We have measured the difference between the free energies of an isolated superconducting electrode with odd and even number of electrons using a Coulomb blockade electrometer. The decrease of this energy difference with increasing temperature is in good agreement with theoretical predictions assuming a BCS density of quasiparticle states, except at the lowest temperatures where the results indicate the presence of an extra energy level inside the gap.PACS numbers: 74.50.+r, 73.40.Rw, 74.25.Bt The key concept of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity [1] is the pairing of electrons. A surprising feature of the theory appears when one considers a macroscopic piece of superconducting metal with a fixed number of electrons TV. If TV is even, all the electrons can condense in the ground state. If TV is odd, however, one electron should remain as a quasiparticle excitation. In principle, if one would measure the energy required to add one electron to the superconductor, there should be a difference between the cases of even and odd TV. This fundamental even-odd asymmetry, which might vanish due to sample imperfections [2], does not manifest itself in conventional experiments on superconductors because these experiments are only sensitive to a finite fraction of quasiparticles. In this Letter, we report a new experiment based on singleelectron tunneling [3] with which we measured the evenodd free energy difference introduced by Tuominen et al. [4].Consider a superconducting-normal (SN) tunnel junction in series with a voltage source U and a capacitor C s (see Fig. 1), a basic Coulomb blockade circuit whose normal-normal junction version has been nicknamed the electron "box" [5,6]. The superconducting electrode which is common to both the junction and the capacitor is surrounded everywhere by insulating material. When the junction tunnel resistance R t is such that R t^> R[(=h/e 2 J the number n of excess electrons on this "island" is a good quantum number [3,7L The w-dependent part of the ground-state energy of the circuit, including the work done by the source £/, is given by En^Ecin -C s U/e) 2 + <£", where E c = : e 2 /2Cz is the electrostatic energy of one excess electron on the island, Cz the total capacitance of the island, and &" is the nonelectrostatic part of the energy of the island. For a normal island G n = 0 [ Fig. 2(a)], whereas for a superconducting island, one has S n -DoPn where Do is the energy difference between the odd-n and even-/? island ground states, and p n =nmod2[ Fig. 2(c)]. The BCS theory yields D 0 =A where A is the superconducting gap of the island. In equilibrium at zero temperature, n will be determined by the lowest E n and is therefore given by a staircase function of U [Figs. 2(b) and 2(d)]. In the normal case, the steps are of equal size, whereas in the superconducting case even-AZ steps are longer than odd-Ai steps. For Do> E c , the odd-n steps disappear, while for Do
