the distance from the Earth to the Sun. So a receiver on a planet around Alpha Centauri receives only a small fraction of the emitted photons. The rest flies past the receiver into the depths of space. On the other hand, photons are quite cheap to buy: You already get about 1019 photons from a laser that shines with one watt for one second.
In the sum of these effects, every photon is precious in interstellar communication. Therefore, one wants to encode as many bits of information as possible into each transmitted photon. How to do that?
Photons (without directional information) have three degrees of freedom: their arrival time, their energy (= wavelength or frequency), and the polarization. Based on this, an alphabet can be agreed upon, so that, for example, a photon arriving at time 11:37 with wavelength 650 nm (“red”) and polarization “left” corresponds to the letter “A”. The number of bits, which can be encoded per degree of freedom, scales unfortunately only logarithmically: 1024 modes result in 10 bits per photon. In practice, one still has to take losses and noise into account, so that with this classical communication it is rarely possible to transmit more than on the order of 10 bits per photon.
Quantum communication, however, offers the possibility to increase the information density. There are several ways to realize this, but a good illustration is based on the fact that one can “squeeze” light (more on this later). Then, for example, the time of arrival can be measured more accurately (at the expense of other parameters). There are analytical models, and also already practical demonstrations, which show that the information content can be increased by up to 50 percent. In our simple example, about 15 bits per photon could be encoded instead of only 10 for the classical case.
2. Information security
Encryption of sensitive data during data transmission is an important issue for us humans. Of course, we don’t know if this is the case for other civilizations. But it is plausible that future colonies on Mars (or Alpha Centauri…) will also want to encrypt their communications with each other and with Earth. In this respect, encryption is quite relevant for transmissions through space.
Today’s encryption methods are mostly based on mathematical one-way functions. For example, it is easy to multiply two large numbers. However, if the secret key is missing, you have to go the other way around and calculate the two prime factors from the large number. This is much more difficult. However, the security of this and similar methods is “only” due to the fact that no one has yet found an effective method of calculation. We have in no case the mathematical proof available that such a calculation is not possible. There is always the danger that a clever algorithm will be found which cracks the encryption. Quantum computers could also be used in the future to attack some encryption methods.
In contrast, there is quantum cryptography. The best-known method uses a quantum key exchange, which has also been used in practice over long distances, for example via satellite. This is based on quantum mechanics and is unbreakable as long as no mistake is made during transmission – and as long as no one disproves quantum mechanics.
If there really is a galactic Internet, how to protect it from being spammed by uneducated civilizations? This problem has already occupied Mieczysław Subotowicz, a Polish professor of astrophysics, who wrote in a technical paper on neutrino communication in 1979 that it was: “so difficult that an advanced civilization could intentionally communicate only through it with aliens of its own level of development”.
Now, as mentioned above, neutrino communications are very inefficient. It would be much more elegant and energy efficient to use photons instead. As an entry barrier, it seems plausible not to allow classical photons, but to require quantum communications. This would leave out young technological civilizations like ours, though we would have a good chance of joining in the next few decades.
4. Quantum computing
Konrad Zuse built the Zuse Z3, the first Turing-complete computer, in his Berlin apartment in 1941. This was a single computing machine. It took several decades until the first computers were connected (networked together) in 1969 with the ARPANET. This gave rise to the Internet, in which billions of computers of all kinds are connected today: PCs, cell phones, washing machines, etc. All these devices are classical computers exchanging classical information (bits) on classical paths (for example via photons in optical fibers).
In the future, quantum computers may gain importance because they can solve a certain class of problems much more efficiently. This could give rise to a “quantum Internet” in which quantum computers exchange “qubits,” or entangled quantum bits. These could be intermediate results of simulations, or even observational data that are later superimposed on each other [https://arxiv.org/abs/2103.07590].
Likewise, it is conceivable that quantum-based observational data and intermediate results will be exchanged over larger distances. This is when interstellar quantum communication comes into play. If distant civilizations also use quantum computers, their communications will consist of entangled particles.
Excursus: The (im)possible magic Pandora quantum box
The idea of using quantum entanglement to transmit information instantaneously (without loss of time) over long distances is a frequent motif in science fiction literature. For example, in the famous novel The Three Body Problem by Chinese author Liu Cixin, the “Trisolarans” use quantum entangled protons to communicate instantaneously.
This method sounds too good to be true – and unfortunately it actually contains three fundamental flaws. The first is the impossibility of exchanging information faster than the speed of light. If that were possible, there would be a causality violation: one could transmit the information before an event happens, thus causing paradoxes (“grandfather paradox” [https://arxiv.org/abs/1505.07489]). Second, quantum entanglement does not work this way: one cannot change one of two entangled particles, thereby causing an influence on the state of the partner. As soon as one of the particles is changed, this process destroys the entanglement (“no communication theorem”).
Third, an information transfer without particles (no particle flies from A to B) is impossible. Information is always bound to mass (or energy) in our universe, and does not exist detached from it. There are still open questions here, for example when and how information that flew in with matter comes out of a black hole again. But this does not change the fact that the communication by quantum entanglement, and without particle exchange, is impossible.
But wait a minute – before we throw away the “magic box of the entangled photons”, we should once more examine the idea. For there is, despite all the nonsense that is written about it, an actually sensible and physically undisputed possibility of use: known under the term “pre-shared entanglement” [https://arxiv.org/abs/quant-ph/0106052].
