Re latest "quantum computers are coming!" article

["Dave Farber" <dave () farber net>]

---------- Forwarded message ---------
From: Rodney Van Meter <rdv () sfc wide ad jp>
Date: Mon, Jan 23, 2017 at 6:07 PM
Subject: latest "quantum computers are coming!" article
To: Dave Farber <farber () gmail com>
Cc: Rodney Van Meter <rdv () sfc wide ad jp>


Dave, for IP, if you wish:



You asked me about this news article:

http://www.trendintech.com/2017/01/15/now-quantum-computers-can-send-information-using-a-single-particle-of-light/

The actual Science article:

http://science.sciencemag.org/content/early/2016/12/21/science.aal2469?rss=1



As is unfortunately common, the science is good (Petta's team at

Princeton is one of the world's best), but the article overhypes the

result.



A little context:



There are a bunch of candidate technologies for physical

implementation of qubits: single photons, ions or neutral atoms

suspended in a vacuum, or several solid-state technologies.  Petta's

group works on quantum dots (QDs) made using relatively standard

silicon fabrication, and controlled via electrical and magnetic fields

(other groups work on electrons in QDs controlled using light or QDs

fabricated in exotic ways).  Broadly, up through about the year 2010,

several of these technologies were on roughly even footing in

development, with ion traps leading but facing significant scalability

& deployability challenges and the others with short-term problems but

long-term prospects.  From about 2010 to 2014, superconducting qubits

(also fabbed like chips) really took off, with a lot of success for

computation, and nitrogen vacancies in diamond (defects in the diamond

lattice capable of trapping an electron) showing promise for

communications.  But in the last couple of years, QDs have come

roaring back, with good success in the lab.



For QDs, scalability is a big issue: the control structures for the QD

are large, making it difficult to put multiple QDs close enough to

each other to use the most obvious kinds of interaction to compute on

more than one qubit at at time.  Thus, it becomes imperative to find a

way to couple QDs that are farther apart, in nearby sites, across the

chip, and ideally between chips.  This is the area where the new work

lies.



Effectively, Mi et al. have built one (two?) two-dot structures into a

1-D "resonating cavity," which in this case is basically just a long

wire where light will reflect off the ends, confining the photon.  If

you have two QD structures at different positions along the cavity,

the light resonating in the cavity can be used to transfer information

from one QD to the other, or to put the two QDs into a mutual state.

It's possible for that light to be strong, but in this case Mi et

al. are using a single photon.



So the first step in demonstrating that their connection works is to

demonstrate that they can couple the QD to a photon in the cavity.

That's what they've done in this work.  So, it's a valuable step, but

there are still one or two steps just to get to two QDs to couple.



And once it's done, there are still a lot of questions about

scalability with this particular approach.  One thing the news article

didn't mention is that this is a microwave photon & cavity, which are

inherently large devices.  High on the list is that the resonating

cavity they are using is several *millimeters* long.  Using this

technology, you could fit, at most, a few dozen quantum dots into a

one-inch-square chip.  So, you need to couple off-chip, as well, which

is harder.



And since it's microwave, single photon detection is very hard.

Waveguides are straightforward, but big and lossy, so this won't carry

over long distances.  As is, it's not a candidate for building quantum

networks, despite the (correct) use of the word "photon" in both the

news article and the technical paper.



But, it's a valuable step, if only one of many in recent years, and

there are still many to go before we have a full-scale functioning

quantum computer in quantum dots or any other technology.



But progress is accelerating, both technically and business-wise; I

can name a dozen startup companies that are getting into either

quantum computing or quantum communications.  The revolution is

beginning!



—Rod



-------------------------------------------
Archives: https://www.listbox.com/member/archive/247/=now
RSS Feed: https://www.listbox.com/member/archive/rss/247/18849915-ae8fa580
Modify Your Subscription: https://www.listbox.com/member/?member_id=18849915&id_secret=18849915-aa268125
Unsubscribe Now: 
https://www.listbox.com/unsubscribe/?member_id=18849915&id_secret=18849915-32545cb4&post_id=20170123183509:911D5AB8-E1C4-11E6-91AF-4450BDDDB970
Powered by Listbox: http://www.listbox.com