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Google, IBM and Rigetti use transmon qubits; these are basically fancy LC circuits where there is a josephson junction coupled to a superconducting island. Because of this, they are also often referred to as superconducting qubits. The qubit states are the various charge levels that can exist on the circuit; since the lowest two levels are separated in ...


10

There is not anything that you cant do with U3 so ideally there is no reason for U1 and U2. Eventually, as the transpilers gets better we may remove them and just have U3 and CNOT. So why did we make U1 and U2? It is because of the hardware. The U1 is done using a frame change (see https://arxiv.org/abs/1612.00858) which means they are done in software (...


6

Let us look at each observation and question in perspective. Before delving deep into the questions, please let me share a few reference architecture diagrams on the components of a quantum computer. We need to review the mentioned observations and understandings from a practical implementation vantage point. When we consider the quantum realm in its ...


6

There is a platform Quantum Inspire provided by QuTech, an organization co-founded by Delft University of Technology. Currently, the platform offers two real quantum processors: Spin-2 - 2 qubits processor with qubits implemented as electron spins Starmon-5 - 5 qubits transmon processor The platform uses a programming language based on QASM. It is also ...


4

Already mentioned is: IBM Quantum Experience, the widest known and largest platform for small quantum computing experiments. There are various physical backends containing upwards of 20 qubits for free users, and more for 'IBM Q partners'. The chips have varying qubit connectivity (architecture), which can be of vital importance to low-level experiments. ...


4

In relating quantum computing to classical computing there may be a small conceptual hurdle that needs to be overcome. Although a classical $\mathsf{NAND}$ gate may be implemented in hardware (say CMOS with a set of N- and P-type transistors), the idea of a quantum gate such as a $\mathsf{CNOT}$ or an $\mathsf{H}$ gate used in quantum computing most often ...


4

It cannot be the case that $H=e^{i\pi/4}\sqrt{iNOT}$. Whatever your interpretation of $iNOT$ (I'd agree with your definition), just square the thing. $H^2=I$, the identity, and so it is certainly not the case that $$ I=e^{i\pi/2}iNOT. $$ It is true, however, that if you perform the sequence that would give you an operation such as $NOT$ or $iNOT$, and you ...


4

I think you would benefit from realizing that quantum gates are an abstraction of the actual operations that we perform on qubits. Just as a qubit is an abstraction (a mathematical model to describe the state of a two-level quantum mechanical system) a quantum (logical) gate is a mathematical construct that we use in the study of quantum algorithms & ...


3

It does, unless you have a way to tune your Hamiltonian such that $\Delta$ becomes zero. Since a tunable Hamiltonian is something you usually want in a quantum computer implementation, this should not be a problem. If this term is non-switchable, it just means that the basis in which you are working is continuously rotating, and you have to keep track of ...


3

You can look up work by Gil Kalai, who is a longstanding and outspoken critic of quantum computing (his most recent essay: Kalai, 2019). He often bases his view on assumptions that I entirely disagree with, but its a refreshing reminder that certain ideas are taken for granted in the industry, namely that NISQ computers will yield practical applications. ...


3

The clock time (speed) for a quantum computer would be related to the T1 relaxation times (1/T1 rate) of the specific qubits and their architecture. The T1 relaxation time is essentially the time it takes for the qubit to return to an equilibrium and to be in the position to re-initialize. While T2 and the gate time tells you how many operations you can ...


2

Typically, quantum search algorithm searches through a very large solution spaces that can represented by logarithmic scale of data, not directly over huge amount of data. An example is travelling salesman problem where the input data are the distances between cities, but each solution is a path and the space to be searched is exponential in terms the number ...


2

Qiskit and other programming languages allow to write a hybrid algorithms, i.e. combination of classical and quantum algorithm. Inputs to quantum algorithm can be pre-processed on classical computer and results produced by quantum computers can be post-processed again classically. You are right that storage is still a problem. So far quantum RAM (qRAM) was ...


2

I read the article and it seems too rudimentary to provide a manual how to build a quantum computer home. It also contain some "half-truth" about quantum computers, e.g.: Quantum computers take advantage of strange properties from quantum mechanics to filter through possible solutions much more quickly than conventional computers. When someone asks about ...


