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5 October (2026)
and back to online meetings...
A lot to catch up.
Finally back home.
Now at the gate, two hours before boarding, but I couldn't check in online as the plane appeared overbooked (no seat selectable) and I got scared they would deny me boarding if not arriving very early. I'm now 30F, last seat at the back of the plane, but at least in the plane. And for the time being at the gate.
These airports are the most anguishing places I actually go to, in real life. You really feel they are queueing you to get you shot at the end of the line. I'm sure the atmosphere of a camp isn't much less human than that of an airport. But I do go to airports, while I've only been to camps in abstraction.
And speak of a concentration of estranged strangers, everybody disfigured by stress, tiredness and frustrations of a failing trip or a lost luggage. Or of an unwanted destination, robing you from this primal need to stay anchored where you were, with your people and your things, and not get lost at the other side of nowhere with nothing but courtesy and civility of slightly less than strangers is waiting for you.
I read recently that Stendhal started at some point to write about everyday of his life, apologizing for a typo in the first sentence, "already", he said, and the first one of many because he was intending not to correct or change or hide anything. A bit like what I'm doing here.
But things need to happen... I was thinking in one of the queuesâthat of the metro, or of the security, or of the baggage drop-off, never mind which, I forgotâbut I was thinking of such a short story, for Stendhal's or my daily journal: you go and ask someone who is in front or behind you in the queue, or at the coffee stand, or in the corridor, you ask "are you the one I'm looking for?" and see how the story goes from there on. In most cases it'll end up on the spot, but I'm sure some would end up in Patagonia or Ulan Bator.
You see if you go somewhere with someone, it's different. It's the place that's coming to you, not you to it.
Many people have to be looking for something or someone. At the look of them, around me, I would say at least a good 20%, although the real figure is probably closer to 100%. Problem is, of course, you're not the one they're looking for, except you might well be, they just don't know, and you don't know either. And most people are so exhausted from waiting for what they're looking for, they don't even know they're looking for something anymore, they only know that they wait. And the waiting is so boring, I'm sure some people would actually bet on adventure, novelty and exploration, for a change. Some would say: "okay, let's do this, let's open the window on the universe together. We have the universe but we don't have the window. You are the window."
And what could happen? "Are you saying this to everybody?" Then it's an easy dialogue: "yes, to literally everybody, since a too-early morning in an airport, but nobody is the person I'm looking for."
The main problem is, of course, if you already found the person you were looking for but you lost this person at the same moment you found them. That's bound to happen to anyone playing this game of grabbing fates as they pass along. Most will escape you, many with a scratch or a bite, furthermore, and when you finally find someone, she's from the Qinghai province in China or the northern territories of Australia. You wake up to the world but in the middle of the night at a train station which goes both ways, and of course the other person is on the opposite platform, waiting for the other train.
Well, the story could still write itself for a few additional sentences: "I've been looking for you everywhere and in everyone, and I've written it like Stendhal would have done, down to the last details, so that you could go and read it, and see that nobody was you, nothing happened but missing you", and then, the story continues in the future perfect, "and then when I will have found you, the journal will stop writing itself, because I'll be writing it on you instead, with you, through you, it'll be your hand holding mine not holding the pen, there'll be nothing to say because everything will be to do. It won't be something to share, to narrate, to remember. It'll be out of time. It'll be us."
Beautiful story.
As I was writing this, the airport music played "Come on, Eileen", and then this music, this supplication, this exaltation drowned in melancholy, these lyrics:
These people 'round here
Were beaten down, eyes sunk in smoke-dried face
They're so resigned to what their fate is
But not us, no never
But not us, not ever
We are far too young and clever
Remember
Too-ra-loo-ra
Too-ra-loo-rye-ay
And we'll hum this tune foreverin addition to my story, all this together, the violins, this got me... and I'm crying, like a child, I'm weeping, I'm crying in the airport, face sunk in the keyboard and dripping all over...
4 October (2026)
The acropolis by night.
Last (Greek) coffee on Paros, waiting for the bus to the airport.
Doing the check-out at the guest-house, it wasn't Dmitry but his sister, Smaragda, who attended me at the reception (which is the kitchen of their house), while Dmitry on the phone was struggling with the CSIC VAT. We had a long chat, waiting that he works it out. She asked me when was my boat. I said that I was taking the plane. She said "really? why? It is very expensive! So when is your plane then?" When I told her the time, she said "so you have a lot of time, the next bus is at 16:10". I told her I was thinking to go by taxi, and of course she said "really? why? It is very expensive! Go with the bus!" I said that all right, I'll go by bus. So now I have to... I wouldn't like that the little interaction we had end up with a lie. It's at 16:10 indeed. Just the time for one last coffee and to write this down. I'm already benefiting from her advices. It's more interesting to write this down than to sit in a taxi.
