Quantum computers and dissipation
Open accessAbstract
We analyse dissipation in quantum computation and its destructive impact on efficiency of quantum algorithms. Using a general model of decoherence, we study the time evolution of a quantum register of arbitrary length coupled with an environment of arbitrary coherence length. We discuss relations between decoherence and computational complexity and show that the quantum factorization algorithm must be modified in order to be regarded as efficient and realistic.
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TL;DR
The paper analyzes how dissipation/decoherence in quantum computers destroys quantum interference and entanglement, causing the probability of successful computation to decrease exponentially with input size \(L\). It argues that Shor’s factoring algorithm must be modified, e.g. supplemented with exponentially efficient quantum error correction, to be regarded as efficient and realistic.
Key findings
- If environmental coupling makes the error rate grow as \(\epsilon(L)=1-A\exp(-\alpha L)\), then the required number of repetitions grows as \(k(L)\approx \exp(\alpha L)\), so the randomized factoring algorithm is no longer efficient.
- In the independent-reservoir limit, the worst-case density-matrix elements decay as \(\exp\{-L\Gamma(t)\}\); for reservoirs with large coherence length, some off-diagonal elements decay as \(\exp\{-\mathrm{poly}(L)\Gamma(t)\}\), which can be more of a hindrance than a help unless selected coherences can be supported.
- Single-qubit decoherence has three regimes: a “quiet” regime for \(t<\omega_c^{-1}\), a quantum regime for \(\omega_c^{-1}<t<T^{-1}\), and a thermal regime for \(t>T^{-1}\). In one dimension at low temperature, \(\Gamma(t)\sim \omega_c^2 t^2\), then \(\sim 2\ln \omega_c t\), then \(\sim Tt\); in three dimensions the decoherence saturates to a value determined by \(\eta=\omega_c/T\).
- For two qubits, collective effects produce “subdecoherent” and “superdecoherent” states, with collective decay constants \(\Gamma_\pm(R,t)\); the paper distinguishes superdecoherence from superradiance and notes that some superradiant states are subdecoherent.
Methods
- Theoretical study of an \(L\)-qubit quantum register coupled to a thermal reservoir, using a decoherence model with Hamiltonian \(H\) containing free qubit, free environment, and qubit-environment interaction terms.
- Single-qubit decoherence is treated semiclassically via stochastic fluctuating fields and numerically simulated, then fully quantized using field modes, displacement operators, and exact expressions for the reduced density-matrix coherence.
- The analysis is extended to two qubits and then to \(L\) qubits, comparing independent reservoirs with shared reservoirs and position-dependent couplings; complexity consequences are assessed through randomized-algorithm error amplification.
Limitations
- The paper states that its model does not describe \(T_1\) processes: populations are unaffected and there is no energy exchange between qubit and environment; it notes that at longer timescales \(T_1\) processes would remove residual coherence in the three-dimensional case.
- The paper states that the semiclassical approach excludes quantum features, most notably qubit-environment entanglement, and that a fully quantum approach is needed for thermal and vacuum fluctuations.
- Evident from the text: the conclusion that any thermal-reservoir decoherence model leads to the same complexity conclusion is presented as a plausibility argument, not a proof; the analysis assumes specific coupling/density-of-states forms and does not include direct two-qubit interactions.
Model-written summary. Check it against the paper before relying on it.
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- Findingnone given§ 1 Introduction · p. 2
The error rate due to interaction with the environment, a thermal reservoir, grows as an exponential function of L regardless of the type and strength of the computer-environment coupling.
“This is because the error rate, which is due to the interaction with the environment (a thermal reservoir), grows as an exponential function of L regardless the type and the strength of the computer - environment coupling.”
- Also stated in: § 2 Errors in Randomized Algorithms · p. 3 · boosted
“Indeed, our results show that the computer-environment interaction leads to this unwelcome exponential increase of the error rate.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The environment or reservoir is modeled as a thermal reservoir.
- Also stated in: § 2 Errors in Randomized Algorithms · p. 3 · boosted
- Findingcitation§ 1 Introduction · p. 2
In the limit of a short correlation length, i.e. coupling to independent reservoirs, the results coincide with those by Unruh (1995).
“In the limit of a short correlation length (i.e. coupling to the independent reservoirs) our results coincide with those by Unruh (1995).”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Unruh 1995
Not checked in the free run.
- Findingreasoning§ 4.2 Fully Quantised Model · p. 10
Thermal fluctuations can affect the qubit dynamics only for times longer than the characteristic thermal frequency T.
“Let us first note that thermal fluctuations can affect the qubit dynamics only for times longer than the characteristic thermal frequency T .”
Quote in the paperSubstantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
- Findingreasoning§ 4.2 Fully Quantised Model · p. 11
In the one-dimensional field case in the low-temperature limit, Γ(t) ∼ Tt for t > T^-1.
“Expression (23) reduces to Γ( t ) ∼ ω 2 c t 2 for t < ω -1 c , Γ( t ) ∼ 2 ln ω c t for ω -1 c < t < T -1 , and Γ( t ) ∼ Tt for t > T -1 .”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Findingreasoning§ 5 Decoherence of Quantum Registers · p. 16
In the independent reservoir case, in the worst case the density matrix elements decay as exp{-L Γ(t)}.
