Archive for the ‘Quantum-Mechanics’ Category

Video: Delayed Choice Quantum Eraser Experiment Explained

Wednesday, June 24th, 2015

Research Links


Research Links: Schrodinger wave equation

Video: Strangeness Minus Three (BBC Horizon – 1964)

Friday, June 19th, 2015

Video: Bells Inequality

Sunday, June 14th, 2015

This video has a decent explanation of the 3 polarizations used in other talks.

Entangled-Light-Emitting Diode

Tuesday, December 9th, 2014

Research Links


  • Toshiba has a quantum information group
  • Likely that a standard LASER's photons are disentangled if they are ever entangled by a "which path" scenario as mentioned below.


Excerpted from Toshiba page:

For some important applications, quantum computers have potentially massive processing power, due to the way data is encoded upon quantum bits (qubits). One of the resources required to operate an optical quantum computer, is entangled light. At Toshiba, our research on entangled light sources has resulted in many important achievements. These include realisation of the first semiconductor source of triggered entangled photons, creation of time-evolving entangled light states, and recently the first electrically driven source of entangled light.

Entangled light possesses the unusual feature that its constituent particles (photons) have inter-related properties, in this case polarisation. Measurement of one photon affects the polarisation of the other, even if they are separated by huge distances. This curious phenomenon was famously declared by Einstein to be “spukhafte Fernwirkung” or “spooky action at a distance”. These properties of entangled light derive from the fact that according to quantum mechanics, the photon pair exist in a superposition state, and the polarisation of the pair is uncertain until measurement of one photon.

Diagram illustrating the effect of measuring one photon from an entangled-LED on the other photon in the pair

We create photon pairs using nanometer-scale regions of semiconductor known as quantum dots. Their small size means quantum dots can capture a maximum of two negative and positive charges (electrons and holes respectively). The electrons and holes recombine to emit a pair of photons.

However, photon pairs emitted by conventional quantum dots are not entangled, as the energies of the emitted photons are polarisation dependent. This means the polarisation of a photon can be determined by measurement of it's energy, providing the dreaded ‘which-path‘ information that is well know to destroy entanglement. We have solved this problem by pioneering a technique to optimise the size and shape of the quantum dot so that the energies of the emitted photons are equal, and entangled light can be emitted. This led to realisation of the first semiconductor source of triggered entangled photon pairs, which we achieved by driving a single quantum dot with a laser.

We have subsequently made many advances in the performance and operation of the device, which include enhanced resolution quantum interferometry, creation of time-evolving entangled states, and improvement of the fidelity, or purity, of the entangled light to 91%. However, entangled light produced previously by us and others requires a laser beam as a power source. For applications such as optical quantum computing that require many entangled photons, the practical advantages of creating entangled light by electrical current are very significant. In collaboration with the University of Cambridge, we now report in the journal Nature, realisation of the first electrically driven source of entangled photons.

Our device is based on a conventional light-emitting-diode (LED) structure, but additionally contains a specially optimised quantum dot. A voltage applied to the LED causes a current to flow, and the quantum dot captures the charge required to emit a pair of photon. In addition, the thickness of the semiconductor material surrounding the quantum dot was optimised to regulate the rate charge is transferred to the dot. Without this feature, entanglement is destroyed by extra charge. We demonstrate that the device works well in both d.c. and a.c. mode, with fidelities up to 82%.

An additional fundamental advantage of the entangled LED is that it has the potential to operate on demand, supplying one entangled pair nearly every cycle. When combined with the practical advantage offered by electrical excitation, the entangled LED will allow simultaneous operation of many entangled light sources on a single chip, opening the path to ultra-powerful semiconductor processors based on quantum computation.

Quantum dot capturing charge supplied by electric current and subsequent emission<br />          of entangled photons

Further Reading



  Type Title and author(s) Source
1 Technical An entangled-light-emitting diode 
by C L Salter et al.
Nature 465, 594–597 (2010)
2 Technical Bell-inequality violation with a triggered photon-pair source 
by R J Young et al.
Physical Review Letters 102, 030406 (2009)
3 Technical Evolution of entanglement between distinguishable light states 
by R M Stevenson et al.
Physical Review Letters 101, 170501 (2008)
4 Technical Biphoton interference with a quantum dot entangled light source 
by R M Stevenson et al.
Optics Express 15, 6507 (2007)
[open access]
5 Technical A semiconductor source of triggered entangled photon pairs 
by R M Stevenson et al.
Nature 439, 179–182 (2006)
6 Technical Inversion of exciton level splitting in quantum dots 
by R J Young et al.
Physical Review B 72, 113305 (2005)


Entangled Photon Pair Sources

Thursday, October 30th, 2014

Research Links

Video: Single-photon detectors – Krister Shalm

Thursday, August 14th, 2014

Research Links

Entangling 2 different color photons

Wednesday, August 13th, 2014

What I think is interesting about this is that since they are 2 different frequencies it becomes much harder to blame the strange behavior of entanglement on phase coherency. Beware I could still be wrong about that.


Link to Article.

PAM Dirac Lectures in New Zealand 1975

Friday, August 8th, 2014

This guy was around for the Solvay conference in 1927.  A good recounting of the history of quantum mechanics and the mathematics underlying.


Note: In this episode he explains the correspondence of the particle and the wave outlook in terms of the harmonic oscillator. 



Research Links

2 Slit with Measurement at One Slit – A quantum view of re-emission

Wednesday, August 6th, 2014


Sequence of Events

  • 50% of the time a photon will be detected at the output of the left hand slit
  • Source emits single photon
  • Photon travels through both slits
  • Due to the right hand path being low probability amplitude the interference pattern at the left slit is not complete interference.  It is lump with squiggle superimposed on it.
  • The photon is measured there.  
  • Since the photon is measured at the output of the left slit due to energy conservation it precludes it also being at the output of the right slit.  Thus it must be considered as mostly re emitted from the measurement location.
  • The photon travels on to the screen where you see very little interference in the pattern.

So maybe the photon always goes through both slits at least somewhat?

Advanced Quantum Mechanics – Freeman Dyson

Tuesday, August 5th, 2014

When Richard Feynman left Cornell Freeman Dyson was asked to fill in for him and teach an advanced quantum mechanics class.  His notes from that class have been made into a book and are available in pdf form.

The story behind the book is told in the video below.  David Bohm taught Freeman Dyson QM.