The More You Know: Qubit

A Qubit, or quantum bit is the fundamental unit of quantum information, it is a mathematical concept satisfying particular properties. The Qubit is the quantum counterpart of the classical bit.

A classical bit can be in one of two different states: 0 or 1, whereas a Qubit can be in the quantum states |1\rangle|0\rangle but also in any superposition of these two. That is, the state |\psi\rangle  of a Qubit will be of the form

|\psi\rangle=\alpha|0\rangle + \beta|1\rangle,

where \alpha and \beta are complex number satisfying

|\alpha|^2+|\beta|^2=1.

The states |1\rangle and |0\rangle form the so-called computational basis \{|1\rangle,|0\rangle\}. Then, as a mathematical concept, a Qubit is a vector normalized to length 1 in a 2D complex vector field.

Just as it’s classical analogue, a Qubit can have many physical realizations: The polarization of a photon, the spin of an electron or a nucleus, the localization of a charge, among other two-state quantum systems.

Go to the Dictionary of Quantum Information and Quantum Computation.

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Announcement: Introducing Entangled Physics New Logo

I am very proud to present Entangled Physics new identity.

Entangled Physics's Logo ©

This logo is composed by two entangled atoms bound by an infinite. It’s design is clean, modern and friendly, and the text “Entangled Physics” is written in an easily readable font. Not only the colors radiate energy, warmth, strength and inspiration, but their intensity brings the image to the foreground.

The logo was created by amazing graphic designer Saida de la Roca.


 All text and Images copyright © Marco Vinicio Sebastian Cerezo de la Roca.

Creative Commons License
Announcement: Introducing Entangled Physics New Logo by Marco Vinicio Sebastian Cerezo de la Roca is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

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The More You Know: Loss-DiVicenzo Quantum Computer

The Loss-DiVicenzo Quantum Computer is a hardware proposal for the implementation of a scalable spin-based quantum computer in a coupled quantum dot system. The qubits of their quantum computer are realized as the two spin states of a confined electron.

In this proposal, initialization is achieved by applying a strong magnetic field and letting the electrons reach their thermodynamic ground state. Single-qubit operations are performed by varying the effective Zeeman interaction on each dot individually.

Two-qubit quantum gate operations are performed by controlling the electrostatic barrier between neighbouring quantum dots. Thus, if the barrier potential is high, the electrons cannot tunnel from one quantum dot to another and the spins are decoupled. When the barrier is pulsed to a low voltage, the two electron wave functions overlap and the spins will be subject to a non-zero Heisenberg exchange coupling:

H_{s}(t)=J(t)\vec{S}_1\cdot\vec{S}_2.

Finally, readout could be performed using spin-to-charge conversion (high-fidelity, single-shot electron charge detection).

Go to the Dictionary of Quantum Information and Quantum Computation.


References:

[1] “Quantum Computation with Quantum Dots”, D. Lossa, and D. P. DiVincenzo. arXiv:cond-mat/9701055

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The More You Know: EPR Paradox

The EPR Paradox is a thought experiment published in a paper in May 1935 by Albert Einstein and two of his postdoctoral research associates Boris Podolsky and Nathan Rosen that was meant to prove that Quantum Mechanics showed internal contradictions in it’s formulation. The authors claimed that if the description of physical reality given by the wave function is complete, then two quantities described by non-commuting operators could have simultaneous real values. We could for instance measure the position and the momentum of two entangled particles with more accuracy than that allowed by Heisenberg’s Uncertainty Principle. Therefore concluding that quantum-mechanical’s wave function description of reality is incomplete.

This thought experiment is considered to be one of the first papers to put the spotlight on quantum entanglement.

For a more extensive explanation you can visit this post.

This apparent paradox was finally solved by an experiment proposed by physicist J. S. Bell (see Bell’s Inequalities).

Go to the Dictionary of Quantum Information and Quantum Computation.


