Questions tagged [physical-qubit]

For questions related to physical realizations of quantum bits i.e. quantum systems living in a 2-dimensional Hilbert space.

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19
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1answer
637 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?
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What is the difference between a qubit and classical bit?

As I understand it, the main difference between quantum and non-quantum computers is that quantum computers use qubits while non-quantum computers use (classical) bits. What is the difference between ...
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6answers
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What is a qubit?

What is a "qubit"? Google tells me that it's another term for a "quantum bit". What is a "quantum bit" physically? How is it "quantum"? What purpose does it serve in quantum computing? Note: I'd ...
16
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2answers
3k views

What is the difference between transmon and Xmon qubits?

Transmon and Xmon qubits are two types of superconducting charge qubits that seem to be often used in superconducting quantum devices. However, I wasn't able to easily find direct comparisons between ...
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3answers
5k views

What does it mean for two qubits to be entangled?

I have done some sort of online research on qubits and the factors making them infamous i.e allowing qubits to hold 1 and 0 at the same time and another is that qubits can be entangled somehow so that ...
15
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2answers
3k views

Is Google's 72 qubit device better than D-Wave's machines, which feature more than 2000 qubits? [duplicate]

Google recently announced the Bristlecone 72 qubit quantum computer. However, D-Wave already announced quantum computers featuring more than $2000$ qubits. Why is Google's new device newsworthy then? ...
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2answers
812 views

What is the longest time a qubit has survived with 0.9999 fidelity?

I am pretty intrigued by the record time that a qubit has survived.
12
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2answers
579 views

How to store qubits while preserving Heisenberg's uncertainty principle?

I know that qubits are represented by quantum particles (for example photons) and that their state is given by one property (for example spin). My question is about the quantum memory: how are the ...
11
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2answers
569 views

State produced by spontaneous parametric down-conversion (SPDC)

I'm researching SPDC's efficacy for use in an optical quantum computing model and I've been trying to figure out exactly what state the photons are in when they come out (as represented by a vector, ...
11
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3answers
420 views

Why do the IBM and Google processors both have 53 qubits?

As I understand from this IBM post both the IBM and Google teams have independently built 53-qubit processors. What is the significance of the number 53? It is purely coincidental, or is there a ...
10
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1answer
1k views

What is the difference between a qubit and a quantum state?

In general, a qubit is mathematically represented as a quantum state of the form $\lvert \psi\rangle = \alpha \lvert 0\rangle + \beta \lvert 1\rangle$, using the basis $\{ \lvert 0\rangle, \lvert 1\...
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What is the physical representation of a qubit?

In regular computers, bits may be physically represented using a wide variety of two-state devices, such as polarity of magnetization of a certain area of a ferromagnetic film or two levels of ...
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5answers
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Resources for quantum algorithm basics

I have just started to learn about quantum computing, and I know a little bit about qubits. What is a resource where I can learn a basic quantum algorithm and the concepts behind how it works?
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283 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 ...
8
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2answers
520 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 ...
8
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2answers
412 views

What limits scaling down the size of superconducting qubits?

There are multiple ways of building a qubit: Superconducting (transmons), NV-centers/spin-qubits, topological qubits, etc. The superconducting qubits are the most well-known qubits and are also the ...
8
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1answer
280 views

How do you apply a CNOT on polarization qubits?

I read that a qubit can be encoded in a polarization state (horizontal or vertical polarization of a photon). How do you perform two-qubit operations on a polarization qubit?
7
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2answers
1k views

What are the pros/cons of Trapped Ion Qubits, Superconducting Qubits and Si Spin Qubits?

Among available qubit technologies for universal quantum computing these 3 come up as promising. NV centers and Majorana qubits also underway but relatively new. I find superconducting qubits and ...
7
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2answers
363 views

What's the difference between a set of qubits and a capacitor with a subdivided plate?

This is probably just a misunderstanding on my part, but everything I've seen on what quantum computers do thus far seems to suggest that the actual process of reading the entangled qubits would be ...
7
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2answers
148 views

How to interpret $-\rvert1\rangle \otimes \rvert1\rangle = -\rvert11\rangle$?

I'm having trouble accepting, intuitively, that $-\rvert1\rangle \otimes \rvert1\rangle = -\rvert11\rangle = \rvert1\rangle \otimes -\rvert1\rangle$. It's my understanding that $ -\rvert1\rangle$ ...
7
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1answer
331 views

Are there measuring standards (and units) for the identification of qubits?

The representation of bits in different technological areas: Normal digital bits are mere abstractions of the underlying electric current through wires. Different standards, like CMOS or TTL, assign ...
7
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2answers
246 views

Does Barkhausen noise affect the measurement of magnetic particle based qubits?

Q1: I've tried to find out if Barkhausen noise affects the measurement of spin-wave excitations in magnetic particle material based qubits. I prefer implementations such as those described in "...
7
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0answers
264 views

How to make qubits more stable towards noise? [closed]

In this paper and this paper, the "Noise Stability of Qubits" (the stability of qubits to external noise) has been discussed. In the first one, Gil Kalai states that it is difficult to create a ...
6
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3answers
715 views

How do we physically initialize qubits in a Quantum register?

