Searching for interstellar quantum communications

Brief

Hippke argues that quantum communications are preferred over classical on grounds of gate-keeping, information security, quantum supremacy, and information efficiency, and then establishes their physical feasibility by comparing column densities: the Earth–Proxima Centauri interplanetary/interstellar path carries roughly 3×10⁻⁸ kg m⁻², eight orders of magnitude less than the 144 km atmospheric column traversed in the longest successful terrestrial entanglement experiment (Fedrizzi et al. 2009). The quantum channel advantage in the photon-starved interstellar regime is quantified at 3–6 bits per photon over classical for 10⁻³ < M < 10⁻²³, converging to 1/ln(2) ≈ 1.44 bits per photon in the noise-limited case. The paper provides the first observing guide for quantum SETI, identifying Fock-state photon statistics and squeezed-light signatures as the primary detection discriminants, achievable with commercially available equipment.

Metadata

Category

Search

Venue

The Astronomical Journal

Type

Peer-reviewed

Year

2021

Authors

Michael Hippke

DOI

10.3847/1538-3881/abf7b7

arXiv

2104.06446

Access

Open access

Length

833.4 K

Programs

Breakthrough Listen

Tags

SETI, quantum-communications, technosignature, OSETI, information-theory

Key points

• More than one hundred dedicated SETI programs over six decades searched exclusively for classical communications and would have missed any quantum signal entirely.p.1
• The combined interplanetary and interstellar column density between Earth and Proxima Centauri is ~3×10⁻⁸ kg m⁻², eight orders of magnitude below the 144 km atmospheric column density used in the record-setting Earth-based entanglement experiment, making interstellar quantum coherence physically feasible.p.4
• Quantum channels deliver a 3–6 bits per photon advantage over classical channels in the regime 10⁻³ < M < 10⁻²³; in the noise-limited case the advantage converges to 1/ln(2) ≈ 1.44 bits per photon.p.4
• Fock-state photon statistics and squeezed-light signatures are proposed as the primary observational discriminants of artificial quantum transmissions, detectable with existing hardware.p.5
• Atmospheric decoherence dominates over interstellar decoherence for all distances less than 1 kpc, except in the Lyman continuum (~50–91 nm) which is opaque to even the nearest stars.p.4
• No joint-detection quantum receiver has been built yet; existing proposals require quantum memory and Hadamard-circuit architectures scaling prohibitively in physical size for high-order PPM.p.5
• Faster-than-light quantum communication is excluded by the no-communication theorem; the physically correct pre-shared entanglement scheme (Bennett et al. 2002) requires classical bit transmission and light-speed delays.p.6
• The 'gate-keeping' hypothesis, that advanced ETI deliberately selects quantum channels to exclude pre-quantum civilizations, traces to Subotowicz (1979), who made the analogous argument for neutrino beams.p.2

Verbatim

• “I show that quantum coherence is feasible over interstellar distances, and explain for the first time how astronomers can search for quantum transmissions sent by ETI to Earth, using commercially available telescopes and receiver equipment.”

  p.1

• “The measurement of Fock state photons or squeezed light would indicate the artificiality of a signal.”

  p.1

• “The combined interplanetary and interstellar column density between the Earth and the nearest star, Proxima Centauri, is about 3 × 10 − 8 kg m − 2 (Mann & Kimura 2000). This is 8 orders of magnitude less than the 144 km through Earth's atmosphere.”

  p.4

• “From an OSETI perspective, the measurement of Fock state photons or squeezing would indicate the artificiality of a signal.”

  p.5

• “post-singularity civilizations may not be communicating by electromagnetic waves but rather by quantum entanglement”

  p.2

Most interesting

• Earth's own atmosphere is the dominant decoherence barrier, interstellar space between here and Proxima Centauri is eight orders of magnitude less obstructive than a 144 km atmospheric column.
• A pure squeezed-vacuum state produces zero contribution from odd-photon-number states, a statistical signature with no known natural astrophysical counterpart and thus an immediate artificiality flag.
• The quantum information advantage (~1/3 boost in bits per photon) is modest by computing standards, but Hippke frames classical channels as energetically wasteful for any civilization that has access to quantum technology.
• The paper explicitly debunks the science-fiction trope of faster-than-light quantum messaging: forcing an entangled particle into a definite state breaks the entanglement, leaving the distant partner in a random classical state.
• Subotowicz proposed the 'gate-keeping' concept in 1979 using neutrino beams; basic neutrino communication was demonstrated experimentally in 2012 at 0.1 bits/s, but photons remain orders of magnitude more practical.
• A quantum receiver capable of achieving the Holevo bound for large-order PPM (>10³) would require a full quantum computer with quantum memory, no such device exists, making the receiver problem harder than the transmitter problem.