How is CNOT gate physically implemented in IBM Q?

How is CNOT gate physically implemented in IBM Q? Ask Question Asked 6 years, 8 months ago Modified today Viewed 5k times 10 This is because if control qubit is in arbitrary state then how can it be made to control the CNOT gate? What is the interface between Control Qubit and CNOT Gate? quantum-gatequantum-state Share Improve this question Follow asked Jul 26, 2019 at 8:07 Ashish 2952 2 silver badges 9 9 bronze badges 2 Hi Ashish! Welcome to QCSE! Do you know, in general, how a CNOT 𝐶 𝑁 𝑂 𝑇 gate works on qubits? One qubit, say the top qubit, "controls" the negation another qubit, say the bottom qubit. If the top qubit is |0⟩ | 0 ⟩ then the bottom qubit does not get negated; otherwise if the top qubit is |1⟩ | 1 ⟩ then the bottom qubit gets negated. The top qubit is the control qubit here. The entire operation is called the CNOT 𝐶 𝑁 𝑂 𝑇 operation. Both qubits can be in a superposition! But this is what the gate does... –  Mark Spinelli Commented Jul 26, 2019 at 17:27 2 Are you asking about the hardware? Like what pulses are used? –  AHusain Commented Jul 26, 2019 at 18:31 Thanks Mark, I have some experience with Quantum Algorithms so I understand the working of CNOT gates theoretically, however when it comes to implementation, I am unable to follow as i find that there are two kinds of gates one are like Hadamard or Rotation gates which essential act upon qubits and does not need input from another Qubit. The other kind of gates are Controlled gates like CNOT which need input from at least one Qubit to act on other Qubit(s). I will continue... –  Ashish Commented Jul 28, 2019 at 14:14 continued part... The moment a gate needs a Qubit's state to decide what operation to perform, it becomes difficult to understand implementation through resonators as given in papers because somehow they are not including explicit steps which I am used to reading in Algorithm literature... If you can point to a literature for layman then it would be great... The implementation of first type of gates like Hardamard makes sense but not Second type of CNOT gates... Pl. help. –  Ashish Commented Jul 28, 2019 at 14:21 Hi AHusain, I want to have a logical understanding of physical implementation of CNOT gate. Pl. refer to my comments on Mark's comment. –  Ashish Commented Jul 28, 2019 at 14:23 Add a comment 2 Answers Sorted by: Highest score (default) Date modified (newest first) Date created (oldest first) 9 First of all let me welcome you to QCSE! I'm new myself. IBM operates superconducting qubits based on josephson junctions. The basic architecture of the chip can be visualized as a graph with qubits at the vertices and superconducting, microwave resonators as the edges. Their native two-qubit operation is something called a cross-resonance gate. Basically, the control qubit is driven at the transition frequency of the target qubit. This induces a coupling between the two that depends on the amplitude of the drive signal. By tuning this amplitude, you can obtain an effective hamiltonian that corresponds to the generator of the clifford group [ZX ] 1/2 [ 𝑍 𝑋 ] 1 / 2 . This, combined with single-qubit rotations, can give you a CNOT gate. For more details I'd recommend this paper. Hope this helps. Share Improve this answer Follow answered Jul 26, 2019 at 22:40 MMK1137 3761 1 silver badge 4 4 bronze badges Thanks, I will read and try to understand the attached paper and request you to go through my comments in response to Mark's comment. –  Ashish Commented Jul 28, 2019 at 14:35 Further, "control qubit is driven at the transition frequency of the target qubit", say if Control Qubit is in arbitrary state then what will be the effect of driving Control Qubit on its state? It would be great if you could point to a literature for layman where a logical understanding i.e. step by step procedure of physical implementation of CNOT is explained with examples on its behavior when Control Qubit is in arbitrary state & controlling multiple CNOTs. –  Ashish Commented Jul 28, 2019 at 14:49 Thank for the answer @MMK1137. I was wondering, is the cross-resonance gate an Ising gate? –  Daniele Cuomo Commented Nov 29, 2022 at 18:06 Add a comment 0 IBM uses a clever combination of cross resonant behavior and gate logic. Here's a simplified explanation... Setup: We need two qubits: a Control Qubit (QC) and a Target Qubit (QT) Both qubits are in close proximity physically Both qubits are coupled/connected in a way that allows them to interact (cross resonance) Step 1: Apply a microwave pulse to the QC qubit at the resonant frequency of the QT. Due to the coupling, this induces a rotation on the QT that depends on the state of the control qubit (ZX rotation). Long story short: if the QC is |0>, we induce a +X rotation on the QT if the QC is |1>, we induce a -X rotation on the QT The cleverness: the IBM engineers use this phenomenon to induce a π/2 rotation (x-axis) on the QT. Step 2: Next, we perform an RX[-π/2] gate directly on QT. The results either add constructively or destructively to Step 1 to create a full rotation or cancel out completely; effectively, conditionally flipping a bit without ever "measuring" it. We can picture this on the Bloch Sphere; however, since we're only using two axis, we only need to picture a clock. For the sake of simplicity, let's say the QT starts at |0>. In the image below, the two columns show two possible states of the QC and the effect of the 2 steps on the QT. Step 1: If the QC is |0>, it induces a π/2 rotation which would leave QT at 3 o’clock. If the QC is |1>, it induces a -π/2 rotation which would leave QT at 9 o’clock. Step 2: We rotate (RX) the QT by -π/2. If QT is at 9 o’clock, a -π\2 rotation would leave it at 6 o’clock (|1> - bit flipped). If QT was at 3 o’clock, a -π/2 rotation would leave it at 12 o’clock (|0> - back where we started). The actual implementation is slightly more complex and includes some additional steps for error correction. Please note: I am not a physicist and this is simply my best attempt at understanding this fascinating topic! If you see anything amiss, please correct me. ❤️ Share Improve this answer Follow answered 1 hour ago Ethan Strider 1011 1 bronze badge New contributor Add a comment Your Answer Sign up or log in Sign up using Google Sign up using Email and Password Post as a guest Name Email Required, but never shown Post Your Answer By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy. Start asking to get answers Find the answer to your question by asking. Ask question Explore related questions quantum-gatequantum-state See similar questions with these tags. 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