 For having me at the meeting, I'm sorry that I couldn't make it in person, my travel schedule was kind of stuck in a way that I really couldn't do it differently. But thanks for accommodating for an online presentation. Anyhow, I hope you see my slide and the pointer and also you hear me well. Yes, we see everything. Good. Excellent. So this is on realizing an universal quantum gate set for itinerant microwave of frequency photons. This is essentially part of a PhD thesis, which is ongoing by Kevin Royer. And Kevin Royer works together with Jean Claude best who works on topics like this in our labs in spite some time. And this project is coordinated by Christopher Eichler. So what we did in this experiment, we essentially made two nominally identical superconducting circuit devices, which can emit and absorb photons and sort of this top left device here can create photons and emit them into a line. And these photons can then travel towards a second chip where they can be absorbed. You can do single photon gates on on on the absorb photons and re-emit them into the transmission line. And then we were also able to realize two photonic qubit gates in this way. Essentially sending one photon down from the first to the second chip is already there. And then having it interact with the second photon that bounces off the second chip here. And we can then characterize the photons using linear detection techniques in the way that Pear had already mentioned by amplifying the radiation fields with microwave frequency amplifiers and then digitizing quadratures and doing tomography on the detected radiation fields. So as usual, I'd like to thank in the in the beginning all the different people who've contributed to the progress and our lab throughout the various years and then you see what they're doing these days here. Like quite a few of them ended up in faculty positions, which is very nice to see. But what is also interesting is that more and more of them really work in the quantum industry, either in startups or in companies that provide tools in and around quantum technology. So that that is very nice to see. And I'd also like to thank all the many collaborators that we have throughout the years. Yeah, why would you care to to realize gates between photons. There's quite a few use cases for those so gates between photons could be used in quantum networks for example when realizing quantum repeaters or quantum routers. The ability to perform gates between propagating photons would also allow one to create interesting many body quantum entangled states or, for example, make cluster states or other 1D 2D or 3D graph states in this way, and maybe also use variational approaches not on stationary qubits but but on propagating photons, for example. There's obviously also the interest in doing photon based quantum computing that if you had an efficient way to realize gates between photons that could also be implemented and not only with optical frequency photons but in the microwave frequency domain. Both with single photons and possibly with with continuous variables like in the way that pair had alluded to in the previous presentation. Good and this talk here we're going to focus on trying to realize a deterministic to photon gate and realizing a deterministic to photon gaiters as and something that is special across all frequency domains so the best of our knowledge so far the even in the optical frequency domain. The two photon gates which were realized were post selected, and our work is certainly strongly inspired by the cavity QED work from from Gerhard Rampus lab who pioneered many of the ways how we put gates between propagating optical frequency photons. And so there's two works from gas rampers group which are quoted here and how ours are set apart from those is is that are the experiments that I'll be discussing today require no post selection so it's a deterministic to qubit gate on propagating photons. And so here you see the sketch of what the setup or the experiment looks like. So there's essentially a source qubit, which is on one chip and that source qubit is shown here in red, we can manipulate it in the usual way with microwave frequency pulses and applying flux pulses to change the transition frequency. And then this source qubit is coupled through a tunable coupler and essentially a converter qubit that allows us to add our at the time that we desire launch a photon from the source qubit into a transmission line and send it along that transmission line. And this transmission line is intercepted or interrupted by a circulator here and that allows us to route those photons towards a second chip on which we have another converter mode realized as a qubit that is coupled with a tunable coupler to our gate qubit that we'll use to both perform single qubit gates on these photons and also to realize a two photon gate between two subsequent photons that were emitted by the source qubit. And then the photons that get reemitted by the gate qubit are again propagating towards the circulator here and then enter our linear detection circuitry where we amplify the fields with traveling wave parametric amplifiers or dosent parametric amplifiers and then digitize the quadratures and do this moment based analysis, which I will remind you of in a second. So this experiment now puts together a number of previous developments that we have made in our lab so one of the developments is to create this capability to create a train of photons from a