Resource-efficient simulation of noisy quantum circuits and application to network-enabled QRAM optimization Luís Bugalho1 2 3 4Emmanuel Zambrini Cruzeiro5Kevin C.

2025-04-29 0 0 5.57MB 28 页 10玖币
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Resource-efficient simulation of noisy quantum circuits and application to
network-enabled QRAM optimization
Luís Bugalho,1, 2, 3, 4 Emmanuel Zambrini Cruzeiro,5Kevin C.
Chen,6, 7 Wenhan Dai,7, 8 Dirk Englund,6, 7 and Yasser Omar1, 2, 3
1Instituto Superior Técnico, Universidade de Lisboa, Portugal
2Physics of Information and Quantum Technologies Group,
Centro de Física e Engenharia de Materiais Avançados (CeFEMA), Portugal
3PQI – Portuguese Quantum Institute, Portugal
4Sorbonne Université, CNRS, LIP6, 4 Place Jussieu, Paris F-75005, France
5Instituto de Telecomunicações, Lisbon, 1049-001, Portugal
6Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
7Department of Electrical Engineering and Computer Science,
Massachusetts Institute of Technology, Cambridge, MA 02139, USA
8Department of Computer Science, University of Massachusetts, Amherst, Massachusetts 01003, USA
Giovannetti, Lloyd, and Maccone (2008) proposed a quantum random access memory (QRAM)
architecture to retrieve arbitrary superpositions of N(quantum) memory cells via O(log(N)) quan-
tum switches and O(log(N)) address qubits. Towards physical QRAM implementations, Chen et
al. (2021) recently showed that QRAM maps natively onto optically connected quantum networks
with O(log(N)) overhead and built-in error detection. However, modeling QRAM on large networks
has been stymied by exponentially rising classical compute requirements. Here, we address this
bottleneck by: (i) introducing a resource-efficient method for simulating large-scale noisy entangle-
ment, allowing us to evaluate hundreds and even thousands of qubits under various noise channels;
and (ii) analyzing Chen et al.’s network-based QRAM as an application at the scale of quantum
data centers or near-term quantum internet; and (iii) introducing a modified network-based QRAM
architecture to improve quantum fidelity and access rate. We conclude that network-based QRAM
could be built with existing or near-term technologies leveraging photonic integrated circuits and
atomic or atom-like quantum memories.
Introduction
A quantum random access memory (QRAM) is an es-
sential computational primitive for many quantum algo-
rithms. The ability to perform a QRAM query in log(N)
time steps, where N= 2nis the number of memory
cells, implies polynomial speed-ups for applications such
as quantum machine learning [1], matrix inversion [2],
quantum imaging [3], and quantum searching [4]. De-
spite its clear importance to quantum information pro-
cessing, a QRAM has yet to be realized experimentally.
Hence, finding a suitable architecture that can be real-
ized in the near-future remains an active research subject
in the theoretical and experimental domains.
In this article, we present a method to simulate large-
scale entanglement accounting for various sources of
noise. We are able to efficiently simulate circuits with
thousands of qubits under dephasing, amplitude damp-
ing, and CNOT errors. Based on our simulation model,
we present a QRAM architecture for photonic network-
based QRAM based on Ref. [5]. The feasibility assess-
ment is based on realistic parameters extracted from re-
cent experiments, which we will refer to throughout the
article.
A classical RAM [6] consists of a binary tree leading
to a final layer of memory cells, each corresponding to an
unique address. The address is represented as a series of
bits, with each bit corresponding to a layer of the binary
tree. Each bit of an address describes how the bus signal
propagates in the layer: to the right or to the left child
node. Hence, the nodes of the binary tree act as switches
for the address. When provided with a n-bit address, the
RAM returns a bit string fkassociated to the memory
cell labeled k. This is called the fan-out scheme [7].
