The concept of quantum interference unveils the hidden beauty of the universe, showcasing how reality—and our existence—can manifest in various forms simultaneously.
On a summer evening, I find myself lying on the grass near the Olympiasee, the lake in Munich”s Olympic Park. Since moving to this city for my doctoral studies in quantum computing, I have often retreated to this serene space, where families play and festivals are held. Yet, it also serves as a reminder of the past, with hills formed from the rubble of World War II. As I listen to “Imaginations from the Other Side” by Blind Guardian, a power speed metal band, I become engrossed in reading an unusual fiction novel, “The Scar” by China Miéville. I am captivated by a passage describing a magical object: the Possible Sword. According to the narrative, this mysterious artifact functions like a machine of possibilities, allowing a skilled swordswoman to draw upon various attack options, with each strike producing “multiple outcomes” and “ghosts of the same sword.” Some potential attacks seem almost real while others feel miraculous. The poetic description of the sword intrigues me, and its mechanism, albeit esoteric, resonates with the theme of my thesis.
Quantum computing investigates how to perform calculations more rapidly by leveraging microscopic phenomena. In my mind, the Possible Sword symbolizes a grand quantum interferometer—a device that creates intricate and lethal attack patterns using quantum interference. My thoughts are interrupted by a loud splash: a young girl has thrown two stones into the lake. Concentric circles emerge on the water”s surface, colliding and creating beautiful interference patterns. “That”s it!” I exclaim, as a spark ignites in my mind, connecting the Possible Sword to the waves on the lake. The energy that powers the sword mirrors quantum interference, a phenomenon characteristic of microscopic systems. I think to myself, “The Possible Sword executes movements akin to waves that interfere, not with water, but with possibilities: potential future paths, possible offensive and defensive maneuvers.”
Like the Possible Sword, quantum interference possesses an air of mystery. For four decades, scientists studying solar neutrinos grappled with perplexing results. Neutrinos are elusive, chargeless particles that are notoriously difficult to detect. In the 1960s, meticulously designed experiments aimed to observe these particles to better understand the solar core. To their astonishment, researchers consistently found that between half and two-thirds of the expected neutrinos were missing, an anomaly not attributable to design flaws or calculation errors; the neutrinos seemed to vanish like ghostly particles. This mystery was elucidated in the 21st century through quantum interference.
Due to a fundamental property of quantum physics known as wave-particle duality, neutrinos—despite being particles—propagate as waves when traveling. When we observe them, we measure their particle-like type, referred to as “flavor”: electronic, muonic, or tauonic. While traversing through a vacuum, neutrinos behave as a superposition of “mass waves,” each with a distinct frequency. As these waves propagate, they acquire a phase shift based on their frequency, leading to interference that can alter the neutrino”s flavor. A surprising implication is that a terrestrial scientist expecting to detect an electronic neutrino may not observe it if it transitions to a muonic or tauonic state.
Neutrino oscillations may seem magical, yet they resemble a common phenomenon that could easily be dismissed if not for its incredible beauty: the appearance of color patterns on the surface of a soap bubble. These fleeting rainbows once captivated even Isaac Newton himself. We encounter them daily, whether in glass reflections or, as a friend from Andalusia might say, in a thin layer of good olive oil. These colors delight our senses due to the interference of reflected light waves both inside and outside these spherical films of water. The light that reaches our eyes after being reflected from inside the bubble experiences a phase shift compared to light reflected from the outside, creating constructive and destructive interferences that filter the colors of the waves arriving from various angles.
Soap bubbles are reminiscent of the famous double-slit experiment, which illustrates the wave-particle duality of matter. In this experiment, a beam of microscopic particles (such as photons or electrons) is projected onto a wall with two slits. A detector is placed behind this wall to observe which particles pass through either of the two openings. When both slits are open, a strange pattern emerges on the wall, displaying constructive and destructive interferences between the two beams. The shape of this pattern can only be explained if quantum particles behave like the waves observed on the surface of the Olympiasee. Neutrinos are akin to soap bubbles; their flavors represent the colors visible on the bubble”s surface, while their oscillations create the interference that forms these colorful swirls.
I am captivated by the Possible Sword because its mechanism mirrors that of a quantum computer. Both are powerful artifacts that generate possibilities if wielded by a skilled user. A quantum computer utilizes quantum interference to perform calculations more rapidly. This acceleration involves both hardware and software. It is often incorrectly taught that a quantum computer can run numerous programs in parallel (an exponential number) to find solutions more quickly. This is a dual misconception. Firstly, quantum computers, as far as science knows, do not offer exorbitant advantages in searching for solutions to unstructured problems. Secondly, quantum interference, which cannot be understood merely as parallelism, plays a crucial role in the speed of calculations. In a quantum computer, programs operate not only as codes but also as waves. Like neutrinos, the waves carrying these programs can interfere, creating patterns defined by a programmer. A skilled quantum computer scientist weaves these patterns to generate constructive interferences that amplify the most useful computational processes—much like the Possible Sword.
Quantum computers do not possess unlimited power; they cannot, for instance, conduct instantaneous searches across any database. However, similar to soap bubbles, they form geometric patterns that highlight certain colors. Quantum computers can provide computational advantages for finding solutions to problems with some structure. A revolutionary application of quantum computers is the clever and subtle use of constructive interference to reveal specific symmetries, such as hidden periods, in data sets (repeating sequences). This capability enables the resolution of algebraic puzzles essential to cryptography, ensuring the security of our everyday telecommunications. The potential for efficiently solving intensive calculations propelled quantum computing into the spotlight in the 1990s.
Another approach to understanding quantum interference is through music, particularly through the study of canons—musical pieces that feature multiple repetitions or imitations of the same voice separated by a time interval (again, a phase shift). One enigmatic type of canon is the “enigmatic canons” found in Johann Sebastian Bach”s “Musical Offering,” BWV 1079. These musical compositions present themselves as puzzles: they are incomplete canons missing voices, requiring us to find a coherent solution formed by the missing voices. A Possible Sword or a quantum computer serves as artifacts for solving problems similar to an enigmatic canon: they execute multiple waves (strikes, programs) like voices in a choral composition that come together, in harmony and disorder, seeking coherent solutions through constructive and destructive interference.
I perceive a transcendental beauty in quantum interference that even helps me recognize my own identity and connect with my gender. As a quantum phenomenon, I am not easily understood by many: I am a non-binary transgender woman, genderqueer and gender fluid. Like neutrino oscillations, my gender is plural. I am not a singular woman but a collective of identities: an activist, a scientist, a mother, a dancer, a painter, an urbanite, a native of a rural town in Extremadura, and many more. I embody a Possible Sword; I am both one and multiple: a woman who navigates and reconciles multiverses traveling together like wave packets. When I speak about this, many people seem confused or do not follow me. I believe that many of those individuals would better understand me if they could experience life as neutrinos, even if just for a day.
In the wave-particle duality of quantum physics, I discover a double irony. On one hand, some physicists, who were denialists in the past, were astounded by the strange implications of quantum theory, all while disregarding comparable wonders that inhabit human reality. On the other hand, there remains a segment of the population that is skeptical and bewildered by gender fluidity; they are unaware that trillions of neutrinos pass through them every second—particles whose gender fluidity is established as a fundamental physical property.