source that was demonstrated by Jean-Claude Bess as part of his PhD thesis a couple of years ago, which is essentially based on controllably coupling photons or excitations that get created in a qubit into a transmission line to create subsequent photon pulses and create in this way a chain of photons. And then we also use another development in that was part of Jean-Claude Bess PhD thesis that relates to photon detection and then this photon detection work which I'll also explain in a little bit of detail. We've essentially made use of a control to Z gate between the propagating photon and the qubit held in a cavity. And this mechanism of both creates a conditional phase shift on the qubit state but also conditional phase shift on the reflected photon. And this conditional phase shift can actually be used to create a two photon gate. And then in the end we do characterize the states that we have created both for single photons and for two photons in this linear detection scheme. Okay, so let me first show to you what our chips look like. So we have essentially two almost identical chips for which contain the source qubit and this converter mode and the gate qubit and the converter mode. And those designs were made for previous experiments and we essentially made a second copy of such as a chip so that we could use it in this context. So here's the picture of this device and the color code is slightly different from what you've seen on the last slide unfortunately so we'll fix that in the future. So on this device here you see two superconducting transparent style qubits. The red one here in the left part is the storage qubit in which we create the excitations. And this storage qubit we can also read out so there's a readout resonator which is on the quarter resonator here that's coupled through per cell filter to a readout line so that this can be used for example to use this chip in a photon detection context as well. And then there is an so-called emitter or converter qubit on the right hand side here shown in blue. And this qubit is strongly coupled to an input and output transmission line shown in purple here. So once we swap the excitation from the storage qubit into this converter qubit it will leak at a megahertz level rate into this transmission line and create a photon in that transmission line. And the storage qubit and the converter qubit are coupled through a tunable coupler. So this has two coupling paths one static coupling path realized by this transmission line here and then one coupling path that is interrupted by a tunable squid that we can then modulate through flux line. And this allows us to essentially do a parametric coupling between the storage qubit and the converter qubit and then this way essentially swap excitations back and forth between the two. So if we want to create a photon we essentially swap the excitation from the storage qubit into the converter qubit from where it will get emitted into the transmission line. And reversely when there's a photon arriving from the transmission line we can essentially switch on this conversion pulse or a parametric coupling between the converter qubit and the storage qubit and absorb a photon into that storage qubit. And yeah you see that every one of the qubits has a charge line to control its state and it has a flux line to control its transition frequency and this third flux line here is used to parametrically drive the coupler between the two qubits. And we have essentially two of these devices which are made nominally. Good so so that's the two devices that we are using in this particular experiment. So then let me remind you briefly of this measurement technique that pair already mentioned to you and we're happy to see that even though that it's now kind of a 10 years old already as a technique. So we still use it in our lab and it also gets used quite actively in this in this community of researchers that investigate the quantum properties of propagating microwave photons and that was part of Christopher Eichler's PhD thesis and probably a good discussion of it is found in this paper from 2012. So essentially when when we want to detect the quantum state of a radiation field. Through first linearly amplifying it and and then detecting its quadrature amplitudes that we essentially in a first set of experiments characterized the input noise to this detection chain and integrate the X and the P quadratures that we can detect by mixing down the signal on a on a microwave frequency mix and then store pair wise components of the X and the P quadrature and form a histogram out of these data points and then, even if you amplify here in the, which was in the initial experiments that we have performed in Christopher thesis was just a hand amplifier that added a large amount of quasi thermal noise to the to the signal that you're actually interested in, even in sort of parametric amplifier settings that since these, there's a little bit of loss between the sample and the amplifier. And the amplifiers are only close to quantum limit that limited but not perfectly quantum limited so this technique is kind of useful to remove the residual noise from from your detected radiation fields. So in this early data that is shown here this histogram that gets formed from this integrated pairs of X and P quadratures has a large width in comparison to the vacuum noise. So you do that essentially without creating any signal and that characterizes your detection chain and and the noise that