A QRAM is the quantum analog of the RAM, similarly
consisting of addresses, quantum switches, and memory
cells in the form of qubits. In particular, with a quantum
address state, over the set of address qubits a, given by
|ψ
in=Pn
j=1 αj|ja, one can retrieve data from a super-
position of memory cells. A QRAM query is defined via
the following transformation,
|ψin=|ψ
in⟩|∅⟩b→ |ψout=
N
X
j=1
αj|ja|Djb(1)
where |∅⟩ represents an ancillary state, over the bus qubit
b, which transforms into the retrieved data state after
querying. In this article, we will restrict our investi-
gations to classical data, i.e. |Djare separable bits.
A direct conversion of classical fan-out protocol to the
quantum realm is inefficient since it requires maintain-
ing quantum coherence over an exponential number of
connections [7].
Three main schemes have been investigated to date:
the fan-out scheme that was already described, the
bucket brigade model, and the teleportation-based
scheme. Important figures of merit for the QRAM are
the fidelity of the above transformation and the query
time. For a detailed study and comparison of the first
two schemes, please refer to Ref. [8].
arXiv:2210.13494v2 [quant-ph] 4 Dec 2023
2
In the bucket brigade (BB) model [7, 9], the number
of qubits of the device scales as O(2n), as does the num-
ber of gates. Moreover, the original protocol [7] includes
an additional third state in each node, called the “wait”
state in order to prevent the exponential scaling of the
amount of decoherence, with respect to the memory size.
However, Hann et al. [8] have shown that the origin of
the noise resilience of the BB model is the amount of en-
tanglement among the memory’s components and not the
presence of the “wait” state, as one can devise a BB model
without the “wait” state that still achieves a polynomial
scaling of the decoherence with respect to the number of
memory addresses n.
More recently, Chen et al. presented a photonic
network-based QRAM scheme [5] that makes use of quan-
tum teleportation of addresses from a quantum computer
to the QRAM binary tree. Such a scheme greatly in-
creases the protocol’s efficiency by teleporting the reg-
isters to the layers (initially prepared in GHZ states) in
parallel as opposed to in series, thereby circumventing the
event of a single qubit loss collapsing the entire tree state.
Additionally, the proposed QRAM maps onto quantum
networks, leading to potential applications in distributed
quantum computing and sensing.
However, Chen et al. left as an open challenge the sim-
ulation of the scheme on large-scale networks since the
computational complexity scales exponentially with the
number of qubits. In this work, we bypass this problem
resorting to more efficient ways of modeling the noise in
stabilizer states. Moreover, this method generalizes to
other quantum networking tasks with similar construc-
tions, such as protocols for distributed quantum compu-
tation.
This comes in line with the fact that distributing en-
tanglement is central in quantum information processing
schemes ranging from quantum computing to sensing to
communications [10–12]. Simulation of distributed en-
tanglement in a network setting, be it a long-distance net-
work such as a possible future quantum internet [13], or
small-distance quantum local area network (QLAN) [14],
is important to assessing the limitations imposed by near-
term quantum technologies. The architecture of the
QRAM considered in this paper, building on photonic
network-based QRAM proposed in Ref. [5], involves a se-
ries of exponentially growing GHZ states, with the largest
having as many qubits as there are memory cells. Each
GHZ state spans across a physical layer in the QRAM
architecture, and the number of nodes per layer grows
exponentially with the number of memory cells 2nN
to be addressed, as shown in Fig. 1.
Computer simulations of noisy quantum processes in
such a system quickly becomes computationally inten-
sive [15–17] due to the density matrices growing exponen-
tially in size with the number of qubits. Even though the
entire QRAM protocol definition, i.e the retrieval of data
given an input address (see Eq. 1), requires more than
just Clifford operations, creating the routing state over
the QRAM architecture only uses Clifford gates. These
operations are the ones used to create these GHZ states
and the teleporting the address state onto the QRAM ac-
cess layers. Moreover, the operations required to access
the QRAM after the routing state is distributed over the
routing nodes only grows with the logarithm of the num-
ber of qubits of the QRAM, in comparison to the linear
amount of operations required to create the routing state.
This is the reason why noise in the system mostly comes
from the GHZ states before access. In particular, this
set of operations to create the routing state can be clas-
sically simulated efficiently [16]. This approach enables
an explicit and efficient description of all the intermedi-
ary states, up to local unitary corrections. In this article,
we develop efficient methods to simulate large-scale noisy
entanglement by characterizing the impact of noise at all
intermediate steps, and apply these tools to simulate a
noisy QRAM.
