Subject: [Qirg] Thoughts on the "quantum interconnects (QuICs)" paper

From: Bruno Rijsman <[email-masked]>
Date: Thu, 11 Mar 2021 11:27:25 +0100

I finally got around to reading the "quantum interconnects (QuICs)" paper that Rodney mentioned on the mailing list and that was discussed in the QIRG working group meeting yesterday.

When we, classical network engineers attending the IETF, talk about networking we tend to implicitly assume that we are talking about interconnecting distributed systems. This could be local area networking (historically the focus of the IEEE etc.) or internetworking (historically the focus of IETF).

One thought that struck me is that the some of the quantum interconnects (QuICs) discussed in this paper are more akin to classical busses than to classical networks or internets.

Two examples of such classical busses are:

1. The system bus which connects multiple CPUs to each other, where the CPUs are physically very close to each other (e.g. on the same board). Examples include Intel QuickPath Interconnect (QPI) and AMD HyperTransport. These are proprietary interfaces.

2. The peripheral component interconnect (PCI) bus, which connects CPUs to peripherals such as memory and network interface cards (NICs). The PCI bus is standardized by an industry consortium: the PCI special interest group (PCI-SIG).

This may be what Rodney had in mind when he spoke of "system area networking".

We will need equivalent interfaces for quantum computers:

1. It seems extremely unlikely that any of the existing quantum computing platforms will reach millions of qubits on a single chip in the foreseeable future. Thus, it seems inevitable that reaching fault-tolerant computing will require interconnecting multiple quantum computing chips (i.e. create entangled states involving qubits on different chips). The paper speaks of "modular quantum computers". For this we will need the quantum equivalent of QuickPath or HyperTransport.

2. Long distance quantum networking requires creating entangled states that involve a memory qubit on the chip and a "flying" photonic qubit on the fiber (perhaps the "media conversion" that Kireeti mentioned in a question). Also, although current quantum computers don't yet have external quantum memory (as far as I know), it is being discussed in literature (e.g. QRAM). For all of these "quantum peripherals" we will need the quantum equivalent of a PCI bus.

One could make an argument that the urgency of building such quantum busses is greater than the urgency of building (long-distance) quantum networks.

— Bruno

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From: Santanu Ganguly <[email-masked]>
Date: Sat, 13 Mar 2021 10:39:01 +0000

Hi,

I wrote privately to Bruno about some open questions surrounding quantum
memories. Afterwards, I thought it would be nice to have perspectives,
thoughts and guidance from the wider community on the following as well, if
anyone will be so kind. Hence, posting part of what I sent to Bruno:

Regarding "quantum memory", there is an option of considering "ballistic
cluster states" . Cluster states and its advantages in QC is a
well-researched area - often referred to as "*one-way*" quantum computing
[1]. As per [1], multipartite entangled cluster states are a sufficient
resource for universal, measurement-based quantum computation. In this
scheme, a multi-party entangled state is first prepared offline and the
actual computation is then conducted solely through single-party projection
measurements on the individual nodes of that resource state – the cluster
state. By choosing appropriate measurement bases in each step, sometimes
(optimisation of this is still a research subject) depending on earlier
measurement outcomes, any unitary gate can be applied to an input state
which typically becomes part of the cluster at the beginning of the
computation.  Local projection measurements on the individual qubits,
including feedforward with a measurement order going from left to right,
are then enough to realize (universal) quantum computation. Whereas this
refers to quantum computing, the same can be considered in communications -
analogy is classical computing where a lot of computing ideas have been
translated into communication space.

One advantage of ballistic cluster states is that they can be generated and
processed (via adaptive measurement) on the fly, *without the need* to
store photons in an optical quantum memory [2]. In this scheme, an array of
sources and simple circuits produce entangled photons at each time step -
these photons are entangled together to produce a 3D cluster where each
layer represents a generation step in time. It has been shown that the
depth of a cluster that needs to exist at any time is only of the order of
a few tens of photons [3].