the input of the detection chain. So then you perform an analysis that is based on calculating the moments of these statistical distributions of the quadratures, and you do that once with your signal being switched off and that creates this noise moments and then you switch your signal that you're interested on. And in this case here we've the example is for Fox state. And then you calculate the moments that are essentially a combination of the noise and the signal and having these two data sets of a noise only moments and noise plus signal moments you can then recover the signal moments and from those signal moments as pair had had discussed you can essentially calculate any quantity that you're interested in either the leader function or order density matrix of the radiation field state that you're interested in. And we had to really use that successfully in our experiments and all sorts of different settings first to characterize propagating Fox states of microwave photons using this method. You can extend it to look at entanglement between qubits and propagating photons you can look at multi modal radiation fields like in this home bundle experiment where you look at to detection paths after beam splitter. And use the same method than on two modes or you can also look at squeezing that as it happens in parametric amplifiers for example. And so exactly that method gets also used in our experiments here. That we have performed in our lab. I see my cameras frozen. Do you actually hear me. Yes, we hear you. Okay, good. Yeah, so I just wanted to verify that I don't don't speak into the void. Okay, that's good. Thanks. All right, so yeah, I have now told you about our devices and and our measurement apparatus. And let me now tell you how we do the, the gates in particular the two qubit gates to photonic qubit gates. And so this, this two photon qubit gate was actually inspired by work from the Rampus lab that I mentioned, which we've also realized which we've made used off of in creating a single photon detector. And that is essentially creating or realizing a controlled phase gate between an incoming propagating photon and and a stationary qubit that maybe may or may not be coupled to a cavity mode. And so I'll explain to you how that works. First in this context of the single photon detection experiment that that John could best it as part of his PhD thesis. And so besides using linear detection schemes for microwave photon detection you can also use nonlinear elements and our sort of experiments pioneering experiments with Rittberg atoms for example and and also at Yale by Blake Johnson initially and also our lab had contributed to that and there's experiments from McDermott's group and Madison. And then a number of groups have thought about this single photon detection problem and trying to detect radiation fields not in the coherent state basis as is done in this linear detection scheme that I had mentioned just before. So let's try to detect in the, in the fuck basis to essentially get a detector that gets clicked does click when, when a single photon gets detected. And also there was a range of interesting experiments done also by Basha McGeohar's group for example in Yasunah Kamuro's group and also by Jean Claude. And now I'll explain to you briefly how this, this technique here from Jean Claude works because that's the one that we use to perform a control phase gate between essentially two propagating photons. Right so in this concept which Jean Claude realized we would like to project our propagating radiation state into the photon number basis instead of the quadrature basis as I mentioned. And we're actually making use by a protocol what that was first suggested by Duane and Kimball. And in that protocol you actually place a two level or multi level system in a, in a cavity. You reflect the radiation field off of that, that cavity and to essentially perform a gate operation between that incoming radiation field and the qubit inside the cavity. And what is special in this case is that the qubit inside the cavity has its grounds to first excited state transition frequency. Omega GED tuned from the cavity frequency but the set first to second excited state transition frequency omega EF in resonance with with a cavity frequency. And then you can create either classical or quantum radiation fields and send them as probe fields to your cavity and see what the interaction looks like with a with a cavity coupled to a qubit. And the radiation field interacting with the cavity in the qubit then bounces off the cavity and in this technique that I've mentioned to you already it gets passed through a circulator and can then be detected using this linear detection scheme that I've just mentioned to you. And in this particular experiment there were two additional features that we were using so we could independently then read out the qubit stage. And this photon detection context and we could also inject coherent fields into the detection mode of the cavity which allowed us to displace this cavity field and in that context we could actually perform beat not tomography on this incoming propagating radiation fields. And in this setting you can essentially measure the parity of the of the input field and if you limit the input fields to single photons or vacuum states that will turn into a single photon detector. But otherwise it's a it's a it's a parity detector that you can use also to beat the tomography and create propagating cat states