Quantum Information Technologies

Quantum Information Technologies

The Next Big Thing is Smaller Than You Think (and Weirder Than You Can Think!)

The Photon
The Qubit of Choice for Quantum Information Processing


1  What is a Photon?
    1.1  Expert Opinions
    1.2  This Just In ...
2  Press Articles
    2.1  SPIE Newsroom, 2006
    2.2  OE Magazine, 2005
3  Imaging Correlated Photons
    3.1  LATSIS Conference, 2008
        3.1.1  Quantum Imaging of Second-order Correlations g(2)(x′,t′,x,t)
4  Photon Design Rules
    4.1  SPIE Optics & Photonics 2006
        4.1.1  Design Rules for Quantum Imaging Devices: Experimental Progress Using CMOS Single Photon Detectors
        4.1.2  Download Materials
    4.2  IEEE LEOS, Santa Clara, CA, 2006
        4.2.1  Quantum Design Rules for Optical Engineers
    4.3  SPIE Optics & Photonics 2005
        4.3.1  Towards Practical Design Rules for Quantum Communications and Quantum Imaging Devices
        4.3.2  Download Materials
5  If You Break the Rules?
    5.1  Afshar's Interferometer
    5.2  Unruh's Critique
    5.3  Afshar Rebuts Unruh's Critique
    5.4  My Criticism of Afshar's Rebuttal

1  What is a Photon?

1.1  Expert Opinions

In case you thought you knew, here are some opinions from those who claim to have real insight.

1.2  This Just In ...

Even after 100 years of the photon, experts are still raising the question, What is a photon? While sitting in the audience (SPIE 2005) listening to these modern experts argue about whether light is really an electomagnetic wave:
"We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena." - J. C. Maxwell (1862)
or a particle:
"Nach der hier ins Auge zu fassenden Annahme ist bei Ausbreitung eines von einem Punkte ausgehenden Lichtstrahles die Energie nicht kontinuierlich auf größer und größer werdende Räume verteilt, sondern es besteht dieselbe aus einer endlichen Zahl von in Raumspunkten lokalisierten Energiequanten, welche sich bewegen, ohne sich zu teilen und nur als Ganze absorbiert und erzeugt werden können." - A. Einstein (1905) [Pardon?]
it suddenly came to me ....
The photon is really a piece of quantum mechanics that accidentally fell into the middle of the 19-th century and some people just never managed to get over it.

2  Press Articles

2.1  SPIE Newsroom, 2006

Photonic Information Processing Needs Quantum Design Rules. (HTML)
Photonic Information Processing Needs Quantum Design Rules. (PDF)

2.2  OE Magazine, 2005

News item regarding the Afshar experiment. (PDF)

3  Imaging Correlated Photons

3.1  LATSIS Conference, 2008

3.1.1  Quantum Imaging of Second-order Correlations g(2)(x′,t′,x,t)

D. L. Boiko, N. J. Gunther, N. Brauer, M. Sergio, C. Niclass, G.B. Beretta and E. Charbon
We report on a spatio-temporal imager of normalized second order correlations g(2)(x′,t′,x,t) between photons. The imager is based on a monolithic 4 ×4 array of single-photon avalanche diodes implemented in CMOS technology and utilizes a simple algorithm to treat multiphoton time-of-arrival distributions. The chip, that enables 100 ps temporal resolution, incorporates integrated high-bandwidth electronics for off-chip processing, operating with photon fluxes as low as 10 photons per second at room-temperature. For any interval of observation, the imager yields a normalized correlation function g(2) calculated from photon arrivals at different detector pairs. The chip can be used to locally probe second-order correlations in the condensate and non-condensed fractions in experiments on transient BEC of cavity exciton polaritons.

4  Photon Design Rules

4.1  SPIE Optics & Photonics 2006

4.1.1  Design Rules for Quantum Imaging Devices: Experimental Progress Using CMOS Single Photon Detectors

E. Charbon, N. J. Gunther, D. L. Boiko and G. B. Beretta
We continue with our previous program where we introduced a set of quantum-based design rules directed at quantum engineers who design single-photon quantum communications and quantum imaging devices. Here, we report on experimental progress using SPAD (single photon avalanche diode) arrays of our design and fabricated in CMOS (complementary metal oxide semiconductor) technology. Emerging high-resolution imaging techniques based on SPAD arrays have proven useful in a variety of disciplines including bio-fluorescence microscopy and 3D vision systems. They have also been particularly successful for intra-chip optical communications implemented entirely in CMOS technology. More importantly for our purposes, a very low dark count allows SPADs to detect rare photon events with a high dynamic range and high signal-to-noise ratio. Our CMOS SPADs support multi-channel detection of photon arrivals with picosecond accuracy, several million times per second, due to a very short detection cycle. The tiny chip area means they are suitable for highly miniaturized quantum imaging devices and that is how we employ them in this paper. Our quantum path integral analysis of the Young-Afshar-Wheeler interferometer showed that Bohr's complementarity principle was not violated due the previously overlooked effect of photon bifurcation within the lens-a phenomenon consistent with our quantum design rules-which accounts for the loss of which-path information in the presence of interference. In this paper, we report on our progress toward the construction of quantitative design rules as well as some proposed tests for quantum imaging devices using entangled photon sources with our SPAD imager.

