Subject: Your brain and quantph/0003084
v3
Date: Tue, 25 Mar 2003 14:48:00 +0000 From: Dimi Chakalov <dchakalov@surfeu.at> To: a.steane1@physics.ox.ac.uk CC: dyakonov@LPM.univmontp2.fr Regarding "the nature of entanglement involving more than two parties", perhaps you may wish to see how your brain stores values in a highly qualified (and in this case totally useful) sense, http://members.aon.at/chakalov/Vecchi.html It can "represent and manipulate correlations directly, rather than indirectly through a manipulation of the correlated entities." We call it 'context', http://members.aon.at/chakalov/Penrose.html#NB It's UNspeakable and real, http://members.aon.at/chakalov/Bassi.html#NB http://members.aon.at/chakalov/Feedback.html#NB and is right above your neck. I'm curious, are EPSRC and the Research Training and Development and Human Potential Programs of the European Union aware of this possibility? Best regards, Dimi
==== Subject: Re: Your brain and quantph/0003084
v3
P.S. I spotted some puzzling omissions in your recent "Quantum Computing and Error Correction", quantph/0304016 (please see below). It seems to me that you have completely ignored the devastating critics by many colleagues of yours, such as Subhash Kak, "Can Qubit Errors Be Corrected?" http://xxx.lanl.gov/abs/quantph/0206144
and Michel Dyakonov, "Quantum computing: a view from the enemy camp", http://xxx.lanl.gov/abs/condmat/0110326
Moreover, your "hierarchical quantum computer" still faces the challenge of reconciling QM with STR, which is wellknown since 1931 but persistently ignored by "quantum computing" community. John Bell was fully aware of these pitfalls. See http://members.aon.at/chakalov/Frasca.html#NB Your tutorial introduction to error correction didn't explain how one could keep the precious entangled state alive and kicking for a *finite* time interval during which you can time the sequence of events of error correction, as read by your good old wristwatch. Please take a good look at your Fig. 1 and tell me how would you position the sequence of error correction events in your reference frame, and then try to map this same sequence onto the "duration" of your entangled state, http://members.aon.at/chakalov/Lloyd.html#2 Perhaps it would be a good idea if you report your results in Nature, as you already did on a bit different occasion. You can read this email also at http://members.aon.at/chakalov/Steane.html#NB I wish you best of luck. Dimi 
Comments: 12 pages, a very simple tutorial introduction to error correction, with emphasis on avoiding misconceptions regarding the treatment of noise "The success of QEC methods in practice will depend on
our having the correct understanding of noise in open quantum systems.
"The first quantum error correcting codes were discovered
independently by Shor [4] and Steane [5]. ([5] A. M. Steane. Error correcting
codes in quantum theory. Phys. Rev. Lett., 77:793797, 1996.)
"Exactly what is meant by a quantum error correcting code
will become apparent.
"As a numerical example, consider the [[127, 29, 15]]
code. Let us suppose we employ this code to store a general 29qubit state
in a collection of 127 twolevel atoms, and we envisage that the excited
state of each atom is subject to spontaneous decay with a lifetime of 1
second. If the sequence of operations required for syndrome extraction
can be completed in 1 millisecond, then the error probability per atom
is of order p = 10^3. The probability that an uncorrectable error occurs
is approximately (3np)^8/8! = 10^8. Therefore we could repeatedly correct
the state for 10^5 s = 1 day before the fidelity of the stored state falls
to a half, despite the fact that during such a process every one of the
atoms will have decayed on average about 50,000 times!
"I propose that the best construction for a quantum computer is a *hierarchical* one, as illustrated in figure 2. The most important role of the parallelism and communication inside the computer is to get the syndromes out as quickly as possible, so that qubits are not left accumulating noise for too long before they are corrected." ===== Subject: Re: Is Quantum Computation
Possible?
On Mon, 11 Feb 2002 14:21:04 0000, MessageID:
[snip] > However, the relativity of simultaneity is not an issue
here,
During the past sixteen months, I've been trying to understand how could you separate the error correction events in a timelike way while keeping the entangled state alive and unchanged. Perhaps you tacitly imply some sort of nonperturbative continuous measurement that could "communicate" with the entangled state in a timelike way. Sounds like a miracle to me. Again, may I ask you to look at Fig. 1 from your "Quantum Computing and Error Correction", quantph/0304016, http://members.aon.at/chakalov/Steane.html#below "Figure 1: Example of quantum error correction which illustrates the main features. A single qubit is encoded into three, and these are sent down the noisy channel. The receiver introduces further qubits (the ancilla) and uses them to extract a syndrome by applying controllednot gates and then measuring the ancilla bits. The received state can then be corrected. Finally, the single qubit of information may be reextracted (decoded) from the three." Here's a timelike sequence: t_1: A single qubit is encoded into three. t_2: These are sent down the noisy channel. t_3: The receiver introduces further qubits (the ancilla). t_4: It uses them to extract a syndrome by applying controllednot gates. t_5: Then measures the ancilla bits. t_6: Then the received state can be corrected. t_7: Finally, the single qubit of information may be reextracted (decoded) from the three. t_8: Everybody are happy and relaxed: no errors were made during this particular error correction process. Please write down the Hamiltonian and explain this miracle. Perhaps you may wish to consult Martin Plenio, I believe he is also eager to manipulate the entangled state locally, in line with STR. My doubts can be read at http://members.aon.at/chakalov/Steane.html#note Some prerequisites at http://members.aon.at/chakalov/Zurek.html#note Looking forward to hearing from you, Regards, Dimi
====== Subject: Re: Is Quantum Computation
Possible?