for example. In addition to the reflected field you can then characterize using these quadrature detection schemes. Okay, so so why is that important is important because this is in a good way how to characterize how the radiation field that gets reflected off the cavity experiences a phase shift that is conditioned on a state of the qubit coupled to the cavity mode. So the G to E transition of the qubit is detuned from the cavity while the E to F transition is in resonance with the cavity. So when the qubit is in the ground state and a photon impinges the photon essentially sees the Laurentian line of cavity and essentially a resonant with this zero to one transition in the cavity but but since the qubit is detuned to this will not see the vacuum mode spitting of the cavity while when the qubit is in the first excited state and the photon comes in. This will essentially experience the vacuum Rabi mode spitting between the E to F transition and the incoming photon. So when you look at the spectral response in these two cases when with the qubit in the ground state you will essentially see a pie phase wrap. So across the resonance frequency of the of the cavity so that just shows the how the phase behaves around the resonance. While when the qubit is in the first excited state, the cavity mode will split into two in this vacuum Rabi mode splitting and then at the cavity resonance frequency at which you shine photons at the cavity. There will be no phase response. And the reflected field will actually then have the the phase difference between these two situations with the qubit either being the bounce date on the first excited state imprinted on the reflected field. And this you could of course measure with properly prepared probe fields and we did that in this case with with we coherent tones where we've compared. With coherent tone experiences in these two situations and in that way we could just verify that our coherent tone experiences this phase shift of pie when it gets reflected on off the cavity with the qubit installed inside the cavity with this particular frequency configuration that I've mentioned to you. And essentially we use this mechanism now to realize a gate between two subsequent microwave photons used in our device. And this method that I've just presented to you works as a parity detector and with this parity detector. And so we have also created cat states and have used the parity detector for doing directly not tomography. And just as a reminder, these are the two publications that came out of that as part of drunk road basis PhD thesis and so here is the parity detection signal as you see that the parity detector tells you whether you have an even or an odd parity of photons in the imaging radiation field and here these different parity states were created by just subsequent e having a number of photons impinge on the on the detector and you could also herald propagating cat states in this way when you condition the detection of the reflected field on whether your parity detector had indicated even odd parity. Good. So, so let's get back to the to the actual topic of this, namely the universal gates that implementation point to the container and microwave photons. And so now we put all these elements together the troops ships with linear detection and capability of doing control phase gates between a propagating mode and and the qubit coupled to a cavity. So how does that then work. So, let's first look at how we create photons reabsorb them create photons absorb them and then reemit them. And as I mentioned, we have source and gate qubits and those are trans bonds there as you've seen on the device they're coupled with this tunable coupler to a qubit that we call a converter mode. Storage mode states be labeled with G and F and the converter qubit modes as they absorb the photons from the transmission line be labeled with zero and one. And okay in this particular case we mediate the coupling between this gate or storage qubit and the converter qubit by driving the tunable coupler here. At the frequency that corresponds to the detuning between the converter qubit and the gate qubit. And when you drive it at that frequency you essentially mediate couplings between the E zero state and the G one state so essentially that creates a swap between a photon coming in from this transmission line and converting it into an excitation stored in the storage. Whenever this is switched on, I can absorb photons from from the transmission line. Or when I create an excitation in the storage qubit I can launch a photon into the transmission line by by switching on this coupling and then emitting the the excitation through this strongly coupled converter qubit into the transmission line. And then modulating that coupling strength J of T and time also allows me to shape the photon that gets emitted into the transmission line. And that's actually the first thing that we've been trying to characterize the system. So here with the source qubit to be able to measure the field amplitude we've created an equal superposition state between vacuum and a single photon fox state. And we've then switched on this tunable coupler and shape the emitted photon. And we've not done anything with our gate qubit so the photon and just bounces off the gate qubit propagates once more towards the circulator and then is detected. And so here's the the mode structure of that detected photon. So that's one of the reference measurements. And then in a second