Quantum
Computer
...
...
...
Quantum RAM
Access Tree
Memory Cells
Layer 1
Layer 2
Layer 3
D1D2D3D4D5D6D7
D0
FIG. 1. Overview of a teleportation-based QRAM ar-
chitecture. A quantum RAM in the form of a binary tree
comprises GHZ states for each physical layer. The left-most
node of each layer iis entangled with an ancillary qubit in
a remote quantum computer, which hosts the query address
qubits (blue). Bell state measurement in the quantum com-
puter then teleports the address state onto the access tree.
The elementary operations to constructing GHZ states in a
photonic integrated circuit (PIC) QRAM are identical to the
ones over [5].
There are several architectures for a QRAM. Here, we
focus on the optically mediated quantum network-based
QRAM architecture introduced in Ref. [5], as it offers sev-
eral key benefits: implementation in quantum networks
compatible with envisioned quantum internet architec-
ture and quantum data centers, and faster query times
and possibility of executing in a non-local manner by
means of teleportation. Hence, this scheme works un-
der any network-like architecture, be it locally (e.g. on a
chip) or across large distances (e.g. over a quantum in-
ternet). Without loss of generality, we characterize each
node of the architecture as one of a spin-photon network
that could be implemented in photonic integrated circuits
(PIC).
The architecture of the QRAM is similar to previous
models, such as BB and the fan-out models. The main
3
e1 e2 e1 e2 e1 e2
Xp
Electron Photon Interaction
FIG. 2. Probabilistic CNOT. Execution of a CNOT gate
between two electrons, e1 and e2, mediated by a photon.
difference concerns the execution of the protocol and the
resources available at each node. In this architecture, one
considers two agents: the quantum computer, which pre-
pares the addresses, and the QRAM or quantum access
tree (see Fig. 1). The quantum computer must provide
an address state with n= log2Nqubits, where Nis the
total number of memories (for simplicity assume nN).
The QRAM has a binary tree architecture, with nphys-
ical layers, where the kth layer (k∈ {1, ..., n 1}) has
2k1quantum nodes. As we describe next, in each phys-
ical layer, all the nodes share a GHZ state, which is used
to teleport the address state onto the QRAM itself, al-
lowing for an ancilla qubit to access the memories in the
correct superposition.
As for the type of physical implementation chosen,
and without loss of generality, we focus on a QRAM im-
plementation involving solid-state spin qubits integrated
into PICs, an approach that is promising in terms of scal-
ability. In particular, we consider diamond nanophotonic
cavities coupled with silicon-vacancy centers [18, 19] as
each QRAM tree node. Each emitter contains an elec-
tronic spin that directly interacts with the photonic ad-
dress register qubits and an accompanying nuclear spin
acting as a long-lived memory. By entangling the elec-
tronic spin with the photon via cavity reflection, consec-
utive reflection of a photon off two neighboring nodes
and subsequent heralding achieves spin-spin entangle-
ment. This remote entangling strategy is repeatedly used
to generate a GHZ state across each layer. Such opera-
tions are probabilistic (see Fig. 2): the photon has a non-
zero probability of being lost to the environment before
reflecting off two cavities and arriving at the detector.
On the other hand, it is possible to perform close to de-
terministic two-qubit gates between the electronic and
nuclear spin qubits, albeit with a larger error [20, 21].
For this reason, we term this architecture teleportation-
based deterministic QRAM, or TD-QRAM.
In these types of systems, the main contributors to er-
rors are (i) spin phase errors (at rate 1/T2), (ii) spin flip
errors (at rate 1/T1), and (iii) errors in hyperfine gates
between electron and nuclear spins (see Supplementary
Table I). We leave out photon-electron interactions, as
one could conceive trading-off the efficiency ηfor arbi-
trarily high fidelity in the cavity-reflection based scheme
proposed in Ref. [22] in the high-cooperativity and over-
coupling regime.