“In the independent reservoir case therefore in the worst case the density matrix elements decay as exp {-L Γ( t ) } .”
Quote in the paperSubstantial claim
- Findingreasoning§ 4.2 Fully Quantised Model · p. 12
As η decreases, the decay onset moves towards larger t.
“As η decreases, the decay onset moves towards larger t .”
Quote in the paperNot a substantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Findingreasoning§ 4.2 Fully Quantised Model · p. 9
The off-diagonal elements of the reduced density matrix of the qubit decay because the overlap between the different field states with which the qubit becomes entangled diminishes in time.
“Indeed the off-diagonal elements of the reduced density matrix of the qubit decay due to the fact that the overlap between the different field states with which the qubit becomes entangled diminishes in time.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
- Findingnone given§ 4.1 Semiclassical Picture · p. 7
In the time evolution of a single member of the ensemble, the amplitude for the oscillations in s_x and s_y remains constant.
“It should be noted that in the time evolution of a single member of the ensemble the amplitude for the oscillations in s x and s y remains constant.”
Quote in the paperSubstantial claim - Findingown data§ 4.2 Fully Quantised Model · p. 12
For large η, there is an intermediate region where vacuum fluctuations dominate and the decay is roughly linear before the thermal regime takes over.
“However, for large η there is an intermediate region where the vacuum fluctuations dominate and the decay is roughly linear before the thermal regime takes over.”
Quote in the paperNot a substantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Findingreasoning§ 4.2 Fully Quantised Model · p. 11
In the one-dimensional field case in the low-temperature limit, Γ(t) ∼ 2 ln ωc t for ωc^-1 < t < T^-1.
“Expression (23) reduces to Γ( t ) ∼ ω 2 c t 2 for t < ω -1 c , Γ( t ) ∼ 2 ln ω c t for ω -1 c < t < T -1 , and Γ( t ) ∼ Tt for t > T -1 .”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Findingreasoning§ 4.2 Fully Quantised Model · p. 11
In the one-dimensional field case in the low-temperature limit, Γ(t) ∼ ωc^2 t^2 for t < ωc^-1.
“Expression (23) reduces to Γ( t ) ∼ ω 2 c t 2 for t < ω -1 c , Γ( t ) ∼ 2 ln ω c t for ω -1 c < t < T -1 , and Γ( t ) ∼ Tt for t > T -1 .”
Quote in the paperNot a substantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Findingreasoning§ 4.1 Semiclassical Picture · p. 7
There is no real decay of the coherences present in a single ensemble member, but merely a shift in phase.
“Hence there is no real decay of the coherences present in a single ensemble member, but merely a shift in phase.”
Quote in the paperSubstantial claim - Findingreasoning§ 4.2 Fully Quantised Model · p. 10
In Eq. (21), effects due to thermal noise can be separated from those due to purely quantum fluctuations.
“In Eq.(21) we can separate the effects due to thermal noise from the one due to purely quantum fluctuations.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
- Findingown data§ 4.2 Fully Quantised Model · p. 12
For η ∼ 1, the decay is dominated by thermal fluctuations of the reservoir and is therefore exponential.
“For η ∼ 1 this decay is dominated by the thermal fluctuations of the reservoir and it is therefore exponential.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Findingown data§ 4.2 Fully Quantised Model · p. 12
In the one-dimensional field case, the coherence always decays to zero.
“In (a) the coherence always decays to zero.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Findingreasoning§ 4.2 Fully Quantised Model · p. 9
U(t) induces entanglement between qubit states and field states.
“The above discussion makes it clear that U(t) induces entanglement between qubit states and field states.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
- Findingown data§ 4.1 Semiclassical Picture · p. 7
The semiclassical simulations for a single quantum bit demonstrate how the full statistical result is obtained by adding the individual members of the ensemble.
“Our semiclassical simulations for a single quantum bit demonstrate how the full statistical result is obtained by adding the individual members of the ensemble.”
Quote in the paperSubstantial claim - Findingreasoning§ 4.2 Fully Quantised Model · p. 12
In the three-dimensional field case, the decoherence saturates to a value determined by η.
“In (b) the system shows a very different behaviour: the decoherence saturates to a value determined by η .”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Findingnone given§ 4.2 Fully Quantised Model · p. 12
In all cases there is a short t region where almost no decay is present.
“In all cases there is a short t region where almost no decay is present.”
Quote in the paperNot a substantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Interpretationreasoning§ 1 Introduction · p. 2
To obtain factors of N with a prescribed probability of success, one has to repeat the computation k times, where k grows exponentially with L.
“Consequently, in order to obtain factors of N with a prescribed probability of success one has to repeat the computation k times, where k grows exponentially with L .”
Quote in the paperSubstantial claim- The authors state this as an assumption: In the amplification argument, the error rate ǫ is independent of the input.
