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Questions tagged [physical-realization]

For questions about practical computers or processors that run on a quantum architecture. This is for questions about the machines themselves, not just any computing that could take place on one. You may also use this tag for questions on realistic implementations of quantum channels. DO NOT use for questions about simulation or emulation of quantum computers, or cloud-based quantum computing services like the IBM Q Experience.

10
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2answers
961 views

How power-efficient are quantum computers?

As we all know, quantum algorithms scale faster than classical ones (at least for certain problem clases), meaning quantum computers would require a much smaller number of logical operations for ...
11
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1answer
2k views

How are quantum gates implemented in reality?

Quantum gates seem to be like black boxes. Although we know what kind of operation they will perform, we don't know if it's actually possible to implement in reality (or, do we?). In classical ...
5
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1answer
65 views

Physical qubit of optical quantum computer

I was researching how optical quantum computers work and discovered a KLM protocol that allows for building quantum gates using linear optics elements like mirrors, beam splitters, and phase shifters. ...
3
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0answers
53 views

What are the main obstacles to overcome to build silicon-photonic quantum computers?

In superconducting quantum computers, we use mostly superconducting qubits or trapped ions. However, those systems are quite large because their environment either requires near absolute zero ...
4
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1answer
73 views

How is a physical qubit measured and how is the result interpreted?

To my understanding, most of the qubits we use today are just Josephson junctions coupled to a resonator that is triggering the qubits to go to different states by using microwave pulses. The same ...
22
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3answers
1k views

What is the argument that practical quantum computers cannot be built?

An answer to another question mentions that There are arguments that suggests that such machines ["quantum Turing machines"] cannot even be built... I'm not sure I fully understand the problem,...
4
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1answer
142 views

Do the latest D-Wave computers use Helium compounds for cooling?

Do the latest D-Wave machines use compounds of $\require{\mhchem}\ce{^{3}He}$ and $\ce{^{4}He}$ for cooling? If not, how does D-Wave approach cooling its plates low enough to achieve superconductivity?...
8
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2answers
106 views

Dependency of architecture on hardware

How much of a role does the type of hardware used to implement the building blocks (like qubits, the circuits, the communication channels,quantum RAM etc.) have to play when designing the architecture ...
3
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1answer
370 views

IBM's quantum computing capabilities — entangled 18-qubit GHZ states and high coherence times

Here's a blog post that corresponds with what IBM presented at the 2019 March Meeting. In this post, they claim they've measured qubit T1 relaxation times above 500 microseconds, and claim that they'...
2
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2answers
81 views

What quantum volume is needed to represent a single logical qubit?

The quantum volume metric $V_Q$ is a proposed metric for quantifying and comparing the performance of quantum computers1. The quantum volume is defined as $$V_Q = \max_{n<N} \left(\min\left[n, d(n)...
19
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1answer
399 views

What cryogenic systems are suitable for superconducting qubits?

Is a dilution refrigerator the only way to cool superconducting qubits down to 10 millikelvin? If not, what other methods are there, and why is dilution refrigeration the primary method?
4
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1answer
252 views

How are witness operators physically implemented?

Let's take an example of an entanglement witness of the form $W = | \phi \rangle \langle \phi | ^{T_2}$ where $ | \phi \rangle $ is some pure entangled state. If I wanted to test some state $\rho$, I ...
11
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2answers
412 views

What real quantum computers are available for students to use?

I am aware that IBM, Rigetti, and Google have built some small-scale devices. Which of them are available for access by an undergraduate student? For how long? With how many qubits?
3
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0answers
13 views

Counting channel uses of the lossy bosonic channel or definition of channel uses

The PLOB-bound ("Fundamental Limits of Repeaterless Quantum Communications") gives an asymptotic upper bound on the secret-key rate per used lossy bosonic channel. However, I'm not sure how to count ...
3
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0answers
35 views

Is generating usable entanglement difficult?

