Subject: [Qirg] link architectures

From: Rodney Van Meter <[email-masked]>
Date: Thu, 29 Mar 2018 09:41:59 +0900

Now that we have the hardware components, we can talk about link
architectures.

Memories and photons can interact in a few different ways, depending
on the technology:

* Convince the memory to emit a photon that is entangled with the
  state of the memory in some fashion.  (Just to make things
  complicated, you can also convince the memory to *maybe* emit a
  photon, depending on the state of the memory, so that the time slot
  for the photon contains a superposition of yes-there's-a-photon and
  no-there's-not-a-photon states.)
* Convince the memory to absorb a photon you sent from somewhere
  else.  To my non-experimentalist eyes, this looks ridiculously hard.
* Reflect the photon (or a stronger pulse of light) off of the memory
  in a way that depends on the state of the memory.

The first and third give us a photon (in flight) entangled with a
memory qubit (stationary).  Let's look at how to use the first one.

The common approach is to have a memory at each end emit a photon, and
to bring those photons together at an interferer and measure them
(commonly called a Bell State Analyzer, BSA).  The results of the
measurements have to be communicated back to the emitters.

Which means a link consists of memory-->photon at one end,
photon<--memory at the other, and the BSA somewhere along the path.
If you put it in the middle, Cody Jones called that Meet in the
Middle.  I'm not wild about that name because it's too close to Man in
the Middle attacks, and I'm afraid it will cause confusion.  My group
calls it M-->I<--M, or just MIM, memory-interference-memory.

If you put the BSA module in the same box as one of the end points,
then in terms of the timing and everything else I can think of that
affects protocols, it's what we call a memory-to-memory link, which we
write M-->M or just MM.

If you take one of the entangled photon pair sources (EPPS), position
it in the middle of your link, and then couple each of the photons
with the memory at each end (e.g., again using the emit&interfere
technique, or perhaps absorption or state-dependent reflection), you
have a memory-source-memory link, M<--S-->M or MSM.  It sounds more
complicated, with more parts and more optical interfaces (== more
loss), which is true, but it has some positive characteristics that
help overcome this.

So that gives us a taxonomy of three link timing architectures:

* M-->M
* M-->I<--M
* M<--S-->M

where the possible choices of timing is a continuum, not just
insisting that the middle device be exactly halfway in between.

Important questions:

* Are these midpoint devices, either the entanglement source or the Bell
  state analyzer, visible to the routing and session creation system?
  Surely they have IP addresses (or at least the BSA does, the EPPS
  comes pretty close to turn-it-on-and-let-it-run).  But are
  they directly involved in session management or entanglement
  creation farther than their neighbors?  I don't think so.
  (Shota, Shigeya, Satoh and I have argued this point quite a bit.)
* How much of this is visible at a distance of more than the end
  points?  I think the *timing* pattern needs to be visible when
  establishing a connection, for reasons that will take a little while
  to explain.  There are other characteristics, as well, but we'll
  have to discuss errors to get into this more, so put it on your
  mental discussion queue.

In the interest of posts that take less than a month to read, digest
and reply to, I'll stop here for the moment.  References on this
timing discussion:

Cody Jones et al., NJP:
http://stacks.iop.org/1367-2630/18/i=8/a=083015

Van Meter et al., unpublished:
arXiv:1701.04586
https://arxiv.org/abs/1701.04586

		--Rod