Pascal's Technical Things

Delayed Choice Quantum Eraser

2011-12-01T22:30:00.005+01:00

Today I'd like to write something about quantum physics.

Quantum physics has gained my interest since several years and although I've seen all the most important experiments by now, it still peaks my interest when something new is discovered (such as the FTL neutrinos recently). An interesting experiment I read about is the Delayed Choice Quantum Eraser. It is most interesting, because nobody really has a conclusive answer for the behavior of this experiment yet. Many experiments in quantum physics can be explained both mathematically and with good reason. Some experiments can be explained only mathematically and Niels Bohr would say "Don't bother with the reason, the math is all you need to know.", but I always need an answer to the "Why?". This experiment reveals something that is still a complete mystery to us and I would like to know why it behaves the way that it does...

The Delayed Choice Quantum Eraser is an extension of the Double Slit Experiment. If you don't know what that experiment is, go see this movie for the short version and/or this movie for the long and more accurate version. The reason why the interference pattern disappears when we measure through which slit the electron went, is because we interact with the electron, which forces it to make a decision. This is also called the 'collapse of the wave function' and the 'wave function' is the probabilistic nature which causes the particle to look like a wave. Also note that while the movie demonstrates the experiment with electrons, it works exactly the same for photons. In the Delayed Choice Quantum Eraser (see this image below) we use photons.

So here it is. On the left-top we see the Double Slit Experiment where the photon goes through one slit (or both, or none) after which the photon is directly split into two entangled photons. This is called quantum entanglement which allows us to know the state of one photon by measuring the other (they always have opposite state, see Pauli exclusion principle). One photon (which we call the "signal" photon) follows the path at the top to detector D0. This is just a normal Double Slit Experiment. The other photon (which we call the "idler" photon) takes a much longer path to any of the other detectors. With the mirrors in this path we can choose to measure from which slit the photon came (using detectors D3 and D4) or to destroy the information from which slit the photon came by combining both the red and blue paths (through BSc) and use detectors D1 and D2.

When we choose to destroy the which-path information (through BSc) we get an interference pattern. But when we choose to keep (and record) the which-path information, there will be no interference pattern!

This is particularly odd, because when the "signal" photon hits (and is recorded by) detector D0, the "idler" photon is still on its way through the experiment (before it reaches any of the mirrors with which we decide what to do with the path information) the "signal" photon already seems to know what we will be going to do with the "idler" photon and decides to show us interference or not.

First question I hear you ask is "Can we use this to look into the future?". And the answer is simple: No, we can't. Because the result of all detectors is needed to see the interference pattern (or not) so there is no causality violation here. But it is still odd (and a little scary, in a philosophical way) that the particle knows the future...

In a good book that I read, called Quantum Enigma, the author describes something that (I figure) could explain the behavior of the Delayed Choice Quantum Eraser. A quantum particle keeps all options open until it is forced to choose. (I already figured that out) But with that decision, the entire path in the history (yes, back in time) of that particle is also decided. I could set up an experiment as shown in this image:

Where a single photon goes through a half mirror with a 50% chance of reflecting off of that mirror and 50% chance of going straight through the mirror. It reaches one of the boxes which I constructed so that I can keep the particle captured until I am ready to look inside one of these boxes. Until I do, the photon has not made a decision yet and is both in Box 1 and 2 at the same time (this can be demonstrated through yet another experiment). This also means that it has taken both paths! And it means it both reflected off of the mirror and it went through the mirror. At the time I open one of these boxes to look where the photon actually is, the whole history of the particle is "rewritten" (or "collapsed") so that the the particle took only one path and either reflected off of the mirror or went through it, but not both.

This could explain the behavior of the Delayed Choice Quantum Eraser such that the result at D0 (all the particle actions and reactions that take place to record it) is not actually decided until the "idler" photon is recorded. Or even until we actually look at the results with our own eyes.

Another explanation that I came up with for the Delayed Choice Quantum Eraser is Special Relativity: As one approaches the speed of light, time slows down. At the speed of light, time has stopped. Thus, because both the "idler" and the "signal" photons travel with the speed of light, all events in their lifetime are instantaneous and they don't have to "look into the future" to make a decision. While from our point of view it takes time for the particle to travel from point A to B, everything from the particle's point of view is instantaneous no matter how far they have to travel to make a decision. Although I am not sure if this holds when the photon travels through a medium such as air. Then it would still travel at the speed of light, but not with the speed c (the speed of light in a perfect vacuum).

Now I am not quantum physics scientist and my math is not nearly good enough to deal with the mathematics of quantum physics, so please go easy on me when you prove me wrong. But I'd like to hear other theories and links to other interesting articles which can shed some light on these things.

The source release that I already regret before it is uploaded.

2011-09-13T21:21:00.001+02:00

A prototype. Without documentation. Stripped from any usernames, passwords and other private information. Hardcoded magic values. Incomplete and unfinished. Works only on the specified hardware. Missing dependencies. You won't like it. I don't like it. But you repeatedly asked me for this. Now you can download the source code of the Star-Trek-like home computer. Use completely at your own risk! No support. It won't work out of the box and you need to be a programmer to get any of it to run. But even then it is still useless. I warned you. Do not email me about this!

Google Calendar

2011-08-23T22:45:00.003+02:00

A few years ago, when Google worked hard on all sort of internet-based tools, I said "Google shouldn't try doing all sorts of things. They should stick to what they are good at, which is their search engine.". These days I have to admit that some (if not most) of their other works are pretty good too.

