Category Archives: Television, Film, Photography & PC Games

Satellite-to-Ground Quantum Key Distribution

Abstract

Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. In practice, the achievable distance for QKD has been limited to a few hundred kilometers, due to the channel loss of fibers or terrestrial free space that exponentially reduced the photon rate. Satellite-based QKD promises to establish a global-scale quantum network by exploiting the negligible photon loss and decoherence in the empty out space. Here, we develop and launch a low-Earth-orbit satellite to implement decoy-state QKD with over kHz key rate from the satellite to ground over a distance up to 1200 km, which is up to 20 orders of magnitudes more efficient than that expected using an optical fiber (with 0.2 dB/km loss) of the same length. The establishment of a reliable and efficient space-to-ground link for faithful quantum state transmission constitutes a key milestone for global-scale quantum networks.

Introduction

Private and secure communications are fundamental human needs. Traditional public key cryptography usually relies on the perceived computational intractability of certain mathematical functions. In contrast, quantum key distribution (QKD)1 proposed in the mid-1980s—the best known example of quantum cryptographic tasks—is a radical new way to offer an information-theoretically secure solution to the key exchange problem, ensured by the laws of quantum physics. QKD allows two distant users, who do not share a long secret key initially, to produce a common, random string of secret bits, called a secret key. Using the one-time pad encryption, this key is proven to be secure by Shannon2 to encrypt (and decrypt) a message, which can then be transmitted over a standard communication channel. In the QKD, the information is encoded in the superposition states of physical carriers at single-quantum level, where photons, the fastest flying qubits with their intrinsic robustness to decoherence and ease of control, are usually used. Any eavesdropper on the quantum channel attempting to gain information of the key will inevitably introduce disturbance to the system, and can be detected by the communicating users.

Continued…

Original article: Satellite-to-Ground Quantum Key Distribution

Working with a Professional Photographer

You may be lucky enough to work with a professional photographer – if so, here are some hints and tips to make sure the shoot goes smoothly and you get what you want from it.

Always follow the rules below when working with photographers:

  • Be punctual. Being on time is very important.
  • Take a friend or parents along with you or at the very least inform someone of your whereabouts.
  • Make sure you tell the photographer that someone knows where you are.
  • Make sure that you know exactly what you’re supposed to do during the shoot. Never feel pressured to do anything that you do not feel comfortable doing.
  • Whether it’s a professional studio or on location, ensure that there is a separate changing area.
  • Reputable photographers will NOT touch you. Make sure you remind them of this if they get within your safety zone.
  • Make sure you get a modeling agreement “release” signed before the shoot. You must be informed where and how your images are going to be displayed, and give your consent.
  • Read all paperwork closely before signing a contract at a photoshoot. Never sign a contract unless we tell you it’s okay. Sometimes, crooked photographers will try to get you to sign a contract at a shoot and then sell the photographs without your knowing. In this case if you signed a contract so there’s nothing you can do about it!
  • Beware of contracts that prohibit you from working with other photographers. The amateur model should not accept this kind of contract. When you are starting out, you need to make as much money as you can working for as many different photographers as possible.

However, don’t be paranoid – most professional photographers are more concerned about film, make-up, and the position of the sun. To most of them, ‘time is money’.

Types of photography agreements

Before you work with a professional photographer, it’s worth clarifying the nature of the arrangement – who’s paying, who gets copyright in the shots and how they can be used in the future.

Trade for print (TFP)

Most professionals are looking to add variety to their portfolios without the cost. Some might be willing to do a trade for print (TFP): you pose for them and in exchange they will shoot what you want. But this usually means you will have to pay for your prints, and this may be very expensive.

Also watch out for the photographer who says it’s free but goes on to use your images for profit – in this case you should get paid!

Photographer pays

This is the staple of the modelling community. The model (or the model’s agent) and the photographer negotiate a mutually satisfactory rate, which the photographer pays the model in compensation for his/her time and a release to use the images collected from the shoot.

When being paid to pose, remember that the person who pays for the shoot is the one entitled to decide the format and details of the shoot, the time and place, how many rolls of film will be shot and the number of outfit changes that will occur. You also have to be prepared to follow the directions given by that person to get the type of shot that the photographer or client wants. This will often be the kind of print or shot that you already have plenty of as that’s probably why you got hired for the job in the first place.

