Quantum Measurement Devices Research Articles

Cryptanalysis and improvement in semi-quantum private comparison based on Bell states

Recently, Jiang proposed a semi-quantum private comparison (SQPC) protocol without the use of a quantum measurement device (Quantum Inf Process 19:182, 2020). However, this paper highlights a security loophole during the transmission phase of the protocol in which a malicious participant can perform a double CNOT attack to steal partial information of the other participant’s private secret without being detected. With the same assumption of the semi-quantum environment as Jiang’s protocol, this paper presents an improved protocol by adopting a strategy of pre-sharing session keys to overcome the double CNOT attack. Moreover, a security analysis of the collective attack is also provided for Jiang’s and the improved SQPC protocols.

Experimental demonstration of the advantage of using coherent measurements for phase estimation in the presence of depolarizing noise

We report an experimental investigation of the role of measurement in quantum metrology when the states of the probes are mixed. In particular, we investigated optimized local measurements and general global projective measurements, involving entangling operations, on noisy Werner states of polarization entangled photons. We demonstrate experimentally that global measurement presents an advantage in parameter estimation with respect to the optimized local strategy. Moreover, the global strategy provides unambiguous information about the parameter of interest even when the amount of noise is not well characterized. This shows that the coherence in quantum operations, such as the Bell-state projection device used in our protocol, can be used to further boost the quantum advantage in metrology and play a fundamental role in the design of future quantum measurement devices.

Direct approach to realizing quantum filters for high-precision measurements

Quantum noise sets a fundamental limit to the sensitivity of high-precision measurements. Suppressing it can be achieved by using nonclassical states and quantum filters, which modify both the noise and signal response. We find an approach to realizing quantum filters directly from their frequency-domain transfer functions, utilizing techniques developed by the quantum control community. It not only allows us to construct quantum filters that defy intuition, but also opens a path towards the systematic design of optimal quantum measurement devices. As an illustration, we show an optical realization of an active unstable filter with anomalous dispersion, proposed for improving the quantum-limited sensitivity of gravitational-wave detectors.

Cryptanalysis of limited resource semi-quantum secret sharing

Li et al. (Quantum Inf Process 17(10):285, 2018) proposed a limited resource semi-quantum secret sharing protocol, in which the classical participants do not equip any quantum measurement devices. However, this protocol has a security loophole. This study investigates this loophole and indicates that a malicious agent can perform the double-CNOT attack to steal partial information about the secret shadow of the other agent without being detected.

Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography

We propose a simple scheme to reduce readout errors in experiments on quantum systems with finite number of measurement outcomes. Our method relies on performing classical post-processing which is preceded by Quantum Detector Tomography, i.e., the reconstruction of a Positive-Operator Valued Measure (POVM) describing the given quantum measurement device. If the measurement device is affected only by an invertible classical noise, it is possible to correct the outcome statistics of future experiments performed on the same device. To support the practical applicability of this scheme for near-term quantum devices, we characterize measurements implemented in IBM's and Rigetti's quantum processors. We find that for these devices, based on superconducting transmon qubits, classical noise is indeed the dominant source of readout errors. Moreover, we analyze the influence of the presence of coherent errors and finite statistics on the performance of our error-mitigation procedure. Applying our scheme on the IBM's 5-qubit device, we observe a significant improvement of the results of a number of single- and two-qubit tasks including Quantum State Tomography (QST), Quantum Process Tomography (QPT), the implementation of non-projective measurements, and certain quantum algorithms (Grover's search and the Bernstein-Vazirani algorithm). Finally, we present results showing improvement for the implementation of certain probability distributions in the case of five qubits.

Semi-device-independent characterization of quantum measurements under a minimum overlap assumption

Recently, a novel framework for semi-device-independent quantum prepare-and-measure protocols has been proposed, based on the assumption of a limited distinguishability between the prepared quantum states. Here, we discuss the problem of characterizing an unknown quantum measurement device in this setting. We present several methods to attack this problem. Considering the simplest scenario of two preparations with lower bounded overlap, we show that genuine 3-outcome POVMs can be certified, even in the presence of noise. Moreover, we show that the optimal POVM for performing unambiguous state discrimination can be self-tested.

Fundamental relations between measurement, radiation, and decoherence in gravitational wave laser interferometer detectors

As laser interferometer gravitational wave (GW) detectors become quantum noise dominated, understanding the fundamental limit on measurement sensitivity imposed by quantum uncertainty is crucial to guide the search for further noise reduction. Recent efforts have included applying ideas from quantum information theory to GW detection -- specifically the quantum Cramer Rao bound, which is a minimum bound on error in parameter estimation using a quantum state and is determined by the state's quantum Fisher information (QFI) with respect to the parameter. Identifying the QFI requires knowing the interaction between the quantum measurement device and the signal, which was rigorously derived for GW interferometer detectors in [Phys. Rev. D 98, 124006]. In this paper, we calculate the QFI and fundamental quantum limit (FQL) for GW detection, and furthermore derive explicit reciprocity relations involving the QFI which summarize information exchange between the detector and a surrounding weak quantum GW field. Specifically, we show that the GW power radiation by the detector's quantum fluctuations are proportional to the QFI, and therefore inversely proportional to its FQL. Similarly, the detector's decoherence rate in a white noise GW bath can be explicitly related to the QFI/FQL. These relations are fundamental and appear generalizable to a broader class of quantum measurement systems.