Ongoing theoretical and experimental research on photonic clusters and
ballistic schemes is also addressing many technical details (e.g. optimal
cluster geometry, error correction schemes, sources designed for cluster
generation). However, it is emerging that photonic cluster schemes, and
closely related ideas, may be plausible approaches for realising universal
quantum computing and communication [4].

AFAIK, this is one of the approaches by Xanadu.

"Flying qubits" are areas of active research at several institutions (UC
Berkeley for example). Interesting times ahead! :-)

References:

1.      Raussendorf, R. and Briegel, H.J. , (2001) Phys. Rev. Lett., 86,
5188

2.      Gimeno-Segovia, M., Shadbolt,P., Browne,D.E., and Rudolph, T.
 (2015) “From three photon greenberger-horne-zeilinger states to ballistic
universal quantum computation,” Phys. Rev. Lett. 115, 020502 .

3.      Morley-Short, S., Bartolucci, S., Gimeno-Segovia, M., Shadbolt,P.,
Cable, H. and Rudolph,T. (2017)  “Physical-depth architectural requirements
for generating universal photonic cluster states," Quantum Sci. Technol. 3,
015005.

4.      Rudolph, T. (2017) “Why I am optimistic about the silicon-photonic
route to quantum computing," APL Photonics 2, 030901.

Thanks and best regards

Santanu

  --
"Education is what remains after one has forgotten everything he learnt in
school" - Albert Einstein

On Thu, Mar 11, 2021 at 10:27 AM Bruno Rijsman <[email-masked]>
wrote:

> I finally got around to reading the "quantum interconnects (QuICs)" paper
> that Rodney mentioned on the mailing list and that was discussed in the
> QIRG working group meeting yesterday.
>
> When we, classical network engineers attending the IETF, talk about
> networking we tend to implicitly assume that we are talking about
> interconnecting distributed systems. This could be local area networking
> (historically the focus of the IEEE etc.) or internetworking (historically
> the focus of IETF).
>
> One thought that struck me is that the some of the quantum interconnects
> (QuICs) discussed in this paper are more akin to classical busses than to
> classical networks or internets.
>
> 1. The system bus which connects multiple CPUs to each other, where the
> CPUs are physically very close to each other (e.g. on the same board).
> Examples include Intel QuickPath Interconnect (QPI) and AMD HyperTransport.
> These are proprietary interfaces.
>
> 2. The peripheral component interconnect (PCI) bus, which connects CPUs to
> peripherals such as memory and network interface cards (NICs). The PCI bus
> is standardized by an industry consortium: the PCI special interest group
> (PCI-SIG).
>
> 1. It seems extremely unlikely that any of the existing quantum computing
> platforms will reach millions of qubits on a single chip in the foreseeable
> future. Thus, it seems inevitable that reaching fault-tolerant computing
> will require interconnecting multiple quantum computing chips (i.e. create
> entangled states involving qubits on different chips). The paper speaks of
> "modular quantum computers". For this we will need the quantum equivalent
> of QuickPath or HyperTransport.
>
> 2. Long distance quantum networking requires creating entangled states
> that involve a memory qubit on the chip and a "flying" photonic qubit on
> the fiber (perhaps the "media conversion" that Kireeti mentioned in a
> question). Also, although current quantum computers don't yet have external
> quantum memory (as far as I know), it is being discussed in literature
> (e.g. QRAM). For all of these "quantum peripherals" we will need the
> quantum equivalent of a PCI bus.
>
> One could make an argument that the urgency of building such quantum
> busses is greater than the urgency of building (long-distance) quantum
> networks.
>
> — Bruno
> _______________________________________________
> Qirg mailing list
> [email-masked]
> https://www.irtf.org/mailman/listinfo/qirg
>

-- 
-- 
"Education is what remains after one has forgotten everything he learnt in
school" - Albert Einstein