- Interpretationreasoning§ 1 Introduction · p. 2
Shor's algorithm must be modified, specifically supplemented with an exponentially efficient quantum error correction, in order to be efficient from the computational complexity perspective.
“This paper shows that Shor's algorithm, in order to be efficient from the computational complexity perspective, must be modified (i.e. supplemented with an exponentially efficient quantum error correction).”
Quote in the paperSubstantial claim- The authors state this as an assumption: The environment or reservoir is modeled as a thermal reservoir.
- Interpretationreasoning§ 1 Introduction · p. 2
The exponential error rate is not related to technological limitations but is a consequence of the computational complexity requirements imposed on any realistic model of computation.
“Let us stress that this observation is not related to any technological limitations but is a consequence of the computational complexity requirements imposed on any realistic model of computation.”
Quote in the paperSubstantial claim - Premisecitation§ 1 Introduction · p. 1
Quantum computers can efficiently solve some problems that are believed to be intractable on any classical computer.
“As the result quantum computers can efficiently solve some problems which are believed to be intractable on any classical computer (Deutsch 1985, Deutsch and Jozsa 1992, Bernstein and Vazirani 1993, Simon 1994, Shor 1994).”
Quote in the paperNot a substantial claim0 of 3 sources checked- cites Deutsch 1985
Not checked in the free run.
- cites Deutsch and Jozsa 1992
Not checked in the free run.
- cites Bernstein and Vazirani 1993
Not checked in the free run.
- Premisecitation§ 1 Introduction · p. 1
Factoring an L-digit number N on any classical computer requires an execution time that grows exponentially with L, approximately exp L^{1/3} for the best known algorithms such as the Number Field Sieve.
“The most striking example is the factoring problem: to factor a number N of L digits on any classical computer requires an execution time that grows exponentially with L (approximately exp L 1 / 3 for the best known algorithms such as the Number Field Sieve (Lenstra et al. 1990)).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Lenstra et al. 1990
Not checked in the free run.
- Interpretationnone given§ 6 Discussion and Conclusions · p. 17
As long as the reservoir is in a thermal state, any model of decoherence will lead to the same conclusions.
“We believe that as long as the reservoir is in a thermal state, any model of decoherence will lead to the same conclusions.”
Quote in the paperNot a substantial claim - Interpretationnone given§ 5 Decoherence of Quantum Registers · p. 13
U(t) acts as a displacement operator on the field with a displacement amplitude depending on the logic value of both qubits of the register.
“It should be noted however that U ( t ) acts as a displacement operator on the field with a displacement amplitude depending on the logic value of both qubits of the register.”
Quote in the paperSubstantial claim - Interpretationnone given§ 6 Discussion and Conclusions · p. 16
If one can support quantum computation with some selected coherences, then a large coherence length can be utilised.
“However, if one can support quantum computation with some selected coherences then a large coherence length can be utilised.”
Quote in the paperSubstantial claim - Interpretationnone given§ 5 Decoherence of Quantum Registers · p. 15
The set of states robust against the dephasing action of the environment depends on the specific form of qubit-environment coupling.
“In general the set of states which are robust against the dephasing action of the environment will depend on the specific form of qubit-environment coupling.”
Quote in the paperNot a substantial claim - Interpretationown data§ 6 Discussion and Conclusions · p. 16
Reservoirs with large coherence lengths introduce only asymmetry in the decoherence rates of various off-diagonal elements of the density matrix with the same Hamming distance between its indices.
“Reservoirs with large coherence lengths introduce only asymmetry in the decoherence rates of various off-diagonal elements of the density matrix with the same Hamming distance between its indices (compare Eq. (41) with Eq. (42)).”
Quote in the paperSubstantial claim - Interpretationnone given§ 5 Decoherence of Quantum Registers · p. 14
In a semiclassical picture, if the distance between two qubits is smaller than the correlation length of the bath, they feel the same fluctuation.
“In a semiclassical picture if the distance between the two qubits is smaller than the correlation length of the bath they 'feel' the same fluctuation.”
Quote in the paperSubstantial claim - Interpretationreasoning§ 6 Discussion and Conclusions · p. 18
Even if quantum algorithms for factorisation are not efficient from the complexity theory point of view, they may still permit factoring of numbers which are much bigger than those that can be factored using classical algorithms.
“Even if quantum algorithms for factorisation are not efficient from the complexity theory point of view, they may still permit factoring of numbers which are much bigger than those that can be factored using classical algorithms.”
Quote in the paperSubstantial claim - Interpretationreasoning§ 4.2 Fully Quantised Model · p. 9
In the fully quantised description adopted, this entanglement is responsible for the decoherence process.
“It is precisely this entanglement which, in the fully quantised description we have adopted, is responsible for the decoherence process.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
- Interpretationnone given§ 6 Discussion and Conclusions · p. 17
In the independent reservoirs limit the fastest decoherence is proportional to exp(-L Γ(t)) regardless of the functional dependence Γ(t).
“In the independent reservoirs limit the fastest decoherence is proportional to exp( -L Γ( t )) regardles the functional dependence Γ( t ).”