Hence, we explore different values for T1,T2of both
electronic and nuclear spin qubits, and peand pnfor
the probabilities of error in electronic and nuclear spin
CNOTs. For the remaining of this article, we set Tn
1=
100 Te
1100 T1and Tn
2= 100 Te
2100 T2. Nuclear
spins have a higher coherence time as they are much less
coupled to the noisy spin-bath compared to electronic
spins. Reported values of characteristic times go, ex-
perimentally, up to Te
11 s,Te
210 ms [23], and
there are theoretical predictions of being able to reach
pe, pn= 102104[24, 25]. Moreover, we detail other
important physical parameters of this type of system,
used for the simulations, in Supplementary Table I.
Results
Simulating the effects of decoherence for a TD-
QRAM. To simulate the QRAM initialization protocol,
we use NetSquid [26] under the stabiliser formalism and
extract all the parameters of the noise channels before
implementing them in simulations, for instances: tim-
ing parameters for every qubit used throughout the sim-
ulation, all the noisy CNOTs with corresponding error
probabilities, and to which qubits and at which step it
is applied. From here, we compute the fidelity of the fi-
nal QRAM state by substituting all these values into the
expressions presented in the methods.
We start by presenting the simulation of a 212-qubit
QRAM in Fig. 3. Here, we detail individually the fideli-
ties of the GHZ state distributed at each physical layer
of the QRAM. The fidelity of the full state of the QRAM
is given by:
F(QRAM) =
n1
Y
i=1
FLayeri,|GHZ2i1+1,
where |GHZq=1
2|0q+|1q
(2)
i.e., the fidelity of the entire tree (or the QRAM) is de-
fined as the product of the fidelities of each physical layer
(see Supplementary Methods for more details). We dis-
tinguish access fidelity from tree fidelity, where the for-
mer refers to the fidelity of the state retrieved after ac-
cessing the memory cells (|ψoutin Eq. 1), and the latter
refers to the multipartite state fidelity of the binary tree
constituting the QRAM. Only the access fidelity depends
on the address and bus qubits.
One observes an exponential decrease of the fidelity
with the number of the layer (notice the logarithmic scal-
ing on the y-axis corresponding to the fidelity). This
agrees with the GHZ state size increasing exponentially
with the number of layers, i.e. scaling 2k. When one
qubit in this multipartite state suffers an error, the en-
tire state is affected.
One critical figure of merit that we extract from the
NetSquid simulations is the query time. As demonstrated
in Ref. [5], the query efficiency scales logarithmically with
the number of qubits. Extracting from multiple queries
of the QRAM, we obtain the query times (apart from
4
10110 2103
N um ber of M em or y Qubits
0.0
0.2
0.4
0.6
0.8
1.0
F i de lity
0.0
0.0091
0.6204
0.9525
0.999
CNOT = 10 2
CNOT = 10 3
CNOT = 10 4
CNOT = 10 5
CNOT = 0
10110 2103
N um ber of M em or y Qubits
1 10 2
1 10 3
1 10 4
1 10 5
1 10 6
1 10 7
F i de lity
0.9 692
0.9 967
0.9 967
= 0.5
= 0.6
= 0.7
= 0.8
= 0.9
= 0.5; T2=
1 2 3 4 5 6 7 8 9 10 11 12
Layer N um ber
1 10 2
1 10 3
1 10 4
F i de lity
Layer 2
Layer 3
Layer 4
Layer 5
Layer 6
Layer 7
Layer 8
Layer 9
Layer 1 0
Layer 1 1
Layer 1 2
= 0.5
= 0.6
= 0.7
= 0.8
= 0.9
10110 2103
N um ber of M em or y Qubi ts
105
Quer y t ime ( ns)
= 0.5
= 0.6
= 0.7
= 0.8
= 0.9
FIG. 3. TD-QRAM Simulations. (a) TD-QRAM access protocol for 12 layers, with the efficiency of generating a Bell
pair swept from η= 50% to η= 90%. The noise analysis considers only dephasing and damping errors. The final fidelity is
calculated according to Eq. 2, with T1= 20 ms,T2= 10 ms, and ϵCNOT = 0 for each layer. (b) Query times with varying
sizes from 2 layers to 12 layers, and sweeping the efficiency of generating a Bell pair from η= 50% to η= 90%. There is an
expected logarithmic scaling of the query time with the number of qubits. (c) TD-QRAM noise analysis with dephasing errors,
T2= 10 ms (filled lines) and T2= 100 ms (traced lines), with fixed amplitude-damping error T1= 2 s. We consider different
QRAM sizes from 2 layers to 12 layers as well as various efficiencies of generating a Bell pair from η= 50% to η= 90%. (d)
TD-QRAM noise analysis with noisy CNOTs, pe=pn∈ {0,105,104,103,102}, for a QRAM with the number of layers
ranging from 2 to 12. The dephasing time is fixed at T2= 100 ms, and the amplitude-damping time is fixed at T1= 2 s.