2

According to comment provided by user gIS, there was no progress in implementing qRAM as proposed in the paper. However, some additional information on qRAM physical implementation can be found on this forum here.


2

It depends on technology used for a qubit implementation. For example, here is an explanation provided by IBM Q team: We must perform the qubit measurements in a way that does not destroy the qubit quantum state. One method is to weakly couple each qubit to a microwave resonator whose resonance characteristics depend on the state of the qubit. Once the ...


2

They are always rotating in the lab reference frame, but most quantum algorithms take a rotating reference frame to simply things, so that a z rotation only happens when you want it to. The rotating reference frame spins along with the natural spinning rate of each qubit, so in general in can spin at different rates for different qubits.


2

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 ...


2

Let me for starters address your last comment: Research validating superposition of colors as a Qubit (if not a Qutrit) Computing basis- https://physics.aps.org/articles/v9/135 This has absolutely nothing to do with the problem at hand. That paper talks about how to generate photons with two frequencies, it has nothing to do with how states can be ...


2

Here is a handful of Linear Optical Quantum Computation (LOQC) resources I have found useful in the past: "Linear Optical Quantum Computing" (2005) by Kok et. al.: this is probably the best review paper that came out after Knill, Laflamme, and Milburn's 2001 discovery that theoretically-efficient LOQC was possible. It's a pretty thorough but very accessible ...


2

a good stater i would say is the paper of Knill and Laflamme about LOQC (Linear Optical Quantum computing) from 2001, that says that quantum computing can be achieved with linear optic. Photons are really good as they can be used in many way to create qubits (polarisation of course, but also time, frequency, OAM). A Ph.D Thesis of Laurent Olislager is ...


2

According to so-called threshold theorem, it is possible to get rid of errors in quantum computation with arbitrary precision. However, there is an assumption that you have enough qubits. To ilustrate the idea, you can encode one qubit $|q\rangle=\alpha|0\rangle+\beta|1\rangle$ with more qubits, for example $|q\rangle=\alpha|0000\rangle+\beta|1111\rangle$ ...


2

Small ion trap quantum computers have all-to-all connectivity. Two-qubit gates can be executed between any arbitrary pair of ions in the trap. This has been demonstrated on up to 11 qubits (see, for example, https://www.nature.com/articles/s41467-019-13534-2). But it becomes difficult to maintain this control when the number of ions gets too large. A linear ...


2

Moore's law deals with the number of transistors in an integrated circuit, which is used as a proxy for computational power. In a quantum computing device the analogy would be the number of qubits. However, this by itself would be a poor benchmark, namely because it is easy to build lots of qubits. Building many qubits with properties such as long ...


1

There are several different issues all bundled together here. First is the concept of classical universality: whatever gate set you choose to specify your classical algorithm in terms of, there's a polynomial conversion to any other gate set. This includes a gate set that comprises reversible classical gates (such as Toffoli). For this, you just need to ...


1

Microsoft has invested huge resources into engineering topological qubits. Their approach is based on topological Majorana states, which occur at the edges of a topological superconducting chain or at interfaces between such chains. For those who see these words for the first time, a quick mental representation is supplied by a ribbon, which can be twisted a ...


1

The source of noise in your system, and how to reduce it, is really dependent on the quantum computing architecture you're pursuing. As stated above the most successful quantum computers at the moment are of the superconducting qubit flavor. And as the articles suggest, the way to reduce noise is low temperature and low vibrations, and perhaps chip design(...


1

With magnetic resonance based quantum computing, the amplitude(integrated area) and relative phase of the read-out spectra tells you the state of the qubits. In this particular example, two different carbon qubits are read out simultaneously, while the third qubit, a hydrogen, would need a separate experiment to read out. The spectrum consists of two ...


1

The could be a problem, but it depends on how you're realising your qubits. Some realisations are configured so that $E_0=E_1$, and then there's no problem. There is (at least from the theoretical perspective) a simple fix: if you're supposed to be waiting a time $t$, then, instead: wait time $t/2$ apply bit flip wait time $t/2$ apply bit flip. This ...


1

Quantum computers are susceptible to these errors/noise because of physical disturbances. An example of this is if some molecule in the surrounding air were to bump or approach the qubit it would transfer some kinetic energy and maybe affect the state. Another example is if a qubit interferes with any adjacent qubit, if they "bump" into one another their ...


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