Quote in the paperNot a substantial claim - Interpretationreasoning§ 4.1 Semiclassical Picture · p. 7
When considering thermal and vacuum fluctuations, the semiclassical view of the qubit-reservoir interaction is not adequate, and a fully quantum approach is required.
“However, when we consider the effect of the thermal and vacuum fluctuations, such a semiclassical view on the interaction of the qubit with the reservoir is not adequate, and one has to apply a fully quantum approach, which we shall present next.”
Quote in the paperSubstantial claim - Interpretationnone given§ 4.2 Fully Quantised Model · p. 12
At longer timescales, when T1 processes involving exchange of energy between qubit and environment take place, the residual coherence will disappear.
“It should however be pointed out that at longer timescales, when T 1 processes involving exchange of energy between qubit and environment take place, this residual coherence will disappear.”
Quote in the paperNot a substantial claim - Interpretationreasoning§ 5 Decoherence of Quantum Registers · p. 15
Superdecoherence versus subdecoherence is not the same as superradiance versus subradiance.
“In order to avoid possible confusions it must be stressed that the phenomenon of superdecoherence vs. subdecoherence that we have just described is not the same as the process of superradiance vs. subradiance more commonly encountered in litterature (see e.g Allen & Eberly 1975).”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Allen & Eberly 1975
Not checked in the free run.
- Interpretationreasoning§ 4.2 Fully Quantised Model · p. 11
For t < ωc^-1, the fluctuations are ineffective in the decoherence process (the quiet regime).
“- a 'quiet' regime, for t < ω -1 c , where the fluctuations are ineffective in the decoherence process”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Premisecitation§ 3 Decoherence · p. 4
In some cases populations can be more fragile than coherences.
“However, it should be mentioned that in some cases populations can be more fragile than coherences.”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Kimble & Mabuchi 1995
Not checked in the free run.
- Interpretationreasoning§ 6 Discussion and Conclusions · p. 17
The noiseless encoding does not pose complexity problems since to encode L qubits one needs simply 2L qubits.
“This 'noiseless' encoding does not pose complexity problems since to encode L qubits we need simply 2L qubits.”
Quote in the paperNot a substantial claim - Premisecitation§ 4.2 Fully Quantised Model · p. 8
Hamiltonian (12) is equivalent to the one introduced in connection with the tunnelling problem and used to model decoherence in quantum computers.
“Hamiltonian (12) is equivalent to the one introduced in connection with the tunnelling problem (Leggett et al. 1987, Gardiner 1991) and used to model decoherence in quantum computers (Unruh 1995).”
Quote in the paperNot a substantial claim0 of 3 sources checked- cites Leggett et al. 1987
Not checked in the free run.
- cites Gardiner 1991
Not checked in the free run.
- cites Unruh 1995
Not checked in the free run.
- Interpretationreasoning§ 6 Discussion and Conclusions · p. 18
From the requirement equation it follows that the size L is bounded by the equation given in the paper.
“From the equation above follows that the size L is bounded by”
Quote in the paperSubstantial claim - Interpretationreasoning§ 6 Discussion and Conclusions · p. 18
Technological progress in isolating quantum computers from the environment and suppressing decoherence will increase the size of the biggest number that can be factored by such computers.
“Technological progress in isolating quantum computers from the environment and suppressing decoherence will increase the size of the biggest number that can be factored by such computers.”
Quote in the paperSubstantial claim - Interpretationreasoning§ 4.2 Fully Quantised Model · p. 11
For t > T^-1, thermal fluctuations play the major role in eroding qubit coherence (the thermal regime).
“- a thermal regime, for t > T -1 , where thermal fluctuations play the major role in eroding the qubit coherence.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Interpretationnone given§ 5 Decoherence of Quantum Registers · p. 13
The unitary evolution produces entanglement between register states and environment states.
“Here again the unitary evolution produces entanglement between register states and environment states.”
Quote in the paperSubstantial claim - Interpretationreasoning§ 4.2 Fully Quantised Model · p. 11
For ωc^-1 < t < T^-1, the main cause of coherence loss is quantum vacuum fluctuations (the quantum regime).
“- a quantum regime, for ω -1 c < t < T -1 , where the main cause of coherence loss are the quantum vacuum fluctuations”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Interpretationreasoning§ 6 Discussion and Conclusions · p. 16
Assuming all coherences are equally important and choosing the worst case for complexity analysis, reservoirs with a large coherence length can be more of a hindrance than a help.
“Assuming that all coherences are equally important and that for the complexity analysis we choose the worst case (i.e. the off-diagonal element which decays at the fastest rate) then we are forced to conclude that reservoirs with a large coherence length can be more of a hindrance than a help.”
Quote in the paperSubstantial claim- The authors state this as an assumption: All coherences are equally important.
- The authors state this as an assumption: For the complexity analysis, the worst case is chosen, i.e. the off-diagonal element which decays at the fastest rate.
- Interpretationnone given§ 2 Errors in Randomized Algorithms · p. 3
In a realistic scenario, the probability of error ǫ(L) also has contributions due to the computer-environment coupling.
“In a realistic scenario the probability of error ǫ ( L ) has also contributions which are due to the computer - environment coupling.”
