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Register a new userQuantum Nondemolition Gate Operations and Measurements in Real Time on Fluctuating Signals
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We demonstrate an optical quantum nondemolition (QND) interaction gate with a bandwidth of about 100 MHz. Employing this gate, we are able to perform QND measurements in real time on randomly fluctuating signals. Our QND gate relies upon linear optics and offline-prepared squeezed states. In contrast to previous demonstrations on narrow sideband modes, our gate is compatible with non-Gaussian quantum states temporally localized in a wave-packet mode, and thus opens the way for universal gate operations and realization of quantum error correction.
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The sum gate is the canonical two-mode gate for universal quantum computation based on continuous quantum variables. It represents the natural analogue to a qubit C-NOT gate. In addition, the continuous-variable gate describes a quantum nondemolition (QND) interaction between the quadrature components of two light fields. We experimentally demonstrate a QND sum gate, employing the scheme by R. Filip, P. Marek, and U.L. Andersen [pra {bf 71}, 042308 (2005)], solely based on offline squeezed states, homodyne measurements, and feedforward. The results are verified by simultaneously satisfying the criteria for QND measurements in both conjugate quadratures.
In the field of cavity optomechanics, proposals for quantum nondemolition (QND) measurements of phonon number provide a promising avenue by which one can study the quantum nature of nanoscale mechanical resonators. Here, we investigate these QND measurements for an optomechanical system whereby quadratic coupling arises due to shared symmetries between a single optical resonance and a mechanical mode. We establish a relaxed limit on the amount of linear coupling that can exist in this type of system while still allowing for a QND measurement of Fock states. This new condition enables optomechanical QND measurements, which can be used to probe the decoherence of mesoscopic mechanical Fock states, providing an experimental testbed for quantum collapse theories.
We present a compact experimental design for producing an arbitrarily large optical continuous-variable cluster state using just one single-mode vacuum squeezer and one quantum nondemolition gate. Generating the cluster state and computing with it happen simultaneously: more entangled modes become available as previous modes are measured, thereby making finite the requirements for coherence and stability even as the computation length increases indefinitely.
The power of a quantum circuit is determined through the number of two-qubit entangling gates that can be performed within the coherence time of the system. In the absence of parallel quantum gate operations, this would make the quantum simulators limited to shallow circuits. Here, we propose a protocol to parallelize the implementation of two-qubit entangling gates between multiple users which are spatially separated, and use a commonly shared spin chain data-bus. Our protocol works through inducing effective interaction between each pair of qubits without disturbing the others, therefore, it increases the rate of gate operations without creating crosstalk. This is achieved by tuning the Hamiltonian parameters appropriately, described in the form of two different strategies. The tuning of the parameters makes different bilocalized eigenstates responsible for the realization of the entangling gates between different pairs of distant qubits. Remarkably, the performance of our protocol is robust against increasing the length of the data-bus and the number of users. Moreover, we show that this protocol can tolerate various types of disorders and is applicable in the context of superconductor-based systems. The proposed protocol can serve for realizing two-way quantum communication.
We propose $mathrm{SQiSW}$, the matrix square root of the standard $mathrm{iSWAP}$ gate, as a native two-qubit gate for superconducting quantum computing. We show numerically that it has potential for an ultra-high fidelity implementation as its gate time is half of that of $mathrm{iSWAP}$, but at the same time it possesses powerful information processing capabilities in both the compilation of arbitrary two-qubit gates and the generation of large-scale entangled W-like states. Even though it is half of an $mathrm{iSWAP}$ gate, its capabilities surprisingly rival and even surpass that of $mathrm{iSWAP}$ or other incumbent native two-qubit gates such as $mathrm{CNOT}$. To complete the case for its candidacy, we propose a detailed compilation, calibration and benchmarking framework. In particular, we propose a variant of randomized benchmarking called interleaved fully randomized benchmarking (iFRB) which provides a general and unified solution for benchmarking non-Clifford gates such as $mathrm{SQiSW}$. For the reasons above, we believe that the $mathrm{SQiSW}$ gate is worth further study and consideration as a native two-qubit gate for both fault-tolerant and noisy intermediate-scale quantum (NISQ) computation.
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