Entanglement of two noninteracting qubits via a mesoscopic system

We propose a method for entangling two non-interacting qubits by measuring their parity indirectly through an intermediate mesoscopic system. The protocol is designed to require only global control and course-grained collective measurement of the mesoscopic system along with local interactions between the target qubits and mesoscopic system. A generalization of the method measures the hamming weight of the qubits' state and probabilistically produces an entangled state by post-selecting on hamming weight one. Our technique provides a new design element that can be integrated into quantum processor architectures and quantum measurement devices.

Experimental observation of anomalous trajectories of single photons

A century after its conception, quantum mechanics still hold surprises that contradict many "common sense" notions. The contradiction is especially sharp in case one consider trajectories of truly quantum objects such as single photons. From a classical point of view, trajectories are well defined for particles, but not for waves. The wave-particle duality forces a breakdown of this dichotomy and quantum mechanics resolves this in a remarkable way: Trajectories can be well defined, but they are utterly different from classical trajectories. Here, we give an operational definition to the trajectory of a single photon by introducing a novel technique to mark its path using its spectral composition. The method demonstrates that the frequency degree of freedom can be used as a bona fide quantum measurement device (meter). The analysis of a number of setups, using our operational definition, leads to anomalous trajectories which are non-continuous and in some cases do not even connect the source of the photon to where it is detected. We carried out an experimental demonstration of these anomalous trajectories using a nested interferometer. We show that the Two-state vector formalism provides a simple explanation for the results.

Continuous-variable quantum process tomography with squeezed-state probes

We propose a procedure for tomographic characterization of continuous variable quantum operations which employs homodyne detection and single-mode squeezed probe states with a fixed degree of squeezing and anti-squeezing and a variable displacement and orientation of squeezing ellipse. Density matrix elements of a quantum process matrix in Fock basis can be estimated by averaging well behaved pattern functions over the homodyne data. We show that this approach can be straightforwardly extended to characterization of quantum measurement devices. The probe states can be mixed, which makes the proposed procedure feasible with current technology.

Tomography of quantum detectors

In quantum mechanics, measurement has a fundamentally different role than in classical physics. Now a general method has been devised to characterize a quantum measurement device, completing the suite of so-called tomography techniques required to fully specify an experiment. Measurement connects the world of quantum phenomena to the world of classical events. It has both a passive role—in observing quantum systems—and an active one, in preparing quantum states and controlling them. In view of the central status of measurement in quantum mechanics, it is surprising that there is no general recipe for designing a detector that measures a given observable1. Compounding this, the characterization of existing detectors is typically based on partial calibrations or elaborate models. Thus, experimental specification (that is, tomography) of a detector is of fundamental and practical importance. Here, we present the realization of quantum detector tomography2,3,4. We identify the positive-operator-valued measure describing the detector, with no ancillary assumptions. This result completes the triad, state5,6,7,8,9,10,11, process12,13,14,15,16,17 and detector tomography, required to fully specify an experiment. We characterize an avalanche photodiode and a photon-number-resolving detector capable of detecting up to eight photons18. This creates a new set of tools for accurately detecting and preparing non-classical light.

Experimental realization of a programmable quantum-state discriminator and a phase-covariant quantum multimeter

We present an optical implementation of two programmable quantum measurement devices. The first one serves for unambiguous discrimination of two nonorthogonal states of a qubit. The particular pair of states to be discriminated is specified by the quantum state of a program qubit. The second device can perform von Neumann measurements on a single qubit in any basis located on the equator of the Bloch sphere. Again, the basis is selected by the state of a program qubit. In both cases the data and program qubits are represented by polarization states of photons. The experimental apparatus exploits the fact that two Bell states can be distinguished solely by means of linear optics. The outcome corresponding to the remaining two Bell states represents an inconclusive result.

Quantum dissipative dynamics of the magnetic resonance force microscope in the single-spin detection limit

We study a model of a magnetic resonance force microscope (MRFM) based on the cyclic adiabatic inversion technique as a high-resolution tool to detect single electron spins. We investigate the quantum dynamics of spin and cantilever in the presence of coupling to an environment. To obtain the reduced dynamics of the combined system of spin and cantilever, we use the Feynman-Vernon influence functional and get results valid at any temperature as well as at arbitrary system-bath coupling strength. We propose that the MRFM can be used as a quantum measurement device, i.e., not only to detect the modulus of the spin but also its direction.

On DNA Molecules as Quantum Measurement Devices

In this letter we reconsider the proposal of Ref. [1] about a quantum measurement performed by a DNA molecule in aqueous solution as a tool for illustrating specific difficulties of some approach to quantum measurement problem. Our main result is that, when the interaction of DNA and enzymes with aqueous environment is properly kept into account, no real problem appears for any specific model.

Programmable quantum measurement device that approximates all projective measurements on a qubit

AbstractA “universal” quantum measurement device is proposed. It can approximate any projective measurement on a qubit. The measurement basis is selected by the quantum state of a “program register''. The device is optimized with respect to maximal average fidelity. A contra‐intuitive result is that if two qubits in the same state are used as a program the average fidelity is higher than if the second program qubit is in the respective orthogonal state.

Quantum-controlled measurement device for quantum-state discrimination

We discuss a possibility to build a programmable quantum measurement device (a "quantum multimeter"). That is, a device that would be able to perform various desired generalized, positive operator value measure (POVM) measurements depending on a quantum state of a "program register". As an example, we present a "universal state discriminator". It serves for the unambiguous discrimination of a pair of known non-orthogonal states (from a certain set). If the two states are changed the apparatus can be switched via the choice of the program register to discriminate the new pair of states unambiguously. The proper POVM is determined by the state of an auxiliary quantum system. The probability of successful discrimination is not optimal for all pairs of non-orthogonal states from the given set. However, for some subsets it can be very close to the optimal value.