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## Keio University

Skip to 0 minutes and 3 secondsRecently we talked about using single photons as qubits. We are back here again in the laboratory of Professor Akira Furusawa. Professor Furusawa thank you for being with us. sure. And now we are going to talk about the hardware of optical quantum computers. How you can actually operate on – on those qubits? Here we have one of professor Furusawa’s experiments. Can you explain this to us? Yeah. So first of all, we create time-bin qubit here. Okay. And we make teleportation over there. Okay. And teleportation can be regarded as identity operation or, uh, linear transformation. Okay. A linear transformation that’s like addition or subtraction. Is that right? Um-hmm Okay.

Skip to 0 minutes and 53 secondsSo I see lots of lenses and mirrors and the like here on, on the table. What are they used for? so, if we don’t use these mirrors and beam splitters, light beam is just go straight. Okay. So to build a circuit, we need many mirrors and beam splitters. That, that is a reason why we use many optics here. I see. So you mentioned teleportation. Over here at this end of the table, there is an experiment where the qubit is teleported from one end of the table to the other end of the table. Right? Um-hmm So that’s what you are calling the identity operation. Correct?

Skip to 1 minute and 45 secondsAnd it can be used to move data from one place inside of a quantum computer to another place inside of a quantum computer. Yeah correct. Good. Okay so that’s the linear operations on this table here. Over here on this side of the room we have a different optical table with another experiment on it. Right? Tell us about what’s on this table? Okay here we are working on non-linear operation and by using teleportation technology. Okay so on this table we have linear operations which is like addition or subtraction. Over here, we have non-linear operations… …that’s like multiplication or division. So if we have both technologies then we have all of the basic technologies we need to build a quantum computer? Yeah.

Skip to 2 minutes and 32 secondsGreat. So this optical table, I see lots of lenses and mirrors and what not on, on the table. It covers several meters. Um-hmm. Is this the future of how we build quantum computers using single photons or is there another way to do it? So to make a real quantum computer we have to make these guys very small. For that purpose, we are making a chip like this. This is a waveguide chip and in this chip, we have this type of circuit. Okay you said waveguide. Um-hmm. What is a waveguide? Uh waveguide is a kind of, very similar to optical fiber. Uh, so just like the optical fiber that brings the internet to my house. Yeah. Okay.

Skip to 3 minutes and 26 secondsAnd by using that type of light waveguide, we are building a circuit. So inside of this chip, you have those waveguides and then you have along with them something that does the equivalent of what the beams splitters and other devices you have here on the table does. Yeah. I see. How large would a quantum computer that uses this technology be? So even with this chip, this is just a couple of gates. Okay. So, to make a real quantum computer we have to have millions of gates. Okay. And this is it’s several millimeters wide and a couple of centimeters long. So if we need a whole lot of them… Yeah, yeah.

Skip to 4 minutes and 16 secondsSo, to solve that problem, we have now developed the methodology of time domain multiplexing. Time domain multiplexing. Yeah. that is, we use this device plenty of time. Uh, so we use, uh, this one and the output coming back to here and, do this again and again and again. So the light comes into the chip. It gets processed a little bit and then it comes out the other end and then you circulate it back around and bring it through the same chip again. I see. Yeah. And do it many, many, many times. Okay. What are the challenges to building a system like that? How hard is it to build a large scale quantum computer using this technology?

Skip to 5 minutes and 14 secondsso I should say is optical technology itself is matured. So now we have to work on quantum error correction. Quantum error correction. Good. So that is the last, problem for us. So individual photons, do you lose them? Yeah. When we have light scattering around like, scattering around the laboratory here. It is going in all sorts of different directions. Um-hmm, Um-hmm. Is that a problem for a quantum computer? In principle, yes but, to solve that problem… …we use quantum error correction for… for quantum error correction, we have redundancy of information. So if we have, if we lose some part of the information, uh, we can ahh…get it back to the original data.

Skip to 6 minutes and 21 secondsOkay so you can reconstruct the states even when you have lost some of the photons… Yeah …out of some large state of many photons Ah-ha, I see. Okay thank you. So that tells us how we can manipulate the photonic qubits, execute gates on them with your technology. Tell me, what else do you need to build a large scale quantum computer? To make real quantum computer, we need the ability of quantum error correction. Ah. Okay. So with quantum error correction you take a large number of photons and use them to represent one logical qubit of data? Um-hmm, yeah. Okay.

Skip to 7 minutes and 6 secondsAnd by using that logical qubits we make quantum error correction and to create that type of logical qubit we are now planning to use photon number resolving, superconducting photon number resolving detector. Okay. So that means you need a special detector that can count one photon, two photons, three photons. Um-hmm I see. Yeah. What else do you need for – in terms of hardware boarding machine? Anything else? I think that’s it. Great. That sounds good. Thank you. Thank you all for joining us. This has been our segment on optical quantum computers.

# Photonic systems

In an earlier Step, we visited the laboratory of Professor Akira Furusawa at the University of Tokyo, and learned about using photons as qubits. In this video, we continue that visit and learn more about how those experiments are conducted.

To build an optical quantum computer, we must have a source of single photons, and the ability to execute some of the gates we discussed in the second week. Those gates are performed using beam splitters and devices that change the phase of a photon. To create a complete set of gates, we need a nonlinear operation and the ability to measure the photons. Professor Furusawa shows us his setup for these operations.

Of course, the setup on a lab bench is too big to build a large system, so Professor Furusawa also shows us a chip, made out of silicon, that performs the same functions as an entire table top full of mirrors and beamsplitters. It uses wave guides, like the optical fiber that brings the Internet to your house. Rather than having many, many chips, instead the light is recirculated back through the chip, in a loop, until the process is complete.

Professor Furusawa ends by saying that the most important challenge for building an optical quantum computer is quantum error correction. We will discuss QEC in the next Activity.