Additionally, on most professional, paid shoots you are not entitled to any of the prints that result from the assignment; the only way you can get hold of them is by getting shots after they are published (your “tear sheets”).

You pay

The model pays the photographer for his/her time and the images he/she takes, as well as the copyright to those images. This is ideal if you need the photographer to shoot a particular image and you want to receive ownership of that image. The photographer and the model must negotiate a mutually satisfactory rate for the photographer’s time but the model has complete control of the images thus acquired. Because this is typically a ‘no hassles’ deal it’s a great way to start off, creating a strong base that you can build upon.

Original article: Working with Professional Photographers

 

One of my favorite local professional photographers is Bryan Chatlien: Bryan Chatlien Photography

Does a Possible Photon Mass Pose a Threat to Creation Models?

In my last blog post I described the consequences for particle and cosmic creation models if the photon, rather than possessing a zero rest mass, actually had a small nonzero rest mass. The consequences are sufficiently severe that most physicists and astronomers believe that the photon really must have a zero rest mass. However, they lack the experimental and observational tools to prove that the photon indeed has a zero rest mass or an upper limit on its rest mass. This implies that none of the consequences I described in my last post are of concern in any physical or astronomical context. Thanks to recent measurements, all that has changed.

During the last several years and even weeks, astronomers and physicists have developed new tools that now place strong limits on the maximum possible value of the photon rest mass that, to a significant degree, alleviate concerns about the consequences I raised. Better yet, the new tools can potentially be made much more robust within the next few years. I will devote the remainder of this blog post to describing the new tools, the limits established so far, and the limits that will be forthcoming.

Wavelength Independence of Light’s Velocity

As I explained in my previous blog post, if the photon has a nonzero rest mass, that mass implies that the velocity of light will be a function of the frequency or wavelength of the light. If scientists can place strict constraints on the limits of the fractional variation of the velocity of light with respect to frequency, those limits will translate into an upper bound constraint of the rest mass of the photon.

In one study, several pulsar measurements indicated that light’s velocity was constant to within 10-20 throughout the ultraviolet, visible, and near infrared parts of the pulsar spectra.1 This constancy limit corresponded to an upper limit on the photon rest mass = 3 x 10-46 grams. (For comparison, the electron rest mass = 9.11 x 10-28 grams.) In a later study, observations of bursts from the gamma ray burst object GRB 980703 covering the range from radio to gamma ray wavelengths yielded an upper limit on the photon rest mass = 4.2 x 10-44 grams.2

Possible Deviation from Coulomb’s Law

We might remember Coulomb’s Law from our junior high science education. Coulomb’s Law states that, just like with gravity, electromagnetism obeys an inverse square law. That is, the magnitude of the electromagnetic force declines with the square of the distance from the source of the electromagnetic force. However, if the photon rest mass is not zero, the electromagnetic force declines at a greater rate than that predicted by the inverse square law. The most sensitive laboratory measurement of possible deviations from Coulomb’s Law establishes that the photon rest mass must be less than 8 x 10-48 grams.3

Torsion Balance Experiments

The best of these experiments uses a rotating torsion balance to detect the product of the square of the photon rest mass and the ambient cosmic magnetic fields. A team of four Chinese physicists used this method to obtain an upper limit on the photon rest mass = 1.2 x 10-51grams.4 However, another team comprised of one American and two Chinese physicists pointed out that uncertainties in the magnetic field levels of the Coma Cluster of galaxies and the Milky Way Galaxy reduce the upper limit to 2.6 x 10-50 grams.5 Yet another team demonstrated that uncertainties in the homogeneity of the magnetic fields and plasma densities of the Coma Cluster and the Local Group of galaxies make the 2.6 x 10-50 gram limit optimistic at best.6

Fast Radio Bursts

Fast radio bursts (FRBs) are the newest discovered phenomenon in astronomy. FRBs are high-energy transient radio pulses that last for only a few milliseconds. While many FRB objects have been found, astronomers have observed only one such object with repeating FRBs.