Quote in the paperNot a substantial claim - Interpretationreasoning§ 5 Decoherence of Quantum Registers · p. 14
The same phase shifts produce a reduction or enhancement of the decay rates depending on whether the phase shifts on the two qubits add or subtract.
“This produces a reduction - or an enhancement - of the decay rates depending on whether the phase shifts on the two qubits add or subtract.”
Quote in the paperSubstantial claim - Interpretationreasoning§ 2 Errors in Randomized Algorithms · p. 3
From the computational complexity point of view, it is irrelevant how weak the coupling is for a fixed L; its dependence on L matters.
“From the computational complexity point of view it is irrelevant how weak this coupling is for a fixed L , it is its dependence on L which matters.”
Quote in the paperSubstantial claim - Interpretationnone given§ 4.2 Fully Quantised Model · p. 12
The cutoff frequency ωc determines the extent of the quiet regime.
“This is because the cut-off frequency ω c determines the extent of the 'quiet' regime.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Interpretationreasoning§ 5 Decoherence of Quantum Registers · p. 14
Random phase shifts due to bath fluctuations of wavelength shorter than the inverse of the qubit separation are the same for both qubits.
“More generally the random phase-shifts due to the bath fluctuations of wavelength shorter than the inverse of the qubit separation are the same for both qubits.”
Quote in the paperSubstantial claim - Interpretationreasoning§ 4.2 Fully Quantised Model · p. 10
Quantum vacuum fluctuations contribute to the dephasing process only for times t > ωc^-1.
“It is evident that the so-called quantum vacuum fluctuations will contribute to the dephasing process only for times t > ω -1 c .”
Quote in the paperSubstantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
- Interpretationreasoning§ 4.2 Fully Quantised Model · p. 10
This formal separation allows identification of the existence of various timescales in the decoherence process.
“This formal separation allows us to identify the existence of various timescales in the decoherence process (see also Unruh 1995).”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Unruh 1995
Not checked in the free run.
- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
- May combine two claims. 25
- Classified as probably not a substantial claim. 7
May combine two claims.
Findingreasoning§ 4.2 Fully Quantised Model · p. 8Since [σ_z, H] = 0, the populations of the qubit density matrix are not affected by the environment, and the coupling with the environment in the model simply erodes quantum coherence.
“Since [ σ z , H ] = 0 the populations of the qubit density matrix, ρ ( t ) = Tr R ̺ ( t ) are not affected by the environment and the coupling with the environment in our model simply erodes quantum coherence.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
May combine two claims.
Findingnone given§ 6 Discussion and Conclusions · p. 17States for which ia = ja and ib = jb do not get entangled with the reservoir and therefore maintain their coherence.
“It is obvious that states for which i a = j a and i b = j b do not get entangled with the reservoir and therefore they mantain their coherence.”
Quote in the paperSubstantial claimMay combine two claims.
Findingreasoning§ 5 Decoherence of Quantum Registers · p. 14Matrix elements of the form ρ_{ia ia, ib jb} show no collective decay; for ib = jb they are constant, while for ib ≠ jb they decay as e^{-Γ(t)}.
“Matrix elements of the form ρ i a i a ,i b j b show no collective decay, in fact”
- Also stated in: § 5 Decoherence of Quantum Registers · p. 14 · plain
“which for i b = j b reduces to ρ i a i a ,i b i b ( t ) = ρ i a i a ,i b i b (0) while for i b = j b we have ρ i a i a ,i b j b ( t ) = ρ i a i a ,i b j b (0) e -Γ( t ) .”
Quote in the paperSubstantial claim- Also stated in: § 5 Decoherence of Quantum Registers · p. 14 · plain
May combine two claims.
Findingreasoning§ 4.2 Fully Quantised Model · p. 8In the fully quantised model there is no exchange of energy between qubit and environment and consequently no T1 type of decay takes place.
“This means that also in our fully quantised model there is no exchange of energy between qubit and environment and consequently no T 1 type of decay takes place.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
Probably not a substantial claim: may not pass S1 (asserted), S3 (load-bearing), S4 (based).
Findingnone given§ 5 Decoherence of Quantum Registers · p. 14Collective decay is apparent in matrix elements of the form ρ_{ia ja, ib jb}(t) with ia ≠ ja and ib ≠ jb.
“Collective decay is instead apparent in the matrix elements of the form ρ i a j a ,i b j b ( t ) with i a = j a , i b = j b :”
Quote in the paperNot a substantial claimProbably not a substantial claim: may not pass S3 (load-bearing).
Findingown data§ 4.2 Fully Quantised Model · p. 11For a single qubit in a one-dimensional field with ωc/T = 100, the three decoherence regimes can be identified in Fig. 4.
“These three regimes can be easily identified in Fig.(4), which shows the decoherence of a single qubit induced by a one-dimensional field for the particular choice ω c /T = 100.”
Quote in the paperNot a substantial claim- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
May combine two claims.
Interpretationreasoning§ 1 Introduction · p. 2Technological progress in suppressing dissipation will help to factor bigger and bigger numbers but will not change the exponential increase of the error rate.
