# Quantum computation

A couple of days back, I received an interesting email from a rather curious mind.

The author of said email apparently found my contact details from one of the conference proceeding where I had submitted a paper.

Now, the author of the email posed rather curious questions, namely

…what exactly makes a quantum computer different from normal?…numerous articles point that quantum computer are superior because they can exist in two simultaneous states, but how does that exactly make a difference?…Lastly, every machine has its limits, so why is it being flaunted around as super machine?

Admittedly, given this was second email to me from a curious student so I was rather existed to answer it. Mind you, this person was also first one to continue conversations with follow up emails.

Anyways, while I did replied answers to all his questions, to the best of my abilities, it was later that I stumbled around this excellent article “What Quantum Computers Do Faster, with Caveats“. It is excellent articles that explains in short about the limitations of quantum computation. The author of this articles also uses Quantum Fourier Transform as example to explain the limitations.

One of the main idea about quantum world that I found hard to explain to him was that of superposition, something which he found surprisingly difficult to grasp; which may be attributed to his completely non-physics background.

When I had started to study about quantum information processing, I used to note down every interesting example or problem that would be capable of explaining a specific concept in a flash. Following is one of those noted example:

For example related to computer programming for understanding the superposition, one may look at a data structure called a linked list. Each data node in the list contains a pointer, to the next data node. The program traverses the list by jumping to the next data node indicated by the pointer. In a doubly-linked list, the data node contains two pointers, one for traversing to the top of the list, and another for traversing to the bottom of the list.

Another way of implementing a doubly-linked list is to use a single pointer space that contains the exclusive-or (XOR) or the two adjacent pointers. Figure below shows a link list node with pointer S that is the XOR of reference A (before) and reference B(after). To traverse the link list upward, the program XORs the current pointer (S) with the one it just left (B), and the result is the pointer of the next node (A). The same process works when traversing down the list. This superpositioning of node pointers is analogous to how the quantum states are maintained simultaneously in a quantum bit.

We can define those lists mathematically as follow:

and

Earlier on, I also had bad habit of never noting down important points without due citation or source for the information. So credit for this example to original poster or author of blog post or paper, respectively. If anyone is aware of where this appears, kindly comment.

Lastly, there are two excellent articles on the Limits of Quantum Computers by Scott Aaronson here and here.

# Computer from a living cell designed at Stanford

Been busy since last almost 2 months, attended two National conferences as participant which was extremely exciting as I made new friends along with new contacts and of course learned a new thing or 2. This followed the unfortunate problem with uncertainty of examination (the teaching staff is boycotting the exams). Anyways now I am resuming blogging normally. This was brought to my attention by a friend.

Bio-engineering Department at Stanford University has managed to create a biological device that demonstrates the properties of conventional transistors and performs standard logical operations such as AND, NAND, OR, XOR, NOR, and XNOR. In fact, such transistors based on DNA and RNA – this is a tiny computer, in the classical definition. The authors call it the working logic of Boolean Integrase Logic, or abbreviated BIL gates.

Like the standard electronic transistor, a new biological transistor quite versatile, can work in different “schemes” of biological circuits. his opens up the opportunity for scientists designing new programmable biological devices from biological sensors and detectors to bio-fuels. For example, you can program a cell to conduct the event counter, for example, how many times she met with a particular substance. You can program it to respond in a special way to other external events or stimuli.

The scientific results scientists in the Science Journal.

The illustration shows the calculated result of computer simulation (top) and the actual output of the logic operations in a biological transistor (bottom).

# Google Scholar Manipulation and Boson Sampler

It is possible to manipulate the data and bibliometric indicators offered by Google product. Researchers from University of Granadas conducted experiment in which they created fake researcher and posted fake papers to inflate citation counts for the paper.

Manipulating Google Scholar Citations and Google Scholar Metrics: simple, easy and tempting

On the quantum news, four papers have reported their research progress with experimental boson sampler. Check the papers below

On the same Scott Aaronson has posted excellent blog post titled ‘The Boson Apocalypse‘, in which he has a collection of all articles along with a small FAQ regarding the boson sampler, worth checking it out.

# Quantum Routers

Recently, a team of physicists from Tsinghau University in China has succeeded in creating a device that is capable of routing quantum information. The device is capable of routing one qubit, for now. While the paper does not claim to be a solution for quantum internet, it certainly proves that routing the quantum data is possible and indeed bring dream of quantum internet one step close to reality.

Experimental demonstration of an entanglement-based quantum router

ABSTRACT

We report an experiment that demonstrates full function of a quantum router using entangled photons, where the paths of a single-photon pulse are controlled in a coherent fashion by polarization of another single photon. Through a projective measurement, we prepare the polarization of the control photon in arbitrary superposition states, leading to coherent routing of the target photon in quantum superposition of different paths. We demonstrate quantum nature of this router through optical measurements based on quantum state tomography and show an average fidelity of $(93.24\pm 0.23)%$ for the quantum routing operation.

arXiv.org

via Phys.org

# Research Paper

I have decided to post once every week, weather and time permits, research paper or essay that are highly interesting.

Quantum Money with Classical Verification

Abstract

We propose and construct a quantum money scheme that allows verification through classical communication with a bank. This is the first demonstration that a secure quantum money scheme exists that does not require quantum communication for coin verification.
Our scheme is secure against adaptive adversaries – this property is not directly related to the possibility of classical verification, nevertheless none of the earlier quantum money constructions is known to possess it.

Quantum Money was propsed by Scott Aaronson and Paul Christiano in there paper Quantum Money from Hidden Subspaces

Here is the video of Q+ Hangout with Scott Aaronson:

# Interesting physics paper

Quantum superpositions of the speed of light by Sabine Hossenfelder

While it has often been proposed that, fundamentally, Lorentz-invariance is not respected in a quantum theory of gravity, it has been difficult to reconcile deviations from Lorentz-invariance with quantum field theory. The most commonly used mechanisms either break Lorentz-invariance explicitly or deform it at high energies. However, the former option is very tightly constrained by experiment already, the latter generically leads to problems with locality. We show here that there exists a third way to integrate deviations from Lorentz-invariance into quantum field theory that circumvents the problems of the other approaches. The way this is achieved is an extension of the standard model in which photons can have different speeds without singling out a preferred restframe, but only as long as they are in a quantum superposition. Once a measurement has been made, observables are subject to the laws of special relativity, and the process of measurement introduces a preferred frame. The speed of light can take on different values, both superluminal and subluminal (with respect to the usual value of the speed of light), without the need for Lorentz-invariance violating operators and without tachyons. We briefly discuss the relation to deformations of special relativity and phenomenological consequences.