Loop state-preparation-and-measurement tomography of a two-qubit system

Abstract

We experimentally demonstrate that loop state-preparation-and-measurement (SPAM) tomography is capable of detecting correlated errors in a two-qubit system. We prepare photon pairs in a state that approximates a Werner state, which may or may not be entangled. By performing measurements with multiple different detector settings we are able to detect correlated errors between two single-qubit measurements performed in different locations. No assumptions are made concerning either the state preparations or the measurements, other than that the dimensions of the states and the positive-operator-valued measures describing the detectors are known. The only other needed information is experimentally measured expectation values, which are analyzed for self-consistency. This demonstrates that loop SPAM tomography is a useful technique for detecting errors that would degrade the performance of multiple-qubit quantum information processors.OCIS codes: (270.5585) Quantum information and processing; (270.0270) Quantum optics

Self-consistent tomography and measurement-device independent cryptography

“…The main point was to extend results of Refs. [10][11][12][13][14], which were meant to provide tests for small correlated errors in quantum computing devices. While debugging small quantum computers forms one application of our scheme, it fits measurement device independent cryptography [15][16][17][18][19] very well, and so we suggest our scheme can be fruitfully integrated with that protocol, too.…”

“…With multiple quantum systems there may be correlations between different measurements or between different state preparations as well. Those correlations, too, can be detected (again, without the need for reconstructing state and measurement operators) [12][13][14]. We focus here on the case of two qubits A and B and as usual we assume these are controlled by Alice and by Bob, respectively.…”

“…Scenario (i) was implemented experimentally recently [14]. Even if ξ is a product state, a Bell inequality may be violated [a "fake violation"] if Alice's and Bob's measurements are sufficiently strongly correlated.…”

Self-consistent state and measurement tomography with fewer measurements

“…Quantum tomography is an important tool for characterizing quantum systems and is useful for a diverse range of quantum information processing applications. It is useful not only for characterizing quantum gates [1,2], but also for tasks such as detecting errors in quantum key distribution [3,4] and quantifying the randomness or privacy of quantum-random-number generators [5,6].…”