experiment, we've created a zero plus one superposition in the in the gate qubit and have emitted that excitation through this tunable coupler and the converter mode into our transmission line, but then through the circulator gets detected as we as you see the path of that photon is now shorter so it suffers less loss. And therefore the photon amplitude is a bit higher and the integral under that curve is a bit larger and from the difference you can look to conclude on the on the difference in the last that the photons emitted either from the source qubit or from the gate. So what is then important in the context of doing both single and two qubit gates is is the capability of creating a photon in the source qubit, and then actually absorbing it in the gate qubit and reemitting it from the gate qubit and the curve is exactly that sequence we emit a photon absorb it in the gate qubit keep it there and then reemitted into our detector and measure it. And you see that in that instance, the amplitude of the zero plus one superposition state is even a little bit smaller and that has to do with both the decoherence that happens during this process and also with the loss and the transmission in the circulator that the photons propagate through. Okay, and so then you can check whether whether our controlled phase gate between the two propagating photons work so what you do first is you send the first photon towards the gate qubit, you either absorb it in the gate qubit or not and then dependent on whether you had absorbed the first photon in the gate qubit or not the second photon will actually interact differently with this gate qubit there if there was no photon absorb in the gate qubit. The second photon will just get reflected with with a high phase shift. But when you have absorbed the first photon in the gate qubit. The second photon will actually not suffer this pie phase shift and this we can distinguish here by looking at the field amplitude and if there's a pie phase shift between the field amplitudes we actually see the sign of the field amplitude change. And so this is a direct evidence between this imprinted phase shift that happens, conditioned on whether the gate qubit is in the ground or excited state, which then is determined by whether the first photon was absorbed or not absorbed. And this is the basis for creating the C phase gate between two subsequent photons. What we also see is that there is a little bit of distortion in these pulse shapes and the distortion has to do with the with the fact that the bandwidth of the photons is a bit too large for the coupling strengths of the, of the converter qubits to their respective transmission lines and so if that coupling strength was the was larger I think we would hope that that mode shapes would be closer to what we would expect theoretically. And so then there's the two types of gates that one can realize in this way so there's first single photon gates. So here's a like the gate sequence to realize gates on on individual photons so we create a photon one and we apply a unitary to it so that happens and then the following way you create photon one in the source qubit. You swap it into the gate qubit then you apply a gate operation while the photon is absorbed in that cube then they swap it back out into the transmission line and then this way you applied a single qubit gate to that propagating photon after absorbing and then you can do a process tomography on on on the gates that you realize and here's an example for an X gate on that photon and that process tomography. You just create all possible input states and put tomography on the output states and from that you reconstruct the chi process matrix matrix for the X gate acting on that photon. And we see that the fidelity the total fidelity is about 76% and that's mostly limited by the loss that the photons experience on the path from the source to the gate qubit. And if we kind of correct for that loss we can would conclude that the process as an internal fidelity of maybe 87% or so, which is still not perfect and the main reason for that is that the coherence of the qubits on this device is, is not so great I think the coherences were on on the out of 10 microseconds, roughly with face coherences at the operating points even being a bit smaller. So then we've realized not only X gates but also identity gates and why gates and also the key gate for a single photon and determine the process fidelity on both as as measured and then corrected for the loss that it experiences and we find roughly 75% single qubit gate fidelity and corrected for loss around around 87%. And then we looked at how well the photon photon gate performs by doing quantum process tomography on that as well and and the photon photon gate is essentially the C phase gate between two subsequently emitted photons P1 and P2. And the first photon gets created converted into the gate qubit and then stored there and then the second photon gets created and then interacts in the way that I've explained with the first photon that is now absorbed in the gate qubit. And essentially, you can now do a quantum process tomography on this on the C phase gate and in the way I've explained previously and determine the process fidelity for the C phase gates. And because of the limited coherence properties and now the fact that both qubits or both photons suffer from from loss the fidelity here not correcting for for losses about 57% and correcting for losses about 75%. And so