While we were waiting, after the banalities, she told me of her solitude on the island. She didn't tell me "I'm lonely", of course. People don't tell you that. She told me about how she has to pretend she's upset with her son, with whom she's got arguments because he doesn't want to listen. Boys never want to listen, she told me, their brain is not as developed as the brain of girls. I asked what it is he didn't want to listen about, and she said "reading; he doesn't want to read". Also that he's difficult because he's bored. I said that surely he must have friends to play with. She told me they have no friends on the island (they're all from Paros for generations), no time to get coffee anymore, no time to chat outside, only work and to take care of her son and very young daughter. So her son's missing friends were really her own. So I had the feeling she was feeling very lonely... That's how people tell you. At least that's my understanding. I'm not telling them "you seem lonely" anymore. Now I just say "hmm-hmm".
She wanted to be a hairdresser. She got a training in Athens, but hairdressing ladies is a nightmare, she told me (she exclaimed that last wordânightmareâin the most Greek way you can imagine, detaining the first syllable in the air with a parabolic motion of her head before she let it crash on the second syllable). Finally the brother called and said he had finally solved it. He had mistaken the last D of the CSIC VAT for a 0 (it was from my handwriting, to be fair), but now it was all right, he just needed my "occupation". I toyed with the idea of making something up, but I told her, and so she told him: "researcher". We thus had to discuss a bit about this too, what it is exactly, that I'm looking for. I told her I wasn't sure, something that ancient Greek people started to look for and nobody found it yet. I toyed with the idea of telling her something more inspired or inspiring but decided not to, she was keeping the conversation going well without me having to be imaginative. But it takes less time to put "Researcher" on a tax-entry form than it does to have a VAT code recognized. So Dmitry called quickly again to say that I could check my email, I was checked out.
I did, and indeed an invoice was there in my inbox. I said "thank you" but Smaragda insisted that I check, so I did. She showed me the price for the room, the extra taxes, she showed me the VAT that was causing so many problems, and so, side by side, we examined this form together with utmost scrutiny, as if we were two tax inspectors.
I feel she would have liked that we spend a bit more time chatting like this, after she showed me everything that there was to show on the invoice, so that she could tell me more about the time of before when she was going to Athens, or about the time of now when she doesn't even go to the coffee shop down the road anymore. I would actually have liked that too, I was not saying much, but she was interesting to listen to, and was making a lot of faces, nodding with a wink, twitching her lips, humming as if in possession of a great wisdom.
I didn't have much to tell her, besides the VAT of CSIC. I could have told her about the monastery, about the candle I had put there in the morning, which was, also, alone when I left if there, and that was already gone by now. About the color of the sea, from up there, about how close the summit looked although there was no road towards it anymore, and if only I had good shoes, how I would have conquered a mountain today. And she could have told me about when she used to go there too, when she was young, when she had friends and time, about how she did reach the top many times and probably will never return now. About how she remembers the color of the sea from up there. How she's not looking at the sea anymore. But we did none of this. After an awkward pause, she extended her hand, I shook it, we both smiled, and back to our loneliness. It seems like lonely people recognize each other. They both get a little bit more lonely afterwards.
Back to Parikia.
The monastery, hanging on a steep flank of the mountain.
Icons and light.
Parikia from the monastery.
Leaving Parikia towards the Agioi Anargyroi monastery.
I arranged the rest of my escape. Fly to Athens and spend the night at Hotel Marina at 13, Voulgari str in central Athens (I stayed at Piraeus port previously). Access by Metro Omonia after ~40 min on blue metro Line 3 until Monastiraki from airport, at which point change to green Line 1 toward Kifissia.
I should have time to wander the old town. Maybe Psyrri (Iroon square), Monastikari and Thissio.
Departure next day in the morning for Madrid, with business in town, before getting back to ICMM. Still in Parikia, I'll try to have a last walk, maybe on the flank of the mountain (I don't have good walking shoes), so as to not spend the whole time doing administration (I still progressed on on referee report and ran some hybrid simulations of Gerace gates with part Fock/part coherent state inputs, so not entirely wasted time).
Last day on the island.
3 October (2026)
That was the sunset for today. The sun disappeared a bit earlier than scheduled, it hid behind the clouds before it plunged into the sea, draped itself with water from the sky not to go totally naked into that from the horizon. As it finally took its dive behind the cloudy curtains, colors splashed from the dark sea and wet all the firmament with burning light: red light, orange light, yellow light. The clouds absorbed the droplets of Sun almost entirely in one instant, and when they had finally soaked all of it, the night washed them out, mixing the blue with the saffron into a purplish black that it lit up with the stars into total darkness.
