30 research outputs found
High dimensional measurement device independent quantum key distribution on two dimensional subspaces
Quantum key distribution (QKD) provides ultimate cryptographic security based
on the laws of quantum mechanics. For point-to-point QKD protocols, the
security of the generated key is compromised by detector side channel attacks.
This problem can be solved with measurement device independent QKD (mdi-QKD).
However, mdi-QKD has shown limited performances in terms of the secret key
generation rate, due to post-selection in the Bell measurements. We show that
high dimensional (Hi-D) encoding (qudits) improves the performance of current
mdi-QKD implementations. The scheme is proven to be unconditionally secure even
for weak coherent pulses with decoy states, while the secret key rate is
derived in the single photon case. Our analysis includes phase errors,
imperfect sources and dark counts to mimic real systems. Compared to the
standard bidimensional case, we show an improvement in the key generation rate.Comment: 6 pages, 3 figure
Multi-partite entanglement detection with non symmetric probing
We show that spin squeezing criteria commonly used for entanglement detection
can be erroneous, if the probe is not symmetric. We then derive a lower bound
on squeezing for separable states in spin systems probed asymmetrically. Using
this we further develop a procedure that allows us to verify the degree of
entanglement of a quantum state in the spin system. Finally, we apply our
method for entanglement verification to existing experimental data, and use it
to prove the existence of tri-partite entanglement in a spin squeezed atomic
ensemble.Comment: 7 pages, 2 figures (Include Supplemental material
Quantum nondemolition measurement of mechanical motion quanta
The fields of opto- and electromechanics have facilitated numerous advances
in the areas of precision measurement and sensing, ultimately driving the
studies of mechanical systems into the quantum regime. To date, however, the
quantization of the mechanical motion and the associated quantum jumps between
phonon states remains elusive. For optomechanical systems, the coupling to the
environment was shown to preclude the detection of the mechanical mode
occupation, unless strong single photon optomechanical coupling is achieved.
Here, we propose and analyse an electromechanical setup, which allows to
overcome this limitation and resolve the energy levels of a mechanical
oscillator. We find that the heating of the membrane, caused by the interaction
with the environment and unwanted couplings, can be suppressed for carefully
designed electromechanical systems. The results suggest that phonon number
measurement is within reach for modern electromechanical setups.Comment: 8 pages, 5 figures plus 24 pages, 11 figures supplemental materia
Quantum manipulation of a two-level mechanical system
We consider a nonlinearly coupled electromechanical system, and develop a
quantitative theory for two-phonon cooling. In the presence of two-phonon
cooling, the mechanical Hilbert space is effectively reduced to its ground and
first excited states, thus forming a mechanical qubit. This allows for
performing quantum operations at the level of individual mechanical phonons and
preparing nonclassical mechanical states with negative Wigner functions. We
propose a scheme for performing arbitrary Bloch sphere rotations, and derive
the fidelity in the specific case of a -pulse. We characterise detrimental
processes that reduce the coherence in the system, and demonstrate that our
scheme can be implemented in state-of-the-art electromechanical devices.Comment: 6 pages main text + 7 pages supplemental material, 3 figure
Enhanced osteogenic differentiation in zoledronate-treated osteoporotic patients
Bisphosphonates are well known inhibitors of osteoclast activity and thus may be employed to influence osteoblast activity. The present study was designed to evaluate the in vivo effects of zoledronic acid (ZA) on the proliferation and osteoblastic commitment of mesenchymal stem cells (MSC) in osteoporotic patients. We studied 22 postmenopausal osteoporotic patients. Densitometric, biochemical, cellular and molecular data were collected before as well as after 6 and 12 months of ZA treatment. Peripheral blood MSC-like cells were quantified by colony-forming unit fibroblastic assay; their osteogenic differentiation potential was evaluated after 3 and 7 days of induction, respectively. Circulating MSCs showed significantly increased expression levels of osteoblastic marker genes such as Runt-related transcription factor 2 (RUNX2), and Osteonectin (SPARC) during the 12 months of monitoring time. Lumbar bone mineral density (BMD) variation and SPARC gene expression correlated positively. Bone turnover marker levels were significantly lowered after ZA treatment; the effect was more pronounced for C terminal telopeptide (CTX) than for Procollagen Type 1 N-Terminal Propeptide (P1NP) and bone alkaline phosphatase (bALP). Our findings suggest a discrete anabolic activity supported by osteogenic commitment of MSCs, consequent to ZA treatment. We confirm its anabolic effects in vivo on osteogenic precursors