“Any technological progress in suppressing dissipation will, of course, help to factor bigger and bigger numbers but will not change an exponential increase of the error rate.”
Quote in the paperSubstantial claimProbably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.
Interpretationnone given§ 1 Introduction · p. 2The analysis covers cases where qubits interact with a single reservoir and the distance between the qubits is comparable with the correlation length of the reservoir.
“Our analysis is quite general and covers cases where qubits interact with a single reservoir and the distance between the qubits is comparable with the correlation length of the reservoir.”
Quote in the paperNot a substantial claimProbably not a substantial claim: may not pass S3 (load-bearing).
Premisecitation§ 1 Introduction · p. 1Shor has shown that quantum computers require an execution time that grows only as a polynomial function of L, approximately L^2.
“In contrast Shor (1994) has shown that quantum computers require an execution time that grows only as a polynomial function of L ( ≈ L 2 ).”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Shor 1994
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.
Study design and conductnone given§ 4.2 Fully Quantised Model · p. 10Equation (19) is exact; no approximation has been made to obtain it.
“Equation (19) is exact , i.e. no approximation has been made to obtain it.”
Quote in the paperNot a substantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
May combine two claims.
Interpretationreasoning§ 4.2 Fully Quantised Model · p. 8The absence of T1 decay is not a limitation of the model because the model describes adequately the decoherence mechanism, which usually takes place on a shorter timescale than energy dissipation.
“This is not a limitation of the model since, as will became clear in the following, it describes adequately the decoherence mechanism which usually takes place on a shorter timescale than the energy dissipation.”
Quote in the paperSubstantial claimMay combine two claims.
Study design and conductnone given§ 4.2 Fully Quantised Model · p. 8The model is exactly soluble and allows a clear analysis of the mechanism of entanglement between qubit and environment.
“Furthermore this model is exactly soluble and allows a clear analysis of the mechanism of entanglement between qubit and environment which is believed to be at the core of most decoherence processes.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The dynamics of a single qubit plus environment is described by Hamiltonian (12).
May combine two claims.
Premisenone given§ 3 Decoherence · p. 4When a reservoir is in a thermal state, coherences change on a much shorter time scale than populations, and decoherence is responsible for destroying quantum parallelism.
“When a reservoir is in a thermal state the coherences change on a much shorter time scale than populations and it is decoherence which is responsible for destroying quantum parallelism.”
Quote in the paperSubstantial claim- The authors state this as an assumption: The environment or reservoir is modeled as a thermal reservoir.
May combine two claims.
Interpretationreasoning§ 5 Decoherence of Quantum Registers · p. 13In the limit ra ≈ rb, the subdecoherent state |Φ(-)> has qubits not entangled with the field, while the superdecoherent state |Φ(+)> has qubits collectively entangled with a coherent state whose amplitude is twice that of the single-qubit case.
“In particular, in the limit r a ≈ r b we have ξ a k ∼ ξ b k and therefore in the 'subdecoherent' state | Φ ( -) 〉 the qubits are not entangled with the field, while in the 'superdecoherent' state | Φ (+) 〉 the qubits are collectively entangled with a coherent state with an amplitude which is twice that of the single qubit case.”
Quote in the paperSubstantial claimMay combine two claims.
Interpretationnone given§ 5 Decoherence of Quantum Registers · p. 15Superdecoherence requires the qubit to be in a region of space smaller than the coherence length of the environment, a much more stringent condition than that needed for superradiance, namely that the atoms are at a distance smaller than the wavelength of the resonant field modes.
“Indeed the conditions for the two processes to occur are quite different: superdecoherence requires the qubit to be in a region of space smaller than the coherence length of the environment, which is a much more stringent condition that the one needed for superradiance to take place, i.e. that the atoms are at a distance smaller than the wavelength of the resonant field modes.”
Quote in the paperSubstantial claimMay combine two claims.
Interpretationreasoning§ 6 Discussion and Conclusions · p. 18The ratio t/τ, which depends on the technology employed, determines the limits of the algorithm, and it is unrealistic to assume that this ratio can be made infinite.
“Thus the ratio t/τ , which depends on the technology employed, determines the limits of the algorithm and it is unrealistic to assume that this ratio can be made infinite.”
Quote in the paperSubstantial claimMay combine two claims.
Interpretationnone given§ 6 Discussion and Conclusions · p. 17For reservoirs with a large coherent length the fastest decoherence will be proportional to exp(-poly(L)Γ'(t)), leading to the same unwelcome exponential increase of the error rate.
“For reservoirs with a large coherent length the fastest decoherence will be proportional to exp( -poly ( L )Γ ′ ( t )) leading to the same unwelcome exponential increase of the error rate.”
Quote in the paperNot a substantial claimMay combine two claims.
Premisecitation§ 2 Errors in Randomized Algorithms · p. 3Shor's quantum factoring algorithm in a single run provides factors of N with probability 1 - ǫ(L), and the probability of error ǫ(L) grows only as a polynomial function of L.
