Quantum Vampire Effect

Bela Lugosi as Dracula

Quantum mechanics is full of subtle and unusual processes that challenge our common-sense understanding of the world, for instance simultaneous particle/wave behavior, quantum non-locality, quantum entanglement and instant quantum teleportation. Now, thanks to four Russian researchers in Moscow and Calgary, Canada, a new example of quantum weirdness has been added to the list, the peculiar phenomenon called Quantum Vampire Effect (QVE).

Ilya Fedorov, Alex Ulanov, Yury Kurochkin and Alex Lvovsky announced the discovery of the Quantum Vampire Effect in a recent ArXiv post. Here I will attempt a brief description of the four Russians' discovery.

Quantum Vampire Effect: Removing a photon from part of a state removes the photon from the entire state.

The picture above illustrates an INTERFEROMETER, one of the physicist's most sensitive measuring devices. It works like this. A quantum state ψ at the left is split into two parts by beam splitter BS1 and then later recombined at beam splitter BS2. If the two paths suffer no change, the state ψ re-emerges intact. However if one of the two paths is perturbed by even the most minuscule change, reconstruction fails and some of the state ψ will end up in detector A.

Possible changes that might hinder the perfect reconstruction of ψ include changes in the density of the air, vibrations of the mirrors caused by a car passing outside or by people talking in the room. The INTERFEROMETER is a very very sensitive device.

So now we take this change-sensitive INTERFEROMETER and remove one photon from one of its paths by means of a weakly-transmitting mirror and a single-photon detector (see above). What do you think will happen? The answer to this question is the basis of the Quantum Vampire Effect.

What happens if we remove one photon from one of the beams is NOTHING. Well not exactly nothing, but almost nothing. The original state is preserved at the final detector; sensitive-change-detector A does not click. But the final state is missing one photon.

The final state is missing one photon. This means that taking ONE PHOTON from the PARTIAL STATE (one of the two beams into which the original state was split) is entirely equivalent to taking ONE PHOTON from the ENTIRE STATE. As far as the final result is concerned, the photon-eating device (weakly-transmitting mirror and detector) in partial beam φ1 could just as well have been placed at the beginning of the experiment where it would grab a photon from the whole beam instead of just a part.

In physics slang this photon-grabbing device is the material realization of the "photon annihilation operator" (symbolized by lower case a) which removes one photon from the associated quantum state. If operator a is applied to partial beam φ1 , this operation is written a(φ1). If the full state ψ has one photon removed, this photon removal operation is written a(ψ). The Quantum Vampire Effect amounts to the discovery that under very general conditions:
 

a (φ1) = a (ψ). 

That is, taking a photon from part of the state is exactly equivalent to taking a photon from ALL OF THE STATE.

So what?

Here's what.

If ordinary light absorption worked this way, objects would not cast shadows (hence the Dracula-inspired name). Instead the light as a whole would be dimmed.

The Quantum Vampire Effect does not cast a shadow but reduces the intensity of the light as a whole.

Ordinary Optical Absorption is much more complicated than the simple removal of one photon at a time by physical application of the annihilation operator "a". The realization of "a" is a DIP & CLICK operation which rarely happens in real life but is easy to do in an optics lab. What you do is DIP a mirror into the beam. And if you happen to catch a photon, your detector CLICKS. The Quantum Vampire Effect illustrated above can only be demonstrated if you throw away all ordinary absorptions and just keep the few chance events associated with DIPS & CLICKS.

But however artificial the Vampire Effect might seem, it has enormous philosophical import because it is a concrete example of an action on the part being equivalent TO THE SAME ACTION acting on the whole. One more little puzzle piece in the Great Quantum Mystery.

And perhaps the key to a brand-new super technology. If an action on a part can instantly affect the whole, can one perhaps use the Quantum Vampire Effect to send signals faster-than-light (FTL)?

Since I have been devising impromptu FTL signaling devices for most of my physics career, it was not difficult to see how to exploit the Quantum Vampire Effect to achieve ultra-fast telegraphy.