That's where my two October lovers were hanging from.
Sunset hunting.
I have successfully simulated the entangling Gerace gate (H+CNOT) with coherent states. It fails in a different way than I'd have thought. If fails gracefully. It's not immediately useful for anything I can see (no obvious quantum supremacy) but it's not trivially useless either. There might be scheme to push it into a computational resource. And at a fundamental level, it's a beautiful thing to characterize.
The most interesting is that the block we're interested in remains shielded all down the circuit, so it remains good for whatever comes next, that's what the deterministic scheme brings us. Next is to implement a real application, Deutsch, Simon, GHZ, teleportation, whatever... and show that with probability $e^{-2}$ or $e^{-3}$ depending on what we do, we get our result. It'll certainly won't beat a direct classical approach, but it's a first foot into quantum computation with only resources already available to our experimentalists (interacting waveguides). It'd be interesting to also consider hybrid approach, e.g., a Fock state + a coherent one. That might bring us over the edge.
The sea is not calming down (I've spent the afternoon at the seafront in a restaurant, with Baklava and more Greek coffees) but the sun is coming down. I'll have the chance today to see the sunset from the castle. I'll see if the other prisoners would be interested in that.
The Gerace gates could be useful as deterministic even on Gaussian inputs, and thus semi-scalable, shifting the indeterminacy to the number of qubits as opposed to the number of gates. This seems to be $e^{-n}$ for the $|\alpha|^2=1$ (optimum) case.
I'm not sure how well known this is. See for instance Descamps et al.[1] It would be interesting to devise a practical problem that could be implemented and actually solved demonstrating a quantum advantage. Maybe using Grover's algorithm, for instance.
There is only one story. This dead man on a 2nd century BC funerary stele is sitting in front of a reclining woman, who, even if she has been worn out by time, is clearly escaping his supplication. It's been going on for over 22 centuries for them.
At the archeological museum. C. 550BC.
Truncated coherent states to $\nu$ photons per rail quickly degrade the LOQC CNOT probability of success from $P_{\rm succ}=1/9$ for $\nu=1$ (Fock states) to, already with $\nu=2$, $P_{\rm succ}\simeq|\alpha|^2/27$ (ratios $0.111$ and $0.037$ at $|\alpha|^2=0.01$): already $3.7\times10^{-4}$ at $|\alpha|^2=0.01$.
Besides, which should also be looked at closer, it's not only that the gate fire less, but it fires wrong, in contrast to Gerace gates, which fail gracefully. Anyway, it's now established that while Fock states are an essential resource for LOQC (as they should), it's not the case for Gerace scheme, surprisingly.
The same arguments work for Gerace than for the earlier gates. And then this calls to ponder what have people propagating coherent states in Kerr medium been up to. These are:
- Nearly Deterministic Linear Optical Controlled-NOT Gate. K. Nemoto and W. Munro in Phys. Rev. Lett. 93:250502 (2004).
- Weak nonlinearities: a new route to optical quantum computation. W. Munro, K. Nemoto and T. Spiller in New J. Phys. 7:137 (2005).
- Quantum control gates with weak cross-Kerr nonlinearity. Q. Lin and J. Li in Phys. Rev. A 79:022301 (2009).
Interestingly, Shapiro and Razavi[2] stayed alert and rebuked this implementation too.
There's been an experimental implementation by Feizpour et al.[3].
I also found this paper which has to do with special properties of coherent states (any optical process is fully characterised by how it responds to coherent states), by Lobino et al.[4], from Lvovsky, who was on the island at least until yesterday. In the bus bringing us from the conference venue to Parikia, he told me that they had an old work on quantum vampires, that was probably related to what I showed. This is their Fedorov et al.[5] paper, which might or not have some amount of overlap, the photon detection (or subtraction) indeed bearing on the prior (see also Ref. [6]). But he missed the main point, since when I told him about ghost imaging with thermal light in the absence of single-shot correlations, he said that the underlying mechanism was «like EPR» (which is a surprising statement in any interpretation of the mechanism); so then I repeated the spurious correlations of thermal light but he shook his head to say that no, no, there are correlations. Like EPR. So he understood nothing of my talk. Only that after you detect one photon, you can detect another one.
As for the nonlinear variations Ă la Gerace, that rely on cross-Kerr instead:
- Quantum optical Fredkin gate. G. J. Milburn in Phys. Rev. Lett. 62:2124 (1989).
- Simple quantum computer. I. L. Chuang and Y. Yamamoto in Phys. Rev. A 52:3489 (1995).
- Nonlinear Optics Quantum Computing with Circuit QED. P. Adhikari, M. Hafezi and J. Taylor in Phys. Rev. Lett. 110:060503 (2013).