I'm a theorist and I assume that I can use entanglement resources for practical use freely, but is this really the case? Is generating entanglement and storing it for long enough periods of time such ...
1
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0answers
29 views

What is the difference in computational power between qubit and bit (transistor)? [duplicate]

If I use for example 5-qubit IBM-Q quantum computer and we accept the law of quantum parallelism ($n$ qubit = $2^n$ ordinary bits), so does it mean that computational power of this computer is equal ...
24
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4answers
2k views

How do I add 1+1 using a quantum computer?

This can be seen as the software complement to How does a quantum computer do basic math at the hardware level? The question was asked by a member of the audience at the 4th network of the Spanish ...
12
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4answers
875 views

Do real commercial quantum computers exist?

We have been reading about quantum computers being developed and tested in labs. And also, we have quantum simulator programs that use limited virtual qubits (up to 30-40 qubits if cloud-based). And ...
3
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1answer
64 views

How do frequency-tunable microwave resonators work?

In superconducting quantum computing resonators are typically an LC circuit. If L and C are fixed the resonator will have a fixed resonance frequency. However, I heard that in the group of Prof. John ...
2
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0answers
24 views

Can one make NV centred type qubits from analogus materials in the periodic table?

Can one replace Nitrogen with Phosphorus and Carbon with Silicon and so on? What would be the conditions that would preclude such a realisation? Are there any implemenations so far?
31
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7answers
6k views

Why is it harder to build quantum computers than classical computers?

Is it because we don't know exactly how to create quantum computers (and how they must work), or do we know how to create it in theory, but don't have the tools to actually execute it in practice? Is ...
7
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1answer
93 views

Current limits on Grover search space

I was wondering why till date Grover search has been implemented only till 3 qubits (corresponding to size of database = 8). Refer this paper The reason why I ask is that we have much larger sized ...
5
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1answer
72 views

Is there any company that backs and implements diamond vacancy quantum computers?

It is known that there are big companies behind the specific qubit implementations (e.g. ion traps, superconducting loops, topological qubits, etc.). But I have not managed to find the company that is ...
7
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2answers
382 views

How to explain in layman’s terms the significance of the difference of qubits of the D-Wave and IBM QX?

So, @AndrewO mentioned recently that he has had 'encounters' with people wondering why D-Wave has a lot more qubits than IBM. Of course, this comparison is faulty, since the IBM and D-Wave's machine ...
4
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1answer
244 views

D-Wave systems and IBM Q System One

What is the difference between the D-Wave systems and the newly-born IBM Q System One?
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2answers
3k views

How many operations can a quantum computer perform per second?

I want to know what time complexity is considered efficient/inefficient for quantum computers. For this, I need to know how many operations a quantum computer can perform per second. Can anyone tell ...
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2answers
215 views

Scale for Quantum Computing [closed]

Does something like Mohs' scale exist for quantum computing? (eg. classical = 0, hybrid = 5, pure quantum = 10) Mohs' scale: a scale of hardness used in classifying minerals. It runs from 1 to 10 ...
2
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0answers
39 views

Does self-induced collapse limit the size of a quantum computer?

This question is related to this question, but I'm specifically interested in self-induced collapse. Suppose that the size of a quantum system alone somehow leads to a projection of the state (a "...
9
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2answers
561 views

What is D-Wave's “Pegasus” architecture?

How is D-Wave's Pegasus architecture different from the Chimera architecture?
11
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1answer
330 views

How are quantum gates realised, in terms of the dynamic?

When expressing computations in terms of a quantum circuit, one makes use of gates, that is, (typically) unitary evolutions. In some sense, these are rather mysterious objects, in that they perform "...
4
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2answers
136 views

Processing density capabilities in a quantum processor

Given the theoretical infinite quantum states that a qbit can be expressed as is there any practical limit to the processing density in any given quantum processor as compared to the absolute limits ...
8
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3answers
220 views

What does it mean for a quantum computer to have $X$ qubits?

I want to preface with a disclaimer that I am a physicist with minimal knowledge of computer hardware. I have a solid understanding of quantum information from a theoretical standpoint but zero ...
2
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1answer
141 views

CNOT gate - control qubit is in superposition

If a control qubit is in superposition, how it will affect target qubit if it is collapsed or in superposition? Is it true that CNOT works only if the control bit collapsed to 1? Also, is it possible ...
5
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1answer
780 views

Is there any single-logical-qubit physical device out there as of end 2018?