Since a while, my girlfriend and me decided to use Google Calendar for our planning. Previously we used the calendar on my home computer but that didn't synchronize with anything. It was just a simple MySQL database behind that custom-made, Star-Trek look-alike interface and it had no web interface (I could make one, but meh). Google Calendar can be accessed pretty much everywhere as long as you have internet access (which is pretty much everywhere nowadays) and my new Samsung Galaxy S2 even has a beautiful Android app (made by Google obviously) which works perfectly and also notifies me for any events that have a notification set. Also my Apple iPad (which is mostly used by my girlfriend) has a very nice app, CalenGoo, which also does a great job syncing with Google Calendar.

Accessibility is one of the reasons why Google Calendar is a very good choice to use it as your daily agenda/calendar. The other reason why I wanted to use this, is because it uses standards. I love standards because they add to the accessibility of any system, but also because they are well know and often the software libraries for them already exist, are mature and are easy to find. Google's APIs are based on the WebDAV protocol, which allows you to access the tools with a HTTP protocol. Because a calendar is a such a common tool, the Google Calendar uses the CalDAV extensions on top of WebDAV. While this is nice and handy, Google even went a step further and provides their own open-source libraries for different programming languages that makes it easy to access their tools (not just Calendar). The Data API Developer's Guide has it all.

Samsung versus Apple

2011-07-26T23:17:00.001+02:00

As a person with a high interest in technical things I bought myself the Samsung Galaxy S2. When I showed my new phone to my sister, she immediately said "This looks like the iPhone" when she saw it. I noticed in the news that Apple has been blaming Samsung for 'copying' the appearance of the iPhone so I was wondering what this really is about.

When I look at this phone, what do I see? I see icons on a background (I made my background black, but it often has a picture in the background). There are names below the icons. There is some sort of a task bar at the top of the screen. Then I looked at the desktop of my computer: There are icons on a background, with names below them. There is also a taskbar, albeit it is not at the top of the screen but at the bottom, it certainly is similar. Windows has had this design since years ago when the iPhone didn't even exist yet! Who is Apple to say someone copied 'their' design?

Ok, so Windows doesn't scroll icons left and right like the smartphones do when you swipe them, but how else would you do that? That would be a childish thing to whine about. Also both phones have a hardware button at the bottom (Samsung actually has 3) but this can't be all about just that, can it?! No, I don't think this bickering between companies is about what they say it is about. I think it is about fighting their rivalry with every possible means, I think it is childish and I despise it.

Programming on the Samsung Galaxy S2

Enough about that. Let's talk a bit domotics. The Samsung Galaxy S2 runs the Android operating system which seems to be very good with Java. So I tried programming for this Samsung phone and that worked out well. It is relatively easy to write something up in Java, install it on the phone and it runs. It really is that simple. No need to have a special platform, no need to buy an SDK or compiler. This means that when my new home computer software is finished, I will be writing some 'apps' for this phone that interface with my home computer! Talk about remote controls :)

Later I will talk a bit more about my work on the new home computer software (and also some about the hardware). But I also have to cook up a release build for another project (Doom Builder) so I won't be writing blogs every other day, sorry.

Also have a look at this guy's website: bwired.nl. He did an amazing job monitoring pretty much everything in and around his house! Although I really don't need to know when he flushed a large amount and when small amount through the toilet, I really liked most of it so I just wanted to plug that here.

An attempt at blogging

2011-07-18T20:12:00.005+02:00

I started this blog, because I noticed a large number of people interested in my home domotics project. I received a lot of comments and emails with questions which was a bit too much to answer. Through this blog I hope to answer some questions and tell you about my current work-in-progress: my new home computer software which is written from scratch and a whole new take on this.

For anyone who got here and is still wondering what this is all about; you may want to watch this YouTube video.

Read more for the details...

This was also noticed by the Dutch computer magazine Computer Idee and they interviewed me for their article on my home computer. In Dutch language of course.

So let me get back to answering some questions. Most questions were about that hard- and software, so I will give you a short bit on that. The hardware is a Pentium D dual-core CPU (3 GHz per core) and 2 GB RAM. The video card is a NVIDIA GeForce 9500 GT which was chosen for its 2 video outputs and an internal S/PDIF connection to merge audio with the video on the HDMI output. The machine has an internal audio chip (SigmaTel) but also has a SoundBlaster X-Fi audio card which was chosen especially for its internal S/PDIF output which can be wired to the video card mentioned earlier. Some processes on the machine choose to use the SigmalTel audio device, while others (displayed on the other screen) choose to use the SoundBlaster. This way the computer is connected to the Home Cinema Set with a HDMI cable and I can play a video on the secondary screen (which goes along with the SoundBlaster audio to the Home Cinema Set) and have the computer interface software still running on the primary screen with its SigmaTel audio going to the screen on the wall. All this is put away nicely in a closet behind a panel (along with some other devices such as the central network switch). A couple wires power some fans for better air flow and extend the power button to the panel.

The software runs on Windows XP, is programmed in C# and uses .NET Framework 3.5. Everything in there is hardcoded and, contrary to my other projects, not so nicely programmed for customization or flexibility. This is what you would call a prototype. It works, but you can't sell it because it lacks quality. I will not make it open source or otherwise distribute it, because of the lack of quality, but also because it really isn't high-tech-rocketscience. You can learn a lot more from the source code of my other projects which are open source.

The reason why I didn't include control of my room lights and other hardware is because all that I could find was hard to find, didn't really assure me that it would work properly and is quite expensive. In the end the light switch next to the door is more practical anyway. Of course, if I can find reliable and decently priced electronics for this purpose some day I will sure think about this again.

I hope this answers most of the questions asked. If you have any more questions, please post them here and I will see if I can answer them. Now I'm going back to work on my new home computer, which I will post some information about later.