Milburn[7] implemented a Fredkin gate with cross-Kerr. Chuang and Yamamoto[8] implemented the dual-rail logical encoding of Scala et al. (before them). As for Adhikari et al.[9], they rely on superconducting qubits to implement those ideas.
This has been well-known criticized by Single-photon Kerr nonlinearities do not help quantum computation[10] and Gea-Banacloche[11].
Working on Gerace gates, I must first check failure of LOQC ones with coherent states. The standard schemes are:
Greek coffee, an Hamiltonian and a stormy sea, my kind of Saturday.
2 October (2026)
Now in Parikia, in a cheap guest house, of someone Dimitri. Said offered spontaneously to accommodate me in his room, got me a key, had an extra bed fit in... but when it turned out I would have to spend more than one night, I decided to decline the generous offer. Besides, on an October 2, I wanted to be alone.
The guy is super interesting and we had fascinating conversations every time we met. So I also wouldn't want to take the risk of making a best friend at the corner of some bad weather condition. Then Pavlos also offered to shelter me in his room, and Xenia even offered to give me her room in full, and move herself with someone else... When I told her I was staying for two nights, she told me "then I move out for two nights". Everybody's so nice in harsh weather...
I'll go now with some of the other people stranded on the island to this Symposium, which is the prettiest corner of Parikia, and have an Ouzo with Panayotis, which is long time overdue. One should not get too much alone either on an October 2.
But not the last day on the island. Ferries are cancelled due to weather conditions, so I'll be two more nights here, and a third additional one in Athens... The wind is hurling outside, it's ominous. What a 2nd of October. I knew it'd be a tough day... but did it need to be this literal?
Last day at HPM.
1 October (2026)
We spent the rest of the day walking round in the city center, first with Anton and then with Dario, Daniele and Monica. At the restaurant, a candle lit fire to the curtains. Fire goes very fast. It was outside, a Greek island of stones and painted wood, so nothing to propagate, but it was not for lack of time. We discussed with Anton possible activities for Pavel. I still think that checking the breakdown of Gerace gates with weak coherent pulses as the most interesting immediate activity on that topic. An entangling gate, Hadamard + CNOT. It should produce the Bell states with inputs from the basis, and fail to do that with weak coherent states. The mechanism of the failure would be particularly interesting, as well as to what happens with squeezed states.
We walked around the same places several times. When at the feet of the castle from the sea promenade, I got glimpse of this beautiful picture:
I am surprised nobody recognized that it is an instant masterpiece, and that I had to explain. A young couple, suspending their love and youth over the precipice, one looking at the other looking at the world, loving with the insouciance of not knowing that it is already too late, that there is no further ascension from this apex of their journey together, with the church in background of their promises, and the sky in background of the church. I still marvel at it. I wonder if they are still there. It is very windy now. Maybe they have already fallen.
I'm in the selfie business now.
Today I spoke.
Day 4 at HPM. Today I speak.
- â Superselection Rules and Bosonic Quantum Computational Resources. E. Descamps, N. Fabre, A. Saharyan, A. Keller and P. Milman in Phys. Rev. Lett. 133:260605 (2024).
- â Continuous-time cross-phase modulation and quantum computation. J. Shapiro and M. Razavi in New J. Phys. 9:16 (2007).
- â Observation of the nonlinear phase shift due to single post-selected photons. A. Feizpour, M. Hallaji, G. Dmochowski and A. Steinberg in Nature Phys. 11:905 (2015).
- â Complete Characterization of Quantum-Optical Processes. M. Lobino, D. Korystov, C. Kupchak, E. Figueroa, B. Sanders and A. Lvovsky in Science 322:563 (2008).
- â Quantum vampire: collapse-free action at a distance by the photon annihilation operator. I. Fedorov, A. Ulanov, Y. Kurochkin and A. Lvovsky in Optica 2:112 (2015).
- â How quantum is the âquantum vampireâ effect?: testing with thermal light. K. Katamadze, G. Avosopiants, Y. Bogdanov and S. Kulik in Optica 5:723 (2018).
- â Quantum optical Fredkin gate. G. J. Milburn in Phys. Rev. Lett. 62:2124 (1989).
- â Simple quantum computer. I. L. Chuang and Y. Yamamoto in Phys. Rev. A 52:3489 (1995).
- â Nonlinear Optics Quantum Computing with Circuit QED. P. Adhikari, M. Hafezi and J. Taylor in Phys. Rev. Lett. 110:060503 (2013).
- â J. H. Shapiro. Single-photon Kerr nonlinearities do not help quantum computation in Phys. Rev. A 73:062305 (2006).
- â Impossibility of large phase shifts via the giant Kerr effect with single-photon wave packets. J. Gea-Banacloche in Phys. Rev. A 81:043823 (2010).