Jan Gulla

• Rukan, Isak Cecil Onsager; Gulla, Jan & Skaar, Johannes (2026). Truncated Photon. Physical Review Letters. ISSN 0031-9007. 137(3). doi: 10.1103/94pm-hp34. Full text in Research Archive Show summary

  An elementary particle such as a photon cannot be cut in two pieces. Still it must be possible to truncate a photon with an optical shutter. The result is neither another photon nor a mix of a photon and a vacuum. Instead it is a superposition and mix of photon numbers up to infinity. This state is rather complicated, but nevertheless locally equivalent to a single photon or vacuum to the left and right, respectively, of a narrow transition region.

• Gulla, Jan; Ryen, Kai & Skaar, Johannes (2023). Limits for realizing single photons. Physical Review A (PRA). ISSN 2469-9926. 108(6), p. 063708–063708. doi: 10.1103/PhysRevA.108.063708. Full text in Research Archive Show summary

  Exact single photons cannot be generated on demand due to their infinite tails. To quantify how close realizable optical states can be to some target single photon in one dimension, we argue that there are two natural but incompatible ways to specify the target state. Either it can be expressed as a photon with a chosen positive-frequency spectrum, or it can be described as an (unphysical) photon in a chosen positive-time pulse. The results show that for sufficiently short target pulses, the closest realizable states contain substantial multiphoton components. Upper and lower bounds for the maximum fidelity are derived and are expressed as functions of the size of the target state's tail, for negative time or negative frequency, respectively. We also generalize the bounds to arbitrary photon-number states.

• Ryen, Kai; Gulla, Jan & Skaar, Johannes (2022). Strictly Localized Three-Dimensional States Close to Single Photons. International journal of theoretical physics. ISSN 0020-7748. 61(5). doi: 10.1007/s10773-022-05133-7. Full text in Research Archive Show summary

  Abstract A class of strictly localized states which can be made arbitrarily close to single photons is constructed, and expressions for central properties are provided. It is demonstrated that single photon states can be well approximated by these states down to localization scales on the order of a few pulse cycles. The results readily generalize to states close to n -photons. We also provide upper and lower bounds for the fidelity between a given single photon and any state strictly localized to a given volume. These results constitute the limit of photon localization, complementary to the weak-localization limit (Bialynicki-Birula, Phys. Rev. Lett. 80 , 5247, (1998)).

• Gulla, Jan & Skaar, Johannes (2021). Tunneling times of single photons. Journal of the Optical Society of America. B, Optical physics. ISSN 0740-3224. 38(11), p. 3457–3465. doi: 10.1364/JOSAB.437386. Full text in Research Archive Show summary

  Although the group delay of classical pulses through a barrier may suggest superluminality, the information transfer is limited by the precursor, which propagates at the vacuum light speed. Single photons, however, have infinite tails, and the question of causality becomes meaningless. We solve this problem by introducing strictly localized states close to single photons, which are examples of optical states produced by on-demand single-photon sources. These states can be arbitrarily close to single photons while demonstrating causality for their leading edge.

• Gulla, Jan & Skaar, Johannes (2021). Approaching Single-Photon Pulses. Physical Review Letters. ISSN 0031-9007. 126(7), p. 073601–073601. doi: 10.1103/PhysRevLett.126.073601. Full text in Research Archive Show summary

  Single-photon pulses cannot be generated on demand, due to incompatible requirements of positive frequencies and positive times. Resulting states therefore contain small probabilities for multiphotons. We derive upper and lower bounds for the maximum fidelity of realizable states that approximate single-photon pulses. The bounds have implications for ultrafast optics; the maximum fidelity is low for pulses with few cycles or close to the onset, but increases rapidly as the pulse envelope varies more slowly. We also demonstrate strictly localized states that are close to single photons.

View all works in NVA

• Rukan, Isak Cecil Onsager; Gulla, Jan & Skaar, Johannes (2026). Truncating a photon. Full text in Research Archive
• Gupta, Chaitanya; Skaar, Johannes; Rukan, Isak Cecil Onsager & Gulla, Jan (2026). What can cutting a photon in half tell us about causality and local equivalence? – Physics World. [Internet]. Physics World. Full text in Research Archive Show summary

  Calculations identify complicated consequences of removing a mirror

• Skaar, Johannes; Thorsteinsen, Pål Henrik; Rukan, Isak Cecil Onsager & Gulla, Jan (2026). Kan et foton deles i to? Ny studie fra UiO gir et oppsiktsvekkende svar. [Internet]. Instagram. Full text in Research Archive Show summary