“Indeed, Shor's quantum factoring algorithm is of this type; in a single run it provides factors of N with probability 1 -ǫ ( L ) and the probability of error ǫ ( L ) grows only as a polynomial function of L .”
Quote in the paperSubstantial claim0 of 1 sources checked- cites Shor 1994
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.
Interpretationnone given§ 5 Decoherence of Quantum Registers · p. 15Superdecoherence is due to collective entanglement between qubits and environment with no exchange of energy, while superradiance is a process of collective radiation by a group of closely spaced atoms.
“While superdecoherence is due to collective entanglement between qubits and environment with no exchange of energy, superradiance is a process of collective radiation by a group of closely spaced atoms.”
Quote in the paperNot a substantial claimMay combine two claims.
Interpretationown data§ 6 Discussion and Conclusions · p. 16Decoherence destroys quantum interference and entanglement in quantum computers, thus decreasing the probability of successful computation exponentially with the input size L.
“Our analysis shows that decoherence destroys quantum interference and entanglement in quantum computers, thus decreasing the probability of successful computation exponentially with the input size L .”
Quote in the paperSubstantial claimProbably not a substantial claim: may not pass S3 (load-bearing), S4 (based).
Interpretationnone given§ 5 Decoherence of Quantum Registers · p. 15Some superradiant states are subdecoherent.
“Furthermore, it must be noted that some superradiant states are subdecoherent.”
Quote in the paperNot a substantial claimMay combine two claims.
Interpretationreasoning§ 6 Discussion and Conclusions · p. 16In the independent reservoirs limit the fastest decoherence is proportional to exp(-L Γ(t)), whereas for reservoirs with a large coherence length there are off-diagonal elements which decay as exp(-poly(L)Γ(t)).
“In the independent reservoirs limit the fastest decoherence is proportional to exp( -L Γ( t )) whereas for the reservoirs with a large coherence length there are off-diagonal elements which decay as exp( -poly ( L )Γ( t )).”
Quote in the paperSubstantial claimMay combine two claims.
Premisenone given§ 3 Decoherence · p. 4When a quantum computer is in contact with an environment modeled as a thermal reservoir, the resulting dissipation destroys coherences and changes populations.
“When a quantum computer is in contact with an environment, here modeled as a thermal reservoir, the resulting dissipation destroys the coherences and changes the populations (the diagonal elements).”
Quote in the paperSubstantial claim- The authors state this as an assumption: The environment or reservoir is modeled as a thermal reservoir.
May combine two claims.
Interpretationreasoning§ 2 Errors in Randomized Algorithms · p. 3If the interaction with the environment causes an exponential increase of the error rate, ǫ(L) = 1 - A exp(-αL), then k(L) ≈ exp(αL) and the randomized algorithm cannot be regarded as efficient any more.
“If the interaction with the environment causes an exponential increase of the error rate i.e. ǫ ( L ) = 1 -A exp( -αL ), where A and α are positive constants, then k ( L ) ≈ exp( αL ) and the randomized algorithm cannot be regarded as efficient any more!”
Quote in the paperSubstantial claim- The authors state this as an assumption: In the amplification argument, the error rate ǫ is independent of the input.
May combine two claims.
Premisenone given§ 2 Errors in Randomized Algorithms · p. 3Errors in Shor's algorithm are of purely mathematical origin and correspond to random decisions that rely on the prime number theorem.
“However, errors in Shor's algorithm are of purely 'mathematical' origin and correspond to the random decisions which rely on the prime number theorem (i.e. on the distribution of prime numbers).”
Quote in the paperNot a substantial claimMay combine two claims.
Interpretationown data§ 2 Errors in Randomized Algorithms · p. 3Shor's algorithm, as it stands at the moment, is not robust to environmental noise and requires modifications.
“Shor's algorithm, as it stands at the moment, is not robust to the environmental noise and requires modifications.”
Quote in the paperSubstantial claimMay combine two claims.
Interpretationcitation§ 4.2 Fully Quantised Model · p. 12The difference in behaviour between one- and three-dimensional fields is reminiscent of dimensional differences in models of phase transitions and is due to the suppressed influence of low-frequency fluctuations in three dimensions.
“This difference in behaviour is reminiscent of the dimensional differences observed in various models of phase transitions (Ma 1976) and is due to the suppressed influence of low frequency fluctuations in three dimensions.”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Ma 1976
Not checked in the free run.
- The authors state this as an assumption: The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
May combine two claims.
Interpretationreasoning§ 6 Discussion and Conclusions · p. 16For reservoirs with a large coherence length, some coherences are not affected by decoherence while some others are destroyed more rapidly.
“In this case while some coherences are not affected by the decoherence some others are destroyed more rapidly.”
Quote in the paperSubstantial claimMay combine two claims.
Interpretationreasoning§ 1 Introduction · p. 2Any model of dissipation will lead to errors that grow exponentially with L, as supported by a plausibility argument.
“Also we provide a plausibility argument that any model of dissipation will lead to errors that grow exponentially with L .”
Quote in the paperNot a substantial claim
The authors state this as an assumption.
The environment or reservoir is modeled as a thermal reservoir.