The efficiency of generating a Bell pair is fixed at η= 90%. The final fidelity mainly depends on the number of noisy CNOTs
performed throughout the protocol and has little dependence on the efficiency. All the error bars over the data correspond to
the error of the average value over 100 simulations of the protocol.
a logarithmic factor derived from making the bus qubit
traverse the binary tree) in Fig. 3.
Dephasing and Damping Errors for TD-QRAM.
Considering only the effects of dephasing and amplitude-
damping errors in the spin qubits, we take T2=ms for
amplitude-damping errors only, and then T2= 10 ms and
T2= 100 ms with a fixed T1= 2 s [23], see Supplementary
Table I. We also set the CNOT error rate to 0. We present
the simulation results for the TD-QRAM scheme under
memory dephasing for increasing QRAM size, as shown
in Fig. 3.
Looking closely at Figure 3(c), one can observe that
the effect of amplitude-damping shows an identical be-
havior to the one of dephasing and amplitude-damping
combined, i.e., with the same type of scaling. However,
it is residual comparatively to the effect of dephasing.
This is easily explainable by the time-scales of the co-
herence times of the corresponding noises (T1and T2)
in the memory differ by orders of magnitude, with the
first, T1, being usually much longer than the latter, T2,
i.e. T11 s [23]. For this reason, its impact can be
5
neglected relative to other sources of error.
Dephasing, Damping and Noisy CNOTs for TD-
QRAM. The only type of error missing in the analysis
is the error derived from the use of noisy CNOTs. Illus-
trated in Fig. 3, the dephasing and damping errors min-
imally contribute to infidelity. We now analyse the case
for noisy CNOTs on top of fixed T1= 2 s and T2= 100 ms
(note we now switch to linear scale in the y-axis for the
fidelity, due to the set of values present for the different
simulations). For simplicity, we consider equal CNOT
error probability, ϵCNOT, for both electronic and nuclear
CNOTs, and vary ϵCNOT from 105to 102as shown in
Fig. 3:
These simulations show that the CNOT gates domi-
nate the overall error in the QRAM state fidelity in the
TD-QRAM. For instance, to access a 128-qubit QRAM,
one needs fidelities of the CNOT gates to be somewhere
near 99.9% to obtain an access fidelity exceeding 90%. In
this architecture, while the query times do not increase
linearly with the size of the memory, the errors do. Ex-
pectedly, applying an error to a single qubit of a GHZ
state contributes in the same order for the entire state.
The price to pay for performing CNOTs with such large
error rates deterministically could be circumvented by
near-perfect yet probabilistic CNOTs [22, 27] via cavity-
based electron spin-photon interactions, as opposed to
deterministic yet error-prone nuclear-electron spin cou-
pling. In light of this, we explore a hybrid teleportation-
based QRAM architecture in the following section.
Teleportation-based Stochastic QRAM . In the
TD-QRAM protocol, the entanglement generation and
swap (Fig. 8) operation are still probabilistic given the
finite chance of photon loss. Hence, these probabilistic
CNOTs are done in parallel throughout each physical
layer to improve efficiency. After an EPR pair is created
between two electron spins, however, transferring entan-
glement onto the nuclear spins is a deterministic proce-
dure. Thereby, the query time grows sub-linearly. As
noted before addressing the TD-QRAM scheme, this de-
terministic CNOT based on nuclear-electron spin inter-
action mainly dominates the infidelity of the GHZ state,
motivating us to contemplate an alternative solution.