we've tried to compare that to what we would expect based on on master equation simulations which you see here as the open bars in this plot of the of the kind matrix of the process. Okay, so, so we've now done single qubit gates and two qubit gates with, I think, respectable fidelity but certainly for for having better applications or so with this need to be improved. Good. So, for the summary we've demonstrated that the domestic universal gate set on internal microwave photons. I think the fidelities are pretty decent but could be improved further and probably need to be improved if one wants to use it and further applications. So, one of the ways to reduce or improve those fidelities by reducing the loss, for example in a circulator so one could use a circulator approaches that make use of superconducting circuits for example, what would also help is to increase the bandwidth of the coupling of the qubit of the converter qubits to their transmission lines that would allow us to do the process faster and also improving the coherences of the qubits used in this experiment would certainly help. And then if one wanted to, to deal with the loss in other ways one could for example use time been encoded photons and use heralding to reject events in which photons were lost. And that's something that we had presented or demonstrated in a previous experiment. Good. And, and if this is then successful one could go ahead and use these two photon gates and networking applications with superconducting circuits. And maybe just as a, as a reminder or just for those who are interested in so like similar techniques we have used to make multi photon entangled states of the cluster gc and w type and that's discussed in this nature communications paper. So, from, from fairly recently where you where we've made states with with up to four photons and done photomography on them but then we've used other methods to characterize the quality of those states with up to 10 photons and so I invite you to look at that. Thank you, which is also from Jean-Claude best and I'd also remind you that these techniques are then also applicable for systems where you have qubits separated by larger distances and this is a picture of one of our experiments where we have to Christ that's linked over this big distance and now we have at ETH a nice setup where the two Christ that are separated by 30 meters and where we can do explorations of non-local quantum physics with superconducting circuits. Okay, with that I'd like to thank you for your attention and I'd like to thank everyone in our lab for doing great work on this and other topics and I would be happy to take some questions if there's any time left. Thank you very much. Thank you. Thank you Andreas for this interesting talk. Do we have questions? Yeah, great results. Congratulations. My question is that can you say like what are the main advantages of your kind of this type of photon source slash receiver than compared to the one you used earlier with this foggy pulse technique? Yeah, I think they're very closely related and in the end it depends on details I think. I do think that for example the fidelities that we've presented with these foggy techniques at 0G1 were actually a little bit better because I think the impedance matching and everything was a bit better on those devices. Yeah, so I'm not sure whether this one is particularly better but it just mediates the interaction in a different way and you couldn't essentially pick different ways to interact the two systems with each other. I think the main difference here now is that in this f0G1 approach is the second quantum system was essentially a second, like there was a qubit and a second cavity from which you then launched photons into a transmission line and here this is generalized with two qubits and instead of using a side fan transition, now one uses a parametric drive between the two systems and I think then which one does better depends on the details of the implementation but I think generally they should be rather equivalent. But our new sort of the more complicated devices that we're currently making, they actually make use of these parametric coupling techniques and we're looking into creating more complicated graph states from photons created in these superconducting circuits. More questions? Just out of curiosity, this tunable coupler, like when you don't drive parametrically, is there a special point you want to see that like do you turn off the coupling then or? Yes, exactly. So in this device here you have these two coupling paths and this actually allows you to zero the coupling and this is important because you want to make sure that the storage qubit is well protected from the strong coupling into the transmission line where you create the photons and that really then works better when you zero the coupling and in this combination where you have a static coupling and a dynamic coupling path you can just adjust the flux to zero the coupling to essentially store the photon as well as possible in this storage mode. Okay, thank you. So there are no more questions so let's thank Andreas again. Thank you and I hope you enjoy your conference and I'm really sorry that I couldn't make it this time it was totally on my end that I was, I just didn't get my travel plan out well. All right, thank you very much for having me and feel free to reach out to me if you have further questions and I'll work in one of them. Thank you. So is there any announcement?