To perform this operation, we must first assume that we can entangle and store a large number of photons. This is not so easy: the current world record for a quantum memory preserves entanglement for only six hours. And even that requires considerable effort: It uses a ground-state hyperfine transition of europium ion dopants in yttrium orthosilicate using optically detected nuclear magnetic resonance techniques [https://www.nature.com/articles/nature14025]. But it is conceivable that technological advances will make longer storage possible. Conditions are particularly good for interstellar travel, because space is dark and cold, which slows decoherence caused by particle interactions.
So let’s assume such a quantum memory is available – what do we do with it? We take one half of the magic box on board a spaceship! And the counterpart remains on earth. Now the spaceship flies far away, and wants to communicate home. The trick is then not to send the bits of the information transmission simply on a photon letter to the earth, but to superpose each classical signal photon first with one (or more) stored entangled photons. The result is one classical photon per superposition, which is then sent “totally normally” to the receiver (for example the earth). Upon arrival, the receivers opens their own magic box and bring their part of the entangled particles with it to superposition. This allows the original message to be reconstructed.
The advantage of this procedure is increased information content: The amount of information (in bits per photon) increases by the factor log2(M), where M is the ratio of the entangled to the signal photons. Even a very large magic box is therefore of limited use, because unfortunately log2(1024), for example, is only 10. Losses and interference (due to noise, for example) also have a negative effect on the amount of encodable information. Nevertheless, “pre-shared entanglement” is a method that can be considered, because it is physically accepted – in contrast to most other ideas in popular literature.
Quantum communication in practice
But what does quantum communication look like in practice? Is there even a light source for it on earth? Yes, for a few years now this has actually been the case! When gravitational waves from merging black holes were detected for the first time at the Laser Interferometer Gravitational-wave Observatory (LIGO) in 2016, “squeezed light” was used. This is laser light traveling through a very precisely controlled crystal (an “OPO” for “Optical Parametric Oscillator”). This converts one green photon into two entangled red photons, to what is called a squeezed vacuum. This reduces phase uncertainty at the expense of amplitude fluctuation. And it is the former that matters: One would like to measure the arrival time of the photons very precisely in order to compare the length of the path with and without gravitational waves. The brightness of the photons is not important.
Such a squeezed light, with lower fluctuations compared to classical light, also improves interstellar communication. It still remains unresolved what is the best way to modulate the actual data. Signal strength is also still low, with just a few watts of squeezed light in use at LIGO. By comparison, there are classical lasers in the megawatt range. So the development of quantum light is several decades behind classical light. But more powerful quantum light sources in the kilowatt range are already planned for next-generation gravitational wave detectors. This would also mark the entry threshold for meaningful interstellar quantum communications.
Detection of quantum communication
Entangled photons are also just photons – shouldn’t they already be detectable in optical SETI experiments anyway? In principle this is correct, because for a single photon it is in principle not determinable who or what has generated it. If it falls on the detector at 11:37 a.m. with a wavelength of 650 nm (color red), we cannot possibly say whether it came from a star or from the laser cannon of the Death Star.
However, a photon rarely comes alone. If we receive one thousand photons with 650 nm within one nanosecond from the direction of Alpha Centauri in our one-meter mirror telescope, then we can be sure that they do not come from the star itself (the star sends only about 32 photons of all wavelengths per nanosecond into our telescope). Classical optical SETI is based on this search assumption. It is thus very sensitive to strong laser pulses, but also very insensitive to broadband sources.
Quantum SETI extends the search horizon by additional features. If we receive a group of photons, they no longer have to correspond to a specific wavelength, or arrive in a narrow time interval, for us to assume an artificial origin. Instead, we can check for quantum properties, such as the presence (or absence) of squeezed light. Indeed, there is no (known) natural process that produces squeezed light. If we receive such, it would be extremely interesting in any case. And there are indeed tests for squeezed light that can be done with existing telescopes and detectors. In the simplest case, one tests the intensity and its variance for a nonlinear (squared) correlation, which requires only a good CCD sensor [https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.113602].
There are numerous other tests for quantum properties of light that are applicable to starlight. For faint sources from which only a few photons are received, one can measure their temporal separation. Chaotic starlight is temporally clustered, so it is very likely to reach us in small groups. Classical coherent light, i.e. laser light, is much more uniform. For light with photon “antibunching”, in the extreme case, the distance between every two photons is identical – so their arrival times are perfectly uncorrelated. This quantum mechanical effect can never occur in natural light sources, and is thus a sure sign of a technical origin. The technique is used from time to time because it is useful for determining stellar diameters (“intensity interferometry”).
For a few stars we can already deduce on the basis of existing data that they are of natural origin: Arcturus, Procyon and Pollux [https://academic.oup.com/mnras/article/472/4/4126/4344853]. In the future, however, the method can be applied to a large number of “strange” objects to test them for an artificial origin: impossible triple stars [https://academic.oup.com/mnras/article/445/1/309/988488], hyperfast globular clusters [https://iopscience.iop.org/article/10.1088/2041-8205/787/1/L11], or generally all interesting objects listed in the “Exotica” catalog by Brian Lacki (Breakthrough Listen) [https://arxiv.org/abs/2006.11304].
Current status and outlook
The idea to extend SETI by quantum effects is still quite new. However, one can fall back on known search procedures and must adapt these only slightly. Thus, dubious light sources can be effectively checked for an artificial origin in the future. We can be curious what the next observations will show, and ask the question: “Dear photon, are you artificially produced?”
The paper is Hippke, “Searching for interstellar quantum communications,” in press at the Astronomical Journal (preprint). See also the video “Searching for Interstellar Quantum Communications,” available at https://www.youtube.com/watch?v=Kwue4L8m2Vs.