Since the decoherence errors from T1and T2contribute
much less to the infidelity relative to electron-nuclear spin
CNOT, replacing some of the noisy deterministic CNOTs
with probabilistic CNOTs helps improve the fidelity de-
spite reducing efficiency. As we will show, this leads
to higher QRAM tree state fidelities, albeit with longer
query times. We call this architecture ‘teleportation-
based stochastic QRAM’, or TS-QRAM.
Relying solely on probabilistic CNOTs in every step of
the protocol would be very inefficient since the probabil-
ity of generating a GHZ state diminishes exponentially
with the number of nodes. In other words, if one entan-
glement attempt fails during construction of a GHZ state,
the entire state collapses. Since each linking process is
heralded, there are ways to circumvent this by choosing a
specified order to perform the CNOTs, similar to entan-
glement swapping in a repeater chain [28, 29]. Here, the
probabilistic swapping operations are equivalent to the
probabilistic CNOTs, and measuring the middle node is
analogous to joining smaller GHZ states to form a larger
GHZ state. Abstractly, they describe the same problem,
which allows us to use the solutions provided by Ref. [29].
Next, we present an in-depth analysis of the trade-off be-
tween fidelity and query rate as a function of error rates
and physical implements.
Increasing T1and T2.To decrease the number of
employed deterministic CNOTs, and taking into account
that these always happen when the electronic spins in-
teract with the nuclear spins, it is natural to consider
dropping the nuclear spins altogether. This is motivated
by the fact that we can perform CNOTs, albeit proba-
bilistically, between the electron spins. The downside is
that electron spins suffer from having shorter coherence
times than their nuclear counterparts. Still, it is advanta-
geous to consider such schemes to avoid the use of noisier
deterministic CNOTs.
To minimize the consequently increased decoherence,
one could conceive schemes for increasing the T1and T2
times for the electrons, since these are the ones now caus-
ing the fidelity bottleneck, together with the required
time to query the memory.
Presently, the SiV’s electronic spin’s T1time is shown
to be longer than 1 s [23], thereby posing no concern
over depolarisation. On the other hand, its T2coherence
time is limited to tens of milliseconds [23] even under dy-
namical decoupling. The main dephasing mechanism is
attributed to the surrounding nuclear spin bath, which is
weakly coupled to the electronic spin of interest via hy-
perfine interaction [30]. A potential avenue to improving
the electronic spin’s T2is therefore to “purify” its environ-
ment by materials engineering [31]. By producing SiV in
a carbon-13 free matrix, for example, the coherence time
may be further extended.
Nevertheless, our numerical analyses of the hybrid
scheme show fidelities still exceeding 60% for a reason-
able CNOT error rate of 103and 1024 memory cells,
using a T2of 100 ms. For such a result, a probability of
success of about 70% for the CNOT is required.
The Teleportation-based Stochastic QRAM
Protocol. In the TD-QRAM protocol, there are
two steps occurring in parallel across each layer in the
QRAM: one for generating EPR pairs across every other
node and another for linking all the states into a larger
GHZ state, via sharing EPR pairs in-between nodes hold-
ing the previously shared EPR pairs (see Fig. 8). This
could be made in parallel because the linking operations
are deterministic.
In the TS-QRAM protocol, however, we must now con-
sider an order for the linking step that depends on the
node’s position, similar to the quantum repeater chain
problem [26, 29, 32]. If a linking process fails, the sub-
set of qubits that would have become entangled must
摘要:

Resource-efficientsimulationofnoisyquantumcircuitsandapplicationtonetwork-enabledQRAMoptimizationLuísBugalho,1,2,3,4EmmanuelZambriniCruzeiro,5KevinC.Chen,6,7WenhanDai,7,8DirkEnglund,6,7andYasserOmar1,2,31InstitutoSuperiorTécnico,UniversidadedeLisboa,Portugal2PhysicsofInformationandQuantumTechnologie...

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