The trick is this: to start with a quantum state that possesses very few photons, so that the hyper-holistic DIP & CLICK operation results in an enormous change. For this purpose, the best input state one could imagine would be a 2-photon state. Then the DIP & CLICK operation would maximally switch the state (non-locally?) from a state consisting of two photons (symbolized |2>) to a state consisting of just one photon (symbolized |1>).

Accordingly, my new FTL design consists of a 2-photon Diagonally-polarized (D) input state which is split by a polarized beam splitter (PBS) into a Vertically-polarized (V) beam sent to ALICE and a Horizontally-polarized (H) beam sent to BOB.

For starts the initial D beam possesses two photons, which in each pulse are shared between ALICE and BOB. If ALICE detects two photons, BOB detects none. If ALICE detects one photon, BOB gets one too. And so on.

But now ALICE introduces a DIP & CLICK machine into her beam which locally removes one photon from her beam. But according to the Quantum Vampire Effect, Alice's act is NOT ONLY LOCAL BUT GLOBAL. Suddenly instead of sharing a two-photon D state, both ALICE and BOB are sharing a one-photon D state.

And these two kinds of state are easily distinguishable -- mainly by the utter lack of any two-photon counting events.

Is this it? Has the fair Muse of Physics finally delivered Nick his long-sought FTL signaling scheme? Please read on.

VLAD: a proposed FTL signaling scheme based on Quantum Vampire Effect

I decided to call it VLAD (for Vampire-Licit Ansible Device) -- "Ansible" being a famous fictitious FTL signaling device invented by sci-fi writer Ursula Le Guin.

And sure enough it works. The above illustration shows the VLAD scheme, including ALICE'S use of a partially-reflecting mirror to DIP & CLICK single photons out of her beam and record every such events with her "a Detector". ("a", you will recall, stands for the quantum photon annihilation operator).

For the VLAD setup, the Quantum Vampire Effect works as advertised: when nothing is done, the input is a two-photon state. But whenever ALICE snatches a photon from her local beam, the ENTIRE SYSTEM acts as though the input was just one photon all along.

The two equations accompanying the drawing represent the quantum wavefunctions for these two cases. 1. When there are two photons in the starting state, ψ(1) describes the situation; 2. When there is only one photon in the starting state, ψ(2) is the correct description.

VLAD "works" in the sense that BOB could easily distinguish which of these two wavefunctions describes the situation by looking at the pattern of photon counts at his detector. VLAD "works" in the sense that ALICE can select, by deploying her DIP & CLICK device or not, whether the wavefunction shall consist of two photons (ψ(1)) or of only one photon (ψ(2)).

But, alas, VLAD finally fails to work after all, because the output of ALICE'S DIP & CLICK occurs at random. Only if Alice sends BOB a signal (at light speed or slower) whenever her "a detector" clicks, can BOB know for sure that that he is looking at a one-photon state. Absent news of ALICE'S "a detector" click, BOB sees no change whatsoever in his pattern of photon clicks. Even through ALICE'S action has randomly embedded a one-photon pattern in the full photon stream, without a decoding signal from ALICE, BOB cannot extract these special events from the original two-photon situation. So says the mathematics. Once again clever Nature has prevented us from using Her marvelous quantum entanglement to send signals faster than light.

Goodbye to VLAD as an FTL signaling device. Farewell, VLAD. Da svedanya.

And the four Russians conclude their QVE paper thus: "We expect the quantum vampire effect to find applications in quantum information technology...The ability to "steal" a photon without casting a shadow may prove useful for eavesdropping in quantum key distributions as well as developing quantum cloaking devices. We also believe the effect to be of fundamental interest, as quantum action at a distance that is not associated with a local state collapse has not yet been studied."

I wish to thank Doctor Alex Lvovsky for patiently clarifying for me many subtle features of the Quantum Vampire Effect. Without his help this post would have been impossible.