References:

[1] “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?“, A. Einstein, B. Podolsky, and N. Rosen, Phys. Rev. 47, 777 (1935).

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The More You Know: DiVicenzo Criteria

The DiVicenzo Criteria for implementing a quantum computer are a set of requirements that any candidate (circuit model) quantum computer must satisfy. These five requirements, plus two relating to the communication of quantum information were formulated by David P. DiVincenzo in 2008 and are stated as follows:

  1. A scalable physical system with well characterized qubits.
  2. The ability to initialize the state of the qubits to a simple fiducial state, such as |000...\rangle.
  3. Long relevant decoherence times, much longer than the gate operation time.
  4. A “universal” set of quantum gates.
  5. A qubit-specific measurement capability.
  6. The ability to interconvert stationary and flying qubits ^1.
  7. The ability faithfully to transmit flying qubits between specified locations.

^1 Flying qubit:  qubits that are readily transmitted from place to place.

Go to the Dictionary of Quantum Information and Quantum Computation.


References:

[1] “The Physical Implementation of Quantum Computation”, D. P. DiVincenzo. arXiv:quant-ph/0002077

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The More You Know: Boson Sampling

Boson Sampling is a model for non-universal quantum computation which performs specific tasks that are thought to be hard to efficiently perform on a classical computer. By exploiting the processing power of quantum mechanics, the output photon number distribution of a linear optical interferometer with single photon inputs can be sampled. Unlike other models for quantum computation boson sampling does not require any special quantum gate, and this is one of the main reasons for which boson sampling has attracted so much attention among physicists.

Go to the Dictionary of Quantum Information and Quantum Computation

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New Article Series: “The More You Know”

In the spirit of writing a “faq / Dictionary of Quantum Information and Quantum Computation” for the blog I’m starting a new article series called: The More You Know.

In each article I’m gonna give a brief definition of terms which are of interest to those beginning to study (or currently studying) Quantum Information and Quantum Computation.

Additionally, I’m also gonna add a new page to the blog called “Dictionary of Quantum Information and Quantum Computation” where all the articles will be listed for easy access.

Hope you like it!

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Luis Masperi’s Price Special Mention

Every year  during the annual Argentinean Physicists Meeting (Reunión anual de la AFA), all students that got their physics degree can participate in the Luis Masperi Price by presenting the work they did for their thesis. The presentations are graded by a panel of  three judges and results are announced by the end of the meeting.

Last month, during the 100th Annual Meeting I participated for this award, presenting my work on “Non-transverse factorizing fields and entanglement in finite spin systems” (in collaboration with my director Dr. R. Rossignoli and co-director Dr. N. Canosa) and got the second place (special mention)!

You can check my paper based on my thesis here.

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Entangled particles, faster than light communications and the no-cloning theorem

In 1982, a paper entitled “FLASH – A Superluminal Communicator Based Upon a New Kind of Quantum Measurement” was published by Nick Herbert, an American physicist who meant to prove that by using EPR entangled pairs and quantum effects, a superluminial communicator could be built to transmit information at speeds faster than the speed of light [1]. Such a statement caused quite some uproar in the scientific community, because, as you may know, there are no signals than can travel faster than light.

The story of Herbert’s paper is not a story of an experimental proposal with a fundamental error, but the story of scientific progress and how, by studying the possibility of a superluminial communicator, the no-cloning theorem came to be. We will see in this article how the resolution of this problem improved our understanding of physics by showing that we cannot copy an unknown quantum state.

Now, before proceeding to review Herbert’s paper, we should first answer the question:

Could there be superluminial velocities?

The answer to this questions is no… and yes… (of course it is, isn’t it?). Let’s first try to understand why it is that the answer to this question should be no.

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My latest paper: Non-transverse factorizing fields and entanglement in finite spin systems

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A couple of weeks ago we uploaded to the arXive our latest paper: Non-transverse factorizing fields and entanglement in finite spin systems.

Let me explain what this paper is about.

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