Hybrid variational quantum eigensolvers: merging computational models
Variational quantum eigensolvers (VQEs) are a highly successful technique for
simulating physical models on quantum computers. Recently, they were extended
to the measurement-based approach of quantum computing, bringing the strengths
and advantages of this computational model to VQEs. In this work, we push the
design and integration frontiers of VQE further by blending measurement-based
elements into the gate-based paradigm to form a hybrid VQE. This facilitates
the design of a problem-informed variational ansatz and also allows the
efficient implementation of many-body Hamiltonians on NISQ devices. We
experimentally demonstrate our approach on a superconducting quantum computer
by investigating the perturbed planar code, Z2 and SU(3) lattice gauge
theories, and the LiH molecule.Comment: 5+18 pages, 2+4 figure
SQEM: Superposed Quantum Error Mitigation
Overcoming the influence of noise and imperfections is one of the main
challenges in quantum computing. Here, we present an approach based on applying
a desired unitary computation in superposition, either on the system of
interest or some auxiliary states. We demonstrate that parallel applications of
the same operation lead to significant noise mitigation when arbitrary noise
processes are considered. We first design probabilistic implementations of our
scheme. These are plug-and-play, are independent of the noise characteristic
and require no post-processing. We then show that the success probability can
be enhanced (up to deterministic) using adaptive corrections. We provide an
analytical study of our protocol performance and demonstrate that unit fidelity
can be achieved asymptotically. The approaches introduced are suitable to both
standard gate-based (GB) and measurement-based (MB) computational models.Comment: 4+ pages, 3 figure
Enhancing Quantum Computation via Superposition of Quantum Gates
Overcoming the influence of noise and imperfections in quantum devices is one
of the main challenges for viable quantum applications. In this article, we
present different protocols, which we denote as "superposed quantum error
mitigation" (SQEM), that enhance the fidelity of single gates or entire
computations by performing them in coherent superposition. Our results
demonstrate that via our methods, significant noise suppression can be achieved
for most kinds of decoherence and standard experimental parameter regimes. Our
protocols can be either deterministic, such that the outcome is never
post-selected, or probabilistic, in which case the resulting state must be
discarded unless a well-specified condition is met. By using sufficiently many
resources and working under broad assumptions, our methods can yield the
desired output state with unit fidelity. Finally, we analyze our approach for
gate-based, measurement-based and interferometric-based models, demonstrating
the applicability in all cases and investigating the fundamental mechanisms
they rely upon.Comment: 38 pages, 15 figure
A square-root speedup for finding the smallest eigenvalue
We describe a quantum algorithm for finding the smallest eigenvalue of a
Hermitian matrix. This algorithm combines Quantum Phase Estimation and Quantum
Amplitude Estimation to achieve a quadratic speedup with respect to the best
classical algorithm in terms of matrix dimensionality, i.e.,
black-box queries to an oracle
encoding the matrix, where is the matrix dimension and is the
desired precision. In contrast, the best classical algorithm for the same task
requires queries. In addition, this
algorithm allows the user to select any constant success probability. We also
provide a similar algorithm with the same runtime that allows us to prepare a
quantum state lying mostly in the matrix's low-energy subspace. We implement
simulations of both algorithms and demonstrate their application to problems in
quantum chemistry and materials science.Comment: 17 pages, 6 figures, all comments are welcome, additional references
adde