Thirteen months ago a team of six astronomers pointed out that the frequency time delays in FRBs—if the distance to the FRB object is known—can be used to place an upper limit on the photon rest mass. That same team used data from FRB 150418 to establish that the photon rest mass can be no greater than 3.2 x 10-47 grams.7 Using data from FRB 121102, the same team later determined that the photon rest mass must be less than 3.9 x 10-47 grams.8 In addition, two Chinese astrophysicists, in a paper that appeared just two weeks ago, used a Bayesian analysis of a catalog of FRBs to constrain the photon rest mass to less than 8.7 x 10-48 grams.9

Pulsars

Pulsars are highly magnetized neutron stars or white dwarfs that emit a focused beam of electromagnetic radiation. This radiation is seen only when the beam is pointing toward Earth in the same way a beam of light from a lighthouse can be seen only when the beam is pointed in the observer’s direction.

If the photon has a nonzero rest mass, it will distort the “dispersion measure” of pulsars. The pulsar dispersion measure refers to the broadening of the sharp, or highly focused, pulse when the pulsar is observed over a certain bandwidth of wavelengths as opposed to just a single wavelength. Pulses emitted at higher frequencies (shorter wavelengths) arrive earlier than those emitted at lower frequencies (longer wavelengths).

By determining a limit on the distortion of pulsar dispersion measures, astronomers can establish an upper bound on the photon rest mass. Four astronomers did just that through their measurements of radio pulsars in the Large and Small Magellanic Clouds. They established that the photon rest mass cannot be greater than 2.0 x 10-45 grams.10 While their limit was ten thousand times lower than previous constraints based on the Crab Nebula pulsar, it ranked about a hundred times inferior to limits established from FRBs.

All pulsars are neutron stars except for one object, AR Scorpii. AR Scorpii is a binary pulsar that contains a white dwarf and a red dwarf (see featured image). The discovery of the pulsing nature of the white dwarf was announced at the beginning of this year.11 Astronomers do not directly observe the highly focused light beam from the white dwarf pulsar. The pulsation they see occurs when the focused beam from the white dwarf sweeps across the surface of the red dwarf. The red dwarf reprocesses the beam into the observed electromagnetic energy. You can watch a short video of an artist’s impression of the pulsing nature of both the white dwarf and the red dwarf components of AR Scorpii here.

The AR Scorpii pulsar gets its energy from its spindown, not from accreting any material from its red dwarf partner. If photons have nonzero rest mass, the spindown rate of the AR Scorpii pulsar will be lower than for the zero rest mass case. Measurements made by two astronomers yield a stringent upper limit for the photon rest mass.12 Assuming a vacuum dipole spindown for the AR Scorpii pulsar, the photon rest mass must be less than 6.3 x 10-50 grams. If the spindown arises from a fully developed pulsar wind, the photon rest mass must be less than 9.6 x 10-50 grams. Realistically, the spindown behavior will be between these two extremes. Therefore, the two photon rest mass limits bracket the true photon rest mass upper limit.

Future Prospects 

The upper limit value of 7–8 x 10-50 grams for the photon mass from measures of the spindown of the AR Scorpii white dwarf pulsar is the most stringent limit, to date, within the secure methods. However, several white dwarfs with magnetic fields ranging from one million to one billion Gauss with periods on the order of an hour are now known to exist.13 Observations of the spindown behavior of these white dwarfs could easily push the upper limit for the photon mass below 1 x 10-51 grams.

A method based on the solar wind magnetic field recently has been refined to where it very likely has become a secure method for constraining the photon rest mass. The best measurements show that the photon rest mass must be less than 8 x 10-52 grams.14 Finally, the galactic magnetic field structure, if mapped to sufficient precision and certainty, could establish a limit for the photon rest mass below 1 x 10-52 grams.15

Secure Creation Models

With an upper bound on the photon rest mass as low as 7–8 x 10-50 grams and potentially much lower, no astronomer, physicist, or any other member of the human race needs to worry about the validity of cosmic or particle creation. Thus, any possible extremely tiny rest mass for photons poses no threat to the biblical creation model for the universe and the particles that comprise it.

Original article: Photon Mass Implications