  166 likes, 6 comments - unioslo_mn on September 4, 2026: "Kan et foton deles i to? Ny studie fra UiO gir et oppsiktsvekkende svar.   Alt lys og all stråling består av fotoner. Elementærpartikler som man tidligere har tenkt ikke kan deles i to. Men forskere ved Universitetet i Oslo stilte seg et ganske grunnleggende spørsmål: Hva skjer om vi prøver å kutte av enden til et foton? Forskerne eksperimenterte med å sende et foton mot et speil i vakuum. Ettersom fotoner oppfører seg både bølger og partikler, går det an å se dem for seg som bølger med en viss lengde.  Så fjerner de speilet midt i bølgen. Det blir som å prøve å kutte det i to. Men i stedet for å få to fotoner får du en blanding av mange ulike tilstander, som for eksempel null, ett, ti eller tusen fotoner. Studien har fått stor oppmerksomhet i fagmiljøet og blitt publisert i anerkjente tidsskrifter. 🎤 Johannes Skaar 📸 Henrik Thorsteinsen ✂️ Henrik Thorsteinsen".

• Borgan, Eldrid; Skaar, Johannes; Sudbø, Asle; Rukan, Isak Cecil Onsager & Gulla, Jan (2026). Kan et foton deles i to? Svaret er ganske sprøtt, viser en ny norsk studie. [Internet]. forskning.no. Full text in Research Archive Show summary

  Dette er viktig å vite for mange som jobber med kvanteteknologi, mener forsker.

• Harris, Rory; Rukan, Isak Cecil Onsager; Gulla, Jan & Skaar, Johannes (2026). 'A mixture from zero to infinity': Physicists split apart a photon — and ended up with an improbable swarm of particles. [Internet]. LiveScience. Full text in Research Archive Show summary

  Physicists have found that splitting a photon would lead to a complex state that may change the way we think of particles.

• Farkas, Ivan; Rukan, Isak Cecil Onsager; Gulla, Jan & Skaar, Johannes (2026). Physicists Find a Way to 'Split' Particles of Light in Half to Make Infinite Photons : ScienceAlert. [Internet]. ScienceAlert. Full text in Research Archive Show summary

  Photons act in freaky ways that remind us that nothing is as it seems at the smallest scales and most fundamental levels of existence.

• Stephens, Marric; Rukan, Isak Cecil Onsager; Gulla, Jan & Skaar, Johannes (2026). Cutting the tail of a photon. [Internet]. Physics Magazine. doi: https:/physics.aps.org/articles/v19/s91. Full text in Research Archive Show summary

  Removing a mirror while a single photon is in the process of reflecting creates a quantum state of countless photons, theorists say.

• Temming, Maria; Rukan, Isak Cecil Onsager; Gulla, Jan & Skaar, Johannes (2026). Here’s what would happen if you tried to break a photon in half. [Internet]. ScienceNews. Full text in Research Archive
• Padavic-Callaghan, Karmela; Rukan, Isak Cecil Onsager; Gulla, Jan & Skaar, Johannes (2026). Photons behave very strangely if you try to cut them. [Internet]. New Scientist. Full text in Research Archive Show summary

  Particles of light cannot be divided into smaller particles, but if you try to snip off the end of one, instead of shortening it multiplies

• Gulla, Jan (2024). Photon localization: Local measurements, localized states, and on-demand sources in quantum field theory. Universitetet i Oslo. Full text in Research Archive Show summary

  Photons and other fundamental particles in physics allow us to predict many phenomena in nature with very high accuracy. However, a fundamental limitation of these particle states is that they cannot be localized in space and time, meaning that they cannot describe any signal generated on demand. In this work we analyze the issue of particle localization in quantum field theory. To formalize what it means for a relativistic particle to be localized, we take an operational approach based on expectation values of local measurements. In this framework it is clear that single photons cannot represent any causal signal, since their energy density is nonzero for all space. To solve this problem, we introduce a new type of optical state that is localized while simultaneously being close to a single photon in many realizable situations. This new state may represent a non-classical signal generated on demand in quantum field theory. Furthermore, we establish constraints on the maximum possible fidelity between any localized state and a single photon. The results show that any ultrafast optical signal produced on demand must inevitably contain substantial multiphoton components! These methods also make it possible to demonstrate causality for the much-debated phenomenon of single-photon tunneling.

View all works in NVA