# Tag Info

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I was looking for why optical quantum computers don't need "extremely low temperatures" unlike superconducting quantum computers. Superconducting qubits usually work in the frequency range 4 GHz to 10 GHz. The energy associated with a transition frequency $f_{10}$ in quantum mechanics is $E_{10} = h f_{10}$ where $h$ is Planck's constant. Comparing the ...

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Firstly, a classical computer does basic maths at the hardware level in the arithmetic and logic unit (ALU). The logic gates take low and high input voltages and uses CMOS to implement logic gates allowing for individual gates to be performed and built up to perform larger, more complicated operations. In this sense, typing on a keyboard is sending ...

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Is a dilution refrigerator the only way to cool superconducting qubits down to 10 millikelvin? There's another type of refrigerator that can get to 10 mK: the adiabatic demagnetization refrigerator (ADR).$^{[a]}$ why is dilution refrigeration the primary method? To understand that, let's talk about one of the main limitations of the ADR. How an ADR ...

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Here is my process for doing arithmetic on a quantum computer. Step 1: Find a classical circuit that does the thing you're interested in. In this example, a full adder. Step 2: Convert each classical gate into a reversible gate. Have your output bits present from the start, and initialize them with CNOTs, CCNOTs, etc. Step 3: Use temporary outputs. If ...

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The transmon is a Josephson junction and capacitor in parallel. Originally, transmons were differential circuits, i.e. two transmons on the same chip were not galvanically connected in any way. In other words, transmons didn't share a ground reference. Furthermore, in the early days, transmons were almost always embedded into the middle of a harmonic ...

7

Because light, at the right frequencies, interacts weakly with matter. In the quantum regime, this translates to single photons being largely free of the noise and decoherence that is the main obstacle with other QC architectures. The surrounding temperature doesn't disturb the quantum state of a photon as much as it does when the quantum information is ...

7

Disclosure: while I am not an experimental physicist, I am part of the NQIT project, which is aiming to develop quantum hardware which is suitable to realise scalable quantum computers. The architecture that we're investing most heavily in is optically linked ion traps. Ions represent some of the physically best understood systems to experimental and ...

7

Getting enough capacitance and maintaining coherence essentially set the size limit. A superconducting qubit, for the purposes of answering this question, can be imagined as an oscillator consisting of an inductor and a capacitor. The frequency of the oscillator can't be too high otherwise controlling the qubit becomes difficult. At Google, we typically work ...

6

Kirchhoff to Lagrangian Let's approximate the transmon as a parallel LC resonant circuit. Suppose we connect a voltage source through a coupling capacitor $C_d$ (d for "drive") to a transmon qubit. If the voltage of the source is $V_d(t)$, then Kirchhoff's equations for the circuit are $$\frac{1}{C/C_d} \dot{V}_d(t) = \ddot{\Phi}(t) + \frac{\omega_0^2}{1 + ... 6 It think the (very) short answer is that there is not a preferred platform yet. This is why there are very active research communities around each of these technologies. Often if someone says otherwise they are probably working on one of the platforms :) 5 The resonance frequencies of TLS fluctuate due to their interaction with neighboring TLS, which occurs through electric dipole interaction or the local mechanical strain in the material. If a TLS at low energy (below kB*T) is involved, this one may change its state randomly due to thermal activation. The resulting change in local electric field or strain can ... 5 In one sense, the Xmon qubit is a transmon qubit, in that they both operate in the E_J>>E_c regime of the CPB Hamiltonian and take advantage of the exponentially suppressed charge noise vs. polynomial decrease in anharmonicity effect discussed in (Koch, 2007). You could work out the dynamics of a superconducting qubit-resonator system without ever ... 4 Yes, they use \require{\mhchem}\ce{^3He} and \ce{^4He}. No, they do not use compounds of these but instead a solution of these two (at the operating temperature) liquid nobel gases. The details can be found in the wikipedia article on dilution refrigerators. 4 Here's a paper comparing Trapped Ion and Superconducting (the main competitors right now) from the group at UMD which compares their trapped ion system with IBM's transmon (superconducting) system. If you want to look at a more algorithm-focused line of thought. If you are looking for a more general summary of the strengths and weaknesses this paper seems ... 4 What follows turned out to be a rather technical explanation, so I'll start with the main point: The qubit state can change the resonator's state, and the resonator's state can be easily measured only if there is a large different in frequencies between the qubit and the resonator. Let's model a qubit as a two-level system and a resonator as a harmonic ... 3 There's a superconducting circuit element called Josephson junction, which is roughly a nonlinear inductor. The inductance of a Josephson junction depends on current via the relation$$L(I) = \frac{L_0}{\sqrt{1 - (I/I_c)^2}}$$where L_0 is the inductance of the junction with no bias current and I_c is the so-called "critical current" which is the maximum ... 3 For superconducting qubits, x and y rotations are usually both done with microwave pulses, and as you said the phase of the pulse determines the rotation axis. See mathematical details in this Physics Stack Exchange post: How do we perform transverse measurements in a two level system? Rotations about the z axis are quite different; they are done by ... 3 Each of the two spins, q\in\{L,R\}, has a bunch of energy levels \{|n\rangle_q\}, each at energy \omega_{n}^q. In other words, the basic Hamiltonian of the spins is:$$ H=\sum_{n=0}^{N}\omega_{n}^L|n\rangle\langle n|_L+\omega_{n}^R|n\rangle\langle n|_R  Written like this, the two spins are not interacting, so we won't get a two-qubit gatewithout ...

2

DanielSank is correct, but I think the answer is actually even more subtle. If there was no loss there would also be no way the background radiation leaking into your quantum device. Even if it was initially thermally excited, one could actively reset the state of the qubits. Thus, in addition to thermal excitations of microwave qubits, the fundamental ...

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Whilst we normally talk about $\left|0\right>$ and $\left|1\right>$ as unchanging states in quantum computing, this is not usually the case in a physical realization where there tends to be an energy difference $\Delta E$ between these states such that $\left|1\right>_\mathrm{logical} = e^{-it \Delta E / \hbar} \left|1\right>_\mathrm{physical}$. ...

1

According to [1]: Readout error is the error in measuring qubits. You read the figure correctly (44 out of 1000 measurements fail on reading). Note there is yet another, though minor, error there: gate error. It is about errors in quantum gate operation, and is one 10th less than the error of measurement. So, actually there may occur more erroreus ...

1

I think the subject matter of supercondcuting qubits is rather broad and diverse, making it challenging to accurately capture it in a 'brief explanation'. With that said, this recent review (Krantz et al., Applied Physics Reviews 6, 021318 (2019)) - "A Quantum Engineer's Guide to Superconducting Qubits" (arXiv:1904.06560) from the MIT group may be a good ...

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One advantage of the transmon design is the additional loop you gain from what you called two-island-design. The yellow flux bias line changes the Josephson energy, thus the resonance of the qubit. You can imagine this as changing the (Josephson) inductance of the SQUID loop being a non-linear LC-resonator. This helps for example in two-qubit gate ...

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