By my first impression, there are many-qubits computers out there and more to come, as to follow the press. Now a closer look reveals that it's all about designing and building physical qubits. ...
7
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2answers
271 views

The process for transferring qubits between locations

I understand that right now qubits are physical entities in a Quantum Computer and I am playing around on the IBM Quantum Computer as well as the Q# language and dipping my toes into the Quantum world ...
26
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2answers
3k views

How does a quantum computer do basic math at the hardware level?

On reading this Reddit thread I realized that even after a couple months of learning about quantum computing I've absolutely no clue about how a quantum computer actually works. To make the question ...
3
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1answer
545 views

What are some popular myths or common misconceptions about quantum computing? [closed]

The most down-voted question at the moment is about using entanglement for faster-than-light communication. Much like how the word “laser” replaced “magic” in the vernacular not too long ago, what ...
6
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1answer
146 views

Transmons and cooper pair box qubits: two islands or a single island and a reservoir

In scientific literature, one typically describes Cooper pair boxes as a small superconducting island coupled to a superconducting reservoir (say, a large ground plane of superconducting metal, or a ...
10
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3answers
3k views

What countries are leading this “Global Quantum Computing Race”?

The terms Quantum Computing Race and Global Quantum Computing Race have been used in the press and research communities lately in an effort to describe countries making investments into a "battle" to ...
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2answers
439 views

What's the current status of quantum computers? [closed]

Is it real now or not completed yet? Can I buy one? / How many quantum computers have been manufactured in the world? Is quantum computing independent (e.g Quantum+LCD+Keyboard) or it add-on for ...
9
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2answers
1k views

Allowed CNOT gates for IBM Q 5 quantum computer

I trying to do some tests in the IBM Q5 computer of IBM quantm experience for some simple error correction protocols, but as I can see, some operations between the qubits are not allowed. For example,...
2
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0answers
82 views

Is it possible to make our own quantum computer? If yes, what will it take? [closed]

I am thinking as quantum computing is just a new word in tech, so let to explore it by building it or a prototype based on it so I am just wanted help in setting up and initial steps which should I ...
2
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3answers
864 views

Is it possible to purchase quantum computing hardware? [closed]

Are there schematics and/or diagrams out there to build a very basic quantum system?
3
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3answers
232 views

Are qubits the only elements required to build a quantum computer?

The power of a quantum computer is often described in terms of how many qubits it has. Does that mean that quantum computers can be run using only qubits, and nothing more? Wouldn't we require some ...
6
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1answer
118 views

Randomness from deterministic machine

It is known that in a classical computer we can't generate a purely random number by deterministic process. I am taking a course in Quantum Computing, recently I learnt that using state we can ...
6
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1answer
79 views

How scalable are quantum computers when measurement operations are considered?

From a high-level point of view, given a quantum program, typically the last few operations are measurements. In most cases, in order to extract a useful answer, it is necessary to run multiple ...
10
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1answer
257 views

Does the D-Wave 2000Q satisfy DiVincenzo's criteria?

DiVincenzo's criteria for quantum computation are the following: A scalable physical system with well characterized qubits. The ability to initialize the state of the qubits to a simple fiducial ...
19
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3answers
1k views

Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?

This is a follow-up question to @heather's answer to the question : Why must quantum computers be kept near absolute zero? What I know: Superconducting quantum computing: It is an implementation of ...
4
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2answers
152 views

Why D-Wave 2000Q requires such a low pressure?

On the D-Wave 2000Q website, it is stated that the processor environment is kept "in a high vacuum: pressure is 10 billion times lower than atmospheric pressure". Why the pressure has to be such low? ...
6
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2answers
215 views

Are quantum simulators like Microsoft Q# actually using quantum mechanics in their chips?

Unlike Google's Bristlecone or IBM's Qbit computer, do simulators like Q# or Alibaba really use quantum mechanics anywhere in their physical chips? Are they just defining properties using a classical ...