Quantum Physics

Demonstrations of quantum advantage have largely focused on computational speedups and on quantum simulation of many-body physics, limited by fidelity and capability of current devices. Discriminating laser-pulse-modulated classical-communication codewords at the minimum allowable probability of error using universal-quantum processing presents a promising parallel direction, one that is of both fundamental importance in quantum state discrimination, as well as of technological relevance in deep-space laser communications. Here we present an experimental realization of a quantum joint detection receiver for binary phase shift keying modulated codewords of a 3-bit linear tree code using a recently-proposed quantum algorithm: belief propagation with quantum messages. The receiver, translated to a quantum circuit, was experimentally implemented on a trapped-ion device -- the recently released Honeywell LT-1.0 system using 171 Yb+ ions, which possesses all-to-all connectivity and mid-circuit measurement capabilities that are essential to this demonstration. We conclusively realize a previously postulated but hitherto not-demonstrated joint quantum detection scheme, and provide an experimental framework that surpasses the quantum limit on the minimum average decoding error probability associated with pulse-by-pulse detection in the low mean photon number limit. The full joint-detection scheme bridges across photonic and trapped-ion based quantum information science, mapping the photonic coherent states of the modulation alphabet onto inner product-preserving states of single-ion qubits. Looking ahead, our work opens new avenues in hybrid realizations of quantum-enhanced receivers with applications in astronomy and emerging space-based platforms.

Faster and more precise physical processing of quantum gate, by suitably designing a pulse sequence to implement the target gate, will greatly improve the performance of quantum algorithms in the presence of noise. In this paper, we demonstrate that, by employing OpenPulse design kit for IBM Q devices, the controlled-V gate (CV gate) can be implemented in about 34.5 % shorter gate time, with 0.66 % improvement in the average gate fidelity, compared to the standard version provided there. Then, based on the theory of Cartan decomposition, we show that the performance of several two-qubit gates containing CV gates can also be improved. Moreover, the average gate fidelity of Toffoli gate can be improved to 96.16 % from 90.23 % achieved in the default IBM Q package. These results imply the importance of our CV gate implementation technique, which, as an additional option for the basis_gate set design, may shorten the overall computation time and consequently improve the accuracy of several quantum algorithms.

Quantum emitters respond to resonant illumination by radiating electromagnetic fields. A component of these fields is phase-coherent with the driving tone, while another one is incoherent, consisting of spontaneously emitted photons and forming the fluorescence signal. Atoms and molecules are routinely detected by their fluorescence at optical frequencies, with important applications in quantum technology and microscopy. Spins, on the other hand, are usually detected by {their coherent response} at radio- or microwave frequencies, either in continuous-wave or pulsed magnetic resonance. Indeed, fluorescence detection of spins is hampered {by their low spontaneous emission rate} and by the lack of single-photon detectors in this frequency range. Here, using superconducting quantum devices, we demonstrate the detection of a small ensemble of donor spins in silicon by their fluorescence at microwave frequency and millikelvin temperatures. We enhance the spin radiative decay rate by coupling them to a high-quality-factor and small-mode-volume superconducting resonator, and we connect the device output to a newly-developed microwave single-photon counter based on a superconducting qubit. We discuss the potential of fluorescence detection as a novel method for magnetic resonance spectroscopy of small numbers of spins.

Genuine multipartite entanglement (GME) offers more significant advantages in quantum information compared with entanglement. We propose a sufficient criterion for the detection of GME based on local sum uncertainty relations for chosen observables of subsystems. We apply the criterion to detect the GME properties of noisy n -partite W state when n=3,4,5 and 6 , and find that the criterion can detect more noisy W states when n ranges from 4 to 6. Moreover, the criterion is also used to detect the genuine entanglement of 3 -qutrit state. The result is stronger than that based on GME concurrence and fisher information.

Current quantum processors are noisy, have limited coherence and imperfect gate implementations. On such hardware, only algorithms that are shorter than the overall coherence time can be implemented and executed successfully. A good quantum compiler must translate an input program into the most efficient equivalent of itself, getting the most out of the available hardware. In this work, we present novel deterministic algorithms for compiling recurrent quantum circuit patterns in polynomial time. In particular, such patterns appear in quantum circuits that are used to compute the ground state properties of molecular systems using the variational quantum eigensolver (VQE) method together with the RyRz heuristic wavefunction Ansatz. We show that our pattern-oriented compiling algorithms, combined with an efficient swapping strategy, produces - in general - output programs that are comparable to those obtained with state-of-art compilers, in terms of CNOT count and CNOT depth. In particular, our solution produces unmatched results on RyRz circuits.

Fast scramblers are dynamical quantum systems that produce many-body entanglement on a timescale that grows logarithmically with the system size N . We propose and investigate a family of deterministic, fast scrambling quantum circuits realizable in near-term experiments with arrays of neutral atoms. We show that three experimental tools -- nearest-neighbour Rydberg interactions, global single-qubit rotations, and shuffling operations facilitated by an auxiliary tweezer array -- are sufficient to generate nonlocal interaction graphs capable of scrambling quantum information using only O(logN) parallel applications of nearest-neighbor gates. These tools enable direct experimental access to fast scrambling dynamics in a highly controlled and programmable way, and can be harnessed to produce highly entangled states with varied applications.

Cavity-optomechanics is an ideal platform for the generation non-Gaussian quantum states due to the anharmonic interaction between the light field and the mechanical oscillator; but exactly this interaction also impedes the preparation in pure states of the light field. In this paper we derive a driving protocol that helps to exploit the anharmonic interaction for state preparation, and that ensures that the state of the light field remains close-to-pure. This shall enable the deterministic preparation of photon Fock states or coherent superpositions thereof.

We propose a straightforward implementation of the phenomenon of diffractive focusing with uniform atomic Bose-Einstein condensates. Both, analytical as well as numerical methods not only illustrate the influence of the atom-atom interaction on the focusing factor and the focus time, but also allow us to derive the optimal conditions for observing focusing of this type in the case of interacting matter waves.

The tensor network variety is a variety of tensors associated to a graph and a set of positive integer weights on its edges, called bond dimensions. We determine an upper bound on the dimension of the tensor network variety. A refined upper bound is given in cases relevant for applications such as varieties of matrix product states and projected entangled pairs states. We provide a range (the "supercritical range") of the parameters where the upper bound is sharp.

Heisenberg's uncertainty principle is often cited as an example of a "purely quantum" relation with no analogue in the classical limit where ?��?0 . However, this formulation of the classical limit is problematic for many reasons, one of which is dimensional analysis. Since ??is a dimensionful constant, we may always work in natural units in which ??1 . Dimensional analysis teaches us that all physical laws can be expressed purely in terms of dimensionless quantities. This indicates that the existence of a dimensionally consistent constraint on ?x?p requires the existence of a dimensionful parameter with units of action, and that any definition of the classical limit must be formulated in terms of dimensionless quantities (such as quantum numbers). Therefore, bounds on classical uncertainty (formulated in terms of statistical ensembles) can only be written in terms of dimensionful scales of the system under consideration, and can be readily compared to their quantum counterparts after being non-dimensionalized. We compare the uncertainty of certain coupled classical systems and their quantum counterparts (such as harmonic oscillators and particles in a box), and show that they converge in the classical limit. We find that since these systems feature additional dimensionful scales, the uncertainty bounds are dependent on multiple dimensionless parameters, in accordance with dimensional considerations.