“When a quantum computer is in contact with an environment, here modeled as a thermal reservoir, the resulting dissipation destroys the coherences and changes the populations (the diagonal elements).”
The authors state this as an assumption.
The quantised environment is modeled as a continuum of field modes.
“The quantised environment is modeled as a continuum of field modes.”
The authors state this as an assumption.
At time t = 0 the combined qubit-environment state is assumed to have a specified product form involving the thermal density matrix of each field mode.
“The state of the combined system i.e. qubit + environment is described by a density operator ̺ ( t ) which at time t = 0 is assumed to be in the form”
The authors state this as an assumption.
The dynamics of a single qubit plus environment is described by Hamiltonian (12).
“We assume that the dynamics of a single qubit + environment is described by the following Hamiltonian”
The authors state this as an assumption.
In the amplification argument, the error rate ǫ is independent of the input.
“Assuming that ǫ is independent of the input the success probability of the algorithm can be amplified (boosted) arbitrarily close to 1 by repeating the computation several times.”
The authors state this as an assumption.
The frequency dependence of the density of states and coupling is assumed to be (dk/dω)G(ω)|g(ω)|^2 ∝ ω^n e^{-ω/ωc}, with n depending on the number of dimensions.
“Therefore we assume ( dk/dω ) G ( ω ) | g ( ω ) | 2 ∝ ω n e -ω/ω c .”
The authors state this as an assumption.
All coherences are equally important.
“Assuming that all coherences are equally important and that for the complexity analysis we choose the worst case (i.e. the off-diagonal element which decays at the fastest rate)”
The authors state this as an assumption.
For the complexity analysis, the worst case is chosen, i.e. the off-diagonal element which decays at the fastest rate.
“Assuming that all coherences are equally important and that for the complexity analysis we choose the worst case (i.e. the off-diagonal element which decays at the fastest rate)”
The authors state this as an assumption.
All modes of the field are in thermal equilibrium.
“We can formulate in more rigorous terms the analysis we have just sketched above by taking into account all the modes of the field in thermal equilibrium.”
The authors state this as an assumption.
The position-dependent couplings have the form g_a k = g_k e^{i k·r_a} and g_b k = g_k e^{i k·r_b}.
“If we suppose g a k = g k e i k · r a and g b k = g k e i k · r b , then, in the continuum limit, the collective decay rates are”
The authors state this as an assumption.
There is no direct interaction between the two qubits.
“In (25) we are not considering any direct interaction between the two qubits which would be necessary for the conditional dynamics (Barenco et al 1995).”
Claims other papers make about this one
Each is checked against this paper’s own text: does it say what the citing paper says it does?
Noise, decoherence, and manufacturing problems make constructing large-scale quantum computers difficult.
“Noise, decoherence, and manufacturing problems make constructing large-scale quantum computers difficult 9-13 .”
From Experimental realization of a quantum algorithm (1998) · cites this paper as [13]
Self-check
Percentages, sums and reported differences re-computed from the paper’s own numbers.
Each contradiction between two passages listed here was confirmed by an independent check.
Conflicting year for the Deutsch and Jozsa citation
MinorThe text cites Deutsch and Jozsa as a 1992 work, but the corresponding reference lists 1989.
“(Deutsch 1985, Deutsch and Jozsa 1992, Bernstein and Vazirani 1993, Simon 1994, Shor 1994)”
“Deutsch, D. & Jozsa, R. 1989 Proc. Roy. Soc. A439 , pp. 553-558.”
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- Authors
- Palma, G. M., Suominen, K., & EKERT, A. K.
- Affiliations
- University of Oxford (GB)
- University of Helsinki (FI)
- Venue
- Proceedings of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences
- Publisher
- The Royal Society
- Published
- 1996-12-31
- Volume
- 452
- Issue
- 1946
- Pages
- 567-584
- ISSN
- 1364-5021 (ISSN-L)
- 1471-2946
- OpenAlex
- W2157754228
- Cited by
- 899
- References
- 15
- Citations by year
- 2026: 10 · 2025: 25 · 2024: 22 · 2023: 26 · 2022: 29 · 2021: 22 · 2020: 23 · 2019: 32 · 2018: 34 · 2017: 35 · 2016: 38 · 2015: 31 · 2014: 29 · 2013: 37 · 2012: 25
- FWCI
- 26.76
- Type
- journal article
- Language
- en
- Open access
- green
- arxiv.org
- Locations
- Proceedings of the Royal Society A Mathematical Physical and Engineering Sciences (publishedVersion)
- arXiv (Cornell University) (submittedVersion)
- CERN Document Server (European Organization for Nuclear Research) (submittedVersion)
- arXiv (Cornell University)
- Topics
- Physical Sciences › Computer Science › Artificial Intelligence › Quantum Computing Algorithms and Architecture
- Physical Sciences › Computer Science › Artificial Intelligence › Quantum Information and Cryptography
- Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum Mechanics and Applications
- Keywords
- quantum computing, energy dissipation, decoherence, quantum registers, environment coupling, computational complexity, quantum factoring, Shor's algorithm, algorithm efficiency
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