Episode #125 of the Stack Overflow podcast is here. We talk Tilde Club and mechanical keyboards. Listen now
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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 ...


13

For a simple example suppose you have two qubits in definite states $|0\rangle$ and $|0\rangle$. The combined state of the system is $|0\rangle\otimes |0\rangle$ or $|00\rangle$ in shorthand. Then if we apply the following operators to the qubits (image is cut from superdense coding wiki page), the resulting state is an entangled state, one of the bell ...


13

A bit is a binary unit of information used in classical computation. It can take two possible values, typically taken to be $0$ or $1$. Bits can be implemented with devices or physical systems that can be in two possible states. To compare and contrast bits with qubits, let's introduce a vector notation for bits as follows: a bit is represented by a column ...


12

There are a few things to distinguish here, which are often conflated by experts because we're using these terms quickly and informally to convey intuitions rather than in the way that would be most transparent to novices. A "qubit" can refer to a small system, which has a quantum mechanical state. The states of a quantum mechanical system form a vector ...


12

You are totally right in your assumption about transporting qubits from Alice to Bob implies something physical. Usually problems/situations that have this setup of a transmission between two parties are called quantum communications. These problems/situations sometimes disambiguate by calling their qubits "flying qubits" which are almost always photons. ...


11

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 ...


11

There are two points I'd make here. D-Wave's computer and Google's computer are fundamentally different. D-Wave's computer is a quantum annealer. Imagine a landscape with some grassy hills. If you put a ball at the top of the hill, it will roll to a local minima, or even the minimum - in this case, a valley. Similarly, a quantum annealer has the qubits as ...


11

Short explanation: D-Wave implements quantum annealing, while Google has digitized adiabatic quantum computation. Lengthy Explanation: D-Wave advertises their line of quantum computers as having thousands of qubits, though these systems are designed specifically for quadratic unconstrained binary optimization. More information about D-Wave's manufacturing ...


11

This section on Wikipedia collects the most important ongoing attempts to physically implement qubits. For physically implementing a quantum computer, many different candidates are being pursued, among them (distinguished by the physical system used to realize the qubits): Superconducting quantum computing (qubit implemented by the state of ...


10

What you call a black box is simply isolating the quantum system that stores (or represents) your qubits from the environment. This can be done in several ways depending on your physical realization. For example, in an ion trap based quantum computer, one uses states of a single ion to represent a qubit, and isolates that from the environment by levitating ...


10

The existing answer does a good job at describing the state that comes from a SPDC configuration at low conversion efficiency, but it's also worth noting that the single-photon behaviour is not all there is to the process. Thus, in particular, if your conversion efficiency (or you detection time / efficiency / SNR) is good enough that you can detect (and ...


9

Background First of all, I'll use $\lvert H\rangle$ as a horizontally polarised state and $\lvert V\rangle$ as a vertically polarised state1. There are three modes of light involved in the system: pump (p), taken to be a coherent light source (a laser); as well as signal and idler (s/i), the two generated photons The Hamiltonian for SPDC is given by $H = \...


9

Well, for the longest coherence time ever, I'm finding this Science from 2013 entitled Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28, which indicates qubits that lasted for over 39 minutes; these, however, only had an 81% fidelity rate. (This is for qubits used in computation, not memory storage. For memory ...


7

A standard reference for linear optical quantum computing is Kok et al. 2009 (quant-ph/0512071). If one qubit is encoded in the polarization degree of freedom of a single photon, and the second qubit in the path degree of freedom of the same photon, then a CNOT gate is trivially implemented by a polarizing beamsplitter. This is a kind of beamsplitter that ...


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 ...


7

A logical qubit is a very fluid concept. You could use physical qubits as logical qubits. Or, you can encode multiple physical qubits as a single logical qubit. The more physical qubits you use, the better the resistance to noise. So, I would suggest that you question isn't exactly the right one to ask, and a better question is whether something useful can ...


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 :)


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 + ...


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 ...


5

By photon qubits, I'm assuming that you meant single-photon qubit systems. Can one use squeezed light to effect multi-qubit operations on photon qubits, or are these completely independent approaches? There are two protocols in quantum communication namely, discrete-varibale (dv) and continuous variable (cv). Squeezed light qubits are a part of cv ...


5

This is a good question and in my view gets at the heart of a qubit. Like the comment by @Blue, it's not that it can be an equal superposition as this is the same as a classical probability distribution. It is that it can have negative signs. Take this example. Imagine you have a bit in the $0$ state and represent it as vector $\begin{bmatrix}1 \\0 \end{...


5

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 ...


4

The fact that a qubit has infinite allowed states can seem as though we could fit more than a bit inside it. However, no matter how fancy our proposed encoding, Holevo's bound shows us that we can never get more than a bit out. This is one effect that provides a bottleneck for how much density we can fit in a quantum processor. given this infinite state ...


4

I'll probably be expanding this more (!) and adding pictures and links as I have time, but here's my first shot at this. Mostly math-free explanation A special coin Let's begin by thinking about normal bits. Imagine this normal bit is a coin, that we can flip to be heads or tails. We'll call heads equivalent to "1" and tails "0". Now imagine instead of ...


4

How is a single qubit in a Bell state $\frac{1}{\sqrt{2}}(|0\rangle+|1\rangle)$ any different from a classical coin spinning in the air (on being tossed)? For both of them, the probability of getting heads is 1/2 and getting tails is also 1/2 (we can assume that heads$\equiv|1\rangle$ and tails$\equiv|0\rangle$ and that we are "measuring" in the heads-...


4

Both models have their potential advantages and disadvantages. The CV model doesn't require energy intensive cooling systems. CV will also work better for continuous-valued problems. Nevertheless, since the model uses photons it brings various challenges to the table as well. Since both models (especially CV) aren't developed, your question may not be the ...


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

This does not warrant a new type of gate. When we write down quantum circuits, each 'wire' corresponds to a single qubit. However, we do not (usually) specify what technology any of these qubits is made out of. You might typically assume that they're all the same technology (e.g. solid state, photonic,...) but there is no need to do so. There are very good ...


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