On Thu, 26 Jun 2003 12:45:13 +0100, Andrew Steane wrote:
Yes I did. Thank you very much. > The quantum network (figure 1) is a diagrammatic representation
If we're talking about entangled state(s), I'm afraid "slow enough" is poetry. > The complete Hamiltonian is thus a function of time
of the
This is *the* crucial point in my note at http://members.aon.at/chakalov/Steane.html#miracle > You can deal with those two ways.
I will definitely use a vonNeumann projection to get "the classical information provided by the measurement" (see below). Replacing each measurement by a controllednot gate to another quantum system is *not* an option. It's a mess. > The corrective operation is either done by using the
classical
NB: I worry about using a vonNeumann projection that must be (1) "slow enough to obey all the standard rules of causality" and (2) deliver the classical information instantaneously. These two requirements form an outstanding challenge, to say the least. I have no idea how are you going to make it, Andrew. Besides, why invent the wheel? Perhaps we have the same gadgets right above our neck. Wishing you all the best, Dimi
====== Subject: quantph/0307062 vs. STR
Dear Dr. Pravia, In your "Robust Control of Quantum Information", quantph/0307062, you and your colleagues wrote: "The common goal is to preserve or manipulate a quantum system so that the effective evolution over a control sequence is precisely the desired process." I'm wondering how would you cope with STR, http://members.aon.at/chakalov/Steane.html#reply As to NMR quantum computing, I'm curious whether you are aware of the opinion of Sam Braunstein who is the chief NAYsayer against liquidstate NMR, see e.g., http://www.sees.bangor.ac.uk/~schmuel/news98/pt00.html I will appreciate the opinion of your colleagues as well. Regards, Dimiter G. Chakalov
====== Note: The problems of error correction in "quantum computing" are not confined to 'noise reduction', as in A. Steane's quantph/0304016 above or in the review article by Emanuel H. Knill et al., "Introduction to Quantum Error Correction", quantph/0207170. Robert Alicki <fizra@univ.gda.pl> (Institute of Theoretical Physics and Astrophysics at Gdansk, Poland) published today a very clear, downtoearth article, entitled: "Remarks on the nature of quantum computation", quantph/0306103. He explained the inevitable problem of error correction, stressing that there are various types of errors, and we can not eliminate all of them. One of the cases that can not be eliminated is the compilation of errors in the final measurement of the output: the probability of correct identification of the output state for the whole register falls down exponentially (Ibid., Eq. 9). This is not an engineering obstacle. The problem is of theoretical origin. My objection is also theoretical: look at Fig 1 in A. Steane's article, and write down the sequence of error correction events. They are uniquely ordered within a finite time interval, as read by a clock. However, to even talk about an entangled state spanned over a finite time interval, we need a theory of Lorentzinvariant nonlocality. This problem is known since 1931 and has been particularly stressed by John Bell. Sorry. Here comes the good news: all we need is a human brain, as I tried to explain above. Look at the definition of 'information' in Sec. 27 of E. Knill's quantph/0207170 of 28 July 2002: "Something that can be recorded, communicated and computed with. Information is fungible, which implies that its meaning can be identified regardless of the particulars of the physical realization. Thus, information in one realization (such as ink on a sheet of paper) can be easily transferred to another (for example, spoken words)." What makes the information in the human brain "fungible"? We call it 'context'. It refers to a holistic state of the human brain. The fact that the human brain does not suffer from causal anomalies, closed time curves, etc., means that its holistic state obeys the laws of special relativity theory. Only we don't know how. This is what we need to understand and explore. Not "quantum computing". No need to 'invent the wheel'. It is right above our neck. I thank Andrew Steane for his thoughtful
reply. It is a great pleasure to communicate with a person who keeps
an open mind and never gives up. In plain English, "If qubits spontaneously
relax whenever there is a 100way entanglement, then quantum computing
will be practically impossible", said Andrew Steane on Tue,
12 Feb 2002 17:35:03 0000. Qui vivra, verra.
Dimi Chakalov