4.1.2  Download Materials

4.2  IEEE LEOS, Santa Clara, CA, 2006

Presented Tuesday March 7, 2006.

4.2.1  Quantum Design Rules for Optical Engineers

Talk presented to Santa Clara Valley chapter of IEEE-LEOS in Sunnyvale, California.

4.3  SPIE Optics & Photonics 2005

4.3.1  Towards Practical Design Rules for Quantum Communications and Quantum Imaging Devices

N. J. Gunther, and G. B. Beretta
A common syndrome in much of the current quantum optics and quantum computing literature is the casual switching between classical concepts (e.g., geometric rays, electromagnetic waves) and quantum concepts (e.g., state vectors, projection operators). Such ambiguous language can confuse designers not well versed in the deeper subtleties of quantum mechanics, or worse, it can lead to a flawed analysis of new designs for quantum devices. To validate that a quantum device can be constructed with the expected characteristics and that its quantum effects are correctly interpreted, a set of unambiguous design rules would be useful. In this paper we enumerate such a set of easily applied quantum rules in the hope that they might facilitate clearer communication between researchers and system developers in the field. In part, we are motivated by recently reported interferometer results that have not only led to flawed claims about disproving fundamental quantum principles, but have elicited equally flawed counter arguments from supposedly knowledgeable respondents. After one hundred years of testing Einstein's photon, it is alarming that such widespread confusion still persists. Our proposed quantum design rules are presented in a practical diagrammatic style, demonstrating their effectiveness by analyzing several interferometers that have appeared in the recent literature. Application to other quantum devices e.g., the quantum eraser, are also discussed. We stress that these rules are entirely quantum in prescription, being particularly appropriate for single-photon devices. Classical optics concepts e.g., refractive index, are not required since they are subsumed by our quantum rules.

4.3.2  Download Materials

5  If You Break the Rules?

Created on Mar 04, 2005

5.1  Afshar's Interferometer

In November 2004, S. Afshar finally made available his manuscript in which he claims to have demonstrated experimentally that one can measure simultaneously the wave character and the particle character on light by using a cleverly modified Young-Wheeler interferometer (Table 1). If this were true, it would mean that a fundamental aspect of quantum mechanics (Bohr's Complementarity Principle) is wrong.
Afshar's manuscript (2004)
Table 1: Dimensions of Afshar's Interferometer
Attribute Dimension Unit
HeNe laser wavelength (λ) 650 nm
Optical total path length 5 meters
Lens distance from pinholes 4.2 meters
Pinhole diameter 0.25 mm
Pinhole cen-2-cen spacing 2 mm
Aperture stop diameter 20.8 mm
Focusing lens diameter 30 mm
Lens focal length 1000 mm
Image cen-2-cen spacing 0.6 mm
Wire comb gauge (200 λ) 127 μm
Wire comb spacing (2000 λ) 1.4 mm
Until 2007, Afshar has not been able to get any manuscript published in a reputable peer-reviewed physics journal. In all likelihood, his claim was summarily dismissed with prejudice by professional physicists on the grounds that it obviously violates Bohr's complementarity principle, which disallows such simultaneous measurements in quantum mechanics; a physical theory that has been tested with the highest precision in the history of physics. However, no one seems to have been able to pinpoint exactly where Afshar made his error. Afshar does have a few defenders, but their supporting arguments generally lie outside the realm of conventional quantum theory.
I argue that Afshar's conclusion is incorrect (i.e., Bohr lives!) but his error is extremely subtle and is easily missed if classical theories of refraction are allowed to creep into the otherwise conventional quantum-theoretic analysis. The key point is that coherent light sources, which are allowed to interfere, refract differently through a lens than does regular incoherent light.
Afshar Explained (Mar 2005 slides)

5.2  Unruh's Critique

Bill Unruh has written a critique of Afshar's claim using a thought-experiment based on an MZ interferometer.

5.3  Afshar Rebuts Unruh's Critique

Afshar dismisses Unruh's argument out of hand.

5.4  My Criticism of Afshar's Rebuttal

Afshar dismisses Unruh's analysis on the grounds that the tandem MZ used in the argument is not a faithful representation of Afshar's Youngian interferometer. There is some merit to that position, but not much. Most significantly, Unruh's analysis puts Afshar in the overall weaker position of arguing that we must use Afshar's special interferometer in order to observe what is otherwise supposed to be a fundamental, and therefore universal, quantum effect.

File translated from TEX by TTH, version 3.38.
On 17 Mar 2008, 14:19.