In quantum algorithms we need to initialize the qubits at the start of our algorithm in some quantum register. Suppose that if we are working with a four qubit quantum register we can initialize the ...
6
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1answer
195 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. ...
6
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1answer
253 views

What is the relation between single photon qubits and squeezed light qubits?

In my constant thrill to know more about Quantum Computing I wanna know what is this relation. Additionally: Can one use squeezed light to effect multi-qubit operations on single photon qubits, or are ...
6
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1answer
595 views

Are qubits preferred over qumode, and if so, why?

Qubit and qumode are different forms of quantum computation. But most existing quantum computers/chips seems to be of discrete variables. I heard that a group chose qubit for a quantum optical ...
6
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1answer
288 views

Difference between coherence transfer, polarization transfer and population transfer?

I asked a question on Physics Stack Exchange but no one answered the question and I didn't get enough views on it. I am asking it on QCSE because the question is related to experimental quantum ...
6
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1answer
391 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 ...
5
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3answers
237 views

Is there anything practical that can be done with a single qubit?

Is there anything practical that can be done with a single qubit? And by "practical," I mean a problem that can be solved or information that can be stored. I realize that one practical thing that ...
5
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3answers
614 views

How is a single qubit fundamentally different from a classical coin spinning in the air?

I had asked this question earlier in the comment section of the post: What is a qubit? but none of the answers there seem to address it at a satisfactory level. The question basically is: How is a ...
5
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1answer
199 views

Interaction of an RF pulse with transmon qubit

When an RF pulse is interacting on resonance with a transmon qubit, it leads to rotation of the qubit around an axis in the XY plane (in a reference frame rotating in the transition frequency of the ...
5
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1answer
829 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. ...
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2answers
64 views

What are some good resources for learning about photonic qubits?

I am interested in resources that would allow me to learn about the physics behind optical quantum computation, and specifically, the physical properties of photonic qubits. I have some good resources ...
5
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1answer
141 views

What is continuous quantum register and how it relates to qubits?

I would like to understand what is a continuous quantum register. I know the direct definition is a quantum register that stores a real number defined by an observable with a spectrum consisting of $\...
5
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1answer
109 views

What are the physical reasons behind noise/errors in quantum computers?

We know that the current quantum computers are noisy/error-prone. They are susceptible to gate errors (single qubit and multi-qubit), decoherence (relaxation and dephasing), readout error, etc. I want ...
5
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1answer
81 views

How do physical implementations of Z gate selectively affect $\lvert1\rangle $ basis vector?

The Pauli Z gate inverts the phase of $\lvert1\rangle $ while leaving $\lvert0\rangle$ unaffected. When I think about how $\lvert1\rangle $ and $\lvert0\rangle$ are physically realized, however, as ...
5
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1answer
91 views

What design considerations set the frequency bounds for superconducting qubits?

Superconducting qubits generally have frequencies within the range of 4 - 8 GHz. What design considerations give the upper and lower bounds for what is a feasible design. I.e, why can't they be higher ...
4
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1answer
90 views

What exactly does it mean to embed classical data into a quantum state?

As the title states. I am a Machine Learning Engineer with a background in physics & engineering (post-secondary degrees). I am reading the Tensorflow Quantum paper. They say the following within ...
4
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1answer
295 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 ...
4
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1answer
72 views

What is the significance of recent demonstration of a passive photon–atom qubit swap operation?

In reference to this recent nature article: https://www.nature.com/articles/s41567-018-0241-6 Specifically, does this warrant a new type of gate?
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2answers
207 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 ...
4
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1answer
195 views

Why are superconducting qubits the most popular approach to build a quantum computer?

There are many different physical implementations of qubits: NMR, spin qubit, N color center in diamond, superconducting qubit, trapped ions, photonics quantum computing, topological qubit, etc. ...
4
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1answer
61 views

Do avoided crossings / CTs /ZEFOZs optimize quantum fidelity in practice?

CTs / ZEFOZs: Energy level structures that include avoided crossings at accessible energies tend to be resilient to noise and therefore present high coherence times, at least in the case of spin ...
3
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2answers
704 views

Polarization and qubit information

It is my understanding that light, and its polarization, is used to transfer information in quantum computers, but how can the information encoded in say, an electron also be stored in light? I ...
3
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3answers
345 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 ...
3
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1answer
69 views

What “states” can a qubit have if it doesn't have values?

I was told that qubits don't have a value, they have a "state". What does that mean? What are the different "states" that a qubit can have (like bits can be either 1 or 0)?
3
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2answers
362 views

Non-layperson explanation of why a qubit is more useful than a bit?

I have a computer science and mathematics degree and am trying to wrap my head around quantum computing and it just doesn't seem to make sense from the very beginning. I think the problem is the ...
3
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1answer
43 views

Rabi oscillations with different energy differences

Please check this Rabi oscillations image1: Now, let's say I want to create a superposition of E1 and E2, so I started an EM field tuned to a frequency related to E2-E1. But, let's say there is ...
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What is the charging energy in relation to a superconducting transmon?

The transmon achieves its reduced sensitivity to charge noise by significantly increasing the ratio of the Josephson energy to the charging energy. What is this charging energy?